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Tropical Depression Eight: the one to watch
By: Dr. Jeff Masters , 2:12 PM GMT on September 13, 2006
Hurricane Florence is now a powerful extratropical storm passing along the south coast of Newfoundland. The storm still has winds of hurricane force, as evidenced by the sustained winds of 76 mph gusting to 93 mph reported at Newfoundland's Segona Island this afternoon. The QuikSCAT imagery from this morning shows an impressive storm with 55+ mph winds affecting a large swath of ocean near Canada.
Gordon is headed out to sea.
Hurricane Gordon became the third hurricane of the season last night, and is expected to remain a Category 1 hurricane for another two or three days until increasing wind shear and cooler waters weaken the storm. Gordon is headed northward out to sea, and is not a threat to land. I'm not going to talk about this storm very much.
Figure 1. Saharan Air Layer (SAL) analysis from 9Z (5am EDT) Sep 13, 2006. The bright orange layers show where very air air laden with Saharan dust lies. Note that TD 8 has to contend with some dry air from the SAL to its north and west, while Hurricane Gordon is in a moister, more favorable environment.
The one to watch: Tropical Depression Eight
The storm we really need to focus on is Tropical Depression Eight. This depression has the potential to grow into a major hurricane that may affect Bermuda or the U.S. East Coast next week. TD 8 is currently having difficulty organizing, due to the presence of dry air to its north and west (Figure 1), and about 20 knots of wind shear. TD 8 also has a very large circulation, and as we saw with Florence, it can take such storm a very long time to organize.
The computer track models all agree on a general westward motion the next five days, taking TD 8 into the middle Atlantic. It appears at this time that the storm will gain enough latitude to pass north of the Lesser Antilles Islands, since a trough of low pressure should pull the storm on a more west-northwesterly track 3-5 days from now. After that, the future track is uncertain. The long-range GFS model shows a more westerly track and an eventual threat to Bermuda late next week, and it is not out of the question that TD 8 could make it all the way to the U.S. However, the odds are against this. History shows that the large majority of tropical depressions that form in this part of the Atlantic end up recurving harmlessly out to sea. With the active jet stream pattern we've seen since early June expected to continue for at least the next two weeks, I expect TD 8 will recurve before reaching the U.S.
Research project studying TD 8
A new research tool is being used to study Tropical Depression Eight. The driftsonde is being used for the first time to aid in hurricane research. The driftsonde is a special high-altitude balloon that floats in the stratosphere at 70,000 feet and can launch special mini-dropsondes that float down on parachutes and radio back information on winds, pressure, temperature, and humidity as they fall to earth. The driftsonde will typically launch two mini-dropsondes per day, but can launch up to one per hour if special high density data is desired. The data from these mini-dropsondes (in theory) should be making it into the global computer models that forecast hurricanes, providing valuable data over data-void ocean regions that should help provide better forecasts. The tricky part is launching the driftsondes at the right time so that they drift from Africa to the Caribbean over a developing tropical cyclone. At least seven driftsondes have been launched since August 28. The research is being done as part of an international field project to help learn about the African Monsoon and hurricane formation called the African Monsoon Multidisciplinary Analysis (AMMA). Here's a sample of the status of some of the driftsonde missions, taken from the AMMA webpage. Note that the acronym AEW refers to "African Easterly Wave", meaning the tropical waves I talk so much about.
MISSION #4 IS TERMINATED: Despite the tilted gondola, gondola #4 allowed the sampling of extra-tropical dry intrusions off the coast, west of Dakar and yesterday we dropped 4 sondes on the southeastern edge of tropical depression #7 (see attached document).
MISSION #5 IS AIRBORNE: Driftsonde #5 is flying over a streak of very moist air that we sample every three hours. It is now heading south and is located at about 11°N and 41°W.
MISSION #6 IS AIRBORNE: After sampling a weak trough of an AEW (1 sonde/3hrs), driftsonde #6 is heading to a more active area immediately off the coast where storm formation is predicted by different models (sampling strategy: 1 sonde/3hrs in the area). It should be over Dakar tomorrow and could be considered as a possible contributor to SOP-3.
MISSION #7 SHOULD BE LAUNCHED TODAY IF POSSIBLE: This driftsonde should be launched in the eastern part of a trough associated with an AEW (no possible launch the last two days due to strong wind and showers). Coordination with SOP-3 is possible with this driftsonde which should be over Dakar the day when SOP-3 begins (in 3 days).
Several of the computers models are forecasting that a tropical low pressure system might form off the Carolina coast on Friday, then scoot quickly northeastward out to sea. We'll have to watch the cold front expected to push off the East Coast Thursday to see if it spawns such a storm.
I'll be back with an update Thursday morning.
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