0-6 km bulk shear (lower to mid levels of the troposphere)
In this blog entry I will get into some of the scientific parameters used to forecast severe weather, so for those who just want the bottom line, here it is:
We are looking at the possibility for severe weather, which could include isolated tornadoes, late Wednesday night after midnight, through the first half of Thursday.
Right now, the Storm Prediction Center only has the western parts of Middle Tennessee under their "Slight Risk" for severe weather. That could shift farther east as time goes on.
Now, scientific part for us "weather geeks" (since this blog is generally geared for people who want the "insider info").
First of all, models vary somewhat on the timing of the cold front, the earliest being the GFS around 8-9am Thursday morning, and the latest being the ECMWF at around 6pm in the evening. Of course, storms could extend out ahead of the actual front.
As shown yesterday, in the 2nd graphic you can see that by 5,000 ft. the winds have increased to 50-70 knots, creating that "shear".
Many of you read the Storm Prediction Center's (and other NWS office's) forecast discussions, where they talk about atmospheric parameters such as "0-6 Kilometer Bulk Shear" and "0-3 Kilometer Storm Relative Helicity:.
Our good friend Tom Johnstone, The Warning Co-coordinator for the National Weather Service in Nashville is one of the teachers of the Advanced Storm Spotter's Class and he does a great job in explaining these parameters to lay people.
The 0-6 Kilometer Bulk Shear measures the difference in wind speed between he surface and 6 km above the ground. The greater the shear, the more likely storms will become severe.
He considers anything below 35 knots to be UNLIKELY to cause severe storms. Notice that the 3rd graphic shows 50 knot shear moving through the area at 7am Thursday.
The 0-3 Kilometer Storm Relative Helicity measures the potential for a storm to acquire a rotating updraft. The greater the SRWH, the more likely storms may become severe and also "supercellular" meaning they contain rotating updrafts.
He considers anything below 150 (meters squared per seconds squared), to be unlikely to cause supercell development (with caution that with high instability, 100-200 m2/sec-2 could cause supercells). Notice that in the 4th graphic we have 200-250 entering the area and up to 350 near the Tennessee River at 7am Thursday. By the way, the NAM does not bring the front through until around 1PM Thursday.
That brings me to instability or the thermodynamic side of severe weather.
I always tell people that the wind dynamics (shear, SREH, etc.) for tornadoes are going on all the time during the winter, when the jet stream and strong weather systems are farther south.
Luckily, there is no warmth and humidity to work with or there would be so many tornadoes that we couldn't live here!
I will get into parameters such as CAPE and Lifted Index at another time.
But for our possible upcoming severe weather situation, the CAPE is not extraordinarily high (possibly because the early morning hours have the least heating), but it is just high enough for that shear we mentioned to work with.