Latest on TS Karen from Dr. Jeff Masters

Hurricane Watches are flying along the U.S. Gulf Coast as Tropical Storm Karen heads north-northwest into the Gulf of Mexico. Karen, the eleventh named storm of the 2013 Atlantic hurricane season, formed about 8 am EDT Thursday in the Southeast Gulf of Mexico. It's not often that one sees a new storm start out with 60 mph sustained winds, but that's what an Air Force hurricane hunter plane found this morning near 7:30 am EDT, when they sampled the northern portion of the storm. A ship located about 50 miles northeast of the northeast tip of the Yucatan Peninsula measured sustained winds of 51 mph near the same time. Satellite loops show that Karen is a medium-sized storm with an area of very intense thunderstorms along its northern and eastern flanks. Wind shear has risen since Wednesday, and is now a moderately high 20 knots, thanks to strong upper-level winds out of the west-southwest. These strong winds are keeping any heavy thunderstorms from developing on the west side of Karen's center of circulation, by driving dry air that is over the Yucatan Peninsula and Western Gulf of Mexico into Karen's core. As a result, Karen has a lopsided comma-shape on satellite imagery. Karen has a strong upper-level outflow channel to its north that is helping ventilate the storm, though, and ocean temperatures are a very warm 29°C (84°F). Between 7 am and 9:30 am EDT the Hurricane Hunters made three passes though the center of Karen, and the central pressure stayed roughly constant at 1004 mb, so Karen is not undergoing much change.

Figure 1. Odds of receiving more than 4" of rain over a five-day period beginning at 2 am EDT Thursday October 3, 2013, as predicted by the experimental GFDL ensemble model.

Forecast for Karen
Wind shear will steadily increase as the storm heads north-northwest, and shear will reach a high 25 knots by Saturday morning as Karen closes in on the U.S. Gulf Coast, according to the latest 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 Friday evening and Saturday morning. The higher shear at that time should be able to induce weakening, and the 8 am EDT Thursday wind probability forecast from NHC gave a 28% chance Karen will be a hurricane at 2 am EDT Saturday, down from 44% on Friday afternoon. Most of the models predict landfall will occur along the western Florida Panhandle Saturday afternoon or evening. The usually reliable European model has Karen making landfall over Eastern Louisiana, though. If Karen does follow this more westerly path, the storm will be weaker, since there is more dry air and higher wind shear to the west. Since almost all of Karen's heavy thunderstorms will be displaced to the east by high wind shear, there will be relatively low rainfall totals of 1 - 3" to the immediate west of where the center makes landfall. 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 (45 - 55%) are along the coast from Buras, Louisiana, to Pensacola, Florida.

Read more at http://www.wunderground.com/blog/JeffMasters/tropical-storm-karen-forms-in-the-gulf-of-mexico#zw02wfBdKhId8gbk.99