Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 5:56 PM GMT on July 31, 2010
The deadliest weather disaster of 2010 is unfolding in Pakistan, where heavy monsoon rains have triggered flooding that has left at least 817 people dead. A death toll may reach 3000, according to the local head of Pakistan's largest rescue service, and more monsoon rains are on the way. Monsoon floods have also hit southeastern Afghanistan hard, where at least 64 have been killed. The heavy rains were caused by a monsoon depression (also called a monsoon low) that formed over the Bay of Bengal on July 24, crossed over India, and reached Pakistan on July 27. The rains increased in intensity over the next two days, peaking on July 29 and 30, when a low pressure system that moved across Pakistan from the west enhanced rainfall from the monsoon depression. Over the 3-day period July 28 - 30, torrential rains in excess of 8 inches (203 mm) fell in many regions of northwest Pakistan, resulting in that nation's worst floods since at least 1929. Rainfall amounts at two stations in the catchment basins of the Jhelum River and Indus River reached 19.49" (495 mm) for the month of July, and 7.56" (192 mm) fell in a single day, July 30, at Tarbela.
A monsoon depression is similar to a tropical depression, but forms in the Indian Southwest Monsoon over the Bay of Bengal. Like tropical depressions, monsoon depressions are hundreds of miles in diameter, have nearly calm winds near the center, and can have sustained winds of 30 - 35 mph. Four monsoon depressions originated in the Bay of Bengal in 2009; the average is 6 - 7. A new monsoon depression developed over the Bay of Bengal yesterday, and is headed westward towards Pakistan. Heavy rains from this new monsoon depression will begin affecting Pakistan on Monday, according to the Pakistan Meteorological Department.
Figure 1. The heavy thunderstorms of a monsoon depression lie over northwestern Pakistan near Islamabad in this visible satellite image taken by NASA's MODIS instrument on July 29, 2010. Image credit: NASA.
Tropical Atlantic may get active by mid-week
The Invest 90L tropical wave off the coast of Africa has grown disorganized, and NHC is no longer generating forecast tracks for the system. A tropical wave that moved of the coast of Africa Thursday is in the eastern Atlantic near 10N 25W, a few hundred miles south of the Cape Verdes Islands. This morning's 12Z run of the NOGAPS model predicts that this wave will develop into a tropical storm by Wednesday, and reach the Lesser Antilles Islands Friday. This morning's 12Z run of the GFS and ECMWF models predict that an area of disturbed weather near 8N 37W, in the east-central Atlantic, will develop into a tropical storm that will move through the Lesser Antilles on Thursday. Wind shear is low to moderate, sea surface temperatures are at record highs, and the dust and dry air of the Saharan Air Layer (SAL) are far enough to the north of these disturbed areas to potentially allow formation of a tropical storm. However, the Madden-Julian Oscillation currently favors downward motion over the tropical Atlantic, which will act to decrease the chances of tropical storm formation. NHC is giving a 30% chance that a tropical depression will form by Monday afternoon from one of these areas of disturbed weather.
I'll have an update Monday morning at the latest.
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