Tropical weather analysis - August 1, 2010

Invest 91L

A well-defined tropical wave located several hundred miles west-southwest of the Cape Verde Islands (dubbed "Invest 91L" by the National Hurricane Center) continues to gradually become better organized, and a tropical depression could be forming, according to a statement from the NHC's 2PM EDT Tropical Weather Outlook:

A LARGE LOW PRESSURE SYSTEM CENTERED ABOUT 875 MILES WEST-SOUTHWEST
OF THE CAPE VERDE ISLANDS IS MOVING WESTWARD TO WEST-NORTHWESTWARD
AT 5 TO 10 MPH. SHOWER AND THUNDERSTORM ACTIVITY IS GRADUALLY
BECOMING BETTER ORGANIZED AND A TROPICAL DEPRESSION MAY BE FORMING.
IF THIS CURRENT DEVELOPMENT TREND CONTINUES...THEN ADVISORIES ON
THIS DISTURBANCE COULD BE INITIATED AS EARLY AS THIS AFTERNOON OR
EVENING. THERE IS A HIGH CHANCE...80 PERCENT...OF TROPICAL CYCLONE
FORMATION DURING THE NEXT 48 HOURS.

Based on the last of the visible satellite animations, it appears that the center is located at 10N 36W. Deep convection appears to be firing near or over the estimated center, and the aforementioned satellite animations depict that a closed surface circulation is rapidly trying to form, though this is not a particularly well-defined one as of yet. Also, upper-level outflow appears to be developing to the north and west of the system, but it is not well-defined as of yet. There is currently no outflow along the south and eastern sides of the circulation, which is to be expected due to competing influence from the ITCZ to the south, as well as moderate easterly to vertical wind shear that the system has been experiencing for the last couple of days.

However, this shear may be relaxing now over the estimated center position, based on low cloud motions indicated in various satellite animations as well as CIMSS wind shear analysis, which depicts only 5 kt of shear over the system, amidst a developing anticyclone aloft. As far as the track forecast is concerned, both CIMSS steering data and animation of water vapor imagery depict a gradual weakness developing in the Atlantic subtropical ridge to the north of the system. This weakness is being caused by a mid- to upper-level trough over the western Atlantic, and a developing upper-level low near 23N 61W that has recently pinched off from the trough. This weakness occurred a little sooner than I had expected this time yesterday, but nevertheless it is quite evident at this point.

It is because of this slight weakness that 91L has turned WNW this evening. Analysis of mid- to upper-level steering layers, along with data from the 18z GFS, indicates that this trough is not particularly well-defined, and more importantly, that the majority of the energy associated with this trough should remain far to the north of Invest 91L. This, along with the fact that the aforementioned upper low is forecast to gradually de-amplify and weaken, will not allow the subtropical ridge to weaken significantly over the next couple of days. Hence, 91L should continue moving toward the WNW over the next couple of days, with an increase in forward speed expected to begin over the next day or so.

This will allow 91L to gain some additional latitude and take greater advantage of the Coriolis force, producing greater cyclonic vorticity. By three days, the models unanimously forecast a rather large break to occur in the Atlantic subtropical ridge as another trough, located over the western United States, emerges into the western Atlantic.

My current thinking though, based on the current large-scale pattern across the United States, is that the overall strength of this trough is being vastly overestimated by the models. Even if it does end up being that powerful, I think it will lift out far faster than what most of the models are indicating this evening. I agree most strongly with the 12z ECMWF, and I expect more ridging than is currently being indicated by most of the models. This puts the greatest United States risk area at the east coast of Florida. However, this is a long-range forecast track, and as such, is highly uncertain. Nevertheless, residents along the east coast from Florida to North Carolina should carefully monitor the progress of this system over the next week or so.

First and foremost though, residents across the northern Leeward Islands, the Virgin Islands, and Puerto Rico, should closely monitor the progress of this system over the next several days, as the system could pass just north of that area in about four days.

Interestingly, the models are rather slow in developing this system, with only the CMC and ECMWF developing the system within the next two days. I have opted to go with these models, based on current organizational trends observed with Invest 91L. Upper-level winds favor additional development of this system, and the 18z GFS strengthens the aforementioned anticyclone, keeping it intact even as the system runs into strong westerly vertical shear associated with the TUTT north of the Leeward Islands in four days. This anticyclone may not allow much in the way of weakening when the system encounters shear from the TUTT.

Thereafter, as the system nears the western Atlantic, the upper air pattern is forecast to continue being zonal and unfavorable, with a positive twist: the GFS suggests that the anticyclone should begin to fade after four days. This would tend to weaken the system by day five as it approaches the western Atlantic. However, this shear is forecast to lift out relatively quickly, and it is not likely to completely destroy the system.

All in all, this system has developed much quicker than I thought it would. As such, I expect this system to become a tropical depression tonight or tomorrow morning. Again, residents across the northern Leeward Islands, the Virgin Islands and Puerto Rico, should closely monitor the progress of this system over the next several days.

Southwest Caribbean tropical wave

A tropical wave located a couple hundred miles east of the coast of Nicaragua continues to produce disorganized showers and thunderstorms. This system has become a little better organized this evening, owing to the long foreseen relaxation of the upper-level winds. However, proximity to land is expected to inhibit any significant development, and the system should move inland across Nicaragua sometime tomorrow morning. Regardless of development, locally heavy rainfall and strong gusty winds will overspread portions of Nicaragua, Honduras, Panama, El Salvador, and Costa Rica tonight and tomorrow.