Severe Weather Threat Tonight

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SEVERE STORMS POSSIBLE TONIGHT: A vigorous early season winter storm will lift out of the Southwest U.S. today, headed toward the Great Lakes. This is the one that dropped two feet of snow on the Arizona mountains, and it will bring a big blanket of snow to the broad area from Denver to Milwaukee today and tonight. The deepening surface low will run from near the Texas panhandle to a point south of Chicago, which of course will have us in the warm sector of this big mid-latitude cyclone. Periods of rain will continue during the day, and as the air becomes unstable and upward vertical velocity really increases, severe thunderstorms will be possible tonight.

TIMING: The severe weather threat will begin over Mississippi this afternoon, and we now believe our greatest severe weather risk here will come from 6:00 p.m. until 2:00 a.m. The RPM brings surface based CAPE values to near 500 j/kg around the I-20 corridor, and the shear parameters are really off the chart. It is your classic case of a cold season system with low CAPE and high shear, which are always problematic. If sustained updrafts can get going, then there is no doubt a few tornadoes will be possible. Watch the Weather Xtreme video and you will see the STP values (significant tornado parameter) are maxed out around Tuscaloosa and Demopolis tonight between 6:00 p.m. and 12:00 midnight.

The greatest tornado threat is along and west of I-65, where instability values will be higher. Still, we can’t rule out a tornado somewhere over East Alabama as well. Storms tonight also have the potential to produce damaging straight line winds and some hail. Rain amounts of 1 to 2 inches are likely, and for now we don’t expect the rain to be enough to create major or widespread flooding.

We encourage everybody to be close to a good source of severe weather information tonight in the event watches and warnings are needed. Remember, never rely on outdoor sirens; a NOAA weather radio is the best way of getting warnings in your home.

TOMORROW: The rain and storms will end during the pre-dawn hours, and the sky becomes partly to mostly sunny during the day. The high tomorrow will be in the low to mid 60s with a gusty wind out of the west.

COLDER THURSDAY: A sharp change to cold weather is likely Thursday; we won’t get out of the mid 40s with a chilly north wind. Most likely, model output statistics (MOS) products will be too warm due to the shallow nature of the cold air; I figure some North Alabama communities won’t make it out of the 30s with that air coming down over the big snow pack to the north.

FRIDAY AND THE WEEKEND: The next impulse will bring a cold rain to Alabama beginning during the day, and continuing Friday night into at least part of the day Saturday. It is a close call, but for now it looks like surface temperatures will stay a little above freezing Friday night and we won’t have to deal with freezing rain, but we will fine tune this as the event gets closer. No risk of snow at all here since the cold air will be very shallow; if we have any problems it will be icing due to freezing rain. And, again for now we are going with just a cold rain. Sunday will be brighter and warmer with a high in the 50s.

NEXT WEEK AND BEYOND: Yet another impulse in the very active southern stream will bring another risk of rain by Monday night. The rest of December looks rather cold and unsettled with the El Nino enhanced southern branch of the jet stream, and lots of cold air to the north over what should be an increasing snow pack over the nation. It will be fun to watch the weather around here as we head down the stretch toward Christmas.

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We will have frequent updates here on the weather situation here on the blog tonight tonight; the next Weather Xtreme video will be posted by 3:30 this afternoon. Enjoy the day!