Tuesday, April 16, 2013

Another potential round of severe weather on Thursday evening

One week after our last round of storms, which was expected to bring high round and isolated tornadoes, another squall line scenario is in the forecast for Thursday evening.  Last week's front moved through slowly, with thunder for several hours overnight Wednesday night and rain much of the day Thursday.  This week's front will be much more progressive.  No, it has no interest in social reform - that simply means it will be moving much more steadily across the region, which will result in conditions improving faster behind it!

Before it gets here, a potentially significant severe weather outbreak will unfold tomorrow afternoon and evening across the Southern Plains, especially from Branson, MO to Oklahoma City to Wichita Falls, TX.  Tornadoes, large hail, and damaging wind are all possible in these areas. As the cold front begins to move into western areas of the Mid-South (AR) overnight Wednesday night, storms should form into a squall line.

Locally, the cold front will likely move through the metro Thursday evening.  In fact, output from 3 models valid at 7pm Thursday, are below and all are in generally very good agreement on the timing of the precipitation.  Scattered storms are possible ahead of the front Thursday afternoon, which the NAM seems to pick up on moreso than the other models. However the main event will occur after 7pm with rain lasting for maybe 6-8 hours total, making for a dry rush hour Friday morning (though streets could be wet from overnight rain).

Forecast precipitation from the European model for Thursday late afternoon. In this, and the model output below, the leading  edge of the heavy precipitation is near the MS River by 7pm.

Forecast precipitation from the GFS model for Thursday late afternoon. Very similar (though with higher amounts) than the European.

Forecast precipitation from the NAM model for Thursday late afternoon. The NAM seems to be picking up on some afternoon storms ahead of the main line.

As far as severe weather, the Storm Prediction Center has placed a huge area from Detroit to Houston (including the Mid-South) under a Slight Risk of severe storms. However, the probability map below highlights the area of greater concern - from southern Illinois across the western Mid-South to northern Louisiana.  In these areas, including the Memphis metro, there is a 30% chance of severe weather (damaging wind in excess of 58 mph, 1" or larger hail, or a tornado) within 25 miles of a point.  [NOTE: This area is VERY similar to the 30% risk area on the Day 3 map for last week's event, though these are two different systems and the results of one should not be forecast onto the other.]

The primary severe weather threats will be damaging wind from a squall line, some large hail, and isolated tornadoes.  In addition, though "progressive" in nature, abundant moisture from the Gulf will cause torrential downpours, which can lead to possible flash flooding under the heaviest storms.

Probability of severe weather within 25 miles of a point. The chance is 30% in the metro.

Forecast rainfall amounts from NOAA for Thursday 7pm through Friday 7am indicate  well over an inch in the metro.

We'll have more on the threats and updated timing as the system draws near.  In the meantime, with our second severe weather setup in 8 days, we're definitely into the heart of severe weather season. Keep your safety plans fresh and practiced!  We'll keep you updated on social media. You should download our mobile app and activate StormWatch+ for immediate push notifications of impending severe weather with street-level detail!  More on our offerings below.

--Erik Proseus, MWN Meteorologist

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