The lull ends / Tropical Depression Three forms / Is Four on the way?
July 30, 2007
After two and a half days of scrutiny that involved a sudden appearance followed by peaks and valleys in appearance, and seemingly missing its window for development, the area of low pressure northwest of Bermuda was designated as Tropical Depression Three by the National Hurricane Center this evening
This comes as a bit of a surprise, because while the system became organized, it did not have much of the appearance of a pure tropical system. The analysts at the Satellite Services Division (who, along with the Tropical Analysis and Forecasting Branch analyze satellite imagery to provide estimates of storm position and intensity), had classified it as being sub-tropical this afternoon and extra-tropical this evening.
The depression, which is not a threat to the United States, is given a 24 hour window to have a chance at becoming Tropical Storm Chantal before it completely sheds its tropical characteristics. I’m doubtful of this occurring… however, I was doubtful of this depression forming in the first place, so we shall have to wait and see.
In the middle of last week , a couple of the global forecast models started to sporadically forecast tropical cyclone formation in the Tropical Atlantic. While it is rare that one of those forecasts actually verifies (i.e. it actually forecasts a storm that forms exactly as predicted), such forecasts are a useful indicator that conditions are changing to become more favorable for development and while maybe none of the particular tropical waves in existence will actualy develop, its likely that one of the next few will.
The first such candidate is currently about 800 miles west of the Windward Islands and is described thusly in this evening’s Tropical Weather Outlook:
A WESTWARD-MOVING TROPICAL WAVE LOCATED ABOUT 800 MILES EAST OF THE
SOUTHERN WINDWARD ISLANDS CONTINUES TO GENERATE SHOWERS AND A FEW
THUNDERSTORMS. ENVIRONMENTAL CONDITIONS APPEAR FAVORABLE FOR SOME
DEVELOPMENT OF THIS SYSTEM OVER THE NEXT COUPLE OF DAYS.
This afternoon, and early this evening, visual and infrared satellite imagery indicated a circulation center along 10° North, that was fairly devoid of convection (with Quikscat indicating that there was somewhat of a surface circulation). As the evening progressed, convection started to spring up 80-100 miles to the northwest and it appears that the circulation is reforming to the north-east of what existed earlier.
The system is in a somewhat favorable enviroment; wind shear is under 20 knots and the dreaded Saharan Air Layer, which had a strong grip on the basin for much of the month is not a factor at this time.
The big factor to its advantage, which is what causes the SHIPS intensity model to forecast it eventually become a hurricane, is the big jump in oceanic heat content as the system moves westward. Its current motion would bring it into the area of increased heat content on Wednesday. The area where the heat content starts to increase is coincidentally near the eastern edge of the area of responsibility for the Hurricane Hunters. If this system shows signs of development, a plane will check it out on Wednesday afternoon.