New Discussion From the Storm Prediction Center
What does this mean for us? Simply put, it is discussing the northern movement of a better severe weather environment. The closer that warm front gets to Davidson/Williamson counties, the more unstable our atmosphere could potentially become tonight. We still think the *best* ingredients for tornado development is ever-so-slightly to our south, but this does not mean we are free from seeing severe weather or possibly a tornado.
Issuance of a tornado watch doesn’t appear likely *right now* for our two counties, but this could change as the night progresses.
NashSevereWx.com will continue to monitor this developing situation and updates will be provided on Twitter @NashSevereWx. Please monitor multiple sources for your weather information!
Tonight’s Severe Weather Overview
The threat for severe weather tonight is higher than it was yesterday.
Latest HRRR Model Loop (see below) shows storms developing after dark, then quickly intensifying and forming a line running from the southwest to the northeast.
Update: We think our severe thunderstorm potential will begin around 9 PM and end around 2AM. Remember, we are only talking about Davidson and Williamson Counties.
Once the storms form, they should pretty quickly race off to the east and leave us alone to sleep overnight.
The Storm Prediction Center included us in its 5% probability of a tornado happening within 25 miles of any point, which is in the brown-shaded region.
Tornadoes need lots of things to happen in the atmosphere to form. Two vital ingredients are wind shear and instability.
Like yesterday, there will be plenty of wind shear in place to rotate the storms that form.
Unlike yesterday, when there was no instability, tonight we expect instability (measured in “CAPE”) in Middle Tennessee. The big question tonight is whether that area of instability will rise far enough north and west to include us. To answer that question, we look to the weather models. There are several weather models, and each has its own answer to this question.
First, there is the NAM4 model. This model has a reputation for overdoing things. It brings sufficient CAPE to cause severe thunderstorms — and possibly tornadoes — south of I-40:
Second, there is the more-reliable HRRR model. By 9 PM, it also has CAPE running up to I-40:
If this model is correct, it’s probably not enough CAPE to cause problems. I still think we will see some thunderstorms, and probably some strong/severe thunderstorms, but I will not be very worried about tornadoes if this model is correct about the CAPE.
The “Significant Tornado Parameter” is one (albeit, not a sure-fire) way of quickly identifying areas where tornado development could be greatest. The higher the value, the greater the probability of tornado development (but not a guarantee).
Here is the Significant Tornado Parameter from the HRRR model:
So, for now, I’m comforted that the instability and tornado potential is further to our south; however, one concern I have is that the last two runs of the HRRR model have brought the greater instability further north and northwest, which would only increase our tornadic thunderstorm potential.
The truth is we won't have a great indication of how bad the storms may be tonight until data returns from the 6 PM weather balloon. pic.twitter.com/p7gGgT3uWw
— NashSevereWx (@NashSevereWx) November 29, 2016
Tonight, you should closely monitor multiple sources of reliable weather information, to include local TV meteorologists and our Twitter feed @NashSevereWx. We do not post warnings to this website. Find them on Twitter, TV, your local radio, and your alert app. (If you look for them on Facebook, they might pop up on your feed in a couple of days).
The Storm Prediction Center has included us in its 15% probability of damaging winds within 25 miles of any point in yellow-shaded region:
Even without tornadoes, we could still see damaging straight-line winds via intense downdrafts from the thunderstorms we expect to develop.
The Storm Prediction Center thinks the hail potential is to our southwest:
Severe weather potential is slightly impressive today and is definitely something to watch. Stay in touch with @NashSevereWx on Twitter for intermediate updates. Further discussion and/or severe weather watches will be posted here if warranted, while warning information will be available on our Twitter, through the Nashville National Weather Service, and other media outlets.
This website supplements @NashSevereWx on Twitter, which you can find here.
Categories: Forecast Blogs