Monday Morning Isaac Update
At 4 am CST Tropical Storm Isaac continued to move to the WNW at 14 mph. Maximum sustained winds were 65 mph and the central pressure was hovering around 990 mb. Ironically, we are just two days shy of Hurricane Katrina’s anniversary. Originally I thought an east coast track was more likely and clearly I was wrong. My biggest error was believing Isaac would become stronger sooner, allowing upper level steering currents to take it north. The upper air analogs from a week ago did include several Gulf of Mexico storms including Hurricane Elena of 1985 and Hurricane Katrina of 2005.
Hurricane Warnings are posted for much of LA, MS, and AL shorelines. I want to caution against focusing on where the center of Isaac will make landfall. A best case scenario would be for a track to the east of New Orleans, but that will mean heavy impacts from Bay St. Louis east through Biolixi and Mobile. The worst case scenario is for New Orleans is for Isaac to track to the southwest as a major Hurricane. This would really test the levees and would affect the entire countries wallet because there are many pipelines and oil rigs in this vicinity. However regardless of where Isaac makes landfall, the wind field is large and impacts will be widespread ranging from minor to extensive (or in worst case, extreme).
Isaac has much in common with the last two Hurricanes to strike the United States, Irene and Ike. Ike and Irene were both large hurricanes with large wind fields. Each storm had much deeper pressures than their wind speeds indicated. Ike for example approached Texas with a pressure of 945, but a wind speed of 105 mph. A 945 mb pressure is consistent with that of a high end Category 3/low end Category 4 hurricane. Hurricane Irene had a pressure of 950 (a solid category 3) but winds of 75 mph. Isaac has struggled to develop an inner core and appears to be dispersing its energy over a wide area. The NHC discussion notes wind shear on the south west side of the system and still a bit of dry air battling the system. Given the warm waters of the Gulf, pretty good anticyclonic outflow and relaxing wind shear in the next 12 hrs, Isaac should become a strong Category 2/low end Category 3 hurricane.
Isaac has proven throughout its life history that it is a heavy rain producer. Jiang, Halverson et al studied the differences in rainfall potential in Hurricanes Isidore and Lily in 2002 and found that storms that produce heavy rain early in their history will continue to do so. With that in mind here is what the HPC is predicting for Isaac.
Freshwater flooding ripped apart New York, New Jersey and Vermont last August as Irene and then Tropical Storm Lee moved through the region. The storm surge is worrisome as the gulf coast is vulnerable due to geography.
To our friends on the gulf coast we are thinking of you today, prepare and listen to local authorities if/when its time to evacuate. Stay tuned to Tornado Titans for updates all day, as well as your local NWS office and the NHC.