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African wave 99L may develop; Felicia threatens Hawaii; record rains from Morakot

By: Dr. Jeff Masters, 3:20 PM GMT on August 09, 2009

A strong tropical wave with a high amount of spin moved off the coast of Africa yesterday, and has a good chance of becoming the first named storm of the 2009 Atlantic hurricane season. The wave was designated 99L by NHC Sunday morning. Wind shear is moderate, about 10 knots, and sea surface temperatures are moderately warm, about 27 - 28°C. There is a large area of dry air to 99L's north, but this is not currently interfering with the storm's organization. This morning's QuikSCAT pass showed a nearly closed surface circulation, stretched out along two axes. The storm can now be classified as tropical depression using the satellite-based Dvorak classification technique, but the surface circulation will have to show better definition before 99L can be classified as a tropical depression. Top winds were in the 20 - 30 mph range, as estimated by QuikSCAT and the Dvorak satellite estimates.

Figure 2. Visible satellite image of disturbance 99L.

Wind shear is expected to be low, 5 - 10 knots, tonight through Tuesday. SSTs will remain relatively constant at 27°C, so conditions favor development. NHC has given 99L a moderate (30 - 50% chance) of developing into a tropical depression by Tuesday morning, which is a reasonable forecast. The UKMET, GFS, and NOGAPS models all develop 99L into a tropical depression 1 - 4 days from now. The dry air of the Saharan Air Layer may become a problem for 99L 2 -4 days from now, as the storm moves slightly north of due west. It is too early to say if 99L will recurve north of the Lesser Antilles Islands or not, since it will be at least 6 days before the storm makes it that far.

The GFS model predicts development of a new tropical wave coming off the coast of Africa about 6 - 7 days from now.

Felicia weakens to a tropical storm, still a threat to Hawaii
Tropical Storm Felicia has weakened steadily over the past 24 hours, thanks to cool sea surface temperatures and increasing shear. Recent satellite imagery shows that wind shear is beginning to affect the storm, with strong upper-level winds from the west pushing the storm's heavy thunderstorm activity all to the east side of the center.

Figure 2. History of hurricane activity over Hawaii since 1950. Hawaii islands have been hit by only 9 tropical cyclones of tropical depression or greater strength, with 4 others passing withing 75 miles of an island. Image credit: NOAA Coastal Services Center.

Sea surface temperatures (SSTs) under Felicia are 25°C, well below the 26°C threshold typically needed to sustain a hurricane. SSTs will slowly increase to 26°C by the time the storm reaches Hawaii, though. Wind shear has increased to a moderate to high 20 knots, and is expected to increase further to 30 knots by Monday morning. The high shear combined with the cool SSTs should reduce Felicia to tropical depression strength by the time the center reaches the Hawaiian Islands Monday night. The wind speed probabilities forecast shows about a 40% chance Felicia will still be a weak tropical storm when it moves through the islands Monday night, and 40% chance it will be a tropical depression. If Felicia does hold together that long, it would be only the tenth tropical cyclone of tropical depression or higher strength to affect the islands since 1950 (Figure 1). Large swells from Felicia are already affecting the Big Island, and a high surf warning has been posted for the Big Island. Felicia or its remnants should bring heavy rain, flash flooding, and mud slides to the islands beginning Monday morning, and a Flash Flood Watch has been posted for most of the islands.

Typhoon Morakot hits China
Typhoon Morakot hit China this morning as a tropical storm, lashing the mainland with very heavy rain. Yesterday, Morakot hit Taiwan, dumping up to 80 inches of rain on the island over a 2-day period, according to news reports. The world 2-day rainfall record is 98.42", set at Reunion Island on March 15 - 17, 1952, so the Typhoon Morakot 2-day totals are some of the highest ever measured in the world. The highest 1-day rainfall total ever recorded on Taiwan occurred Saturday at Weiliao Mountain in Pingtung County, which recorded 1.403 meters (4.6 feet or 55 inches) or rain. Nine the ten highest one-day rainfall amounts in Taiwanese history were reached on Saturday, according to the Central Weather Bureau. Three are dead and 31 people missing on the island, and Morakot also killed 23 people in the Philippines.

I'll have an update on Monday.

Jeff Masters


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