Isaac bearing down on Haiti

Tropical Storm Isaac has remained approximately level in intensity this afternoon. An Air Force Reserve hurricane hunter aircraft measured surface winds of 65 mph at 3:55 pm EDT, with winds at the aircraft's flight level of 5,000 feet of hurricane force, 80 mph. The surface pressure is falling, and is down to 994 mb. Tropical cyclones have a warm core, and the Hurricane Hunters typically find that a storm's lowest pressure is also where the warmest temperature are. That was not the case this morning, but is the case this afternoon. The Hurricane Hunters found a modest 4°C increase in temperature as they penetrated Isaac's core at 2:43 pm EDT (that difference fell to just 1°C by 3:55 pm, though.) A ship located about 100 miles southwest of Santo Domingo in the Dominican Republic measured sustained winds of 58 mph early this afternoon, and wind gusts of 46 mph have been measured on the south coast of the D.R. at Barahona this afternoon. Infrared and visible satellite loops show that Isaac is starting to close off a center, and eyewall formation will likely begin early this evening. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow has improved considerably since this morning.


Figure 1. True-color MODIS satellite image of Isaac taken at 11:10 am EDT August 24, 2012. At the time, Isaac had top sustained winds of 60 mph. Image credit: NASA.

Latest model runs for Isaac
The latest set of 12Z (8 am EDT) model runs is similar in spread to the previous set of runs. Final landfall locations range from Alabama (ECMWF model) to the Florida Panhandle south of Tallahassee (GFS model.) It is possible that the trough of low pressure pulling Isaac to the north may not be strong enough to pull Isaac all the way to the northeast and out to sea, and the ECMWF model indicates that Isaac could stall out after landfall, and spend several days over the Tennessee Valley.

Impact on South Florida, Cuba, Jamaica, and the Bahamas
Residents of South Florida, Cuba, Jamaica, and the Bahamas should not focus overly much of the exact track of the center of Isaac, because the storm's heaviest winds and rain will be spread out over a large area, and will not be focused near the core of the storm. Tropical storm-force winds currently extend outwards 185 miles to the right of Isaac's center, and will continue to extend outwards about this far as the storm passes by the Bahamas and South Florida. Heavy rains of 4 - 8 inches will be common along this swath. I recommend using the latest wind probability forecast from NHC to see your odds of receiving tropical storm-force winds of 39 mph or higher.


Figure 2. Daily Oceanic Heat Content or Tropical Cyclone Heat Potential (TCHP) for Issac. The forecast points are from the 11 am EDT NHC advisory; the 24 hour forecast point shown here is for 8 am EDT Saturday, and the 72 hour forecast point is for 8 am Monday. For tropical cyclones in favorable environmental conditions for intensification (i.e., vertical wind shear less than 15 kt, mid-level relative humidity >50 %, and warm SSTs [i.e., >28.5C]) and with intensities less than 80kt, values of ocean heat content greater than 50 kJ/cm^2 (yellow and warmer colors) have been shown to promote greater rates of intensity change. Isaac will be in such a region when it is over water between its current location and the Florida Keys. Once Isaac goes beyond the Keys, total ocean heat content will fall to levels not as conducive for rapid intensification. Image credit: NOAA/RAMMB.

Intensity forecast for Isaac in the Gulf of Mexico
Isaac will likely be a 50 - 55 mph tropical storm on Saturday and Sunday as it moves over Cuba, but once the storm pops off the coast of Cuba into the Florida Straits, it will likely intensify. Isaac will be over very warm waters of 31 - 32°C (88 - 90°F) in the Florida Straits, wind shear will be light to moderate, and the upper-level wind pattern will feature an upper-level anticyclone over the storm, aiding its upper-level outflow. If Isaac makes landfall near the tip of South Florida, as the latest 12Z (8 am EDT) runs of the GFS, HWRF, and GFDL models are suggesting, the 24 hours it has over water before landfall will probably allow it to intensify by 15 - 20 mph. I think the storm is too large for it to increase its winds more than that in just 24 hours. If Isaac spends a full two days over water after passing the Florida Keys, it is possible that it will have enough time to develop a full eyewall and undergo rapid intensification into a Category 2 or 3 hurricane, though none of the models are currently calling for this to happen. The 5 pm EDT NHC wind probability forecast gave Isaac an 11% chance of becoming a Category 2 or 3 hurricane in the Gulf. While the surface waters in the Gulf of Mexico are very warm, near 30 - 31°C, the total heat content of these waters is unusually low for this time of year. We got lucky with the Gulf of Mexico Loop Current this summer, as it did not shed a big warm eddy during the height of hurricane season, like happened in 2005 (I discuss this in my Gulf of Mexico Loop Current Tutorial.) Without the type of super-high heat energy we had in 2005 in the Gulf of Mexico, I doubt we can get a Category 5 hurricane in the Gulf in 2012.

Invest 97L off the coast of Africa
A tropical wave that moved off the coast of Africa on Thursday has been designated Invest 97L by NHC this morning. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear of 10 - 20 knots. In their 2 pm EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Sunday afternoon. The 2 pm EDT SHIPS model forecast predicts that 97L will track west-northwest over the next few days, and now predicts moderate shear for the next 5 days over 97L. However, the storm will have trouble with dry air, and none of the reliable models currently foresee that 97L will develop over the next five days.

I'll have a new post sometime 10 am - 1pm Saturday.

Jeff Masters