Mid-South severe weather chances on Wednesday night

As we blogged on Saturday, the Mid-South is under the gun for severe weather conditions on Wednesday night.  Multiple days of warm southerly wind and temperatures at or above 70 degrees (in fact, we could near 80 on Tuesday and Wednesday) will set the stage as a strong cold front pushes through the region late Wednesday night.

The Storm Prediction Center has placed the entire region in a Slight Risk for severe weather. (What is a Slight Risk?)  In fact, the slight risk (map below) encompasses most of the Mid and Lower Mississippi Valley, part of the Lower Ohio Valley/Midwest and central Gulf Coast as severe storms are possible from Indianapolis to Houston.

A Slight Risk encompasses a huge section of the Mississippi Valley, including the Memphis metro, on Wednesday.

The Storm Prediction Center also issues probability maps, including the chance of experiencing severe weather within 25 miles of any location. Below, the Wednesday probability map shows that there is at least a 30% chance of severe weather in the metro.

SPC graphic indicating the probability of severe weather within 25 miles of any point.  

Computer models still differ a bit on when the cold front will move through.  The GFS model, and to an extent the NAM model, maintain a fairly progressive front, meaning that it maintains steady forward progress.  They bring the front through in the midnight-3am timeframe early Thursday morning.  The European model on the other hand has been very consistent in it’s forecast that the front will slow down as it gets into the Mid-South and leans towards passage around dawn Thursday with slower progression east.  The GFS solution is shown below, but some weight is given to the European model in our forecast as it has been consistent and also has shown skill in the medium range. We are leaning towards the NAM as a happy medium for frontal passage.

GFS computer model projection of the location of the cold front and associated precipitation at 1am Thursday.

As for timing and threats, the main severe weather threat will be strong to damaging wind just ahead of the front, or probably around midnight, in association with an extensive squall line. Wind at 2-3,000′ will be screaming at 65-75 mph and any of this wind that gets transported to the surface on thunderstorm downdrafts could cause damage. An isolated tornado threat will exist along the line as well, as wind shear will be sufficient. For several hours prior to the arrival of the line, or during the evening hours, scattered storms could form over the area. Any rogue storms ahead of the line could pose a threat for strong wind, large hail, or tornadoes, though the possibility of scattered storms forming is much lower than the squall line, which is near certain. All timing forecasts are still subject to change by a few hours either way.

Finally, there is a fair amount of disagreement between the GFS and European models on how long the rain lasts behind the line (and subsequently how much we get), particularly on Thursday. The European indicates a wet day on Thursday, while the progressive GFS dries us out.  We’ll indicate a likelihood of rain, mainly Thursday morning, but not quite to the extent of the Euro model.  Flash flooding will be a concern overnight with the main batch of precip as amounts of 1-2″ in a couple of hours is plausible.Another 1″ is possible behind the front for totals in the 2-3″ range from 6pm Wednesday to 6pm Thursday.

Projected rainfall totals from Wednesday evening through Thursday, courtesy NOAA.  Amounts of 2-2.5″ are indicated for the metro.

In sum, while this is a storm system that deserves our attention, we do not expect a widespread tornado outbreak, though widespread 50-60 mph wind with areas of stronger wind are a good bet.  Hail is possible but not likely, and the flash flooding concern is moderate.

We’ll continue to keep you updated over the next 48 hours and will nowcast the event itself on our social media channels.  Prepare now for the possibility of severe weather after dark Wednesday night and how you will be notified if you are asleep when storms move through. Our StormWatch+ smartphone solution will wake you up if your location is in the path. All links are below.

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