Karen Weakens Significantly; 4 Feet of Snow in South Dakota; 18 Tornadoes in Midwest

Tropical Storm Karen has weakened to a minimal-strength tropical storm with 40 mph winds as it heads towards landfall in Southeast Louisiana. Karen continues to struggle with high wind shear of 25 knots, due to 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. Satellite loops show the classic appearance of a sheared storm, with the low level center exposed to view, and the heavy thunderstorms pushed to one side by the high shear. A spiral band on the north side of Karen's center of circulation moved over Southern Louisiana, Mississippi, and Alabama Friday afternoon, bringing a few scattered areas of 1" of rain. Long-range radar out of New Orleans shows a few thunderstorms over land, with the bulk of Karen's rain offshore. Karen brought a storm surge of up to 1.6' above normal along the Louisiana and Mississippi coast Saturday morning, as seen on our wundermap with the storm surge layer turned on.


Figure 1. MODIS satellite image of Tropical Storm Karen, taken at approximately 12:30 pm EDT on October 5, 2013. At the time, Karen had top winds of 40 mph. Image credit: NASA.

Forecast for Karen
The computer models have come into good agreement on the track of Karen, with the storm expected to make landfall in Southeast Louisiana and pass near or to the south of New Orleans early Sunday morning. With wind shear showing no signs of letting up, any strengthening of Karen on Saturday will be slow, and it is more likely that the storm will weaken to a tropical depression with 35 mph winds before landfall. NHC's 11 am EDT Saturday wind probability forecast shows the highest odds of tropical storm-force winds to be at the tip of the Mississippi River at Buras, Louisiana: 47%. New Orleans has a 38% chance, and the rest of the coast from Mississippi to Pensacola, Florida has odds ranging from 20% - 30%. Karen should cause mostly minor damage at landfall, with flooding rains, storm surge, and a few weak tornadoes of concern.


Figure 2. iWitness Anne Zollinger captured this photo of snow accumulation in Wright, WY on October 4, 2013.


Figure 3. Radar reflectivity image of the supercell thunderstorm that spawned the tornado that hit Wayne, Nebraska (marked by the circle with "+" symbol in it.)

A blizzard and a severe weather outbreak in the Midwest
A storm far more intense and dangerous than Tropical Storm Karen is Winter Storm Atlas, which continues to pound the Midwest with a variety of extreme weather today. Blizzard conditions enveloped much of Wyoming and South Dakota on Friday, with an astonishing 48" (4 feet!) of snow falling in Deadwood, South Dakota. Check out this amazing photo of the snow there. The 43.5" of snow that fell in Lead, South Dakota was that city's fourth heaviest snowfall on record. In Rapid City, South Dakota, the airport recorded thundersnow and sustained winds of 44 mph, gusting to 55 mph at 4 pm Friday, before communication were lost. The snow tally so far in the city is 18.3", making it the sixth largest snowfall in recorded history. Casper, Wyoming received 16.2" of snow, their tenth greatest snow storm in recorded history. The storm brought a significant outbreak of severe thunderstorms with very large hail and eighteen preliminary reports of tornadoes, with the most damaging tornado hitting Wayne, Nebraska on Friday afternoon near 5:30 pm CDT, causing millions in damage, and injuring fifteen people. The severe weather threat is much less for Saturday and Sunday, with only a "slight" risk of severe weather being predicted by NOAA's Storm Prediction Center. 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, as detailed in his latest blog post.

Extremely critical fire threat continues for Southern California
A Santa Ana wind event is in its second day over Southern California in Los Angeles and Ventura Counties, where numerous locations saw wind gusts in excess of 60 mph on Friday. The top wind gust was 75 mph in Santa Paula at 4:45 am Saturday, October 5; the Naval Air Station Point Muga had a gust of 74 mph at 1:54 am Saturday. Strong wind gusts of up to 60 mph, combined with humidity levels of 5 - 10%, will make for extremely critical fire conditions again on Saturday afternoon. Fortunately, no fires were sparked on Friday. Let's keep it that way on Saturday!

I'll have a new post Sunday.

Jeff Masters