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Tropical Weather Discussion, Monday August 2, 2010 at 635 am EDT/535 am CDT

Invest 91-L:
Invest 91-L was located about 975 miles or so west-southwest of the Cape Verde Islands this morning. Latest imagery indicates that a low-level well defined circulation has not yet formed, even though it is pretty well organized. I suspect that within the next 24 to at very most 36 hours, this system will be able to form a low-level circulation and be classified as Tropical Depression #4.

The satellite imagery as a whole this morning says plenty. It is pretty obvious that this is a fairly large system in its overall size and development should be slow to occur; however, it will be harder in the long run for outside influences like troughs to significantly affect this system.

A quick look at the model guidance from overnight shows that the GFS model is further west in its track, as is the Canadian model and the UKMET model. One significant change in the model guidance is that the European model is forecasting a track that is significantly further east than the last two model runs. Abrupt model changes like the European model should be found as suspect.

The key to what the model guidance is trying to resolve is the synoptic pattern of two high pressure systems, one over the western Atlantic west of Bermuda and the other over the south-central United States. The reason why some of the model guidance is turning this system out into the open Atlantic is that they are seeing a weakness in the ridge near 65 West Longitude. Remember what I said about this being a large storm that may be slow to develop? Well, I’m not buying into the fact that this weakness will turn the storm, but I think this system will end up missing the weakness altogether. In the long-term the ridge of high pressure over the western Atlantic may build westward and link up with the other ridge of high pressure system over the south-central United States. If this synoptic setup pans out, then a track towards the west-northwest towards Florida and the Gulf of Mexico would be likely. Basically, the bottom line this morning is that the longer it takes to develop in the short-term, the greater the threat to the Leeward Islands, Virgin Islands and Puerto Rico. Do not be surprised to see the model guidance shift further south and further west today and tonight. Also, if this ends up further south than what the model guidance suggests, then the weakness in the ridge probably will not influence this system and this is basically what the Teleconnections suggest.

As I mentioned yesterday, teleconnections between what is going on here in the Atlantic Basin and what is going on in the upper levels over Asia lead me to believe that this system will not curve out into the open Atlantic or even ride up the US East Coast. Currently, a strong ridge of high pressure extends from southern Japan southwestward into Southeast Asia. This type of pattern should translate into the Atlantic Basin by this weekend and I expect a strong ridge of high pressure to extend from the coast of North Carolina southwestward into the Gulf of Mexico. This says to me any approaching storm will not curve up the East Coast, but instead take a track very similar to Bonnie and this is not good news at all!! Could I be wrong in my analysis and interpretation?? Absolutely! but this is what I’m looking at rather than just blindly following the model guidance.

As I already mentioned, the Canadian model has shifted its forecast track further west and forecasts that this system will impact the Leeward Islands late Wednesday night into Thursday morning, the Virgin Islands during the day on Thursday and Puerto Rico Thursday afternoon into Thursday night. After Thursday, the Canadian model forecasts a west-northwest track through the Bahaman island chain from Friday night through Saturday night before curving northward up the coast; however, notice that by Sunday morning a ridge of high pressure is building in from the Great Lakes, this causes this storm to track into the coast of North Carolina next Monday.

As for intensity, I strongly suspect that this system will be classified as a tropical depression either sometime today or at the very latest tomorrow with it becoming a tropical storm by about Wednesday. Now, how much this intensifies depends on how low latitude this stays at. Wind shear analysis this morning showed favorable wind shear conditions up to about 60 West Longitude. West of 60 West Longitude, there is an area of 20 to 40 knot shear that runs basically from 60 to 70 West Longitude. Now, if this system stays at a lower latitude like the Canadian model suggests, then it would avoid the strongest of the shear, however, if it curves further north like the European model suggests, then it would encounter a much more hostile environment. The reason the SHIPS guidance is forecasting so much shear after Thursday is that it forecasts a sharper curve northwest along the 63 to 65 West Longitude line right into the strongest shear. My take is that this system may either track right over the Leeward Islands like the Canadian model suggests or just north of the islands, lets say 19 or 19.5 North Latitude; this would take the system into an area of 30 knots of shear for about a day before the shear relaxes again in the Bahamas.

So, all interests in the Leeward Islands, Virgin Islands and Puerto Rico should closely monitor the progress of this system. It seems possible that weather conditions will go downhill across the Leeward Islands during Wednesday afternoon into Wednesday evening with tropical storm and possibly hurricane conditions later Wednesday night into Thursday morning. Over the Virgin Islands, weather conditions are forecast to go downhill on Wednesday night with tropical storm and possibly hurricane conditions during the day Thursday. Across Puerto Rico, weather conditions are currently forecast to go downhill during Thursday morning with tropical storm and possibly hurricane conditions during Thursday afternoon into Thursday night.

After Thursday, the forecast track and intensity of future Colin becomes more unclear, however, I think a track across the Bahamas is possible Friday through Saturday as a hurricane with this storm becoming a risk from North Carolina to Florida late this weekend into early next week. The reason I am laying out this geographic range is because there is a chance that the teleconnection method of forecasting may not turn out correct in the end. Right now though, the most pressing and immediate concern is for the Leeward Islands, the Virgin Islands and Puerto Rico.

Needless to say, I will be monitoring things very closely and will keep you all updated.

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