Chris Becomes a Hurricane, Large Disturbance in the Gulf of Mexico (Now Invest 96L)

Tropical Activity for June 21st, 2012

I took an extended break over the past couple of weeks, and now that I’m back the tropics are starting to heat back up despite being relatively early in the season. Tropical Storm Chris which formed yesterday has just been upgraded to the first hurricane of the 2012 hurricane season as it meanders well into the open waters of the north Atlantic. Meanwhile, there is a new disturbance organizing in the Gulf of Mexico which could be a more substancial player in the days to come. Read all about it in this tropical weather update.

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Hurricane Chris

Enhanced Infrared Imagery of Hurricane Chris

The hurricane season thus far has proven to be an early bloomer. After recording our first two tropical cyclones of the season in May, the season has continued its early season by providing us with the third named system in June and our first named hurricane, which is much earlier than normal. For a comparison, Irene was the first named hurricane in the 2011 hurricane season, which occurred in late August! In any event, the origin of Chris can actually be traced back to a non-tropical low pressure that developed along a decaying frontal boundary. As the front continued to decay, an upper level potential vorticity (PV) maximum moved over the non-tropical low pressure, and quickly allowed the low pressure to occlude and undergo warm seclusion as it detached from the frontal boundary and became vertically stacked with the upper level disturbance. Deeper convection then began to form around the circulation as the temperature gradient between the marginal sea surface temperatures (SSTs) compared to the anomalously cold upper level temperatures was sufficient for deeper convection. At this point, the system had a decent warm core, and the National Hurricane Center (NHC) felt that the deep convection and overall more tropical nature of the storm was sufficient for an upgrade. Beyond this time, the system has only become better organized, with an eye feature developing on IR as early as last evening. The visible imagery this morning is particularly impressive with spiral bands and moderate deep convection surrounding the eye in all quadrants. Despite the current well organized appearance, the system is quickly moving towards colder waters even less supportive of a tropical cyclone, so its unlikely Chris will be able to continue to intensify as an tropical system. However, there is a 60-70 m/s jet streak located to the west of Chris which is aiding in divergent upper level flow on the left exit region of the jet streak. This will probably allow Chris to transform into a powerful extra-tropical cyclone as it moves over the cooler waters of the north Atlantic.

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Gulf of Mexico Disturbance (Now Invest 96L)

Visible Imagery of the Gulf of Mexico Disturbance

With Chris moving out of the picture, the next threat turns to something closer to home that is beginning to organize in the Gulf of Mexico. A very large circulation is developing in the Gulf of Mexico. Currently, we have a broad area of 850 hPa vorticity that is elongated from the Yucatan Peninsula up into the east Gulf of Mexico. This area of vorticity is associated with deep convection, although it is displaced somewhat to the south and east of the vorticity, which is obvious looking at the visible loop above. The convection is also associated with a developing upper level anticyclone, which is helping to ventilate the convection in the upper levels. The biggest questions in the coming days is what will this large broad circulation do, and who will be affected.

ECMWF Forecast for the large-scale circulation that develops over the Gulf of Mexico. Plotted are 500 hPa absolute vorticity (shaded) and geopotential heights (white contours). Annotated features are highlighted.

Animated above is the 00z ECMWF showing absolute vorticity and geopotential heights. I have highlighted the relevant features that will influence the development and track of this feature for the next 5 days. First, I want to draw your attention to the orange shaded areas of cyclonic vorticity. These represent the multiple mesovorticies that are rotating around the larger circulation that is centered over the Gulf of Mexico. While initially these areas of vorticity are small and diffuse, they start to merge over the Eastern Gulf of Mexico east of the mean low level circulation center by day 3. This type of circulation evolution is similar to how tropical cyclones develop in large gyre-like circulations that traditionally occur in the West Pacific Basin. For more reading on these types of cyclones, there are several good papers by Lander (1994) and Molinari and Vollaro (2012) that document the development and evolution of these types of systems. While the gyre circulation is relatively large, it moves very slowly for the first 3-4 days, as the upper level ridge attempts to develop eastward across the deep south in the wake of a weak trough. However, by the end of the forecast period, the upper level ridge gives way, and the westerly flow associated with a series of weak upper level troughs starts to pick up the developing tropical cyclones in the Eastern Gulf of Mexico. A word of caution. While this is one solution depicted from the ECMWF, the forecast genesis of a tropical cyclone (TCG) is still a big unknown. It all depends on which area of vorticity becomes the dominant feature and where these individual mesovorticies decide to merge. The spatial scale of most of the global models is simply too coarse to predict which convective elements will influence the development of this feature. The GFS provides a different solution, with vorticity becoming focused east of the Florida coastline rather than in the Gulf of Mexico. With the potential TCG of this event still a good 2-4 days away, there will probably be large jumps in the modeling, as the models get a better handle of the convective elements rotating around the larger circulation. At this point, there is no done deal that this system will evolve into a tropical cyclone, and I’m in support of the NHC’s outlook of a 30% chance of tropical cyclone development in the next 48 hours. I do think the chances for TCG do increase beyond that time frame, however. Given the broad nature of this disturbance, development will be very slow

GFS forecast at 54 hours with Standardized Anomalies of Precipitable Water (shaded) and 700 hPa winds (wind barbs)

One thing that is probably a safe bet though is torrential rainfall from the Yucatan coastline of Mexico across into Cuba and most of Florida. This circulation will be tapping into a precipitable water environment that is 3-5 standard deviations above average. This moisture originates from moisture originally drawn up from the intertropical convergence zone (ITCZ) that moved northward across Central America after the landfall of Hurricane Carlotta. Thus, while TCG may still be up for debate at this point, the threat for life-threatening flooding rainfall across the large area is a very real concern on the south and eastern regions of this large scale circulation. Even this morning, a sounding from Key West, FL (KKEY) showed total precipitable water at 2.43″ which is near its record value. This is very impressive considering how moist south Florida typically is this time of the year. This very deep and moist atmosphere will only continue to spread northward with time as the low level flow associated with the circulation continues to spread an influx of moisture from Central America northward.

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Aside from these two features, there isn’t much else happing, and the East Pacific is quiet after Carlotta hit last week. I’ll keep you updated on the latest in the Gulf of Mexico, and until then enjoy the rest of your work week!

