Thursday, April 18, 2013

4/18/13 - Severe weather scenario in the Memphis metro

UPDATE 4:50pm CDT:

A Slight Risk zone continues across the entire area as storms begin to enter the western portions of the metro.  Fortunately, no storms have prompted severe thunderstorm warnings any time recently. An atmospheric sounding taken near Cleveland, MS (in the MS Delta) by a Mississippi State University team indicated that instability over the area was poor, contributing to the lack of strength in the storms, however wind shear was still very high. We'll see if a new watch box is issued.  In the meantime, the east AR box is scheduled to expire at 5pm.

UPDATE 2:50pm CDT:

The Storm Prediction Center has dropped the entire Moderate Risk area in favor of a high-end Slight Risk as the storm system is not panning out to be as intense as forecast.  The Mid-South can still expect to have a straight-line wind threat, along with possible hail and an isolated tornado (as wind shear remains as a favorable ingredient that is in place).

3pm Convective Outlook drops the Moderate Risk in favor of a Slight Risk for the entire region.

UPDATE 2:30pm CDT:

The NWS in Memphis is downgrading the Moderate Risk to a Slight Risk. While the threat of strong to severe storms is still present, it does not appear to be high enough to support a moderate risk any longer. A potent line of storms is still expected during rush hour. Reference the NWS (technical) forecast discussion for more.

UPDATE 2:15pm CDT:

Although the tornado threat is not particularly high with this event, wind shear in the atmosphere is sufficient to support tornadoes should they form, thus we expect a Tornado Watch to be issued very soon.  The main threat continues to be damaging straight-line wind with a squall line that is marching towards the area from AR.

MWN StormView Radar shows storms moving east into eastern AR at 2:09pm.
We expect this line to reach the Mississippi River around 5pm and continue moving east through the metro. Rush hour will see torrential rain, possibly areas of standing water, the threat of strong gusty wind, and dangerous lightning. If you can leave early, it's probably a good idea to do so.

In addition, Memphis City Schools has cancelled all after-school activities due to the threat of inclement weather. This is a good move. If you have activities planned outdoors this evening, follow suit and cancel them.


Severe weather is likely in the Mid-South today. We've covered the reasons in previous posts. Today, we focus on what will happen and when. Time permitting, this post will become a live blog with updates as new information is released from NWS-Memphis and the Storm Prediction Center. For the latest, follow us on Facebook and Twitter though. Of course the website will also have current info, especially in the MWN Storm Center, where watches/warnings, SPC products, etc. are updated in realtime.

As of 10am, the entire region is under a MODERATE RISK of severe weather with significant severe storms possible. A Moderate Risk implies substantial severe storm coverage, or an enhanced chance for a significant severe storm outbreak. It also implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather and greater forecaster confidence as compared to a Slight Risk. Below are the SPC graphics for today, issued at 8am.  They will be updated a couple of times during the day.

Slight and Moderate Risk areas for today/tonight

Probability of tornadoes. The metro is in the 10% area, which is fairly high for tornadoes, and also in the hatched area, indicating a few tornadoes could be intense (EF-2+).

Probability of damaging wind. The metro is in the 45% area, which is high. Basically, our chance is nearly 50/50 of seeing 60 mph+ wind today.

What to expect

Scattered storms will be possible in the metro, though mainly in eastern AR, after about 3pm. These storms could become supercells, producing strong wind, hail, and a few tornadoes. They will be moving to the northeast at about 35-45 mph. West of any scattered cells, a squall line will be moving east into the metro between 5-8pm.  This is the most likely time for severe weather.  The squall line will be capable of producing damaging wind of 60-75 mph, hail, and isolated tornadoes as well.  The predominant threat today is straight-line wind damage from the squall line.  Rain, occasionally heavy, will continue through the evening behind the main line, tapering off by midnight. Lightning will also be possible throughout the evening. Flash flooding is a concern as some places could see 1.5"-2.0" of rain in a few short hours. Any storms that move over the city between 4-6pm could produce urban flooding on local streets and make rush hour travel difficult.

Preparation and safety tips

This is likely the greatest risk from severe weather that we have faced since early last spring, before it got hot and severe weather threats shifted well north of us. Have a severe weather plan in place and ready. Prepare for the likelihood of very strong wind and possible tornadoes, including during rush hour.  We highly recommend proactively cancelling all outdoor activities scheduled after 5pm. 

  • Charge your cell phones before you head home from work if possible. Keep ID on your person at all times.
  • Prepare for the possibility of power outages that could last into the night if they become widespread. 
  • Have your safe place ready. This should include a way to monitor the weather, flashlights, activities for the kids for a short time (with fresh or charged batteries), and bicycle helmets for the kids (to protect from any falling debris).
  • Don't count on outdoor warning sirens to warn you of possible tornadoes! You probably won't hear them, even though you think you will.
  • NOAA Weather Radio, local media sources and smartphone apps should be your primary sources of severe weather information. We highly recommend StormWatch+, found in a stand-alone app for iPhone/iPad as well as in the app for Android and iOS devices (links below).


--Erik Proseus, MWN Meteorologist

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