9 PM Update:
Main severe weather threat now pushing East out of Arkansas. Still a few isolated storms around through this evening, generally ending before Midnight.
5:30 PM Update:
Tornado Watch issued until 2 AM for most of Arkansas. A tornado watch means conditions are favorable for storms that may produce a tornado 3 PM Update:
Nothing showing up on radar in Arkansas as of 3PM. Storms are moving East through Missouri with wind damage reported around Springfield. Winds were estimated to 70 MPH. Storms will rapidly increase across Arkansas after 4 PM.
(Radar & Warnings at 3 PM)12 PM Update:
Updated tornado probabilities from the Storm Prediction Center. Highest tornado threat for Eastern and Southeast Arkansas through this evening, although still a risk for Central Arkansas as storms begin to fire up.
(SPC Probability of a tornado within 25 miles of a given point - issued 1130AM Wed Oct 17)
Given the strong wind field expected to develop ahead of the cold front, an enhanced threat of strong to violent tornadoes is indicated mainly for Southeast Arkansas. Below is the area at greatest risk for EF2 or stronger tornadoes.
(SPC EF2+ tornado probability within 25 miles of a point exceeding 10%)11 AM Update:
Storms still on track to take shape late this afternoon into this evening. Here is the latest high resolution computer model forecast for 6 PM today showing a line of storms rapidly intensifying over Central Arkansas.
(4km NAM simulated radar forecast for 6 PM Wed Oct 17 2012)Wednesday Severe Weather Discussion:
A fast moving cold front will sweep into Arkansas Wednesday Afternoon bringing the risk of some strong to severe thunderstorms. Most of Wednesday will be dry, but storms will begin to fire up along the cold front by late afternoon. Storms should initiate around 4-5 PM and increase in coverage and intensity through the early evening.
(Computer model projection for 5 PM Wed Oct 17 2012)
Storms will quickly intensify during the evening hours as they push across Central Arkansas. This will be the time frame for the greatest threat of severe storms mainly between 5 PM and 7 PM.
(Computer model projection for 7 PM Wed Oct 17 2012)
The storms will organize into a line as they push Eastward through Eastern and Southeast Arkansas later in the evening. The main threat from the storms will be strong wind gusts, however, a brief large hail and tornado threat may take shape as the storms first begin to intensify over Central Arkansas.
Later in the evening, a squall line will rapidly push across Eastern and Southeast Arkansas. Most storms will likely exit the State by about 10 PM
(Computer model projection for 10 PM Wed Oct 17 2012)
The Storm Prediction Center has placed most of Central Arkansas under a SLIGHT risk for severe weather for Wednesday.
(Storm Prediction Center Day 2 Outlook for Wednesday Oct 17th - issued Tue Oct 16 1230PM)
Below is the severe threat associated with the storms for Wednesday evening. As the storms first begin to initiate there may be a few individual cells before the storms consolidate into a line. Given an environment of strong shear (changing wind speed/direction with height), these cells will begin to rotate leading to a hail/tornado threat. Once the storms organize into a line, strong straight line wind gusts will be the primary threat. Due to the fast movement of the cold front, flooding is unlikely
Although the severe threat will not last long due to the fast moving nature of the cold front, conditions will briefly come together for severe storms including tornadoes. The front will also be associated with a strong jet stream moving into Arkansas Wednesday evening. At the same time, warm humid air will increase ahead of the front. The graphic below shows where the warm unstable air moves into an environment of strong upper level winds.
(NAM model 250mb winds/Surface Based CAPE)
In addition, winds in the lower level of the atmosphere will be out of the South to Southwest while the winds aloft will be out of the West to Northwest. this creates the shear allowing for an environment in which storms can rotate. This is why there will be a brief tornado threat Wednesday evening as the storms begin to strengthen.