Tonight/Overnight – Maybe a Little Rain
There’s nothing on our radar right now, and the HRRR model isn’t too impressed by our overnight rain chances:
We may see a few light showers squeeze out in the middle of the night while the bulk of the rain passes to our SE, clearing out the SE US for tomorrow’s event.
Severe Weather Concerns Wednesday Afternoon & Evening – Early 60°, High 74°
During the day Wednesday, all weather models agree dew points will rise into the mid-60°s. The GFS model has a 65° dew point by 6 PM:
Such dew points create oppressive summer humidity, and here they’ll be tomorrow afternoon, 2 days before Christmas. Something’s obviously not right.
Joining these dew points is CAPE – the amount of potential energy which can be released to create powerful updrafts and strong/severe thunderstorms. When moisture-rich energy at the surface combines with temperatures rapidly cooling the higher up the atmosphere you go, CAPE values increase. In a cold-weather season, 500 j/kg of CAPE is all you really need to set off severe weather. By 6 PM Wednesday, models indicate CAPE values in excess of that: 669 j/kg (GFS), 763 j/kg (NAM), 1360 j/kg (NAM4 – the dramatic sibling of the model family), 950 j/kg (Euro), and 1071 j/kg (SREF). This model consensus creates confidence in the forecast.
Any rising thunderstorm will encounter substantial shear, which is another essential element for strong/severe thunderstorms. Shear is measured in terms of helicity, or bulk shear — both are way up there in every single weather model. Shear blows a thunderstorm updraft at an angle, allowing it more room to grow/strengthen. It also will rotate the thunderstorm.
Another key severe weather ingredient, especially for the formation of tornadoes, has to do with how low to the ground the clouds will form. Nerds refer to this as LCL heights. Studies in our area indicate most tornadoes form when the LCLs are under 1,000 meters. Every single weather model thinks tomorrow’s LCLs will be well under that 1,000 meter threshold. In fact, they are at/below 500 meters, most in the 200 meter range.
After all that, you need something to trigger these storms. If using a cooking analogy, you’re not going to have a very good meal if you put all the ingredients in the pan and forget to turn the stove on. The “on” switch tomorrow will be a cold front which will slowly approach during this event. It’ll set everything in motion.
All that to say this — something’s cooking.
These are the things the best forecasters in the world, at the Storm Prediction Center in Norman, OK, have been pouring over. Their job is to take this information and issue regular outlooks for severe weather. The latest one (below) shows their level of concern for the red, shaded (they call it “hatched”) area. It’s red and shaded to denote a “significant severe” weather event:
The hatched/shaded area means there is a 10% or greater probability of significant severe weather happening to or within 25 miles of anyone in the area. “Significant” means:
- A tornado that produces EF2 or greater damage.
- Wind speeds of 75 mph (65 knots) or greater.
- Hail 2 inch in diameter or larger.
(BTW, I added the black arrows to the above image to show the expected motion of the storms).
Those residing and traveling through these areas should be weather aware at all times. This afternoon, our NWS cautioned travelers and shoppers: “We know that many folks will be traveling to see loved ones or finishing up that last-minute shopping during this time. If you can complete these tasks before or after the severe weather threat, that would be best, but if you have to be out during this time, please remain weather aware.”
What Kind of Severe Weather?
- damaging straight-line winds
- large hail
- flash flooding
- a few tornadoes
When you put the above ingredients into the models, they try predict possible hazards, then display the results in forecast soundings and/or maps.
Every forecast sounding we have indicates the existence of a tornado-friendly environment. Significant Tornado Parameter is an algorithm to identify tornado friendly environments, and it loves this system. Here’s one of the more conservative forecast soundings:
This can also be expressed in a more traditional map. This time, lets use the over-dramatic model:
The more conservative models are not that far away from the above. Like this one, showing a 50% chance of a Significant Tornado Parameter of 5.
In addition, the models predict large hail, and very heavy rainfall. Flash flooding might end up being the most dangerous part of this storm system.
What’ll They Look Like on Radar?
These storms aren’t going to form a beautiful, continuous line, and sweep across the Mississippi and then Tennessee Rivers like a regiment of Civil War soldiers, for all to see. Instead, they’ll take the form of single storm cells, or small groups of storm cells, racing as fast as 60 MPH from southwest to northeast, tracking from central and north Mississippi into Middle Tennessee.
Here’s the NAM4 model, which — again — seems a little dramatic (and maybe a little too soon timing-wise).
Here’s another take, later in the day, from a different model:
The point is not to make a timing estimate, but to illustrate storm size/shape and direction. Here is one more:
That’s a fair question. The reality is that atmospheric science is not good enough to provide specific times and locations right now.
The ETA range is wide, from noon until midnight, with the main threat being 4 PM until midnight.
Of particular note, from our NWS: “This severe weather threat will carry into hours of darkness Wednesday night. The combination of nocturnal severe weather and tornadoes can be deadly.” NWS is speaking to the statistics — we lead the nation in nocturnal tornado fatalities.
How Worried Are You?
The last time I was this worried was March 2, 2012, when that tornado-producing supercell lifted its tornado when it crossed into Davidson County, but still dropped giant hail across I-40. It feels like that. That event, frankly, was us dodging a big bullet, and somewhat of a forecast “bust,” even with all the damage we got. Dewpoints didn’t get to where we thought they would, and it spared us. Those to our north were not as fortunate.
To be clear, lots can “go wrong” with this forecast, and we hope it does. For example, CAPE could be wildly overestimated (although, with the models consistently advertising plenty of CAPE, we don’t think so). Or, maybe we don’t get those crazy high mid-60°s dew points. “Boundaries” laid down by earlier storms may initiate storms prematurely or otherwise mess with the local severe setup. Or, massive thunderstorms to our south could block our CAPEability (a pun I recently invented). Severe weather is by no means a certainty, but all signs look like something’s going to happen to someone. So, please be prepared.
We do not think you should be driving anywhere in the red/shaded area after noon Wednesday, if not before. Will flights be delayed? That’ll depend on what’s happening at the airport. Those with flights in/out from 4 PM to midnight should be prepared for delays.
For those new/rusty on tornado safety, please see this page.
Storms aren’t going to hit everyone in the red shaded area. The hassle to get ready for this event is worth the relatively low risk of an potentially dangerous event.
So, take a deep breath, have a plan, and stay tuned to multiple reliable sources of weather information. Be sure to tune into your favorite local TV weather team tonight (2, 4, 5, and/or 17) and through the even tomorrow. You can find more info from us on Twitter @NashSevereWx.
Remember, we do not post Warnings to this website, but they auto-post to our Twitter account. You are encouraged to check out the app banner at the bottom of this page for an alert app.
Christmas Eve & Christmas Day
Because of the gravity of tomorrow’s event, and because your eyes are probably bleeding from reading all this by now, we’re going to hold off on peeling back all the layers for Christmas Day. However, you should know our NWS has announced strong storms — likely not severe — are a concern Christmas Day afternoon/evening. If that wasn’t enough, another system is expected to eject out of the Plains late this weekend, and deliver another potential severe weather setup.
I wish I was making all this up.
This website supplements @NashSevereWx on Twitter, which you can find here.
Categories: Forecast Blogs