Busy evening at the National Hurricane Center

August 1, 2006

After enjoying a few days of quiet in both the Atlantic and Pacific basin, the National Hurricane Center forecasters found themselves with plenty of work to do this evening. The fun started this afternoon with the formation of the seventh Eastern Pacific tropical depression, which was upgraded to Tropical Storm Fabio this evening. That was not the only action in the area as, the eighth tropical depression formed (and is expected to become Tropical Storm  Gilma over the course of the next day). Neither are threats to land.

The headliner was the formation of Tropical Depression Three in the Atlantic. Readers of Dr Masters blog are undoubtedly familiar with the saga of the tropical wave that has been approaching the Lesser Antilles and persevering against unfavorable conditions. After being rather unnotable for a few days, the wave started to become better organized last night and the trend continued into today. As explained in the first paragraph of the first forecast discussion for TD-3, forecasters received evidence of a circulation at the surface this afternoon and saw that the thunderstorm activity associated with that circulation has persisted for several hours. Those two items are the pre-requisites for classifying a system as a tropical depression. With the conditions met, the first advisory was released this evening.

The intensity forecast is a little bit surprising to me in its conservativism. It maintains the depression at a strength of 30 knots (34 miles per hour, just under tropical storm strength) for the first 48 hours of the forecast, followed by weakening and dissipation. The forecast discussion notes the unfavorable enviroment (dry air, followed by destructive quantities of shear) in the path of the depression and points out that all of the global forecast models dissipate the system. The forecaster does point out that the SHIPS model does intensify the depression gradually but discounts this with the comment that “perhaps this model cannot appreciate just how fragile a system the depression is right now”.

That statement is reasonable, SHIPS does seem to be over-optimistic with the development of storms in a dry air enviroment, so one would not bet the house on its forecast verifying in this situation. However one could counter the statement with “perhaps the global models cannot appreciate how resilient the depression is right now”. After all, its formation was far from a high probability event. While there is probably little chance of surviving the upper level low that resides over the southern Bahamas, which would put destructive shear in the path of the depression, I would argue that there is a window of opportunity for a tiny bit of intensification, just enough to make this Tropical Storm Chris (up to say, 40 mph).

Ultimately that argument is an insider pool one, of little importance to the rest of the world. The depression is expected to move steadily to the west and pass north of Puerto Rico and Hispanola, negating the only threat that this system could pose (heavy rainfall leading to mudslides, etc). At this point, I can’t really advise the need for anyone to keep a close eye on it; just checking in with the National Hurricane Center once or twice over the next couple of days to make sure things are going as expected should suffice.

I am reminde of the remark that Dr Masters made when Beryl formed, that there were no indications that her formation presaged the beggining of a more active period in the Atlantic. I would say that the formation of this depression is something more significant, either an “end of the beginning” (the last weak system before the ‘real storms’ come out) or the beginning of the ‘real season’ (as the first system do develop in the classical manner, albeit weakly). I suspect that the quiet days for the NHC won’t be coming back any time soon.

EVER SO SLIGHTLY LATER: I wrote my post without having it in mind that the depression was near enough to land to warrant the issuance of intermediate public advisories (i.e. advisories every three hours). Perhaps a good thing, as had I thought of that, I would have held back on my thoughts until afterwards… but I digress. The public advisory written by Mr. Franklin at 11 PM stated “little change in strength is forecast during the next 24 hours”. Stacy Stewart wrote the intermediate advisory released at 2 AM, which states “some strengthening is possible during the next 24 hours … and the depression could become a tropical storm later today”. The latter statement, of course, is in line with my thinking as explained above. This may be due in part or in whole to an apparent reformation of the center further east than it was estimated to be at earlier. We’ll have to wait for Stewarts 5 AM discussion for his thinking, (which he is usually quite generous with offering in his discussions).

One really should be getting this sort of information direct from the NHC, and not rely on me, but I will point out that there is a Tropical Storm Tatch up for the islands of Saba, St Eustatius, St Martin and St Barthelemy and that watches may be posted for Puerto Rico and the Virgin Islands. Also, Tropical Storm Warnings could be posted for Antigua, Barbuda, Anguilla, St. Kitts, and the British Virgin Islands at short notice.

Finally, a link to the nice forecast track map courtesy of the Navy.

Corrected the name of the eighth tropical storm in the Pacific, which was upgraded at 11 AM on the 1st.

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