Little Change to Karen; U.S. HIt By a Blizzard, Severe Weather, and Santa Ana Winds

Published: 1:44 PM GMT on October 04, 2013

Tropical Storm Karen is proving resilient in the face of dry air and high wind shear, as the storm heads north-northwest at 10 mph towards Louisiana. A NOAA hurricane hunter plane is in the storm this morning, and found top surface winds near 60 mph and a central pressure of 1001 mb, a pressure 2 mb higher than on Thursday evening. NOAA buoy 42001 located about 60 miles (95 km) north-northeast of the center reported a sustained wind of 38 mph, gusting to 49 mph, at 8:45 am EDT. Satellite loops show that Karen has maintained a vigorous circulation this morning in the face of high wind shear of 25 knots from strong upper-level winds out of the west. These winds have driven dry air from the Western Gulf of Mexico into Karen's core, making it difficult for heavy thunderstorms to build on the west and south sides of Karen's center of circulation. 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 20 - 40 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 pushing tides about 1 - 1.5' above normal along most of the Louisiana and Mississippi coast, as seen on our wundermap with the storm surge layer turned on.

Figure 1. MODIS satellite image of Tropical Storm Karen, taken at approximately 3:30 pm EDT on October 3, 2013. At the time, Karen had top winds of 65 mph. Image credit: NASA.

Figure 2. Brooding clouds from Tropical Storm Karen hover over the waters offshore of Cancun, Mexico, at 11 am EDT October 3, 2013. Image credit: Mindy Saylor.

Forecast for Karen
Wind shear for the next three days is expected to stay high, around 20 - 30 knots, according to the 8 am EDT SHIPS model forecast. The atmosphere is quite dry over the Western Gulf of Mexico, and this dry air combined with high wind shear will retard development, making only slow intensification possible until landfall. 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 bring higher wind shear near 30 knots and 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, may be able to induce weakening, and NHC has sharply reduced its odds of Karen achieving hurricane strength. The 5 am EDT Friday wind probability forecast from NHC put Karen's best chance of becoming a hurricane as a 23% chance on Sunday at 2 am EDT. This is down from the 41% odds given in Thursday afternoon's forecast. Most of the models show Karen intensifying by 5 - 10 mb on Saturday afternoon and evening as the storm nears the coast, as the storm interacts with the trough of low pressure turning it to the northeast. This predicted intensification may be because of stronger upper-level outflow developing (due to diverging winds aloft sucking up more air from the surface.) We don't have much skill making hurricane intensity forecasts, so I wouldn't be surprised to see Karen do the opposite of what the models predict, and decay to a weak tropical storm just before landfall, due to strong wind shear. In any case, residents of New Orleans should feel confident that their levee system will easily withstand any storm surge Karen may generate, as rapid intensification of Karen to a Category 3 or stronger hurricane has a only a minuscule probability of occurring (1% chance in the latest NHC forecast.)

Since Karen is expected to make a sharp course change to the northeast near the time it approaches the south coast of Louisiana, the models show a wide range of possible landfall locations. The European and UKMET models are the farthest west, with a landfall occurring west of New Orleans. The GFS model is at the opposite extreme, showing a landfall about 400 miles to the east, near Apalachicola, Florida. NHC is splitting the difference between these extremes, 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. NHC's 5 am EDT Friday wind probability forecast shows the highest odds of tropical storm-force winds to be at the tip of the Mississippi River at Buras, Louisiana: 66%. New Orleans, Gulfport, Mobile, and Pensacola have odds ranging from 47% - 51%.

Most significant fire threat for Southern California in the past 5 years
A Santa Ana wind event is building over Southern California this morning, where wind gusts in excess of 50 mph have already been observed this morning. From the Los Angeles NWS office:

"Most significant fire weather threat across Southern California in past 5 years as strong Santa Ana wind event unfolds. In addition to the strength of winds being projected...the concerns with this event include the widespread nature and long duration of Santa Ana winds...very long period of single digit humidities...and extremely dry fuels approaching record levels. Red flag warnings are in effect for much of Los Angeles and Ventura counties overnight into Sunday. The onset of the offshore winds are expected to begin across the mountains by late evening...then descend into the lower elevations overnight. The peak of this Santa Ana wind event will likely be late tonight through Saturday morning...with the strongest winds focused across Los Angeles and Ventura counties."

Figure 3. A moderate risk for severe weather is predicted for this afternoon over Iowa and surrounding states.

A blizzard and a severe weather outbreak in the Midwest
The same low pressure system that is expected to turn Tropical Storm Karen to the northeast this weekend is hammering the Midwest with a variety of extreme weather today. Blizzard warnings are flying in Wyoming, Nebraska, and South Dakota from the storm, and a significant outbreak of severe thunderstorms with a few tornadoes is expected over much of Iowa this afternoon. Wunderground weather historian Christopher C. Burt has done some research to see the last time a blizzard, major severe weather outbreak, tropical storm, and extreme fire danger all threatened the U.S. at the same time, and has not been able to find such an event in past history.

Portlight disaster relief charity ready to respond to Karen
The disaster relief charity, founded and staffed by members of the wunderground community, are ready to respond to Tropical Storm Karen, if they are needed. You can check out their progress on the Portlight Blog or donate to Portlight's disaster relief fund at the website.

I'll have a new post this afternoon.

Jeff Masters

Portlight volunteers at Biloxi shelter (Portlight)
Portlight volunteers at Biloxi shelter, by Portlight

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.