OK

The Storm Prediction Center has taken the relatively rare step of issuing a moderate risk for severe weather three days in advance of the predicted event. As is typical for springtime severe weather outbreaks, a potent weather system moving in from the west will clash with a warm, moist airmass pumping in off the Gulf of Mexico and fire off severe thunderstorms over portions of the southern Plains.

While severe weather is expected to occur every day this week through Thursday, the biggest event will occur on Wednesday.

Here's the map from the Storm Prediction Center. Red means moderate risk, yellow means slight risk, and green means general, non-severe thunderstorms.


                                                                 
Risk Area Population Large Cities At Risk
Moderate 3,742,558 Oklahoma City, OK
Tulsa, OK
Norman, OK
Broken Arrow, OK
Joplin, MO
Slight 13,489,424 Ft. Worth, TX
Kansas City, MO
Wichita, KS
Arlington, TX
Overland Park, KS

Here are the associated percentages for the risk zones. The percentages mean that there is an xx% chance of severe weather (damaging wind, large hail, and/or tornado) within 25 miles of any point in the shaded area. The black hatching indicates the risk for significant severe weather (winds greater than 70 MPH, hail larger than golf balls, or strong tornadoes).

15% warrants a "slight" risk, so the 30% and 45% zones are the areas of greatest threat.

As this event is still a few days away, the forecasts can and will change. These risk areas are fluid -- severe weather will likely occur somewhere in the area, but the extent and geographical location will change between now and the event.

Here's some meteorological geekiness to explain why the severe weather on Wednesday could be particularly bad. The following model images are from the NAM's (North American Model) 18z (2PM Eastern) run. All model images are valid for 1PM Central Time on Wednesday.

This is the 250 millibar (25-30,000 feet up) chart showing winds and heights. Winds in the moderate risk zone are 40-50 knots from the southwest.

This is the surface wind chart, showing winds coming in from the southeast over the moderate risk zone.

Here's a hodograph from the Tulsa area to illustrate the wind shift with height. You can read about hodographs here, but the gist of it is that when the line curves clockwise like this, it indicates a risk for supercells and tornadoes. The bigger and longer the curve, the higher the risk.

This wind shift will create what are essentially horizontal tubes of rotation in the atmosphere. When thunderstorms develop and their updrafts hit this horizontal tube of vorticity, it essentially bends the tube sort of like a slinky. The counter-clockwise turning portion of the tube becomes part of the updraft, leading to a rotating updraft.

These rotating updrafts can keep supercells alive for hundreds of miles, producing extremely large hail, strong damaging winds, and violent tornadoes.

Since the atmosphere is expected to be ripe for strong supercells, the SPC went ahead and issued a moderate risk. It could be upgraded or downgraded as more model runs come in, but since the risk exists, folks in Oklahoma, Missouri and Kansas need to take it seriously.

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TwisterData.com's excellent GFS/NAM/RAP model website.

I'll continuously post updates to my Facebook page on this and most other major severe weather outbreaks.

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