Invest 94L Next Threat in the Atlantic, Hurricane Bud Takes Aim at Mexico

Tropical Activity for May 24th, 2012

Despite being a full week shy of the official start of hurricane season, things are once again heading up in the Atlantic basin, with a new invest 94L that has the potential to become our next named system. In the East Pacific, Hurricane Bud has intensified to a category 2 hurricane as it approaches the Mexican coastline. All of this in more in this tropical weather outlook below.

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Invest 94L

Visible Imagery of Invest 94L

Hurricane season won’t officially start until June first, but thats not stopping the most recent development in the Atlantic basin. Invest 94L developed yesterday through a persistant but disorganized area of convection located over the Caribbean Sea and Central America. An upper level impulse has managed to pick up this deep moisture fetch, which has resulted in heavy rain  just offshore of Florida. Some rotation is also noted, although at present, the circulation is not organized enough to qualify as a tropical or subtropical cyclone. Part of this lack of organization is due to over 70 knots of wind shear that is directly overhead of the circulation. At this current value, the environment simply can’t support any type of tropical or subtropical formation. However, convection is currently being supported by upper level diffluence coupled with speed divergence which is providing a forcing mechanism via jet dynamics. So what is in store for this particular invest? After all, the National Hurricane Center (NHC) has already updated their outlook on this system giving it a 40% chance of tropica/subtropical genesis in the next 48 hours.

GFS Forecast from 12-72 hours. Plotted is the dynamic tropopause (shaded every 20 hPa) and the total winds at the dynamic tropopause (wind barbs). 925-850 layer mean relative vorticity is depicted by thin black contours.

Both the GFS and ECMWF are depicting development with this invest, but the evolution that leads to genesis is fairly complicated. I’ll try to break down each 12 hour period in the graphic above. I’ve plotted here the dynamic tropopause which should give a sense of the overall height of the troposphere. Colors that are gray shaded represent a very vertically deep tropopause thats typically experienced in the tropics, while cooler colors represent where upper level impulses have caused southward protrusions of stratospheric air limiting the size of the tropopause. This evening, note the upper level trough that is swinging through the southeast. This feature is associated with the sub-tropical jet which is distinct from the polar jet located further north over North America. Over the following 24 hours this trough will split, with the northern piece riding over a subtropical ridge, while the southern portion will fracture and merge with an already established piece of low-level relative vorticity associated with Invest 94L (red circle).

Figure 16 From Davis (2010) which identifies one potential flow evolution that result in the development of a subtropical cyclone.

Chris Davis wrote a paper in 2010 that discussed the development of subtropical cyclones through the interaction of the subtropical jet with tropical air. In the paper, subtropical cyclone formation results from the initial genesis of convection over the tropical regions that then interacts with an unstable subtropical jet. As the jet becomes unstable, it develops large undulations (partially the result of convection feeding the ridge downstream of the digging jet). This feedback process continues until a the resulting trough fractures from the main flow and becomes a cutoff cyclone.

ECMWF 54 Hour Forecast at 1800 UTC 26 May 2012. Plotted at 925 hPa is relative humidity (shaded, every 10%) and heights (black contours, every 10 m). Plotted at 200 hPa is wind speed (shaded, every 10 knots) wind direction (vectors) and heights (white contours, every 60 m)

This presents a favorable regime for a subtropical cyclone because the upper level cyclone shelters the storm from the higher shear located further north and south associated with upper level jets (pictured above from the ECMWF). The other resulting effect is lowering the dynamic tropopause (DT) by 100-200 hPa. This is significant because this also forces the minimum temperature associated with the top of the troposphere to shift downward in height, resulting in colder temperature anomalies in the mid-upper levels of the troposphere. This is evident by looking at the 500 hPa temperature standardized anomalies, with values being near 1 standard deviation below normal near the cyclone center. Temperatures near the Florida coastline are hovering at around 26-27 degrees Celsius, which is marginal for tropical cyclone development. However, the colder temperatures aloft will compensate somewhat, allowing for convective generation despite the less than favorable thermodynamic environment.

Trackwise, most of the track guidance is taking 94L off to the northeast for the first 24 hours, but as the storm merges with the southern portion of the upper level trough, it will abruptly shift westward, with several models having the system make landfall in the next 72 hours. Now the big question, will this system develop? At this point, we have pretty good agreement between the GFS and the ECMWF. However, both models preformed poorly forecasting the genesis of Alberto. Thus, I remain cautious despite their agreement, and will stay with the NHC’s call for 40% chance of development in the next 48 hours. However, this probability will likely need to be raised if we see the following evolution outlined above.

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Hurricane Bud

Visible Imagery of Hurricane Bud

Hurricane Bud has rapidly intensified over the past 24 hours from a moderate tropical storm into a Major Hurricane. Maximum sustained winds are at 115 mph with a central pressure hovering around 965 hPa. Microwave imagery depicts and impressive storm with a well defined eye embedded underneath the cirrus canopy. No doubt the lower shear from a local anticyclone aided in the intensification of this storm, combined with the warm sea surface temperatures in this region. Due to the storm being more vertically coherent than expected, the storm has been moving more along with the mid-upper level flow, which continues to posses a large weakness in between two sub-tropical highs. This has allowed Bud to maintain its northeastward motion and it now seems likely that the system will make landfall along the Mexican coastline in the next 24 hours. Most of the track guidance has shifted around to this track. Intensity wise, the storm has likely peaked in intensity as vertical wind shear will start to ramp up, as upper-level westerly flow intensifies as the storms forward motion slows. This is reflected in the intensity guidance, with a faster demise commencing beyond 24 hours as land interaction with the high terrain of Mexico causes the low-level circulation to decoupled from the mid and upper level centers of the storm. Overall, even with the expected weakening, Bud will likely still be a hurricane at landfall, making it the first Hurricane to do so in the East Pacific in the month of May since Adrian in 2005. Folks near Manzanillo and points west should hunker down over the next 24 hours, conditions will be rapidly deteriorating from here on out.

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And this wraps up another tropical weather outlook. I’ll provide an update on both Bud and 94L tomorrow. Thanks for your time!

