Monday, May 20, 2013

Plains storm system to make a run at the Mid-South

Spring severe weather season has been fairly quiet for much of the U.S. so far thanks in large part to an abnormally cool transition season.  However, all that changed over the past week as Gulf moisture and warm air finally started moving north into the Plains.  That warm-up and increase in humidity has been noticed here in the Mid-South the past week or so as well, as temps are regularly reaching the 80s, overnight lows are having a hard time dropping out of the 70s and heat indices are near or above 90 each afternoon.

All of this is "setup" for severe weather once Mother Nature started bringing potent storm systems into the warm, soup-y airmass. The past couple of days in the Plains have proven that the ingredients were in place as tornadoes, damaging wind, and very large hail has occurred to our west.  As the slow-moving storm system responsible for the Tornado Alley storms shifts east, the threat of severe weather moves with it.

Storm reports received by the NWS on Sunday. Over 400 severe weather reports were received .

Risk areas and what they mean

As of mid-day Monday, the Storm Prediction Center has issued a Slight Risk for all of the Mid-South for the possibility of severe storms on Tuesday afternoon and night. The higher risk of severe weather, a Moderate Risk, extends from central AR into northeast TX and does not currently include the Memphis metro.

The probabilities map that SPC issues, and is shown below, provides the chance that severe weather will occur within 25 miles of a point on the map.  For most of the metro, except the eastern sections, there is a 30% chance of severe weather (1" hail, 60 mph wind, or tornadoes) Tuesday and a 15% chance east of the Bluff City.  The highest probabilities (45%) correspond with the Moderate Risk zone above. In addition, the western part of the metro (Memphis to Tunica and west) is on the border of a "significant severe" area (what we call the "hatched area" in the business, as it is noted on the map with a black hatching). This means that there is a 10% or greater probability of significant severe weather within 25 miles of a point.  "Significant" severe weather is EF-2+ tornadoes, 75+ mph wind, or hail 2" in diameter or larger.

Remember that a Slight Risk does NOT mean the storms will be slightly, or borderline, severe, only that severe storms are not expected to be widespread, but will very likely occur.  )We believe the term "Enhanced Risk" or "Elevated Risk" would be more appropriate.)  More widespread severe weather is expected in the Moderate Risk zone, which is shown by the probabilities map. Click here for more information on risk zones and their meaning.

What to expect, and when, in the Memphis metro

Overnight tonight, storms in AR will rapidly fall apart as they move east across the state, leading to perhaps a few showers in the morning locally. However, heating of the day (highs in the upper 80s) and plenty of moisture/humidity (dewpoints near 70), along with some wind shear, will promote new storm development to our southwest during the afternoon. A few showers or a weak thunderstorm are possible in the afternoon in our area, but a "cap" of warm air over the lower atmosphere will keep things from getting out of hand.

As we head towards the evening, the storms to our west will move east or northeast into the region.  Some supercell storms, especially west of the river, could produce tornadoes. However, we expect that storms will become more linear as the sun sets and one or more squall lines will be what we deal with locally. These storms will be capable of damaging straight-line wind, large hail, and an isolated quick-hitting weak tornado. If discrete cells make it into the metro in the evening, the tornado risk would be higher.  The most widespread storms are likely to occur after 8pm Tuesday (perhaps even a couple of hours later) and last for several hours.  In fact, general thunder could occur throughout the nighttime hours into early Wednesday.  In addition to storms, there is a threat of very heavy rain producing some flash flooding Tuesday night.  With any luck, a good part of Wednesday will be precip-free.

Hi-resolution model data showing "future radar" that indicates a large area of storms could be moving into through the metro (Memphis is blue star) at 10pm Tuesday. THIS IS A MODEL, NOT A FORECAST.
Prepare ahead of time for the possibility of severe weather after dark Tuesday night. Have multiple ways to receive severe weather information, especially if you are asleep. Weather radios or smartphone apps like our StormWatch+ severe weather notifications (link below) are a good way to receive info that will wake you up.  Outdoor warning sirens are NOT.  Anytime severe weather is possible, we'll provide you with the latest via our social media feeds. Check those out below if you haven't already.

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