It continues to look as though Saturday could bring most of Tennessee — including the northern Cumberland Plateau and East Tennessee — its biggest severe weather threat of the 2015 spring season.
Just a quick update to point out the latest thinking from the National Weather Service’s forecast offices in Nashville and Morristown. I’ll post something a little more in-depth by late Friday morning.
For now, there’s still “only” a slight risk of severe weather outlined by the Storm Prediction Center. I expect that to change when the next update is issued tomorrow morning.
First, here’s the updated hazardous weather outlook from NWS-Morristown:
Severe thunderstorms are expected Saturday and Saturday evening across the Tennessee Valley and Southern Appalachian region along and ahead of a cold front. Damaging winds and large hail are expected to be the primary impacts with this activity, but isolated tornadoes will also be possible.
That’s sort of a cookie-cutter approach. More detailed information is available in NWS-Morristown’s area forecast discussion:
A significant bout of severe weather will be possible across the Southern Appalachian region Saturday afternoon and evening along and ahead of a cold front progged to drop south from the Ohio Valley. Saturday morning, a broad area of showers — some locally heavy — will move northeastward through the region along/ahead of a warm front. This warm front will stall out across southwest Virginia and far northeast Tennessee by the early to mid afternoon. In wake of the warm front, precipitation will taper off — and perhaps some partial clearing will occur — as winds veer and increase from the southwest. As a result, an unstable environment will develop within the warm sector with CAPE values are progged to be as high as 2000 to 3000 J/KG by 21z. This, combined with bulk shear values as high as 60-70KTS will result in very good storm organization with strong/rotating updrafts later in the day.
The main question then becomes storm mode. Currently, most solutions are showing an MCS/derecho feature dropping southeastward out of Kentucky during the late afternoon and early evening. This environment would certainly support a long-lived convective derecho event, with derecho composite values between 1 and 4 observed on the latest NAM/SREF runs. Within this line, the potential for supercell structures will exist as well. Some solutions even show an initial band of discrete cells ahead of the main line during the mid-afternoon. The combination of high helicity values, high 0-1 KM shear, and STP values as high as 3 to 5 — shown by the NAM — would definitely support some tornado threat — especially near areas where a more backed surface wind profile and in areas near any lingering residual outflow boundaries from earlier convection. Overall though, the main threats will be widespread convective damaging winds in excess of 60-70 MPH and large hail. Persons that have any outdoors activities planned should particularly monitor the latest forecast information.
This is the latest hazardous weather outlook from NWS-Nashville:
Some storms may be strong or possibly severe early Saturday morning as a warm front moves across the area, with a higher risk for severe storms on Saturday afternoon along and ahead of a cold front. Very large hail, damaging winds, a few tornadoes, heavy rainfall and localized flooding all appear possible.
It seems almost surreal that we’re under a frost advisory until 9 a.m. tomorrow morning and we’re talking about a severe weather threat that will begin just 24 hours later. Our severe weather here in this part of the world more often occurs after warm weather has well established itself, but that just goes to show how potent this setup promises to be on Saturday. A lot of the classic ingredients for spring severe weather appear to be coming together. It’s too soon to say this is going to be a widespread severe weather outbreak for Dixie Alley, but at the very least it appears that some damaging hail and damaging winds will be possible for some folks on Saturday.