Tropical weather analysis - June 15, 2010

Began writing at around 10:30 PM eastern

Invest 92L did not become a tropical depression today, just as I had expected. However, over the last couple hours, deep convection, the deepest of its life (with cloud tops to -80C in some locations) thus far, has developed and expanded in both intensity and areal coverage. The center is difficult to pinpoint, but I estimate it is near 14N 47W, based on animation of shortwave infrared satellite imagery. This is not certain, however. An earlier ASCAT pass indicated only a sharp wind shift associated with the system, not a closed circulation. Obviously, unless this occurs, this current impressive burst of convection means absolutely nothing. Satellite imagery also confirms the findings of the aforementioned ASCAT pass, with a noticeable lack of westerly winds along the south side of the circulation. Animation of water vapor imagery indicates a large area of dry rapidly advancing toward 92L from the southwest, due to strong southwesterly upper-level shear, diagnosed at 20 to 30 kt as per CIMSS data.

As this shear continues to gradually push the dry air toward 92L, I expect the current burst of convection to wane overnight and into the morning, as has been the case throughout the system's existence. I honestly don't see a conceivable way for the system to maintain its current deep convection, as southwesterly shear will continue to impinge upon the system, completely preventing the establishment of upper-level outflow along the southwest quadrant of the system, which will in turn allow the dry air to tirelessly ingest into the system. Analysis of vorticity data from CIMSS indicates that the system's cyclonic vorticity has become much better organized over the last 12 hours, but also indicates that the aforementioned southwesterly shear has decoupled the low- and mid-level circulations.

92L is currently moving NW based on my estimations, though this is highly uncertain. I am most in agreement with the NOGAPS' steering forecast, which is for the current NW motion to end shortly, probably within the next 12 hours or so, after which a continued general WNW movement can be expected. On this forecast track, 92L will be passing through the north-central Leeward Islands in about four days, and could bring some locally heavy rainfall and gusty winds to those islands, as well as Puerto Rico and possibly Hispaniola, as well. Thereafter, the system should traverse north of the Greater Antilles, and could eventually find its way to the Bahamas or south Florida. However, this is highly speculative and should be treated as such.

More importantly, the models, the GFS in particular, are hinting at a substantial relaxation of the shear in the Caribbean beginning in about two days and persisting throughout the forecast period. This is because these models lift the TUTT off to the north, out of the Caribbean. The models have been consistent with this, and so this is definitely worth keeping an eye on. So by the time 92L approaches the Leeward Islands and enters the eastern Caribbean Sea, assuming that it retains a decent overall structure, organization into a tropical cyclone is not outside the realm of possibility. Its structure has to survive the upcoming shear first, though. I give this a 20% chance of ultimately becoming a tropical cyclone. Residents across the Leeward Islands, Puerto Rico, and Hispaniola should carefully monitor the progress of this system.

After the system passes the Leewards and Puerto Rico, it will once again encounter unfavorable westerly to southwesterly flow associated with the TUTT, whose axis will be centered north of the Greater Antilles at that time.

Elsewhere

Elsewhere, a tropical wave, as per the 1800 UTC surface analysis from the National Hurricane Center, is located south of 11N along 27-28W, moving W at 5 to 10 kt. Yesterday, I was quite concerned with this wave, but given its currently disorganized state (most of the precipitation is not associated with the wave, but with the ITCZ), along with the forecast of 15 to 20 kt of westerly shear, I am not expecting development of this feature.

More of note, the CMC, GFS, NOGAPS, and ECMWF forecast a tropical wave could pose a threat to develop into a tropical cyclone as it nears or enters the eastern Caribbean by day 6 to 10. The latter is quite a long range forecast. Also, the fact that each model offers a different timing for the system leads me to believe that we do not know the specific date at which this tropical wave will enter the area, if it does at all. However, I do believe that this is worth keeping an eye on, since the ECMWF has been forecasting it since last night. Upper-level winds should be at least marginally conducive for some development of the system.