Midwest's February Thaw to be Followed by a Return of the Polar Vortex
A major February thaw is underway across the Midwest U.S., where high temperatures are above freezing for a second consecutive day in cities like Chicago, Minneapolis, and Detroit. The welcome thaw is bringing significant melting of the heavy snowpack over the Midwest, raising flooding concerns. Much of Illinois, Iowa, Wisconsin, Michigan, Minnesota, and Indiana are covered by snows of 10 - 30" that if melted, would be equivalent to 2 - 6" of rain. With temperatures in the 40s and low 50s coming on Thursday, accompanied by up to 1" of rain, flash flooding is expected in many regions of the Midwest. Soils are still frozen, which will limit the infiltration of the rain into the ground, providing faster run-off and greater flooding potential. Also of concern are ice jams on rivers, caused when the ice on top of rising rivers breaks up and jams together, creating blockages that back up river waters. NOAA's Storm Prediction Center has placed much of the Midwest under their "Slight Risk" area for severe weather on Thursday, with portions of Kentucky and Tennessee in their first "Moderate Risk" area of the year. Severe thunderstorms with strong winds, hail, and few tornadoes will make it seem like spring. However, it will be winter on Thursday over Northern Iowa and Southern Minnesota, where blizzard warnings are posted.
Figure 1. The equivalent amount of rain stored in the Midwest snowpack as of February 19, 2014, ranges from 2 - 6" over much of Iowa, Illiinois, Indiana, Wisconsin, Michigan, and Ohio. Image credit: NOAA/NOHRSC.
Return of the Polar Vortex Next Week
Fortunately (?) for the Midwest, this week's thaw will be short-lived, preventing the kind of major flooding that would result if all of the snowpack were to melt in a week. This morning's runs of the GFS and European models were better able to handle the evolving upper-air pattern over the Pacific Ocean, and it appears that their earlier runs seriously underestimated the strength of a ridge of high pressure forecast to build over the Western U.S. 6 - 10 days from now. This ridge will be accompanied by a return of the cold "Polar Vortex" over the Midwest and Northeast U.S., bringing bitter cold temperatures and strong winds. Temperatures 20°F below normal will likely invade the Upper Midwest on Sunday, and gradually spread southeastwards during the week. The peak cold is predicted to occur late next week, with temperatures 20 - 35° below normal covering much of the eastern 2/3 of the country. As a result of these new model runs, the natural gas market has been soaring ever since early this morning, and is now approaching a five-year high of $6.
Figure 2. The 6 - 10 day forecast for February 24 - 28, made on February 19, 2014, calls for a high probability of below-average temperatures over the eastern 2/3 of the contiguous U.S., as the dreaded "Polar Vortex" drops southwards over the U.S. again. Image credit: NOAA/CPC.
Thanks go to WSI's Mike Ventrice for the info on the natural gas market. He posts to @WSI_Energy on Twitter.
Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
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