It feels weird to be talking about Severe Weather in August but with our fall semesters starting up in just under two weeks and the workload that comes with that we’ve been keeping an eye out for any chase setups before then. Below are all the graphics I will be referring to in this blog post.

The first thing we see when analyzing the setup is the trough coming in at 500 MB. Both the NAM and GFS are consistent in bringing in a vort-max with an area of 40 to 50 knot area of winds. Both models are good in that aspect of their consistency. We wont be getting winds at 500 MB that strong until the fall season returns in the southern plains. 40 knots or more is favorable for organized supercells so that would not be an issue. Moving down to 850 MB both models are showing a low level jet from Eastern Nebraska north into South Dakota. The GFS and NAM has a 35-40 knot LLJ on their 18Z runs but the NAM has backed down to 25-30 knots with the 0Z run. The GFS has remained consistent and continues to show a 35 to 40 knot LLJ across E. Nebraska into South Dakota. Both models show 850 MB winds out of the south-southeast which will enlarge the hodographs to support low level mesocyclones. Instability should be more then favorable with 3,500 to 4,500 J/Kg of Surface Based Instability with Mixed Layer values over 3,500 J/Kg. That’s the basic data regarding the setup.

But nothing in severe weather is that simple to forecast and that is even more true in the summer months when the cap comes into play. The NAM is showing an unusual picture of a major storm complex underway in South Dakota by 0Z thus contaminating a majority of the sounding locations and thus the graphical outputs. For this discussion we will side with the GFS and discuss it’s output. The GFS is showing a majority of Nebraska remaining capped into the evening with a weakness in the cap developing over the NE side of the state along a boundary northwest into South Dakota. In this area conditions would appear to be favorable for low level mesocyclones and associated tornadoes. Dewpoint values should be in the mid 70′s with temperatures forecasted to be in the upper 80′s to mid 90′s. This would create a 15-20 degree dewpoint spread which would allow cloud bases to remain low enough to produce tornadoes. If the temps got much higher then that or the dewpoints mixed out then the problem would become elevated thunderstorms with bases too high to produce tornadoes and tap into the low level energy. The GFS and NAM are showing LCL’s (Lowest Condensation Level or Cloud Base) at 1000 Meters which is in the ‘green zone’ for tornadoes. The question then turns to will convection initiate and if it does how long will it remain discrete or will it even be discrete at all. If we were to head to South Dakota that would be a 1500 mile round-trip not counting any miles added on during the chase, so it would have to look pretty good in our eyes for us to commit to that chase. Obviously it is possible that we will chase as I’m writing up a discussion on the setup. Right now the Storm Prediction Center has a 15% contour out for our target area with general wording. We wont know until late tomorrow evening if we will chase. If we do its a 12 hour drive up from Dallas so we would be leaving pretty early Thursday Morning, but I’d say chances are increasing that we will make the commitment and head up there. The chase status will remain on slight for now but may be raised to moderate later this morning depending on how things turn out.

I’ll have another update later this afternoon with the 12Z runs and further information. Thanks for dropping in and as a reminder you can view our live streaming video on our streaming page. We also have past video up in the chases section. I was just watching the Yazoo City Tornado video a few days ago and still cant believe how close we came to that monster. It’s been a fun year and here’s hoping we get one more chase in before the fall semester starts!

- David Reimer