Tropical weather analysis - June 18, 2010

Began writing at approximately 6:20 PM EDT

Invest 92L continues to thrive, generating persistent but highly disorganized convection. Most of the convection is displaced well to the east of the center at the moment, owing to strong upper-level westerly shear, analyzed at 30 kt by the University of Wisconsin's CIMSS.

Based upon analysis of the last visible satellite animations of the day, along with recent buoy observations and recent microwave imagery, courtesy of the Navy Research Lab (NRL), I estimate that the center is located near 17N 64W, along the extreme western edge of the disorganized convection in that vicinity. This isn't certain, however, given the system's current disorganization. These same observations, along with surface observations from the Leeward Islands, U.S. and British Virgin Islands, indicate that there are no signs of a circulation at this time.

As for movement, it appears to me, based on satellite animations along with doppler radar animations from Martinique, that 92L is moving due W. A rather meager thermodynamic environment continues to plague 92L, as animation of water vapor imagery indicates dry air encroaching upon the system from the west and southwest, owing to the aforementioned vertical shear, though the dry air does appear to be lessening some -- gradually.

However, that, along with an almost complete lack of upper-level divergence near the estimated center, should prohibit the development of organized deep convection near the center, at least for the next several hours. As the environment continues to gradually moisten, however, and divergence increases, we might see another brief bout of deep convection that persists for several hours. Should this predicted flareup occur as it has for the last several nights, it could generate some rather heavy rainfall across Puerto Rico tonight and into this morning, particularly along the southern coastline. Orographic lift will also act to contribute to heavier rainfall across the mountainous portions of that island.

I do not expect a serious flash flood disaster, however. Notwithstanding, Puerto Rico, along with the Leeward Islands, U.S. and British Virgin Islands, could continue to see some locally heavy rainfall, along with wind gusts in excess of 30 to 35 kt. Given that vertical shear has been steadily decreasing in the path of 92L, if we get another impressive convective burst tonight like we have the past couple nights, it may become a little more organized and longer lasting, which would tend to enhance the rainfall threat across the aforementioned areas.

Based on steering data from CIMSS, 92L should continue moving W in the short-term. Also, if you're wondering why I used the low-level steering flow as a basis for the above prediction, it's because presently, 92L isn't a deep enough system to feel the effects of the mid-level steering. Obviously, a weakness in the subtropical ridge is evident at both 700 mb and 500 mb as per the aforementioned steering maps, and 92L will eventually feel this weakness, deep or not. However, based upon steering forecasts from PSU,

I do not think that the weakness will begin pulling it more poleward until around around four days, when the system should be exiting the northern coast of Cuba and entering the southeastern Gulf of Mexico.

My forecast track takes 92L across the entire length of Hispaniola beginning in about 18 to 24 hours, briefly emerging back into the Caribbean and making landfall across extreme eastern Cuba by around 42 to 48 hours. By around 80 to 86 hours, whatever is left of the system should begin to emerge off the northern coast of Cuba, and into the southeastern Gulf of Mexico. Thereafter, the models are unanimously indicating a turn to the WNW as 92L begins to feel the effects of the aforementioned weakness in the subtropical ridge. Beyond four days, there are some indications that the ridge could rebuild westward a bit.

There is an equally likely possibility, however, of 92L continuing westward, completely missing the Greater Antilles, nearing 85W, and then turning more poleward. Perhaps needless to say, there is a large degree of uncertainty in my current forecast.

Should 92L make it into the Gulf of Mexico, upper-level winds will be favorable for development. However, I give 92L's chances of developing into a tropical cyclone in the Gulf at only about 10%.

Elsewhere

Elsewhere in the tropics, the GFS and NOGAPS are suggesting that a tropical cyclone could develop east of Nicaragua in about three days. This system would quickly run westward into Central America, and does not presently appear to be a threat to the United States. Upper-level winds will be favorable for development here.

The models have been less bullish recently on the prospects of a tropical cyclone developing near and then subsequently passing through the Windward Islands in about a week. However, they still indicate an active ITCZ in the area in about 6 to 7 days, so this will still need to be carefully monitored. Upper-level winds will be extremely favorable for development in this area.