Hurricane Rina in the Northwestern Caribbean, Invest 97L in the Central Caribbean

Tropical Activity For October 25th, 2011

Its been a while since my last update, but the tropics have heated up again for perhaps one last hurrah in the Atlantic Basin. Currently we have one intensifying hurricane, Rita, which is slowly moving towards the west-northwest in the Northwestern Caribbean. In addition, we also have Invest 97L which is right on the heels of Rina. I’ll go into each system in detail.

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Hurricane Rina

Enhanced Infrared Loop of Hurricane Rina

After the late September activity of Ophelia and Philippe, Rina is the first tropical cyclone to develop in nearly a month in the Atlantic basin. The storm rapidly intensified from a tropical depression Sunday Afternoon into a category 1 hurricane by early yesterday afternoon. The reason for such abrupt intensification could be attributed to both the very high Sea Surface Temperatures (around 30 degrees Celsius), oceanic heat content, combined with lowering vertical wind shear over the system thanks to low 200mb wind flow. Currently, the storm is in the process of developing an eyewall on microwave imagery. As this eye continues to become better defined, we should see Rina continue to intensify over the next 12-24 hours. It is beyond this period where both the track and intensity forecast for Rina become more tricky.

500mb Vorticity and Heights from the 00z ECWMF

First of all lets look at how the synoptic environment evolves over the next 4 days. First of all Rina is being steered by a subtropical ridge to the northeast over the Western Atlantic Ocean. With the absence of any major troughs currently, Rita is expected to slowly move towards the northwest. Most of the track computer model guidance shows this solution in the short term. However, beyond 48 hours, things diverge quite quickly. There are still a handfull of models that recurve Rina into Florida, although at what state the storm in is a very big question mark. The ECWMF literally has no reflection of the system at 500mb by 96 hours, a good sign that the system has either become shallow with its convection stripped off, or dissipated. This will cause the system to be steered by the low level flow, which at the later time frames becomes light and variable. There is some question as to if the ECWMF is loosing the system too quickly, because it hasn’t had a very good representation of the system thus far, and its already had a hard time with moderate intensity systems this year (Philippe was a good example). Thus, I’m putting a little more weight towards the GFS solution in the long term, which shows the low level vortex of Rina stalling out just north of Cuba towards the end of the forecast period. As for any potential landfall impacts, Rina may get close to the Yucatan coastline, but in the end, I think it will be a near miss, with the small circulation passing through the Yucatan Channel between Cuba and Mexico.

200mb Winds and Heights from the 00z ECWMF

Intensity wise, things also become more tricky beyond 48 hours. In the short term, intensification seems likely as the upper level winds are likely to remain low enough to allow steady development. Most of the intensity guidance is showing a peak between category 2-3 and I see no reasons to go against this forecast given the current favorable conditions in place. Beyond 48 hours, however, conditions start to slowly go downhill. Shear will gradually increase as the system moves out from under the protective shield of the upper level ridge. The edge of the ridge will actually begin to induce southeasterly shear before the upper level westerlies further north start to cause a more substancial impact on the system. Thus, I think we won’t see a major decline in intensity until the system reaches the Yucatan Channel, where it appears the storm will get ripped apart. While the GFS maintains a robust system even by the end of the forecast. This doesn’t seem physically possible as the storm meanders in the face of 40+ knots of shear.

My First Call For Hurricane Rina

With all that said, here is my forecast for Rina for the next 120 hours. In the short term, intensification should continue, although I’m a little bit less optimistic on rapid intensification due to the slight bit of southeasterly shear already restricting the outflow on Rina’s southern flank. Beyond 48 hours we should see a gradual weakening trend as the storm makes it closest approach with the Yucatan near 72 hours. Note that my cone of uncertainty does have a decent portion over the Yucatan, so it wouldn’t take much of a nudge westward to create a landfalling situation. Once the storm reaches the Yucatan Channel, vertical wind shear will really start to do a number to the system, and by 120 hours, I don’t think we will have anything more than a remnant low meandering near Cuba. Overall confidence is high in the short term portion (24-48 hour) of this forecast, but drops off pretty quickly thereafter.

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Invest 97L

Infrared Loop of Invest 97L

In addition to Rina, there is another area of disturbed weather way in the Central Caribbean Designated Invest 97L. Thus far, the system has remained convectively active as seen above, although there are no obvious signs of a developing low level circulation on microwave imagery. Vertical wind shear over the storm is very low, as the system is situated under a strong upper level anticyclone. It seems that the main problem with the system at this time is that most of the low level vorticity is stuck near the Central American coastline, and not co-located with the convection. Thus, it will likely take some time for this system to organize despite the favorable upper level winds and warm Sea Surface Temperatures. While the statistical intensity guidence shows this system developing rather quickly, I’d through caution that these models assume the storm already has a well defined circulation, which is not the case as of yet. I think the system may have better luck when it gets over more widespread open waters in the Southern Caribbean beyond 36 hours. Overall, I’m giving the system a 30% chance of development in the next 48 hours, with the most likely time for the system to develop towards the end of this forecast period.

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And that’s all I have for this update. Until next time!

Ophelia Battling Shear, Invest 90L Forms, Disturbance in the Bahamas could provide source of more East Coast rains; Hurricane Hilary in the East Pacific

Tropical Activity For September 24th

Its been quite a while since my last post, as I have gotten pretty busy lately. However, I did want to post an update this morning on the Atlantic tropics while also taking a quick look a small but potent hurricane ongoing in the East Pacific. Ophelia, which formed a few days ago, is getting sheared pretty decently right now. On its heels is another strong tropical wave labeled Invest 90L, which looks poised to become the next tropical cyclone. Meanwhile there is also a small tropical disturbance over the Bahamas that might enhance rainfall across the East Coast. Lets drill down into the details.

Tropical Storm Ophelia

Infrared Satellite Loop of Tropical Storm Ophelia

First in the Atlantic, lets talk about the moderately sheared Tropical Storm Ophelia. Yesterday in the early to mid morning, Ophelia looked very unhealthy with deep convection displaced well away from the center which was completely exposed in a large region of dry air. However, despite the strong shear and dry air over the system, convection re-developed this afternoon and allowed a short burst of intensification. The reason for this convective resurgence might have a bit to do with a convective coupled phase of a kelvin wave (CCKW) moving overhead of the system currently. During the active phase there are low level 850mb westerly anomalies that precede 200mb easterly anomalies which help to lower the net shear over a particular area with divergence aloft aiding in convective development. This wave was rather strong over the East Pacific, but has been weaker more recently. None the less, the relatively unexpected reduction in shear along with divergence assisting convection aloft may have helped to spur a resurgence of convection over Ophelia today.

