Tropical Storm Isaac is pounding Haiti and the Dominican Republic with torrential rains that are causing extremely dangerous flooding and landslides. Isaac’s center passed over Haiti’s southwest peninsula early this morning, tracking about 50 miles west of the capital of Port-au-Prince. As the center pulled away to the northwest, Isaac’s heaviest thunderstorms moved ashore over Hispaniola near sunrise, and are now dumping heavy rains with rainfall rates approaching one inch per hour, according to recent microwave satellite estimates. Barahona on the south coast of the Dominican Republic had received 5.14″ of rain as of 8 am EDT this morning, and it is probable that some mountainous areas in Haiti and the Dominican Republic have already received up to 10″ of rain from Isaac. These rains will continue though much of the day, and have the potential to cause high loss of life in Hispaniola.

Figure 1. A river north of Port-au-Prince, Haiti in flood due to rains from Isaac. Image from Amélie Baron via Twitter.

Latest observations
Isaac built a partial eyewall last night as the storm approached Haiti, but passage over the rough mountains of Haiti has destroyed the inner core, and the surface center of the storm is now fully exposed to view on satellite images. Radar out of Guantanamo Bay, Cuba shows no sign of an organized center, but does reveal some very intense thunderstorms affecting Eastern Cuba, Western Haiti, and nearby islands. Latest data from the Air Force Hurricane Hunters confirm that Isaac has weakened; during their penetration to obtain their 7:08 am EDT center fix, the aircraft reported top surface winds of 55 mph with their SFMR instrument, top flight-level winds at 5,000 feet of 68 mph, and a pressure rise of 3 mb, to 998 mb. Infrared and visible satellite loops show that Isaac remains a large and well-organized storm, though it lacks an inner core. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is still good to the north, but is lacking elsewhere. An impressive large multi-day satellite animation of Isaac is available from the Navy Research Lab.

Figure 2. Rainfall rates estimated by the NOAA F-17 polar orbiting satellite at 6:21 am EDT August 25, 2012. Rainfall rates of 1 inch per hour (orange colors) were occurring in a large area to the south of Hispaniola, and these heavy rains have now moved onshore. Image credit: Navy Research Laboratory.

Latest model runs for Isaac
The latest set of 0Z and 06Z (8 pm and 2 am EDT) model runs are similar in spread to the previous set of runs. Our two best models, the GFS and ECMWF, are virtually on top of each other, with a landfall location in the Florida Panhandle between Fort Walton Beach and Panama City. It is likely that the trough of low pressure pulling Isaac to the north will not be strong enough to pull Isaac all the way to the northeast and out to sea, and Isaac has the potential to drop torrential rains capable of causing serious flooding over the Southeast U.S. The latest 8-day precipitation forecast from the GFS model (Figure 3) calls for 10 – 15 inches of rain over portions of Georgia and South Carolina from Isaac. The ECMWF model, however, predicts that a ridge of high pressure will build in and force Isaac to the west after landfall, resulting in a slow motion across the Tennessee Valley into Arkansas by Friday. Arkansas is experiencing its worst drought in over 50 years, so the rains would be welcome there.

Figure 3. Predicted precipitation for the 8-day period from 2 am Saturday August 25 to 2 am Sunday September 2, from the 2 am EDT August 25 run of the GFS model. This model is predicting a wide swath of 5 – 10 inches of rain (orange colors) will affect portions of Cuba, Florida, the Bahamas, and the Southeast U.S. Image credit: NOAA/NCEP.

Intensity forecast for Isaac
Isaac survived passage over Hispaniola relatively intact. It’s large size aided this, and this will also help it survive passage over Cuba today and Sunday. By the time Isaac pops off the coast of Cuba into the Florida Straits on Sunday, it will likely be a 50 mph tropical storm with a large, intact circulation. Isaac will be over very warm waters of 31°C (88°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. As I discussed in my previous post, Crossing Hispaniola and Cuba: a history, there have been five storms since 1900 with an intensity similar to Isaac, which crossed over both Haiti and Cuba, then emerged into the Florida Straits. These five storms strengthened by 5 – 20 mph in their first 24 hours after coming off the coast of Cuba. Given the relatively intact structure of Isaac so far, and the favorable conditions for intensification, I expect Isaac will intensify by 15 – 20 mph in 24 hours once the center moves off of the north coast of Cuba. 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. The latest 06Z (2 am EDT) run of the GFDL model is calling for Isaac to intensify to Category 2 strength, then weaken to Category 1 at landfall in Mississippi on Tuesday. The 06Z HWRF run is calling for landfall in the Florida Panhandle near Fort Walton Beach as a borderline Category 2 or 3 hurricane. The 5 am EDT NHC wind probability forecast gave Isaac a 19% chance of becoming a Category 2 or stronger hurricane in the Gulf. I expect these odds are too low, and that Isaac has a 40% chance of becoming a Category 2 or stronger hurricane in the Northern Gulf of Mexico. I doubt the storm has much of a chance of hitting Category 4 or 5 status, though. 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, Category 4 and 5 hurricanes in the Gulf of Mexico in 2012 will have difficulty forming.

Invest 97L off the coast of Africa
A tropical wave (Invest 97L) is located about 350 miles west-southwest of the Cape Verde Islands off the coast of Africa. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Monday morning. The 8 am EDT SHIPS model forecast predicts moderate shear for the next 5 days over 97L, so some development is possible if 97L can fend off the dry air to its north. None of the reliable models foresee that 97L will be a threat to the Lesser Antilles Islands. However, both the GFS and ECMWF models predict that a tropical wave that has not yet emerged from the coast of Africa may develop next week, and potentially take a more westward track towards the Lesser Antilles.