Tuesday, May 24, 2011

Severe weather outbreak possible in the Mid-South Wednesday evening

A strong low pressure system will move very slowly through Missouri Wednesday night and will push an equally strong cold front through the region by early Thursday morning. This low has already been responsible for the deadly tornado in Joplin, MO on Sunday night (which was just the highest profile of several tornadoes that day).

Today, a widespread severe weather outbreak will take place in the Plains in an area the Storm Prediction Center has tagged with a "High Risk" - indicating that destructive tornadoes and large-scale severe weather is likely.  Hopefully the strongest storms avoid highly populated areas, but there are major cities under the gun today as well. By tomorrow, the threat shifts east and the lower Mississippi River Valley and Mid-South will get their turn to deal with this powerful system.

SPC risk areas for Wednesday/Wednesday night
The map above shows the area most at risk from this "potentially volatile situation," as the Storm Prediction Center has stated.  The Moderate Risk area includes the Memphis metro.  The map below shows the probability of severe weather within 25 miles of any point, while the hatched area is the area most likely to experience "significant" severe weather.

Probabilities of severe weather within 25 miles of any point - Wednesday
Several things to cover.  First, the threats... This weather system has already shown (and will do so again tonight) the potential to produce strong tornadoes.  Little will change by the time it reaches the Mid-South.  The subtleties with a system like this are what make the difference though, and these are hard to predict even just 24 hours in advance (they're very subtle), so we prepare for their possibilities.  Bottom line: with this system, tornadoes are possible.  In addition, very large hail and damaging thunderstorm wind (straight-line wind) is also very possible.  The system will be somewhat of a slow mover and thunderstorms could affect the same areas repeatedly, so flash flooding will be another threat.

Next, timing, which is based on the arrival of severe weather ingredients... the map below was issued by the Memphis office of the NWS today and indicates the potential timing for severe weather.  I generally agree with this map and will further refine it for the Memphis metro area specifically. An ongoing round of severe weather will move across Arkansas overnight tonight. I expect this area to be weakening after midnight as it moves east of Little Rock, but we could still see some storms, possibly with strong wind gusts, very early Wednesday morning (perhaps sometime between 2-6am).

Then, clouds will thin and perhaps clear out in the morning and sunshine will help temperatures rise into the upper 80s, possibly to near 90 by mid-afternoon. If you know anything about severe weather, you know that this is not good ahead of an approaching system, as it contributes to high instability in the atmosphere. In addition, sufficient moisture will be in place as dewpoints will be in the upper 60s to near 70.  Plenty of juice. As the afternoon goes on, upper-level winds will pick up and wind at the surface will also be gusty from the south at 25-35 mph. This brings about another required ingredient - wind shear.

Finally, the trigger.  Tomorrow it will be an upper-level disturbance that makes it's appearance over Arkansas during peak heating, or mid-afternoon. Storms will fire in response to the disturbance and quickly become severe, likely supercellular with the threat of tornadoes, by late afternoon.  These storms could affect the Memphis metro area by rush hour Wednesday and will be in the form of scattered supercells or clusters of storms.  By mid-evening, say 7-10pm, the storms will likely begin to form into a large-scale squall line that will move through the metro area around midnight, give or take an hour or two.  The passage of the squall line should end the severe weather threat for the remainder of the night.

Timing of potential severe weather, as indicated by NWS-Memphis
So, in sum, the probability of severe weather is moderate, which means preparations and action plans need to be polished off.  All modes of severe weather are possible, including the risk of tornadoes. Timing appears to be between 4pm and midnight, with the greatest chance of tornadoes ahead of the main line and the greatest risk of damaging straight line wind along the line.

None of this is meant to hype the situation or scare anyone.  We don't hype at MWN - we tell it straight, but also don't sugar-coat it.  Plan ahead and know what to do should severe weather strike where you will be.  Be prepared, not scared, and rest assured that we will keep you informed.  The best way to do that is to follow our social media channels, listed below, stay in touch with MemphisWeather.net on the web, download our apps for weather on your smartphones, or hit our mobile website at http://pda.memphisweather.net/. Also, familiarize yourself with the MWN Storm Center, including the Safety Tips at the bottom of the page.  Finally, HEED ALL WARNINGS as they are issued and don't be afraid to exercise your plan.  More resources and ways to prepare are listed in this blog we posted before the April 26-27 severe weather event.

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