ECWMF 48 Hour 200mb Heights and Winds and 850mb Heights, Winds, and Temperature

However, currently, the system still remains rather sheared, and its likely that shear will remain over the system for the next 48-72 hours as an upper level low develops and then moves in tandem with the system, maintaining westerly upper level flow. Seen above is the ECWMF evolution of the low and upper level features at 48 hours. In essence you can’t get a much more unfavorable vertical wind shear pattern for a tropical cyclone, with a strong upper level low situated just west of Ophelia is combined with a nearly as strong low level ridge which is forcing Ophelia to continue moving west into this upper level low. Thus, I don’t think Ophelia will survive such a hostile environment, and I am predicting dissipation somewhere in the next 24-48 hours as the storm continues to track towards the west-northwest. I don’t have the time of desire to issue a forecast track for Ophelia, and I feel it will pose little threat to the Lesser Antilles or any other major land masses during its lifetime.

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Invest 90L

Visible / Infrared Loop Combo of Invest 90L

After a bit of a lull in the past week, the Atlantic basin keeps producing new invests. The latest one is Invest 90L and it looks already likely to become a tropical cyclone very shortly. As you can see above, convection is starting to take a banded appearance with a well defined curved band forming along the NW side of the budding low level circulation. This presentation is confirmed further by Microwave Imagery. The upper level winds do look favorable for development over the next 24-48 hours, as the system is situated on the southern side of an upper level anticyclone which is producing low shear with easterly upper level flow. High total precipitable water near the system also supports development with most of the dry air confined to at or above the Cape Verde islands. Finally, sea surface temperatures in the region are fairly warm in the 28-29 degree Celsius range although these do drop off a bit further to the northeast. Given all of the positive factors for development along with the improving convective appearance, I think its highly likely a tropical cyclone will form in the next 24 hours, and I am giving Invest 90L a near 100% chance of development in the next 48 hours. Despite my bullish short term forecast, the long term prospects for this system are not nearly as good, as most of the modeling takes the system further northwest into a more hostile environment characterized by high shear and lower SSTs. Thus its window for intensification is short and it will have to develop quickly, potentially rapidly if this system is to become a hurricane.

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Bahamas Disturbance

Infrared Image of the area of disturbed weather over the Bahamas

Yet another tropical disturbances exists over the Bahamas currently. This system formed from a tropical wave interacting with an upper level feature. At the upper level feature slowly died, anticyclonic motion began to increase over the system as the divergence continued to persist. Currently, we see a small anticyclone persist over the convection as the system is sandwiched in between two upper level troughs. While these upper level conditions are favorable for the system at the moment along with warm sea surface temperatures, the 850mb vorticity does not seem very well organized and elongated north-south across the convection. In addition, the ECWMF seems to suggest that this feature is about to be swept up in the strong southerly flow ahead of a strong mid-latitude low. While this is normally not favorable for tropical cyclone development as the vorticity continues to be too elongated for a well defined low level circulation, it will likely aid in moisture transport into the east coast representing another surge in tropical moisture across the I-95 corridor for this weekend. Thus, despite the initial favorable conditions, I am only giving this disturbance a 10% chance of tropical cyclone development, in agreement with the NHC, due to the aforementioned expectation that the flow will continue to elongate the circulation. However, expect torrential downpours and flooding potential once again for the Eastern United States coastline as this feature adds to the total precipitable water and overall rain shield.

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Hurricane Hilary

Enhanced Infrared Satellite Loop of Hurricane Hilary

While I normally don’t discuss much with systems that occur over the Eastern Pacific, this one deserves an exception. Hurricane Hilary has been a very powerful mid range Category 4 hurricane over the last 24 hours. In this time, it has maintained a very small inner core with hurricane force winds estimated to extend no further than 25 miles away from the eye. Microwave imagery has also been spectacular showing a very small eye and storm core representing the maximum winds. The tropical microwave imager (TMI) is a polar orbiting earth satellite (POES) that has both a passive microwave imager (that produces similar imagery to the previous link), as well as an active precipitation radar that is able to actively measure the intensity of precipitation in the tropics in small swaths. This satellite was able to make a direct pass over Hilary today showing a very well defined inner core, although it appears the eye has opened up a bit to the northwest as of 1z. While Hilary looks very impressive right now, it looks like it may have a future as a less organized system as upper level southerly flow increases (shear has already trickled up to 15 knots). In addition, the current forecast track takes the system over much cooler SSTs by the end of the forecast period, which would likely result in rapid weakening. The models are in great disagreement in the long range on if Hilary will or will not re-curve, with a lot of the disagreement steaming from how vertically deep will the system be in that time frame. At this point, I think Hilary despite its intensity is still a rather fragile system, and it may react very negatively to the chances in its forecast environment. If Hilary weakens more than expected in the next 2-3 days, it may take a more southerly track given its weaker vertical structure feeling less of an influence to upper level features such as troughs or a weaker ridge. However, if Hilary maintains major hurricane intensity for a long duration of time, its vertical structure will make it more susceptible to these same upper level features making a re-curving solution more likely. In any event, the potential for a significant hurricane (category 2 or above) impacting Baja California are decreasing given that none of the guidance shows an impact in a region where the storm won’t have to traverse less than ideal sea surface temperatures.

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And this concludes another tropical weather update. I can’t promise on when the next time I’ll be able to update this due to work and other matters, but I promise if anything really interesting is going on I’ll make quick updates. Until then!

Katia Moving Slowly Northwestward; TD#14 Forms; Invest 96L Forms

Tropical Activity For September 6th, 2011

I sound like a broken record at this point, but the hits keep coming. In addition to Hurricane Katia which is likely to pass in between the United States and Bermuda, we now have a new depression, TD#14. In addition, we also now have Invest 96L which looks likely to become the next system in line to become a tropical cyclone.

