Tropical weather analysis - May 22, 2010

Invest 90L

Invest 90L continues to look highly disorganized, and it appears that if there was indeed a closed surface center this time yesterday, it has since dissipated, and the system is now an open trough. However, this is largely negligible because the system is a non-tropical entity, and hasn't even made the transition to a subtropical cyclone. The last of the visible satellite imagery, along with shortwave infrared satellite imagery, and buoy data indicate a complete lack of westerly winds along the south side of the circulation, as well as some northeasterly winds along the western flank of the circulation, indicating that the system has indeed lost the closed surface circulation it once had. The broad and ill-defined center is elongated SW to NE, owing to strong westerly shear in excess of 30 kt. Based on the aforementioned meteorological tools, I have done my best to take a shot at guessing the poorly-defined circulation center, and I have fixed a center at 26N 67W. 90L had a slight center relocation today.

Analysis of water vapor imagery, along with certain CIMSS products, indicates that a weak but elongated upper-level low has pinched off from the shortwave trough to the NW of the system, near 27N 66W. Steering currents within the vicinity of the upper low are weak to nonexistent, and so this low will move very little over the next 24 hours. Even if it attempts to collocate itself about the surface center (a sign of subtropical transition), strong upper-level shear will quickly decouple the two circulations. All of the computer models predict the current shortwave to the NW of the system to completely dissipate within 24 hours. During this time, the models agree that upper-level shear should subside enough to the point where the aforementioned upper low can align itself above the surface center. This is when any potential transition into a subtropical cyclone will start to transpire. Additionally, with the upper low collocated above the surface center, vertical shear will remain considerably lessened beyond 24 hours, because shear is weak along the axis of any system. This will allow 90L to continue deepening as it moves slowly N along the western edge of the subtropical ridge, since shear stays favorable.

This N motion should continue until around 72 hours, at which point a turn to the NW is expected. The transition to a subtropical cyclone should be complete probably by around 36-42 hours. By day four, the system will be making its point of closest approach to the East Coast of the United States, at which point all models foresee it being just east of the North Carolina coast. However, one very interesting trend in the models that started with the GFS this time yesterday, is the absorption of 90L/Alex into a large extratropical low forecast to drop southward from the North Atlantic Ocean on the afternoon of May 26. While I would prefer to see a little bit more consistency, since the models may not foresee any additional center reformations until the system becomes better organized (and thus in turn, the ultimate position of the system relative to the east coast could be a couple degrees off), other models aside from the GFS have been following suit in this possible absorption, and this cannot be completely ignored at this point. We will see how things evolve in the model fields over the next day or so. Said assimilation could occur either before or after landfall. It's too early to tell at this point which scenario is correct.

I do eventually foresee 90L/Alex getting close to the North Carolina coast by the afternoon of the 26th, and it is anybody's guess as to what might happen at that point. It could continue inland before becoming extratropical, owing to the aforementioned Atlantic extratropical low, or it could skip landfall entirely and become absorbed a day or so earlier. Or, it could simply meander slowly off the coast until the low absorbs it.

Synopsis in layman's terms

Invest 90L has the potential to become a subtropical cyclone prior to reaching the North Carolina coast on Wednesday, May 26. I put the odds of this happening at about 40%. Regardless of development, gale force winds will develop in association with the system, particularly on the north side, owing to the large pressure gradient between the low and the subtropical ridge. These winds will produce rip currents and high surf all across the eastern seaboard, from Georgia all the way up to perhaps Virginia, over the next several days. There is large uncertainty in what 90L/Alex will do after nearing the coast on day four. Interests all along the Carolinas and Virginia should carefully monitor the progress of this system over the next several days.

Caribbean low pressure area

In about a week from now, all of the models, save the GFS, foresee an area of disturbed weather across the western Caribbean. The overall trend for this has been further west. I still do not really buy into this scenario, as I would like to see some consistency in the time frame, something we've not had thus far with this potential system. However, models have been consistent in gradually lifting out the vertical shear across the southwest, central, and even a portion of the western Caribbean by around five days. This will allow any potential area of disturbed weather that forms there to take advantage of the favorable upper-level conditions, and potentially spin up. Any system forming here would likely move eastward, owing to the weakened subtropical ridge, imparted by the aforementioned North Atlantic extratropical low, the same one forecast to absorb 90L/Alex. While Haiti may not be a direct target, it still needs to be on guard for possible flash flooding and mudslides associated with anything that might form in this area.

Synopsis in layman's terms

Models continue to predict the possibility of an area of disturbed weather across the Caribbean in about a week. The trend has been further west with regards to location, and it now appears that any system that develops will do so in the western or west-central Caribbean. Anything forming here would head eastward, putting portions of the Greater Antilles at risk for heavy rainfall and mudslides.