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 , 9:22 PM GMT on September 02, 2008
With four named storms going at once--Gustav, Hanna, Ike, and Josephine--the tropics are exceptionally active today. The last time there were four named systems present on the same day was on August 24, 1999, when Bret, Cindy, Dennis and Emily were all active in the Atlantic. Four hurricanes have occurred simultaneously on two occasions. The first occasion was August 22, 1893. The second time was on September 25-27, 1998, when Georges, Ivan, Jeanne and Karl were all hurricanes. There have been five named storms at once--this occurred in 1971, from September 10 to 12.
Hanna has weathered the worst of the wind shear that has been affecting it, and is holding its own. The strong upper-level winds from the north have weakened, allowing the wind shear to fall from 30 to 25 knots this afternoon. Satellite loops show that Hanna has responded by building a little more heavy thunderstorms near its center, although these thunderstorms are still absent on the northwest side of the storm.
The track forecast for Hanna
The current steering flow driving Hanna to the southeast is very weak, and we can expect erratic motion over the next day. By Wednesday, a rather strong high pressure ridge will build over Hanna, forcing it northwest to a landfall in the Southeast U.S. Due to the storm's expected rather random motion over the next day, plus the expected track of Hanna parallel to the Southeast U.S. coast, the location of final landfall has a much higher uncertainty than usual. South Carolina would be the best bet, since it sticks out further than northern Florida and Georgia.
The intensity forecast for Hanna
The wind shear is forecast to gradually relax to 10-15 knots by Wednesday morning, and remain in the 10-20 knot range until landfall Friday. This reduced shear should allow Hanna to intensify, as the storm will be over warm 29°C waters with a Tropical Cyclone Heat Potential (TCHP) of 50-70, just below the value of 80 typically associated with rapid intensification. The GFDL, HWRF, and SHIPS models all respond by intensifying Hanna to a Category 1 hurricane by landfall in the Southeast U.S., which is a reasonable forecast.
Tropical Storm Ike continues getting organized over the middle Atlantic, and has the potential to become a large and dangerous Cape Verdes-type hurricane by Sunday. Visible satellite loops show a large and expanding circulation, with good upper-level outflow developing in all quadrants. Ike is in a very favorable upper-level wind environment, with an upper-level anticyclone overhead, and wind shear less than 10 knots. Heavy thunderstorm activity has increased significantly today, as Ike has moistened the atmosphere around it to wall off dry air that was interfering with its organization. Rather cool sea surface temperatures (SSTs) of 27.5°C. SSTs will gradually warm to 29°C over the next four days, but the shear is forecast to increase above 20 knots by Thursday. The SHIPS model responds by strengthening Ike only to a Category 1 hurricane tomorrow, then weakening it to a tropical storm during the higher shear, then strengthening it again to a Category 1 hurricane by Sunday. The HWRF and GFDL models do not predict the shear will affect Ike as much 3-4 days from now, and intensify the storm into a Category 2 or higher hurricane by Saturday, when it is expected to be entering the eastern Bahama Islands. These models no longer predict landfall in the Dominican Republic or Haiti.
The longer term fate of Ike is highly uncertain. The ECMWF model forecasts that Hanna will be strong enough to create a weakness in the ridge of high pressure steering Ike to the west. Ike will then follow Hanna's path, recurving northwards off the Florida coast, with a pass very close to North Carolina and New England. The NOGAPS model thinks Hanna will not influence Ike as much, and Ike will hit South Florida. The GFS model is no help, it dissipates Ike in the Bahamas. A track across Cuba and into the Gulf of Mexico or the Western Caribbean is also a strong possibility. We'll have to see how strong Hanna gets before the future path of Ike will be apparent.
Tropical Storm Josephine is looking pretty impressive for a tropical storm that just formed today. However, it's currently looking like Josephine may not trouble any land areas, and I'm going to give it short shrift today and move on.
Elsewhere in the tropics
There's a tropical wave over Africa behind Josephine that the GFS model forecasts will develop into a tropical storm next week.
My next blog entry will be Wednesday morning, and I'll have a summary of the devastation Gustav wrought along its path.
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