Wednesday, October 17, 2012

Update on this evening's severe weather risk

UPDATED 3:15pm (all updates in red): As discussed yesterday afternoon, the Mid-South has been placed under an elevated severe weather risk for late this afternoon through the evening hours.  Model data continues to support this scenario and the timing has not changed from yesterday afternoon as further data has remained very consistent.

Below are the late morning graphics from the Storm Prediction Center showing the slight risk areas and the probabilities of occurrence for tornadoes, damaging wind, and hail.  The probabilities represent the chance of the event occurring within 25 miles of a location, thus a 10% tornado risk indicates a 10% chance of seeing a tornado within 25 miles of a location (i.e., your house).  (The mid-afternoon graphics were nearly identical.)

Probabilities of tornadoes, high wind, and large hail within 25 miles of any point.
Hatched tornado area indicates 10%+ chance of an EF-2+ tornado.

The details 
We expect to see scattered storms, some supercells, develop by mid-afternoon over southern Missouri and Arkansas as very strong upper level wind overspreads that area.  As the high upper-level wind spreads east and the cold front moves east, storms will merge into mini-lines that will then congeal into a large squall line.  We now expect there could be two lines that affect the metro before they merge into one larger line to our east.  All storms will occur during the 6-11pm time frame and move quickly east, bringing a threat of strong to severe, possibly damaging and widespread, straight line wind to the metro.  This wind threat is significant enough that you should plan on the possibility of power outages and small trees down.

High-resolution computer model data (RAP model) indicating storms in and around the metro at 8pm.
Actual arrival time could vary by 1-2 hours from this depiction

A note on the tornado threat
An isolated tornado could also form within individual storms in the squall line, though these types of "spin-ups" tend to be short-lived and weak (as tornadoes go).  There is a slightly greater risk of tornadoes, which could be stronger, in any supercells that form separate from the line, which we expect could occur mainly to our west (before any lines come together) and south (where the line may not be as well developed).   The threat to the metro is very small and is most likely between 6-8 pm.  We gauge the risk of individual supercells with tornadic potential to be low, but not non-existent, in the metro.

"Be prepared, not scared"
Please review the safety tips below and prepare ahead of time for the possibility of strong wind and potential power outages.  If you have outdoor plans this evening: we EXPECT everyone in the metro will see at least one line of storms with deadly lightning and strong wind between 7-11pm.  The storms will be moving quickly and weather could deteriorate in just a few minutes.  If you hear thunder or see a shelf cloud (long line of dark low level clouds) approaching, it's time to seek shelter under a sturdy structure or in your vehicle.  If sirens sound or your smartphone app alerts you to a Tornado Warning - SEEK STURDY SHELTER WITHOUT DELAY.

MWN event coverage
We will be nowcasting this weather event live on Facebook and Twitter to keep you up to date with the very latest on timing, threats, and warnings issued.  In addition, we cannot stress enough the need for multiple ways of receiving severe weather information - radio, TV, weather radio, smartphone apps, etc.  Do NOT  rely solely on outdoor sirens to alert you to severe weather!

We certainly hope that you will consider the MWN mobile apps for iPhone/iPad and Android devices, which feature StormWatch+.  For a small one-time fee within the app, you get peace of mind knowing that any warnings issued for your location (not your county) will be pushed to your phone without delay.  Links to the apps are below.

Visit on the web or on your mobile phone.
Download our iPhone or Android apps (click here for mobile link),  now with StormWatch+!
Nowcasting services available on our Facebook page and Twitter feed.

No comments: