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 , 8:19 PM GMT on September 03, 2008
Tropical Storm Hanna is becoming more organized, according to the latest satellite imagery and data from the Hurricane Hunters. The latest center report at 3:44 pm EDT found the pressure had dropped 5 mb in just two hours, down to 990 mb. Satellite loops show that wind shear continues to interfere with Hanna's organization, with most of the heavy thunderstorms limited to the northeast side of the storm. However, the amount and intensity of these thunderstorms has increased in recent hours. Wind shear has dropped to a moderate 15 knots over Hanna--the lowest shear the storm has seen. The direction of the upper level winds creating this shear has shifted from northwesterly to southwesterly today, and this new shear direction should keep Hanna's heaviest thunderstorms on the north side of the circulation center for the remainder of the storm's life. The new shear direction was a welcome change for northern Haiti, where Hanna's flooding rains have killed 26 people. Satellite estimates suggest six inches of rain has fallen on northern Haiti and the northern Dominican Republic from Hanna.
The track forecast for Hanna
Hanna has finally begun its turn to the northwest, after moving farther east than most of the models expected. The next set of 18Z model runs, available tonight at about 8-9 pm EDT, should have a pretty good handle on where Hanna will make landfall, since the storm is done with its erratic movement. A landfall location near the South Carolina/North Carolina border Friday night is my forecast. On Saturday, Hanna will be racing north along the East Coast, bringing tropical storm conditions to the mid-Atlantic and New England states.
The intensity forecast for Hanna
The wind shear is forecast to remain in the moderate to marginal range, 15-25 knots, over the remainder of Hanna's life. There is a large amount of dry continental air lying between Hanna and South Carolina, which will continue to cause problems for the storm. However, sea surface temperatures are a warm 29°C, with a Tropical Cyclone Heat Potential (TCHP) of 40-70, just below the value of 80 typically associated with rapid intensification. The main intensity models--GFDL, HWRF, SHIPS, and LGEM--all keep Hanna as a tropical storm for the remainder of its life. However, given the large size of this storm and its proven resilience to wind shear, I give a 60% chance intensification to a Category 1 hurricane will occur.
Figure 1. Microwave image of Hurricane Ike at 12:29 pm EDT 9/3/08 showing a developing eyewall that looks like the number "6". Image credit: Navy Research Lab, Monterey.
Tropical Storm Ike is probably a hurricane now, and has the potential to become a large and dangerous Cape Verdes-type hurricane by Sunday. Visible satellite loops show that heavy thunderstorm activity has now wrapped all the way around the core of the storm, and microwave imagery (Figure 1) shows that Ike has built an eyewall. An eye has just appeared on visible satellite imagery, as well. Upper-level outflow is good, and Ike is in a very favorable upper-level wind environment, with an upper-level anticyclone overhead, and wind shear less than 10 knots. Ike has moistened the atmosphere around it enough to wall off a large amount of dry air that surround it. Rather cool sea surface temperatures (SSTs) of 27.5°C will quickly warm to 29.5°C over the next two days, but the shear is forecast to increase to 20-35 knots Thursday through Friday. Ike will probably remain a Category 1 hurricane through Friday, then enter a period of rapid intensification to major hurricane status Saturday or Sunday, assuming that it maintains its eyewall through Thursday and Friday's shear. The GFDL model forecasts Ike will hit the Dominican Republic Saturday night as a Category 3 hurricane. The HWRF model has Ike missing Hispaniola, and plowing into the southeastern Bahamas as a Category 4 hurricane on Sunday and Monday. Northern portions of the Dominican Republic and Haiti should anticipate the possibility of heavy rains of 4-8 inches on Saturday and Sunday, even if the storm passes north of the island. I expect Ike will become a major hurricane on Saturday or Sunday.
The longer term fate of Ike remains uncertain--as usual. The three long-range models (ECMWF, GFS, and NOGAPS) all forecast Ike will recurve on Tuesday. This recurvature will happen between South Florida and the central Bahama Islands, taking Ike out to sea or very near the Outer Banks of North Carolina. The timing and strength of trough of low pressure expected to turn Ike north is uncertain at this time. This will depend, in part, how strong Hanna gets, since Hanna will be interacting with this trough. Ike could turn sooner than the models predict, affecting just the Bahamas, or later than predicted, taking it through South Florida, the Florida Keys, or Cuba and into the Gulf of Mexico. The models will have a clearer picture of things by Friday, when Hanna will make its intentions known. I don't have a guess at this point which way Ike will go.
Tropical Storm Josphine is a long way out to sea, and it will be at least a week before it may threaten any land areas.
Elsewhere in the tropics
The GFS model is forecasting that at least two more tropical waves will move off the coast of Africa over the next ten days and develop into tropical storms.
My next blog entry will be Thursday morning.
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