From Rob Lightbown of Crown Weather

Originally written Sat Aug 18^th, 2012* See update

Invest 94L:
Invest 94L, a tropical wave which is located about 250 miles or so to the southwest of the Cape Verde Islands, is of significant concern to me and I will say that right up front. The analysis early Saturday (18) morning showed that shower and thunderstorm activity was gradually becoming more consolidated and was slowly increasing. Additionally, the 850 millibar vorticity chart showed that the amount of spin associated with 94L was also slowly increasing.

Invest 94L is currently tracking to the west at a forward speed of 20 to 25 mph (up from 15 to 20 mph on Sat) and I expect this westward track to continue right into a majority of next week. As for strengthening, I think we will see gradual intensification and this system seems destined to become Tropical Storm Isaac by either Sunday night or during the day Monday. Beyond that, environmental conditions are favorable for strengthening throughout much of next week and I do feel confident enough to say that I think this will become a hurricane by about the middle part of next week.

The latest GFS model guidance has shifted further west with the overall track of 94L and forecasts it to significantly impact Barbados and much of the Lesser Antilles with hurricane conditions from Thursday afternoon through Thursday night and into Friday morning. After that, the GFS model forecasts future Isaac to impact Puerto Rico with hurricane conditions by next Saturday. From there, the longer range GFS model seems to suggest a track north and then northeast about halfway between the US East Coast and Bermuda during the week of August 27th.

The latest European model guidance has a much weaker system that never really closes off into a storm. This leads to a Caribbean cruiser by late next week into next weekend. Right now, given the presentation of how 94L looks and that the overall pattern in the Atlantic Basin is favorable for an intensifying tropical cyclone, I think the European model guidance forecast seems unlikely and I’m leaning much more towards the stronger GFS model forecast.

So, here are my thoughts on Invest 94L: I do think that this system poses an eventual threat to the Lesser Antilles. Given the fact that the ridge of high pressure to the north of 94L is fairly strong, I see no evidence of a turn to the northwest before it reaches the Lesser Antilles. Additionally, the fact that the intensity guidance is insistent on significant strengthening to not only hurricane strength, but major hurricane strength leads me to believe that all interests in the Lesser Antilles, as well as Barbados should closely monitor the progress of this system for you may have a hurricane knocking on your door by Thursday.

Go to http://www.crownweather.com

UPDATE:  (from Weather Underground) “A large area of dry air lies just to the north of 94L, as seen on the latest Saharan Air Layer (SAL) analysis and water vapor satellite loops. This dry air is interfering with development, and this morning’s visible satellite loop shows that 94L’s heavy thunderstorm activity is a bit sparse. The storm does have an impressive amount of spin at middle levels of the atmosphere, though. A pass from the Indian OceanSAT-2 satellite Saturday night at 10:06 pm EDT noted a broad, elongated center of nearly calm winds several hundred miles in diameter at the surface, and nothing resembling a well-organized closed surface circulation. 94L will pass near buoy 41041 on Monday morning. The first hurricane hunter mission into 94L is scheduled for Tuesday afternoon.

“It is unlikely that 94L will be able to organize quickly enough to become anything stronger than a Category 1 hurricane before reaching the islands, given the storm’s current struggles with dry air, and the lack of model support for intensification. With 94L staying relatively weak and disorganized, the chances of it turning to the northwest and missing the Lesser Antilles, as the NOGAPS model has been predicting, are diminishing. The GFS model predicts that 94L will go on to hit the Dominican Republic as a strong tropical storm on Friday, though the storm could also miss the island, passing just to the north or the south. At longer ranges, the storm is capable of going anywhere from Canada to Mexico’s Yucatan Peninsula; it’s too early to tell.”

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2193

For us living in the Cayman Islands we need to watch this storm’s progress very closely