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Hurricane Katia

Visible Loop of Hurricane Katia

Katia has now been with us for more than a week, but it seems that it is now past its peak. Yesterday, Katia looked very well organized with a large distinct eye surrounded by very deep convection. At that time the storm had 115 knot winds (135 mph). Today, however, this does not seem to be the case, as the storm completes an eyewall replacement cycle (EWRC) which had degraded the appearance of the storm, while also stretching out the radius of maximum sustained winds.  Recon recently made a pass into the storm and found the pressure was actually a bit higher at 962 mb with flight level winds that supported minimal Category 3 intensity. However, SFMR winds were much lower around 80 knots. Thus for a storm undergoing an eyewall replacement cycle, the winds are likely lower than the flight level winds would normally suggest, so the storm has been downgraded to a Category 2 storm with 90 knot (105mph) winds. Trackwise things have become much clearer over the last 24-48 hours, with all of the track guidance showing a sharp turn forecasted to occur between the 48-72 hour period.  Thus, the chances of a United States landfall seem to be very low at this point. Intensity wise, most of the guidance suggest that Katia has peaked and should continue a slow weakening trend over the next 24-48 hours. I have no reason to go against this guidance as conditions will be becoming gradually less favorable as sea surface temperatures decrease and vertical wind shear increases from the southwest. Towards the end of the period the storm will be rapidly accelerating to the northeast and will likely become extra-tropical. I won’t issue a new forecast for this time at this time, but will continue to update the progress of the system as it progresses further north. It looks like the only main threat will be to the shipping lanes rather than any land masses as a tropical cyclone.

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Tropical Depression #14

Nighttime Visible Loop of Tropical Depression #14

Hot on the heels of Katia is another Atlantic Basin tropical cyclone. Tropical depression #14 formed this afternoon from a large disturbance associated with a tropical wave that interacted with the inter-tropical convergence zone (ITCZ). The storm looked rather impressive today on visible imagery, with evidence of banding starting to occur around the center. Microwave imagery shows much of the same, although the convective activity still looks a touch disorganized. Shear over the storm currently appears to be around 10-15 knots although a broad anticyclone aloft has persisted over the cyclone. Total precipitable water over the system seems pretty high which should support continued convective development in the short term. The National Hurricane Center is forecasting gradual development of this system into a minimal hurricane by the end of the forecast period in 120 hours as the storm moves mainly on a west-northwest heading. However, I am not in agreement, at least intensity wise.

850mb heights vs. 500mb heights at select time intervals from the 12z ECWMF

First of all, lets look at the track. Here I have compared the 850mb level, which best represents the low level flow that steers shallow systems, with the 500mb level, which represents the mid-level flow that steers more vertically deep systems. Notice that at 24 hours, both the mid-level center and the low-level center of TD#14 are still pretty well stacked, although there is already a bit of separation starting to occur. This separation is due to the low level flow strengthening. Part of the reason why the ECWMF seems to want to intensify the low-level flow may be due to a possible subsident phase of a kelvin wave that is moving through the region over the next 24 hours. A subsident phase of a kelvin wave last week affected Katia in a negative way, creating shear over the system a full 24-48 hours earlier than expected and allowing dry air to advect into the circulation and halt intensification. As the positive outgoing longwave radiation (+OLR) moves over TD#14 it will tend to suppress convective development and allow the low level easterlies to increase in magnitude, hence the stronger low level flow. Thus by 48 hours, the low level circulation as represented at 850mb is running out ahead of the mid-level circulation. This is also obvious at 72 hours. Thus, despite the track guidance showing a path to the west-northwest in agreement with the mid-level flow, I think we will see TD#14 take a more westward path more along with the low-level flow, putting it on the southern end of the track guidance.

200mb wind vectors and magnitude from the 12z ECWMF

Intensity remains tricky as well. While the flow currently over the system seems to be anticyclonic which should promote low shear, the increasing low level flow might cause shear due to the fact that the low level flow might move faster than the upper level flow, even if the wind vector is in the same direction. While most of the intensity guidance points towards a gradually intensifying tropical cyclone over the next 3-5 days, the shear beyond 24 hours might halt strengthening. By 72 hours, the ECWMF is depicting an increasingly unfavorable upper level pattern as a shear axis starts to develop to the north and west of TD#12. Part of this shear axis is caused due to the strong outflow associated with Katia. Overall, this entire setup does not lead me to believe robust development is possible, and I think TD#14 will peak as a weak to moderate tropical storm in the next 24 hours before the shear increases.

My Track and Intensity Forecast For TD#14

Given all of the thoughts above, here is how I think things will play out. TD#14 will likely still intensify modestly in the next 24 hours before the shear gets too strong. However, I’m not expecting more than a weak to moderate tropical storm at best. Beyond 24 hours, the system will gradually weaken and likely maintain a fast west-northwest track as the low level flow steers it further westward than most of the model guidance suggests at this time. There is much uncertainty in the long range beyond 72 hours. The system will likely slow down after it enters the Caribbean, but I don’t know how much will be left of the system by that point. While I have maintained it as a minimal tropical storm at this time frame, the system could easily dissipate beyond 72 hours. It is worth mentioning that the GFS is far more bullish with this system in comparison to the ECWMF. However, looking at the upper level flow, it seems to be unrealistically intensifying TD#14 in the face of pretty strong southerly flow. I’ll keep you guys updated on the latest with this system.

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Invest 96L

Visible Loop of Invest 96L

And now we have Invest 96L. This system was declared earlier today after recently organizing a ball of convection and at least a modest circulation. Based on the visible loop above, you can see it still has a ways to come in the organization department. Microwave imagery also looks rather meager at the moment, with not much deep convective elements near the greatest rotation. Despite this, shear is very low over the circulation (5 knots or so) with a well defined anticyclone over the system. This is important, because there is some very dry air in close proximity to the system, and any northerly shear would likely advect this air into the circulation, choking off the convection. However, as long as the shear remains low, this dry air will stay away from the core circulation and we should see further organization. In fact, having such a large area of dry air may prevent the storm from growing too large, allowing for faster genesis as a small circulation. In addition to favorable shear, sea surface temperatures are very warm near the storm, and would support robust development once an inner core is established. With all of these favorable factors identified, its not surprising to see both the GFS and the ECWMF very bullish for development by 48 hours. Thus, I think its very likely this will become our next tropical cyclone, and I am giving Invest 96L a 80% chance of development in the next 48 hours, which is far higher than the NHC’s current probability of 30% in the next 48 hours.

