Ida not weakening; spreading heavy rains inland

Data from the Hurricane Hunters this afternoon indicates that Ida has not weakened, and remains near hurricane strength. AT 3 pm EST, an Air Force airplane reported that the pressure had fallen to 991 mb, and a small area of hurricane force winds of 75 - 80 mph had developed in the outflow from a intense burst of thunderstorm activity on the southwest side of the eye. Surface winds elsewhere in the storm were 60 - 70 mph, so NHC decided that the hurricane-force winds seen by the Hurricane Hunters were a short-lived phenomena not worthy of upgrading Ida to hurricane strength for. In any case, heavy rains and storm surge, not wind, will be the primary threats from Ida. Heavy rains from the storm are lashing the Gulf Coast from southeast Louisiana to the Florida Panhandle. Rainfall amounts so far have been less than an inch, but radar estimated rainfall over the offshore waters show a core of 5 - 6 inches of rain associated with Ida's heaviest thunderstorms. Winds along the Gulf Coast of Mississippi, Alabama, and the Florida Panhandle were 20 - 35 mph this afternoon, and will steadily increase tonight as Ida draws near. Winds at the Mississippi Canyon buoy 150 miles south of Gulfport, MS were sustained at 65 mph at 1:40 pm EST, with 27 foot waves. This oil rig measures the wind at a height of 122 meters above the water, so one must reduce the winds by a factor of roughly 15% to correct the winds to the standard measurement height of 10 meters (see the journal article, GPS Dropwindsonde Wind Profiles in Hurricanes and Their Operational Implications, for more info on how to do corrections like this). This correction results in a surface wind speed of about 55 mph.


Figure 1. Long range radar out of Mobile, AL shows the eye of Ida, with the heaviest rains to the north side of the storm.

Infrared and visible satellite loops show that Ida has been holding on against high wind shear this afternoon, with heavy thunderstorm activity staying active in Ida's northern eyewall. Most of Ida's heavy thunderstorms have been displaced to the north of the storm, due to strong upper-level winds out of the southwest creating 35 knots of wind shear. Water vapor satellite imagery reveals a large area of dry air to the southwest of Ida, and the shear has now driven this dry air deep into the core of the storm, significantly disrupting it.

The intensity forecast for Ida
There is not much change to the intensity forecast for Ida. The high wind shear of 35 knots currently affecting the storm is forecast to increase to 40 knots early this evening, and 50 knots by midnight. With Ida now over waters near 26°C, which is barely enough to support a hurricane, and with water temperatures decreasing to 23°C near the coast, continued weakening is to be expected. However, Ida is beginning to transition to an extratropical storm (Figure 2). The spiral band on its east side no longer spirals around the storm, but instead hooks to the southeast like a giant comma, looking more like a front than a spiral band. It is often the case that during such a transition the winds will not die down much. I expect another bout of weakening will take Ida's top winds down to the 50 - 60 mph range along the coast tonight through Tuesday morning, though, since Ida is still partially tropical. Regardless of Ida's strength at landfall, the storm will be able to dump 4 - 8 inches of rain along the Gulf Coast and well inland. We can't rule out the possibility of an isolated tornado when Ida makes landfall, but the Storm Prediction Center Discussion maintains that the airmass in place over the Gulf Coast is relatively stable, and prospects for an appreciable severe weather threat appear low.


Figure 2. Afternoon satellite image of Ida, showing the classic signature of a tropical storm undergoing transition to an extratropical storm. There is heavy thunderstorm activity near the center, but the long band of clouds to the east of Ida doesn't look much like a spiral band--it looks more like a front.

The storm surge forecast for Ida
Tides are almost 4.0 above normal at Shell Beach, LA (on the east side of New Orleans) 2.0 feet above normal at Dauphin Island, AL, and 1.5 feet above normal at Pensacola, FL (Figure 3). NHC is calling for a storm surge of 3 - 5 feet above ground level, which is a reasonable forecast even if Ida weakens further. A large stretch of coast will be subject to very high water levels for an extended period of time today and tomorrow, with battering waves on top of the surge likely to cause a significant coastal erosion event.




Figure 3. Observed vs. predicted water levels at three coastal stations. Top: Shell Beach, LA (just east of New Orleans); middle: Dauphin Island, Alabama; bottom: Pensacola, FL. The green line shows how high above normal the water is. For Shell Beach, it was 3.6 feet above normal at 2 pm CST, 2.0 feet at Dauphin Island, and 1.5 feet at Pensacola. Image credit: NOAA Tides and Currents.

Comparing to Hurricane Kate
The last November hurricane in the Gulf of Mexico was Hurricane Kate. Kate struck the Florida Panhandle near Mexico Beach on November 21, 1985 as a Category 2 storm with 105 mph winds. According to Wikipedia, Kate killed five people in Florida and did $300 million (1985 dollars) in damage. Ida will cause relatively minor damage compared to Kate.

Links to follow:
U.S. Severe Weather Page
Wundermap for the Gulf Coast of Alabama.
Long-range radar out of New Orleans, LA.
Navarre Beach, FL webcam.
Coastal observations from the University of South Alabama.
Coastal observations from LSU.

Update on portlight.org charity errorts
The portlight.org disaster relief charity is gearing up to respond to any disaster needs from Tropical Storm Ida, and has sent a self-sufficient kitchen capable of serving 2000 meals per day to the Florida Panhandle region. They have another mobile kitchen ready to respond, if needed, as well as a truck filled with durable medical equipment and clinical/surgical supplies. Donations are welcome!

I'll have an update between 7 - 8 pm tonight.

Jeff Masters