Ida takes aim at the U.S. Gulf Coast

Hurricane Ida burst into the Gulf of Mexico as a Category 2 hurricane with 100 mph winds this afternoon, and is poised to deliver a solid blow to the U.S. Gulf Coast between Southeast Louisiana and the Florida Panhandle on Tuesday morning. Radar imagery out of Cancun reveals that Ida has retained its tight inner core this afternoon, with only limited rain bands affecting Mexico and western Cuba. Top winds at Cancun, Mexico today were only 15 mph, despite the fact that Ida passed just 60 miles east of the city. Infrared and visible satellite loops show little change in the intensity of Ida's heavy thunderstorms this afternoon, but the cloud pattern is beginning to become distorted due to strong upper-level winds from the southwest that are creating 20 - 25 knots of wind shear over the hurricane. Water vapor satellite imagery reveals a large area of dry air to the southwest of Ida, but this dry air has not yet intruded into Ida's core. The latest 5:30 pm EST vortex report from the Hurricane Hunters showed that the central pressure had risen 1 mb, to 977 mb, but that the surface winds were still near 100 mph. They noted that the eyewall was open to the east, a sign that Ida's inner core may be in trouble.


Figure 1. MODIS satellite image from NASA's Terra spacecraft, taken at 1:35 pm EST 11/08/09. Image credit: NASA.

The intensity forecast for Ida
The high wind shear of 20 - 25 knots currently affecting Ida is forecast to persist at that level until Monday night. With the storm now beginning to show a distortion of the cloud pattern due to this shear, it would not be a surprise of the shear managed to inject some dry air into Ida's core Monday morning, significantly weakening the storm. Aiding this process will be cooler waters. Early Monday morning, Ida will be crossing over waters of 26°C, which is barely enough to support a hurricane. By Monday night, wind shear is expected to increase to 40 knots, which ordinarily would begin to tear the storm apart. This wind shear is due to an extratropical low pressure system over the western Gulf of Mexico, and this low will begin dumping cold, stable air into Ida Monday night through Tuesday. This will cause Ida to begin transitioning to an extratropical storm, and it is possible that during the transition Ida's winds will die down relatively slowly, despite the wind shear. The intensity forecast has a high amount of uncertainty, and I wouldn't be surprised at a landfall strength anywhere in the range of 45 mph - 80 mph. My personal best guess is a 50 - 60 mph tropical storm at landfall. Regardless of Ida's strength at landfall, the storm will be able to dump 4 - 8 inches of rain along the Gulf Coast, from New Orleans to Panama City. Isolated tornadoes will also be possible, primarily to the right side of Ida's track.


Figure 2. Maximum storm surge (depth of water above ground) for a Category 1 hurricane with 85 mph winds moving north-northeast at 15 mph. Ida is expected to be moving at this speed and direction at landfall, but will probably be weaker, so surges will not be this high. Image credit: NOAA SLOSH model run for Mobile Bay, 2008 version. Heights are given relative to the NAVD88 datum.

The storm surge forecast for Ida
Storm surge is the other major concern for Ida. With a strong high pressure system anchored over the U.S. today, the pressure difference between this high and Ida is creating a strong pressure gradient that will drive tides 3 - 5 feet above normal from New Orleans to the Florida Panhandle tonight. As Ida approaches on Monday, an additional rise in water will occur, and a large stretch of coast will be subject to very high water levels for an extended period of time. With battering waves building Monday afternoon into Tuesday morning, a significant coastal erosion event is shaping up. A particular concern is the western end of Alabama's Dauphin Island, where storm surges from four hurricanes over the past fifteen years have caused heavy damage to the low-lying heavily developed island. If Ida makes a direct hit on Mobile Bay as a Category 1 hurricane with 85 mph winds, a storm surge of up to six feet is possible there, assuming the storm hits at mean tide. The tidal range along the Gulf Coast varies by about 1.5 feet between low and high tide (Figure 3). High tide is near 2 am EST on Tuesday, but the official NHC forecast currently has the storm hitting between 6 am - 9 am, when the tide will be going out, so the storm may indeed be hitting at about mean tide.


Figure 3. The tide forecast for the Gulf Coast calls for high tide to occur around 2 am EST Tuesday morning. Ida is expected to hit later that morning, while the tide is going out.

I'll have an update Monday morning.

Links to follow:

U.S. Severe Weather Page
Cancun radar

Jeff Masters