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And that’s all I have for this evenings outlook. Check back later this week for another update!

Quick Update: Katia A Major Hurricane; Invest 95L Forms; Gulf of Mexico Disturbance

Tropical Activity For September 6th, 2011

Good Morning! I just wanted to post this quick update on the tropics. I’m pressed for time at the moment, so I’ll just create a quick list of items covered above and will have a more full update later this afternoon.

• Hurricane Katia became a major hurricane yesterday and has peaked in intensity as a 115 knot (135mph) storm. The storm is all but assured to re-curve out into the open Atlantic and not affect the United States, although there is an outside chance it could affect the Canadian Provinces.

• Invest 95L has formed out in the Atlantic, and has the potential to become our next named storm, before the low level flow starts to outrun the upper level flow beyond 24 hours. I am giving it a 50% chance of development in the next 48 hours.

• A new disturbance has formed in the Gulf of Mexico, and also seems to have a decent chance to become a tropical cyclone in the next 48 hours.  I am giving this system a 30% chance of development in the next 48 hours.

Again I’ll have a full update later in the day. Until then!

Quick Update: Katia Struggles Against Shear and Dry Air; Invest 94L Looks to Become a Tropical Cyclone

Tropical Activity For September 1st, 2011

Here is a quick update on the tropics early this afternoon. Katia is still a hurricane, but is battling the combination of shear and dry air, and intensification is no longer expected until these unfavorable conditions can be overcome. Invest 93L remains poorly organized, but a NOAA plane is currently investigating the disturbance to see if it has become organized enough to be considered a tropical cyclone. Finally, Invest 94L sprung up from the area of disturbed weather I mentioned a few days back and is likely to become a tropical cyclone in the next 12-24 hours. I’ll do a quick little update for each storm, but won’t issue any track or intensify forecasts and mainly focus on the current conditions.

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Hurricane Katia

Visible Loop of Hurricane Katia

Katia pretty much did the opposite of what I expected last night. After looking rather impressive on microwave and satellite imagery, the organization of the storm collapsed somewhat last night as a dry air intrusion caused partially by southerly shear helped to cause the low level circulation to become partially exposed. This presentation continues currently as witnessed above in the visible loop. It seems that the southwesterly vertical wind shear that was expected to develop over the storm beyond 24 hours has occurred earlier than expected thanks to the initial dry air intrusion, and I no longer expect Katia to become a Category 2 hurricane. In fact, it will struggle to maintain minimal hurricane intensity for the near future. Katia does have a large circulation though, and I don’t expect the wind shear to become too strong to cause dissipation, but I think its now unlikely that Katia will be able to intensify into a category 2 or beyond hurricane until the current upper level trough to the west of the system has lifted out.

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Invest 93L

Infrared Visible Loop Combo for Invest 93L

Invest 93L continues to remain poorly organized. While convection continues to be robust to the east of the center, the broad circulation is continuing to experience 30-40 knots of vertical wind shear which is preventing any sort of major organization taking place. The circulation does appear a bit better organized on microwave imagery, however. In addition the atmosphere continues to remain very moist and divergence over the convection continues. If the storm can become better organized, it will also be able to tap the very high oceanic heat content located over the Central Gulf of Mexico. As for now though, I remain somewhat pessimistic on the development prospects of tropical cyclogenesis in the next 48 hours, and remain below in probability of the NHC’s forecast.

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Invest 94L

Visible Loop of Invest 94L

Invest 94L has developed from the area of disturbed weather that has been persisting off the Atlantic coast for the last couple of days. I mentioned this system several days ago in my tropical weather outlook. Since then its obvious that the low level circulation has become very well defined on both visible imagery above, and on the latest microwave pass. There was some question earlier asking if this system was warm core, but it appears that in the latest AMSU cross section that the storm has a significant warm core. Given the current presentation on satellite along with microwave imagery, I think its fairly likely that this system will be upgraded to a tropical cyclone, likely a tropical storm given the 35 knot plus winds found on a recent ASCAT pass. Thus, I am giving 94L a 90% chance of tropical cyclone development in the next 48 hours, which is substantially above the NHC’s outlook of 50% currently.

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That’s all for this mini-update today. I’ll try to push forth a more substantial update tonight if I have time.

Katia Becomes a Hurricane; Invest 93L Forms in the Gulf of Mexico

Tropical Activity For September 1st, 2011

The tropics in the Atlantic basin continue to remain active. Katia has attained minimal hurricane intensity, while Invest 93L has developed in the Gulf of Mexico from the area of disturbed weather I mentioned yesterday. First I’ll focus on Katia and then Invest 93L

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Hurricane Katia

Visible Loop of Hurricane Katia

Katia has fought bouts of dry air today, but has still managed to intensify regardless. Last night after I issued my first forecast, it became evidence that the system was going to entrain more dry air than expected looking at a microwave image. These dry air intrusions continued through the day today as it was continually evident on microwave imagery. However, despite this, wind shear remained low over the storm and continued to support slow development. Finally tonight deep convection was able to wrap around the northern part of the storm allowing Katia to become a hurricane. Deep convection continues to be located near the center which should promote development as long as the system does not experience any more dry air intrusions. For now upper level winds remain favorable for continued intensification, but there are upper level westerlies on the horizon. Sea surface temperatures also continue to remain warm (28/29 degrees Celsius).

500mb heights and vorticity from the 12z ECWMF

Trackwise, things continue to remain pretty straightforward. Throughout the period the track of Katia will be dominated by a strong mid-level ridge which will track westward with the storm. This should leave the storm on a west-northwest track on the same heading for the next 72-96 hours. This is the general consensus of most of the track guidance. Towards the end of the forecast period, it does seem like Katia will slow down as the mid-level ridge to its west weakens thanks to it pushing up against a decaying trough. This trough will leave open a weakness that the next trough swinging in from the Great Lakes will replace. Beyond 120 hours several solutions remain on the table. Katia may still be picked up by this second trough, but another possible solution is that the weakness to the west of Katia will not be enough to cause Katia will move far enough north to be picked up by the second trough. Given the latitude of Katia already expected by 120 hours, it still seems likely that Katia will be eventually steered out to sea, but it may take a few more days than expected as the storm essentially slows to a crawl by the end of the forecast period. At this time, I continue to think that the chances of a US impact from Katia are low.

