Tropical Storm Karen
has slowed down a bit to 9 mph but is still heading north-northwest towards Louisiana. An Air Force hurricane hunter plane was in the storm Friday afternoon, and found top surface winds near 50 - 55 mph and a central pressure of 1004 mb. This pressure is 1 mb higher than what the Hurricane Hunters found on Friday morning. NOAA buoy 42001
reported a sustained wind of 44 mph, gusting to 51 mph, at 11:50 am EDT, when the center of Karen passed 40 miles to its southwest. The winds have declined since, as Karen has moved away. A spiral band on the north side of Karen's center of circulation moved over Southern Louisiana, Mississippi, and Alabama early Friday afternoon, bringing up to 1" of rain to isolated areas. 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. 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 12:30 pm EDT on October 4, 2013. At the time, Karen had top winds of 50 mph. Image credit: NASA.Figure 2.
Wind forecast for 5 am EDT Monday October 7, 2013, from the GFS model run done at 8 am EDT Friday, October 4, 2013. The model is showing that Karen will be at tropical depression strength with winds of at most 35 knots (40 mph) at landfall near Apalachicola. This model run was initialized using data from the NOAA jet, so should be of higher reliability than previous model runs. The image was generated using our wundermap with the model layer turned on.Forecast for Karen
Wind shear through Sunday afternoon is expected to stay high, around 20 - 30 knots, according to the 2 pm EDT SHIPS model forecast.
The GFS , European, and HWRF models predict that Karen will weaken Friday night and Saturday morning as the storm slows down and turns to the north. The GFDL model disagrees, showing slow intensification. On Saturday afternoon, an approaching trough of low pressure will turn Karen sharply to the east-northeast. Most of the models show that Karen will intensify by 10 - 20 mph on Saturday evening and Sunday morning as the storm interacts with this trough, because of diverging winds aloft that will suck up more air from the surface. However, ocean temperatures will have cooled to 28°C by then, limiting the amount of intensification that can occur. Wind shear is forecast to rise to 35 - 40 knots on Sunday evening, which may cause rapid weakening. NHC has again reduced its odds of Karen achieving hurricane strength. The 11 am EDT Friday wind probability forecast
from NHC put Karen's best chance of becoming a hurricane as a 14% chance on Sunday at 8 am EDT. This is down from the 21% odds given in the forecast six hours previous to that.
Since Karen is expected to make a sharp course change to the east-northeast near the time it approaches the south coast of Louisiana, the models
show a wide range of possible landfall locations. A double landfall is possible, one over the Mississippi River Delta in Southeast Louisiana, and another in the Florida Panhandle. This is the solution of this morning's 12Z (8 am EDT) runs of the European and UKMET models. Karen may stay just offshore of Louisiana and make only one landfall in Alabama or the Florida Panhandle, as predicted by the 12Z runs of the GFS and GFDL models. 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 makes landfall. Higher rainfall totals of 3 - 6" can be expected to the east. NHC's 11 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: 59%. New Orleans, Gulfport, Mobile, and Pensacola have odds ranging from 35% - 45%. I expect Karen will cause mostly minor damage at landfall, with flooding rains, storm surge, and a few tornadoes all a concern. Most significant fire threat for Southern California in the past 5 years
A Santa Ana wind event has begun over Southern California, where wind gusts in excess of 60 mph have already been observed at three mountain locations Friday morning. Strong winds will spread to the valleys by Friday afternoon, and continue through Saturday. 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 Friday afternoon over much of Iowa.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 conditions enveloped much of Wyoming today, with up to 2' of snow falling at several mountain locations. In Rapid City, South Dakota, where more than a foot of snow is expected, the Police Department has issued a no travel advisory. A significant outbreak of severe thunderstorms with very large hail and a few tornadoes--possibly strong EF2 and EF3s--is expected over much of Iowa early this evening. 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.
I'll have a new post Saturday by noon.