Severe Weather Outbreak Thursday
We have been going over computer model data since the 12z morning runs came in and it now appears that a severe weather outbreak is setting up for Thursday with all modes of severe weather possible including tornadoes, damaging winds, hail and flash flooding. We have been watching this system since last week and now computer models are coming into better agreement on this setup so, let’s jump right in.
Currently the SPC has the entire region under a slight risk of severe weather maxed out at 30% in the red shaded area below on the outlook map. We fully believe parts of the area in the 30% risk will be upgraded to a MODERATE RISK of severe weather by Thursday.
As we have said for days now, wind energy and forcing are not going to be a issue. One main ingredient you look for is mid level winds. For severe weather you want to see mid level winds around 40 mph. Right now the GFS model has the mid level wind forecast to be in the 110-120 miles per hour, with the NAM model painting 80-100 miles per hour. So wind energy is no issue at all. Another BIG PROBLEM is the type of wind shear that is going to be in place. This is called directional shear and is the most dangerous type as it enhances the risk for damaging winds to make it to the surface as well as tornadoes. From the surface through the low level into the mid levels of the atmosphere we have a very high amount of shear that will be in place Thursday, and that is a major concern and a contributing factor to this severe weather outbreak. Here is the latest model forecast from the NAM for 0-6km deep layer shear:
As you can see, 90-100 MPH numbers through the area. The common threshold for severe weather is 35-40mph, so this parameter is actually in the dangerous range.
This will be the key to this severe weather outbreak. If we get more sun than expected, if early morning showers and thunderstorms along the warm front move out faster than expected, instability numbers will rise faster and chances for a severe weather outbreak will skyrocket. Here is the current CAPE forecast maps from the 18z GFS and NAM models.
First map is from the GFS model. It is on the lower end of Cape values with 100-300 across the area.
This one if from the NAM Model – It is showing numbers anywhere from 500-1000 cape values. These numbers fully support a severe weather outbreak.
We will have to monitor dewpoints and surface temperatures Thursday. If dewpoints can get to 60 or above, the NAM’s Cape Forecast will likely be correct or perhaps a bit higher than what it is showing now. If dewpoints make it to 60+ we are going to have serious problems with this severe weather outbreak. Right now models are forecasting dewpoints around 57-58 degrees, so this will be crucial tracking on Thursday. If we can keep early morning rain-clouds in the area, it will lessen the severe weather risk to some degree.
Significant Tornado Parameter from The NAM Model.
The STP index from the Nam model is a combination of multiple weather indicators to give us a good ideal where the atmosphere is best primed for tornadic development. The areas in blue on this map are where this model is picking up the best potential for a few tornadoes. I would expect that if SPC upgrades this to a moderate risk tonight when the new update comes out, most of the area below will be included in it
Timing of Storms Arrivial
Right now with the current model data, it appears that storms will get started in our Far Western coverage area of Western KY and Central Illinois in the 3-5 pm area Thursday, the the line moving into Southwest Indiana around 6-7pm, Southern Indiana and Northern Kentucky along I65 around 9-11pm and East of I65 after 11pm. Louisville metro can expect this line entering around 9-11pm based on current data.
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OVWA – Roger