Friday, December 20, 2013

Severe weather possible Saturday - our thoughts on how it unfolds

As most people are aware by now, a risk of severe weather exists across area on Saturday.  All one has to do is step outside and feel the humidity, gusty south wind, and warm temperatures, then check the calendar, to know things aren't quite right in the atmosphere!

First, what we expect, for those who just want to know when it's going to rain and how bad it's going to get!

Tonight: As the MWN Forecast indicates, showers and thunderstorms are expected overnight tonight with unseasonably mild and breezy conditions.  The precipitation is expected to move in after 9pm and last intermittently throughout the night. No severe weather will occur tonight, though a few rounds of thunder are likely.

Saturday: A break in the precipitation will likely occur in the immediate metro during a chunk of the morning Saturday with rain and storms continuing to our northwest in AR.  No promises on the Ugly Sweater Run in the morning however! This will allow temperatures to climb to near-record levels in the mid 70s by noon.  By late morning or early afternoon, we expect scattered storms to again develop over the area, fueled by the rising morning temps. The severe weather risk with these cells increases as the afternoon goes along.  By mid to late afternoon, the storms should start organizing into waves or clusters of storms, perhaps with a "main round" of storms, or squall line, by late afternoon to early evening. These afternoon storms will pose a damaging wind threat, as well as a tornado threat.  In addition, outside of storms, it will be very windy during the day Saturday with gusts to 40-45 mph possible.  As my friend David in Nashville said, loosen any objects in your yard that you don't want to keep!

Below is a loop of one high-resolution model's IDEA on how storms will play out overnight tonight and through tomorrow evening. This is NOT a forecast, just a possibility. We wouldn't show it though if we didn't like the general idea it renders.

Finally, it should be noted that tomorrow's storms (due to the wind pushing them) will be moving at 50-60 mph.  If a warning is issued for your area, don't wait to see what happens - take action! At that speed, you don't have time to think about it or go look for the storm.  It could be on you before you know it.

A Moderate Risk of severe storms exists Saturday for the Memphis metro and points southwest.
Saturday night: By evening, or behind the first "main line" of storms, additional rainfall is a good bet and models are hinting at another fairly substantial line of storms during the late evening or early overnight hours.  Damaging wind will again be possible and a tornado can't be ruled out.  We expect all precipitation to move east of the metro between 10pm-midnight.  When all is said and done, we should see rainfall totals in the 1.5-2" range in the immediate metro. However, just to our northwest, in northeast AR and southeast MO, totals will be 4-6" with locally higher amounts, Flash Flood Watches are posted for these areas. After the rain ends tomorrow, we begin a stretch of cooler and dry weather that lasts through the Christmas holiday.

Rainfall totals from 6pm tonight through 6pm Sunday. 2" or so is expected in the metro while flash flooding is possible to our northwest where 4-6" is possible.
Turning a little more to the technical side... 

First, tornado threat: This event will be a typical "high shear, low instability" winter event.  While the winds at the surface and aloft are impressive and sufficient shear could be present for tornadoes (even strong ones), we'll have to watch closely if to see if instability can form. The best (or worst) case for this would be a dry morning with peaks of sunshine and temps in the mid 70s by noon.  Most other ingredients are in place, including surface low pressure that moves across AR (a classic transition season severe weather track).  If it can deepen enough, surface wind may back just enough to increase low-level helicity values sufficiently to produce tornadoes.  You'll note that this is not quite my usual spiel in which I completely downplay the tornado threat. Moderate risk areas on day 2 are not common and this setup is not "garden variety." While damaging wind (up to 70 mph) is a more likely threat due to low-level flow of 60+ knots as low as 2000-3000' above us tomorrow, enough shear could be present to spin up tornadoes. The most likely time for tornadoes is mid to late afternoon.

Second, flash flooding: we do not believe the flash flood threat is high in the immediate metro. Though precipitable water amounts are very high (over 1.5" tomorrow), the heaviest and most persistent precipitation will occur to our northwest along a stalled front that low pressure will move along and storms will be moving fast enough that we don't expect copious rainfall over a long period from any one storm or line.  However, all storms can and may produce torrential rainfall for brief periods, temporarily filling low-water crossings, ditches, etc.  If training storms develop, localized areas could see an inch or more of rain in a short period of time.

Plan ahead for what you'll do if you need to seek shelter quickly between early afternoon and late evening.  Where will you go and how will you know you need to take action?  Again, with storms moving at freeway speeds, you may not get much notice.  We HIGHLY recommend you get yourself an early Christmas present and purchase our mobile app (only $0.99 for all of our great info, radar, forecast, etc.) and THEN activate StormWatch+ on the Alerts tab for about 1/4 the cost of a weather radio. A small one-time fee gets you personalized watch and warning information with alerts that will even wake you if you are sleeping - but only if YOU are in the path of the storm. Put the MWN meteorologist on your phone and fill the preparedness void with information you can count on!  Links to the apps are below.

We'll be providing rapid and timely updates throughout the day tomorrow on our social feeds below - which means you'll know exactly what to expect and when to expect it.  It's called social media nowcasting and you won't find it anywhere else in Memphis, with a focus on JUST the Memphis metro.

Erik Proseus
MWN Meteorologist

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