200mb wind vectors and magnitude from the 12z ECWMF

Intensity continues to be the trickier aspect of the intensity. While Katia remains under favorable conditions currently, an upper level trough to the west of the system will begin to impinge on the outflow by 24 hours. From 48-96 hours, Katia will likely experience some southwesterly flow which could halt the intensification process of the first 24 hours or even induce some weakening. By the end of the forecast period, this upper level trough starts to lift out and the anticyclone will be able to rebuild over the system as it slows down. The National Hurricane Center (NHC) has been mentioning in their discussions that Katia outflow will be able to diminish this unfavorable flow. I am not that convinced that Katia has quite a large enough envelope to have this effect, especially considering how much larger the upper level trough is in relation to Katia. Thus, I disagree with the NHC’s intensity forecast beyond 24 hours. While most of the intensity guidance suggests significant strengthening, beyond 24 hours, I am leaning towards the lower portion of this guidance.

My Forecast Track and Intensity For Hurricane Katina

Given the thoughts above, here is my updated forecast for Hurricane Katina. I was incorrect in forecasting rapid intensification in the past 24 hours, but outside of the wild card of dry air intrusions, I think all factors are set, including the small central dense overcast (CDO) that has formed over the center for a quick burst of rapid intensification. However, this process beyond 24 hours will likely be halted as southwesterly upper level flow cuts into the storm and starts a temporary weakening process. I have the storm dropping to 90 mph, but this could be slightly lower or higher based on the amount of shear that transpires. Finally, by the end of the period I have the storm re-intensifying back into a Category 2 storm as the upper level impulse that produces the shear lifts out and allows the ridge to build back in. Track-wise, I’m close to the NHC’s track, except a little faster initially and further to the north. Once again I’m more confident in the track forecast than intensity, but I am reasonably confident that there will be some degree of shear that will halt  the intensification process of Katia.

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Invest 93L

Enhanced Infrared Loop of Invest 93L

And we continue to be on a never ending train of invests. The latest is Invest 93L which developed out of the area of disturbed weather in the Caribbean that has moved northward into the Gulf of Mexico. Thus far, the invest continues to remain poorly organized with plenty of deep convection scattered about across the Gulf of Mexico. Latest microwave also does not show much organization with the convective pattern. The reasoning for the poorly organized appearance currently is very high shear (30-40 knots) over the system which is blowing off the convection to the north well to the east. Divergence over the system remains strong, which explains the convection continuing to fire despite the strong wind shear. The convection is also supported by very high sea surface temperatures (above 31 degrees Celsius) and plenty of moisture. In addition, both the ECWMF and the GFS are showing development beyond 48 hours in the Gulf of Mexico near Louisiana and Texas. However, given the current disorganized nature of Invest 93L and the high shear over the system that is expected to continue to some degree, I am led to believe this will most likely develop after the 48 hour period. Thus, I am only giving Invest 93L a 40% chance to develop into a tropical cyclone in the next 48 hours, which is lower than the current NHC prediction.

 

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And that wraps up today’s tropical weather outlook. I’ll try to have another update later this afternoon or tomorrow if I have time.

Irene Makes First Landfall Near Cape Lookout at 85 mph…. Marching Towards the Northeast

Tropical Activity For August 27th, 2011

Good Saturday afternoon everyone. The system that everyone has been watching for the better part of the last week is Irene, and it finally made landfall around daybreak near Cape Lookout as a 85 mph hurricane. In addition to Irene, TD#10 has finally fizzed into a remnant low. We do still have Invest 91 fighting the outflow of Irene, and now there is a new feature off the African coast. As has been for the last 3-4 days, I’ll focus primarily on Irene since it is the only system affecting land.

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Hurricane Irene

Visible Loop of Hurricane Irene

Hurricane Irene has made landfall. Landfall occurred at around 8am this morning after the system too a slight wobble to the north. Radar can clearly show the storm moving inland very near Cape Lookout. Its interesting to note that the central pressure with Irene is significantly lower than you would typically expect for a category 1 storm at 952 mb. This is because of the shear size of the storm, the pressure gradient that normally generates the winds is significantly reduced despite the low pressure. While I have not seen any confirmed reports of sustained hurricane force winds inland… there were many gusts above hurricane force, along with a significant storm surge as the system plowed ashore.

Radar Loop of Irene as it continues to move to the north-northeast through North Carolina

Since landfall, Irene has actually maintained a pretty organized appearance over land as it has resumed its north-northeast track. Winds have lowered slightly to 80 mph although  most of these winds are likely occurring over open waters. While the storm remains organized now, that likely won’t last for too much longer, as it moves back out over open waters into a much cooler environment. Here sea surface temperatures are much lower (26-27 degrees Celsius) which will make it very difficult of Irene to maintain her intensity in the face of increasing shear. Thus, I am still expecting Irene to start weakening more rapidly in the next 24 hours, and I am still expecting the system to make a second landfall near Atlantic City, NJ as a tropical storm borderline cat 1 hurricane. While the damage threat from these winds will not be very high, the high winds of tropical storm force or greater extending 290 miles from the center will cause a significantly larger storm surge than the storm of this intensity will normally produce. This is why it is prudent to heed the advise of the local law enforcement and evacuate the coastline if you are ordered to do so. My forecast map from yesterday remains relevant, although the timing has sped up about 3 hours or so. If you live anywhere along the New Jersey and New York coastline, the time is now to finalize plans and prepare for evacuation if ordered. There will likely be significant coastal flooding from this system that could extend a significant way inland, so structures near the coast should be evacuated. Further inland, there are indications that there might be a Predecessor Rainfall Even (PRE) which could help to enhanced rainfall totals further even before Irene arrives. Thus the inland flooding threat remains extreme and on the same order of magnitude as when Floyd was moving up the coast. Please take precautions if you live in a flood plain, you might have to evacuate as well.

