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 , 2:42 PM GMT on September 29, 2007
Tropical Depression Karen has weakened to a tropical depression, thanks to ferociously high wind shear levels exceeding 50 knots. Satellite loops show Karen's exposed low-level center of circulation, visible as a swirl of low clouds. Karen continues to generate new heavy thunderstorm activity, but these cells immediately get pushed several hundred miles east of the center by strong upper level winds from the west. This morning's QuikSCAT pass missed Karen.
Wind shear is expected to remain above 30 knots over Karen through Sunday, then decline to 15 knots by Sunday night. Although the storm has shown an unusual ability to maintain a strong wind pattern in the face of high wind shear, it is uncertain if Karen can survive past today. The GFS and ECMWF models predict the shear will destroy Karen. I put Karen's survival chances at about 25%.
If Karen survives, an upper level environment favorable for strengthening is expected to set up 3-5 days from now, and Karen would probably become a hurricane. Steering currents may become weak during that period, and Karen may move very slowly. By the middle of next week, I expect a ridge of high pressure will build over the eastern U.S. and western Atlantic, forcing Karen westward towards the U.S. The path Karen might take late next week is highly uncertain. Most of the models continue to predict the formation of a new tropical or subtropical cyclone somewhere between the coast of North Carolina and the Western Caribbean on Monday. Some of the models take this new storm northeastward out to sea, which would pull Karen northward in its wake. However, most of the models predict that the new storm will take a more westerly path into Georgia or Florida, or possibly the Gulf of Mexico. A storm-storm interaction between Karen and the new storm might ensue, an event the models are poor at handling.
Figure 1. This morning's visible satellite image with wind shear contours overlaid show a very unhealthy tropical depression. Wind shear of 50 knots is over the low-level circulation center of Karen, thanks to strong upper-level winds from the west (denoted by the big white arrow at the plot's bottom). These high winds have pushed all of Karen's heavy thunderstorm activity several hundred miles downwind of the center. Image credit: University of Wisconsin CIMSS.
Lorenzo sets another rapid intensification record for 2007
Hurricane Lorenzo hit Mexico's Veracruz coast near Tuxpan early Friday morning. The storm's heavy rains have triggered mudslides blamed for at least four deaths. Lorenzo has tied the Atlantic record for fastest intensification from a tropical depression to a Category 1 hurricane--twelve hours. Hurricane Blanche of 1969 was the only other storm on record that intensified from a tropical depression to a Category 1 hurricane in just 12 hours. Hurricane Ethel of 1960 may have done so faster, though. Ethel strengthened from a 45-mph tropical storm to a 85 mph Category 1 hurricane in just 6 hours. We don't know when Ethel started as a tropical depression, since this was before the satellite era.
Reliable record keeping of intensification rates of Atlantic hurricanes began in 1970, when regular satellite coverage became available. Since 1970, Hurricane Humberto of 2007 holds the record for fastest intensification from first advisory issued to hurricane strength--18 hours. (Actually, Humberto did the feat in 14 1/4 hours, but this will get rounded off to 18 hours in the final data base, which stores points every six hours). There have been six storms that accomplished the feat in 24 hours.
Since 1970, Hurricane Felix of 2007 holds the record for fastest intensification from the first advisory to a Category 5 hurricane. It took Felix just 54 hours to accomplish the feat. Hurricane Camille of 1969 also took 54 hours to do so, but the first advisory put Camille as a 60 mph tropical storm. It is likely that Camille would have been classified as a tropical depression earlier had reliable satellite imagery been available.
Is it a statistical fluke that we've had three record-speed intensifying hurricanes this year? It could be. Our reliable data records only go back to 1970, and there may have been periods in the past with similar events. No scientist has published a paper linking rapid hurricane intensification rates with global warming. However, three record-speed intensifying hurricanes in one season certainly raises questions, and is very odd.
Tropical Storm Melissa formed this morning, far out in the eastern Atlantic. The storm is under about 10-20 knots of wind shear. Melissa will probably not affect land, since it is starting out too far north and will gain additional latitude in the coming days. The storm's expected track will take it northwest towards a region of high wind shear early next week, which should destroy the storm.
I'll have an update Sunday by noon.
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