Karen Having Trouble With Dry Air and High Wind Shear

Tropical Storm Karen is having trouble with dry air and high wind shear as the storm heads north-northwest at 12 mph into the Gulf of Mexico. An Air Force hurricane hunter plane is in the storm, and found top surface winds near 65 mph between 3:30 - 4:30 pm EDT Thursday, and a central pressure of 999 mb, 5 mb lower than this morning's. Satellite loops show that Karen is a medium-sized storm whose heavy thunderstorms have declined in intensity and areal coverage since this morning. The heavy thunderstorms are all on the northern and eastern flanks of the storm, and the low-level center is exposed to view. High wind shear of 20 knots, due to strong upper-level winds out of the west-southwest, is driving dry air from the Western Gulf of Mexico into Karen's core. Heavy thunderstorms are having difficulty building on the west and south sides of Karen's center of circulation because of the shear, resulting in a lopsided comma-shape on satellite imagery. Karen is attempting to build an eyewall, and has managed to wrap a band of heavy thunderstorm about half way around its center. If this band wraps all the way around, Karen will likely be able to intensify into a Category 1 hurricane. Karen has a strong upper-level outflow channel to its north that is helping ventilate the storm, and ocean temperatures are a very warm 29°C (84°F). Ocean heat content is about 30 kJ per square centimeter, which is fairly typical for this time of year, and does not increase the odds of rapid intensification. Strong southeasterly winds ahead of Karen are already pushing tides 1 - 1.5' above normal along the coast from Eastern Louisiana to Alabama, as seen on our wundermap with the storm surge layer turned on.


Figure 1. Predicted 3-day rainfall totals for Karen, generated at 3:14 pm EDT Thursday October 3, 2013. NHC now puts this product on their website.


Figure 2. Ocean heat content (also called the Tropical Cyclone Heat Potential, TCHP) for October 3, 2013, along the path of Tropical Storm Karen, was about 20 - 40 kJ per square centimeter. This is a fairly ocean heat content for this time of year, and does not increase the odds of rapid intensification. TCHP values above about 75 kJ per square centimeter are typically associated with rapid intensification. Image credit: NOAA/RAMMB.

Forecast for Karen
Wind shear is expected to increase as the storm heads north-northwest, and shear will be quite high, 25 knots, on Saturday, as Karen closes in on the U.S. Gulf Coast, according to the 2 pm EDT SHIPS model forecast. The atmosphere will grow drier as Karen moves into the Northern Gulf of Mexico, and the drier air combined with increasing wind shear will retard development, making only slow intensification likely through Friday. A trough of low pressure and an associated cold front will be moving through Louisiana on Saturday, and the associated upper-level westerly winds will be able to turn Karen more to the northeast as it approaches the coast on Saturday. The higher shear, combined with ocean temperatures that will drop to 28°C, should be able to induce weakening, and the 5 pm EDT Thursday wind probability forecast from NHC gave a 23% chance Karen will be a hurricane at 2 pm EDT Saturday, down from 41% odds at 2 am EDT Saturday.

The models are split into two camps for Karen's track. The European, UKMET, and GFDL models have Karen making landfall over Central or Eastern Louisiana. These models keep Karen relatively weak, resulting in a path that follows the low-level winds more to the west, where there is more dry air and higher wind shear. The GFS model and HWRF model keep Karen stronger, and predict a landfall in the Western Florida Panhandle. NHC is splitting the difference between these two solutions, which is a reasonable compromise. Most of Karen's heavy thunderstorms will be displaced to the east by high wind shear when the storm makes landfall, and there will likely be relatively low rainfall totals of 1 - 3" to the immediate west of where the center. Much higher rainfall totals of 4 - 8" can be expected to the east. To judge the possibilities of receiving tropical storm-force winds at your location, I recommend using the NHC wind probability forecast. The highest odds of tropical storm-force winds (44 - 66%), according to NHC's 5 pm EDT Thursday forecast, are along the coast from Buras, Louisiana, to Pensacola, Florida.


Figure 3. A possible analogue for Karen: Hurricane Ida of 2009 followed a path very similar to Karen's, and was a hurricane just south of Louisiana before suddenly weakening to an extratropical storm with 40 - 50 mph winds as it made landfall in Alabama.

A possible analogue for Karen: Hurricane Ida of 2009
We have little skill forecasting intensity, and I expect that at landfall, Karen has a 20% probability of being a Category 1 hurricane with 75 - 85 mph winds, and a 20% chance of being a minimal tropical storm with 40 - 45 mph winds. One possible scenario is a repeat of what happened with Hurricane Ida of 2009. Ida took a track very similar to Karen's, and was a hurricane with 75 mph winds just south of the mouth of the Mississippi River in Louisiana. High wind shear from an approaching trough of low pressure, combined with cooler ocean temperatures near shore, combined to cause a sudden weakening of the storm. Ida became extratropical a few hours before making landfall on the Alabama coast, and brought top sustained winds of 40 - 50 mph to the coast from Shell Beach, Louisiana to Waveland, Mississippi.

Jeff Masters