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Invest 91L

Visible Loop of Invest 91L

While Irene continues to batter the coastline, there continues to be the small Invest 91L following in its footsteps. This system, while posing a well defined low level circulation, continues to be highly sheared by the outflow of Irene. The shear has actually increased to near 40 knots near the circulation, and its quite remarkable that the system is hanging on like it is right now. Still, its likely this system will start to fall apart as soon as all the convection is ripped away from the circulation center. I agree with the NHC’s assessment that this storm has a near 0% chance of becoming a tropical cyclone in the next 48 hours.

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Strong African Wave

Infrared Loop of New Tropical Wave Exiting Africa

I have added a new feature to the chart for this update, a very strong tropical wave that is in the process of moving off the African coastline. While it seems a bit convoluted currently, mainly due to mountainous terrain near the African coast in Guinea, this system seems to be entering a very favorable upper level pattern characterized by low shear and upper level divergence. In addition to the favorable environment, both the GFS and ECWMF are suggesting tropical cyclogenesis by the 48 hour period just south of the Cape Verde islands. Given the strong model support and expected favorable conditions. I am going to go ahead and give this disturbance a 30% chance of tropical cyclone development in the next 48 hours, although it will likely be by the end of this period.

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And that is all I got… I’ll have another update tomorrow on Irene as it begins to undergo extra-tropical transition and the rest of the features in the Atlantic

Irene Slowly Weakening But Still Has Major Impact Potential for the East Coast

Tropical Activity For August 26th, 2011

Irene remains the top story for this tropical weather outlook, as is less than 24 hours away from a North Carolina Landfall. Also on tap is TD#10 which continues to struggle, and newly declared Invest 91L. First though, I’ll focus on Irene.

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Hurricane Irene

Visible / Infrared Loop Combo of Hurricane Irene

Hurricane Irene has been moving progressively closer to the North Carolina coastline today as it has also weakened slightly. The maximum winds are down to 100 mph and this could be lowered further if recon does not find any higher surface winds speeds. While they have been able to find flight level winds exceeding 100 knots, the standard 90% conversion rate does not seem to be holding water in this case. While the pressure dropped last night to a minimum of 942mb, it has since risen back up to 950mb. A likely consequence for the pressure rise and the lowering of the winds was the collapse of the inner eyewall as observed on microwave imagery with only a fragmented outer eyewall remaining. While this is still a low minimum central pressure (950mb), the pressure gradient has actually decreased due to the storm increasing in size, allowing the winds to drop. The main story likely won’t be devastating winds, but storm surge, generated by the very large radius of tropical storm force and hurricane winds. According to the latest National Hurricane Center advisory on Irene hurricane force winds extend out 90 miles from the center of circulation. This is in addition to to the 290 mile radius of tropical storm force winds. Thus tropical storm force winds are already being felt on the North Carolina coastline at this time, and things will only go downhill from here. Wave heights have already increase to near extreme levels southward from the coast of North Carolina, with one buoy reporting up to 26 foot waves and going up rapidly. Storm surge for the North Carolina coastline is expected to be between 3-5 feet. With 26 foot+ waves on top of this already high water, structures closest to the Atlantic coastline will likely be damaged as waves topple less able structures to withstand the force of the water.

WSR-88D radar loop of Irene as it approaches the coastline.

Track-wise things seem to remain on track from my thoughts from yesterday. As seen on radar imagery above, it does seem that the center of Irene has become to start moving with an eastward component of motion. This will lead to a landfall somewhere between Morehead City, NC and the Outer Banks of North Carolina within the next 24 hours. The track guidance remains in agreement with this solution for the next 24 hours. Beyond this time frame the tracks continue to have some divergence to them, but the center of the guidance continues to skirt the New Jersey coastline with a landfall somewhere near Atlantic City, NJ. Considering the weakening that has taken place earlier today, I am lowering my forecast intensity for the second landfall point a touch, although it will likely be near or just below hurricane Intensity at the second landfall point.

My Updated Track and Intensity Forecast For Hurricane Irene

Thus, here is my updated, and likely final map for Hurricane Irene. I have zoomed in a bit on the 24-48 hour time frame in order to show more clearly where I have the system going. I have once again made some reductions to the current intensity for both landfalls, mainly due to the lower initial winds and expectation that Irene does not have the inner core to take advantage of the warm ocean waters before landfall. Thereafter, land interaction, increased shear, and lower sea surface temperatures should continue the weakening trend and I have Irene making landfall once again near Atlantic City, NJ in about 42 hours as a strong tropical storm. Thereafter the storm continues to accelerate and should be extra-tropical before 72 hours. It seems that beyond North Carolina, the biggest threat associated with Irene will be very heavy rainfall that may cause widespread inland flooding due to 10-15″ rainfall amounts expected along and just to the west of the storm center. There also continues to be major surge potential across the east coast, especially if Irene stays more offshore. This in itself will likely cause coastal flooding across areas of New York City and across Long Island. This is only made worse by the expectation that Irene will be moving up the coast during an astronomical high tide, which will increase the water height further. In any event, stay safe everyone that is in the path of the storm, and I will try to put out an update tomorrow recapping the landfall in North Carolina while I continue to follow the storm as it tracks up the coast.

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Tropical Depression #10

Infrared Loop of TD #10

Tropical Depression #10 is still around with us, but as you can see its even less organized than yesterday. Only a few convective elements well to the west of the circulation remain. The main culprit continues to be 10-20 knots of northerly shear over the system as an anticyclone continues to be displaced to the left of the circulation. Dry air is also evident encroaching on the circulation of TD#10. With these unfavorable conditions expected to continue as the storm moves to the northwest, it seems unlikely that this system will be able to become a tropical storm. The storm will most likely become a remnant low in the next 24-36 hours unless convection increases substantially in that time frame.

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Invest 91L

Visible / Infrared Loop of Invest 91L

The feature that I mentioned yesterday has now officially been recognized by the National Hurricane Center as Invest 91L. The system itself has actually become less organized in the last 24 hours as strong upper level shear of 20-30 knots from the outflow of Hurricane Irene is now blowing over the circulation. This is expected to increase over the next few days, so it seems unlikely this system will have any chance to develop now that the shear has increased. Because of this, I am now only giving this system a near 0% chance of development in the next 48 hours.

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And that’s a wrap for another tropical weather update. I’ll be back tomorrow with more information about Irene as it moves up the coastline.