Well folks, I’m your special guest blogger (Kelton Halbert) from the University of Oklahoma School of Meteorology. I’m back home in Nashville for the holidays and was asked to lend a hand – so don’t blame David if I butcher this!
We’ve been talking about the threat for severe weather over the last couple of days, but it’s time to break down what all we’re expecting this afternoon and evening. It’s certainly not an apocalypse by any stretch, but it’s definitely not a day you want to be caught off guard. Pay attention to the weather later in the afternoon and early evening, because it could get a little bumpy.
The NOAA Storm Prediction Center has us under an Enhanced Risk for severe thunderstorms, with areas further out West under a Moderate Risk. This is means we are reaching into the upper half of the severe weather risk categories, which is highly unusual for this time of year. In fact, it likely comes as no surprise as this whole weather pattern is very unseasonable and is much more reminiscent of something we would see in the Spring. This is why it’s going to be important to pay attention to the weather today, because these types of severe weather setups can potentially be volatile.
When and What? Non-Technical Severe Weather Synopsis
Things will be kicking off further to our West, making things a little bit easier for us since we’ll have time to watch it as it moves our way. Right now, we’re expecting the storms to be on our doorstep anywhere from 6PM-9PM this evening. These storms are expected to be able to produce all modes of severe weather – tornadoes, hail, damaging wind, flooding, etc. This won’t be our normal long-line of fast moving storms that we typically see out here, meaning that not everyone will see or experience severe weather conditions, and that it will be localized to individual storms that develop and move through the area. This is actually part of the reason why we have an elevated risk, because these individual supercell storm structures are more efficient at producing severe weather. Below is a simulated radar image for tonight.
It’s going to be impossible to nail down exact times and county locations/impacts on this one. Since we are dealing with individual cells, we have to wait for them to develop first before we can make any judgements. It’s entirely possible that the severe storms will skirt around our counties and have minimal impact, and it’s equally likely that one will come marching through. It’s also possible that the cloud cover this morning can help tone things down a bit – current visible satellite imagery shows a lot of clouds, and the atmosphere will need breaks in the clouds to help form the initial thunderstorms out West. Right now it’s looking like those cloud breaks are still likely to happen, but it’s something that by monitoring over the next few hours, we can get a better feel for whether or not the threat will materialize.
Severe weather parameters are pretty much off the charts for this system by any standard, much less for December. Convective Available Potential Energy (CAPE) values – a measure of atmospheric instability available to thunderstorms – is forecast to reach 1,000-1,500 J/kg, which is about 500-1000 more than we need to get things going to begin with. An upper-level shortwave trough – a type of upper level low or disturbance – is also helping by providing lift and wind shear. Down at the surface, winds are southerly and transporting moisture to the surface low located in OK/KS, while aloft, winds are out of the West-Southwest as they move around the shortwave trough. This combination of lift, shear, and instability is what helps create organized thunderstorms and the severe weather we are expecting.
Having mentioned the cloud cover already, it should be noted that the clouds aren’t going to be as much of a hinderance as we’d like in order to kill off the severe weather chances. The data from this morning’s weather balloon launch already shows a temperature profile favorable for instability, and that’s only going to increase throughout the afternoon. By evening, when the storms are expected to march through, this combination of shear and instability is expected to be maximized. Below is a forecast sounding just ahead of the thunderstorms as they role through. Before panicking, please know that this does not mean without a shadow of a doubt that tornadoes will happen. What this is intended to tell us is that in these sorts of environments, tornadoes are favored – but this is from a model forecast, and model forecasts are not always exactly right. That aside, this sounding does show us a very favorable environment for tornadoes, which is why this is something to take seriously.
There are a few preparedness notes to touch on. For starters, storms will be moving very fast today – anywhere from 50-60MPH. That’s fast. If a tornado develops near us, that isn’t a lot of time for it to move clear across Davidson/Williamson counties in less than an hour. Therefore, if you hear the call to take shelter (tornado sirens, weather radio, our Twitter, etc), it’s not a good idea to hesitate. If you plan to be out this evening, make sure there’s somewhere you can easily reach shelter in as little as 10 minutes.
Additionally, make sure you have multiple sources of information available. If the cell network goes down or power goes out, access to our Twitter feed may not be possible. Have a radio/weather radio nearby or a television for news outlet information. We’ve got several backups to make sure tweets go out in case one person loses network/power, but it’s always a good idea to have multiple sources of information!
Lastly, if you have travel plans this evening, expect delays – especially if you are flying anywhere or have family members flying in. It’s impossible to say how bad the travel impacts will be, but plan accordingly, or if possible, delay your travel until after the storms have passed.
This website supplements @NashSevereWx on Twitter, which you can find here.
Categories: Forecast Blogs