Tropical Depression Four update and other tidbits

August 22, 2006

– As of this morning Tropical Depression four was in a steady state with no change in intensity since yesterday evening. I concur with The Storm Track’s Adam Moyer’s reason for why TD 4 has not intensified, which is the presence of Saharan dust in the area. I mentioned this dust in my tidbits post a couple of days ago and cited it as a reason for thinking that things were going to be quiet. Well, it’s not so quiet now, but the storm could be more subdued than it would be were it not for the presence of the dust. It will be another three days or so until it is entirely in the clear. It’s best days lay there.
Models that forecast itensity don’t really take the restrictive effects of the dust into account, which causes them to over-forecast how strong the storm becomes when it is being affected by dust. This problem was noted by researchers of the Saharan phenomenon and continues to be worked on.

– Speaking of models, NHC senior hurricane specialist Stacy Stewart had the quote of the season (up to this point) in his 11 pm discussion:

The global models…as well as the GFDL and the BAM models…remain tightly clustered and in excellent agreement on an immediate northwestward motion. Unfortunately… the models have also been excellently wrong thus far.

And indeed the depression has moved more west and less north than the “exellently wrong” models have forecast. The problem for the forecaster has been that when the models in such tight agreement, there isn’t a ‘second opinion’ to fall back on when the immediate move to the northwest proves wrong. One can see this in the model guidance Stewart had to work with last night. The only model not among the cluster was the CLIPER model (labeled CLP5), which is a simple model that the more modern dynamic models should outperform significantly. Stewart’s decision was to make the forecast more westerly, but not quite as westerly as CLIPER, out of respect for the dynamic models.

The storm has moved more northerly since then, and this morning’s model guidance was even more clustered, with the only disagreement being on when a turn to the north will occur. One is skeptical when the models are in such tight agreement, but based on climatology the only realistic objection one could make is that maybe the turn to the north occurs further west than forecast. The overall idea that this will not be a threat to the United States is sound.

– In the Central Pacific, Hurricane Ioke intensified into a category four hurricane and continues to bear down on Johnston Island, with a forecast closest point of approach of 20 nautical miles to occur this evening.

– The tropical depression that formed in the Pacific yesterday became Tropical Storm Ileana immediately afterwards and she is now near hurricane strength. The forecast of a track parallel to, but well off of the Baja Peninsula is unchanged. Refer to the National Hurricane Center for details.

– As I type this, it appears that the majority if not all of the satellite estimates provided to the National Hurricane center this afternoon have the storm at a strength of 35 knots, which would make it Tropical Storm Debby and called as such in the 5 PM advisory package

4 PM UPDATE – Appearances deceiving and all that… the 35 knot estimate ended up being the highest of the three, the agency which had estimated 35 knots previously (and that I thought would do so again) dropped their estimate to 30 knots and the other agency kept their estimate at 30 knots, which is under tropical storm strength. It will be the forecaster’s call based on what satellite imagery shows from the past couple of hours.   Check out the National Hurricane Center page at 5 PM EDT to find out what the call is.


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