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 , 2:32 PM GMT on September 19, 2007
A non-tropical low pressure system (93L) developed yesterday afternoon off the southeast coast of Florida, and is bringing heavy rains to central Florida today. Animations of long-range radar out of Melbourne, Florida and satellite loops show a large, disorganized area of thunderstorms affecting the Florida Peninsula and adjacent waters. The surface center of circulation is right at the coast north of West Palm Beach. Water vapor satellite loops show that the upper level low pressure system that helped spawn 93L has moved off the southwest coast of Florida, and is headed westward across the Gulf of Mexico. This upper level low is bringing about 20 knots of wind shear over 93L, and pulling in some dry air from the northwest. As 93L traverses Florida today and tonight, it will bring heavy rains to the state. Radar estimated rainfall has been less than three inches thus far. No development of 93L is expected until the surface circulation emerges off the Gulf Coast of Florida Thursday.
Figure 1. Current long range radar out of Melbourne, Florida.
Once 93L emerges into the Gulf of Mexico Thursday, all of the models indicate the storm will intensify. This is a very complex forecast situation, since the storm is starting out with the cold core of an ordinary non-tropical low pressure system, and will transition to a warm-cored tropical storm. The transition to a warm core system will probably take at least a day. A storm undergoing such a process cannot intensify rapidly while this is occurring. This means that if 93L hits New Orleans Friday night/Saturday morning as the GFS and GFDL models are predicting, the storm will likely still be below hurricane strength--as predicted by the 8am EDT run of the SHIPS intensity model--or a minimal Category 1 hurricane--as predicted by this morning's 06Z run of the GFDL model. I think a tropical storm is more likely. Such a track would take it just north of the high heat content waters of the Loop Current in the central Gulf of Mexico (Figure 2). If 93L takes a more southerly track as the ECMWF and NOGAPS models predict, it will have an extra day over water, and more time to firmly establish a warm core. A warm core, fully tropical system is capable of must faster intensification rates. A more southerly track would also take the storm over the high heat content waters of the Loop Current, further aiding the transition to a warm core system. Texas could see a Category 1 or 2 hurricane on Sunday in this scenario. Slowing down the intensification will be the presence of plenty of dry air to the northwest, however, and a tropical storm may be all that Texas would see.
The NOAA jet is scheduled to fly this evening and collect data to help with the Wednesday evening (00Z Thursday) model runs. The first Air Force Hurricane Hunter mission is scheduled for Thursday afternoon.
Figure 2. Tropical Cyclone Heat Potential (TCHP) for September 18, 2007. TCHP is a measure of the total heat energy available in the ocean, and values greater than 90 kJ per square centimeter can trigger rapid intensification of tropical cyclones. The very high values of TCHP in the central Gulf of Mexico are associated with a warm ocean current known as the Loop Current. Image credit: NOAA/AOML.
Remains of Ingrid
The remains of Tropical Storm Ingrid are still kicking up some heavy thunderstorm activity a few hundred miles north of the northernmost Lesser Antilles Islands. There is still a bit a spin evident on satellite loops and this morning's QuikSCAT pass. Wind shear has dropped from 30 knots yesterday to 20 knots today, and is expected to decline below 10 knots by Thursday. We will need to watch this area for development. Ingrid's remains are moving slowly northwest. Steering currents are weak in the area, and the system will probably not move much over the next five days.
Typhoon Wipha whiffed
Typhoon Wipha made landfall just south of China's most populous city, Shanghai, at 3 am local time this morning. Wipha weakened significantly to a Category 2 storm just before landfall, and then to a tropical storm as it passed west of Shanghai. Damage from Wipha's winds and rain was far less than originally feared.
Figure 2. Radar images of Typhoon Wipha as it made landfall just south of Shanghai, China. Image credit: Taiwan Central Weather Bureau.
I'll have an update by 4pm EDT this afternoon when the next set of computer model runs become available for 93L.
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