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Surprise tropical storm forms near the Azores; Invest 91L has potential to develop

By: Dr. Jeff Masters, 2:19 PM GMT on October 05, 2009

Surprise! A 70-mph tropical storm popped up seemingly out of nowhere early this morning, in a region of the Atlantic not ordinarily prone to tropical storm formation. Tropical Storm Grace formed at 41.2° north latitude, in a remote ocean area near the Azores Islands. This is the farthest northeast an Atlantic tropical storm has ever formed since satellite observations began in the 1960s. Since 1960, only one tropical storm has formed farther north--Tropical Storm Alberto of 1988, which formed at 41.5°N, off the coast of Cape Cod, Massachusetts. Satellite imagery revealed that Grace formed an eyewall and well-defined eye this morning, though the storm's tropical storm-force winds did not extend out very far from the center. Last night, the center of Grace passed about 20 miles west of Ponta Delgada in the eastern Azores, which recorded sustained winds of 31 mph, gusting to 44 mph. Grace formed over chilly waters of about 23°C, well below the usual threshold of 26°C required for tropical storm formation. Grace's formation was aided by some very cold temperatures in the upper atmosphere (-54°C at 200 mb), which made the atmosphere more unstable than usual. The storm won't be around much longer, as Grace is already over much colder waters of 21°C, and is headed towards even colder waters.

Figure 1. The storm that would later become Tropical Storm Grace passes through the Azores Islands at 14:20 UTC 10/04/09. Image credit: NOAA/CIRA.

Invest 91
A large tropical wave near 12N 46W, about 1000 miles east of the Lesser Antilles Islands, is generating a considerable amount of heavy thunderstorm activity as it moves west-northwest at 15 mph. The wave is under low wind shear, 5 - 10 knots, and is over warm waters, 29°C, which is 3°C above the threshold typically needed to allow tropical storm formation. Last night's QuikSCAT pass showed a large and disorganized region of converging winds in the region, with top winds of 25 mph. There is very little dry air in the vicinity, and conditions appear favorable for some slow development of this wave. The wave is poorly organized at present, with no signs of a surface circulation visible on satellite imagery. None of the computer models develop the wave, but they do show relatively low wind shear along the wave's path for the next five days. The wave should continue to track west-northwest over the next 2 - 3 days, and spread heavy rain showers over the northern Lesser Antilles Islands by Wednesday. By Thursday, the trough of low pressure that is pulling 91L to the north should bypass the storm, allowing a high pressure ridge to build in and force 91L due west. NHC is giving 91L a low (less than 30% chance) of developing into a tropical depression by Wednesday.

Figure 2. latest images of Invest 91L.

Pacific typhoons
In the Philippines, the cleanup continues from Tropical Storm Parma, which hit northern Luzon Island Saturday as a Category 1 typhoon. Parma continues to linger offshore the northern tip of the Philippines' Luzon Island, but its heavy rain is now offshore the Philippines. The storm has been blamed for the deaths of 16 people in the Philippines, but did not have the devastating impact that was earlier feared. Storm chaser James Reynolds took some dangerous looking video of Parma in the Philippines.

Super Typhoon Melor became the second Category 5 tropical cyclone of the year over the weekend, but has weakened slightly to a Category 4 super typhoon with 155 mph winds. Melor is expected to recurve to the northeast and pass within 200 miles of Tokyo, Japan, on Wednesday night or Thursday morning.

Jeff Masters


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