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Subtropical Depression 17 forms; monsoon rains kill over 100 in Asia

By: Dr. Jeff Masters, 1:04 PM GMT on October 06, 2010

Subtropical Depression Seventeen formed this morning, approximately 200 miles north of Puerto Rico. The storm is not a threat to bring high winds to any land areas, but will produce heavy rains over Puerto Rico, the Virgin Islands, the northern Lesser Antilles, and perhaps the eastern Dominican Republic. Radar estimated rainfall over Puerto Rico and the Virgin Islands (Figure 1) shows rainfall amounts in excess of eight inches have fallen near St. Thomas in the Virgin Islands, and the St. Thomas Airport officially measured 6.61" yesterday--its 5th wettest day in history. St. Thomas has picked up an additional 1.48" today as of 9am AST. Not surprisingly, Flash Flood Warnings are posted for the island. Weather radar out of Puerto Rico shows that a large area of heavy rain will continue to affect the Virgin Islands and eastern Puerto Rico this morning. Martinique radar shows somewhat less activity over the Lesser Antilles.

Satellite loops show STD 17 has a broad, somewhat ill-defined center of circulation, with the heaviest thunderstorms 50 or so miles from the center. This is characteristic of a subtropical storm, which is a hybrid between a tropical storm and an extratropical storm. An upper level low pressure system to the west of STD 17 has pumped cold, dry air aloft into STD 17, keeping it from being fully tropical. As the trough gradually weakens today and Thursday, STD 17 should become fully tropical and intensify into Tropical Storm Otto. Steering currents favor Otto being lifted northwards and then northeastwards out to sea by Friday.

Figure 1. Radar-estimated rainfall from Subtropical Depression Seventeen over Puerto Rico and the Virgin Islands shows that rains in excess of eight inches (white colors) have fallen near St. Thomas.

Elsewhere in the tropics
Most of the models indicate the possibility that a strong tropical disturbance or tropical depression forming in the Southern Caribbean, off the coast of Nicaragua, 5 - 7 days from now.

Monsoon flooding kills 83 in Indonesia, 28 in Vietnam
Heavy monsoon rains triggered flash flooding in a remote section of Indonesia this week that killed at least 83 people. Another 68 people are missing, and 3,000 homeless. In Vietnam, heavy rains of up to 51" (1300 mm) have fallen since Friday, resulting in river flooding that killed at least 28 people. Over 34,000 people are homeless from the floods, which hit five provinces from Nghe An to Thua Thien-Hue, a swath of territory starting some 300 km (180 miles) south of Hanoi and stretching south. Heavy monsoon rains also hit nearby Hainen Island in China, forcing the evacuation of 64,000 people.

Jeff Masters


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