Tropical weather analysis - May 24, 2010

Invest 90L

Invest 90L continues to look highly disorganized today, and has changed very little, if any, in overall organization since this time yesterday. Visible satellite animations depict the broad surface circulation is located near 28N 70W. This circulation is plainly seen, as it is completely removed from any thunderstorm activity, which is located well to the north and east of the center. While this is certainly characteristic of a subtropical cyclone, analysis of the Cyclone Phase Diagrams from FSU indicate that this system has not yet developed a low-level warm core. Unless this occurs, this system cannot become a subtropical cyclone. Also, to continue the trend of the last several days, there appear to be several small swirls embedded within the large cyclonic envelope, a further testament to the storm's presently disorganized state. Though it is not certain, as it is quite hard to tell with all the cirrus overhead, I suspect that another vorticity center is located near 30N 71W, though this circulation is also weakening. An earlier ASCAT pass indicated that the overall circulation associated with this system is very broad, covering a large area from the Greater Antilles westward and northward, including the Bahamas and all the way up to New England.

Said pass also indicates that this system's circulation is not entirely closed, and also that a large swath of 25-30 kt sustained winds exist east of the broad center(s). Due to the typical low-bias of ASCAT measurements, it is safe to assume that there is a large portion of gale force winds well to the east of the center, owing to the large pressure gradient between the subtropical ridge and 90L. It is likely that these winds exist along the northern end of the circulation as well, though unfortunately, the recent ASCAT pass missed this area, and there are insufficient surface observations north of the center to really confirm this suspicion. Animation of water vapor imagery indicates that a large amount of continental dry air continues to advance eastward into the broad center, owing to an upper-level low centered near 33N 81W, across central South Carolina. Analysis of winds from 100-500 mb, courtesy of University of Wisconsin CIMSS, depicts this low rather well, and it appears to have deepened somewhat over the last several hours. However, the models unanimously foresee this low has reached its peak and will gradually weaken and move southwest from this point on.

However, I suspect that this low will still be close enough to continue ingesting dry air into 90L for the next day or so. The models all agree on a track east of the Carolinas by around 48 hours, as 90L moves under the influence of the subtropical ridge to the east. Where they diverge is thereafter, as they all handle the timing and strength of the large north Atlantic trough differently. The CMC depicts the system will get picked up by the trough at around 54 hours; the GFS foresees the same, though it loses the identity of this system much quicker than the other models, at just 72 hours; the NOGAPS doesnot foresee 90L ever getting caught up into the aforementioned trough, and instead leave it to meander across the mid-latitudes of the Atlantic Ocean for several days in the wake of the trough. While this solution is possible, the current longwave pattern still favors anomalous troughing across the northeastern United States and Atlantic Canada, and so 90L will eventually be picked up by a trough, even if not the one that the CMC and GFS are predicting. Finally, the ECMWF offers an interesting but unrealistic scenario: 90L never gets picked up by the trough, and instead maintains a completely separate entity throughout the next week and beyond, eventually becoming a powerful extratropical low across the north Atlantic.

I'm going with the GFS scenario, having the system interact with the trough by 54 hours, and completely lose its identity by 72 hours within the trough. Rough seas are still likely from Georgia to New England over the next several days, even if the system doesn't develop, which I don't suspect that it ever will, despite the forecast of gradually decreasing vertical shear. Why? The environment is simply too dry, and the upper low that was previously atop the system has since detached. The NHC only gives this system a 20% chance of developing over the next 48 hours, and I am inclined to agree with them.

Synopsis in layman's terms

Development of 90L into a subtropical cyclone is looking increasingly less likely. However, regardless of development, gale force winds are a certainty north and east of the center. This will produce rough ocean conditions from Georgia northward to New England over the next several days.

Caribbean disturbance

The models have suddenly jumped on the development bandwagon again for the western Caribbean, this time by 120 to 144 hours (5-6 days). The instigator for this system is the remnants of the current Eastern Pacific disturbance centered across the Gulf of Tehauntepec, west of Central America. This situation appears to be very similar to the Alma/Arthur situation in May 2008, and is worth keeping an eye on. However, while still forecast to lessen across the western Caribbean over the next four to six days, vertical shear will still only be at marginally favorable levels for tropical cyclogenesis, and so the CMC and GFS, the only models that foresee development in this area, may be overdoing it. They forecast that the system will move ENE across eastern Cuba by around five days from now. If the system forms further south, it will have a much greater chance of becoming a tropical depression. All in all, I'm still not impressed with this area, though it does bear watching, but I'd rather see some model consistency first.

Synopsis in layman's terms

A tropical disturbance could form in the western Caribbean over the next several days, though development into a tropical depression or storm is not anticipated. Locally heavy rainfall will be possible across portions of the Greater Antilles, however.