Tropical weather analysis - July 29, 2010

Invest 90L

A large but well-defined perturbation in the Intertropical Convergence Zone (ITCZ) was designated Invest 90L this afternoon by the National Hurricane Center. Visible satellite animations suggest that this system is embedded within a large area of convection and cyclonic vorticity associated with the ITCZ. My best estimate for a "center" position, based on satellite data, is 7N 31W. Latest CIMSS 850 mb vorticity data also somewhat supports this.

Upper-level winds are favorable for additional development of this system, with CIMSS wind shear analysis indicating only 5-10 kt of east to northeasterly shear over the system. This favorable upper-level environment is forecast to persist throughout the next several days, with a fairly uniform easterly flow. The two largest impediments I can see for 90L is the large amount of dry air to the north, depicted rather nicely on water vapor loops, and the broad cyclonic gyre that the system is embedded in. However, given the very moist environment within the vicinity of the disturbance, along with the fact that the system should move generally W over the next couple of days, I am not expecting the dry air to completely halt any additional development of 90L.

Analysis of 12z steering from PSU's e-wall suggests that a break in the Atlantic subtropical ridge could occur in about 60-72 hours as a mid- to upper-level trough swings by to the north of the system. However, unless 90L develops significantly during the next three days, this weakness should not be strong enough to turn the system poleward so much so that it completely misses any and all land areas.

All of that having been said, I expect some slow additional development of the disturbance as it moves generally W over the next couple of days. As previously noted, a turn to the WNW could occur in 60-72 hours as the subtropical ridge weakens somewhat. The 18z GFS develops this system just north of Puerto Rico in 168 hours. The 12z CMC develops this in as little as two and a half to three days, and eventually makes it a hurricane. However, such rapid development does not appear likely at this time, and I am discounting both the track and intensity solution currently being provided by this model. Finally, the 12z ECMWF hints at tropical cyclogenesis occurring north of the Leeward Islands in about six days, but quickly loses the system as it moves it WNW toward the southeast United States coast.

I would not expect development to begin occurring in earnest until around 60 hours, as the system begins to gain some latitude. The National Hurricane Center is giving 90L a 20% chance of becoming a tropical cyclone within the next 48 hours, and this is a reasonable forecast.

Windward Islands tropical wave

A tropical wave is producing scattered showers and thunderstorms across portions of the Windward Islands and adjacent eastern Caribbean Sea.

However, the wave axis has not yet passed these islands. Upper-level winds are currently not favorable for development, but could become more favorable over the next several days as the wave moves generally W, well-embedded within the low-level easterly trade wind flow common of the tropics. Surface observations from the Windward Islands, along with buoy data, indicate that nothing of note is occurring at the surface in association with this wave.

I suspect that any development will be very slow to occur. And, though the upper wind flow may improve somewhat over the next several days, the system will run into Central America in about four to five days.

Regardless of development, locally heavy rainfall and strong gusty winds will accompany the wave as it passes through the Windward Islands tonight. None of the models, save the CMC, which is always too bullish with tropical cyclogenesis, significantly develop the wave.