High 68°, Rain Tonight
Much like yesterday was (including wind gusts over 20 MPH), except tonight we expect rain after dark. We aren’t sure when after dark, though.
The NAM4 model thinks it’ll start around 6 PM, the kind you don’t really want to get caught in:
The HRRR model is not impressed by it, though. It thinks rain starts later — around 9 PM — and is light and scattered.
Thunderstorms remain possible, but there is no indication they’ll be strong or severe. The Storm Prediction Center only has us in their “general” thunderstorm risk.
Rain Early Monday, Wake Up 55°, High 60°
We may hear some thunder while we sleep. Rain may extend through the morning commute, and should clear before lunch.
Groundhog Day Severe Weather (Tuesday)
The closer we get to Tuesday, the more confidence we will have about forecast; however, there is still a lot the science does not know about what will happen. This includes exact ETA and precise storm type. Tomorrow we will get in-range of the higher resolution models, which will add to, but not perfect, our understanding of what’s to come. You never really know what you have until you see it coming in from West Tennessee on radar, but we want you to be prepared for what we think it will look like.
Powerful upper-level winds and a cold-front-dragging strong surface low pressure center will conspire to create a good chance of severe weather happening in these areas Tuesday:
The Storm Prediction Center has six severe weather “threat” areas, from 0 (thunderstorms, but yawn) to 5 (high, move to space). I made this handy “guide” last year.
We’re sitting on the line between 2 and 3, although technically, we’re in the 2. That’s not that big of a deal. You can have very bad storms in a 2 and 3.
A squall line capable of severe thunderstorm winds (58+ MPH) and a few tornadoes should race out of Arkansas, cross the Mississippi River into west Tennessee late in the afternoon/early evening, and cross I-65 around midnight Tuesday night.
First, before any of this happens, it’s going to be windy. A Wind Advisory (gusts over 35 MPH) may be needed Tuesday, again, before the storms arrive.
Second, we have to mention the possibility of storms developing out ahead of the main squall line, arriving in the afternoon and early evening. We still don’t have a good handle on this, but right now I don’t see compelling evidence this will happen. I can’t find evidence in the GFS or Euro models for before-the-squall line thunderstorms, and the NAM4 model isn’t showing any updraft-helicity streaks by noon Tuesday (so a little too soon to say).
The latest run of the NAM has a few showers running across our area in the afternoon, but they really don’t look like supercells to me — more like what we call “crapvection” — which are rainers and thunderstorms that soak up all the good CAPE before the strong storm dynamics arrive. Crapvection results in weakers storms later. That said, the NAM doesn’t really show a big ole nasty squall line running across west Tennessee, so we think this model may be smoking weed and pontificating on poetry and life in a 7-Eleven parking lot rather than, you know, like, being smart about the weather and stuff.
Draw no conclusions about the potential for “before the squall line” storms Tuesday. We may see them. The data as to this question looks like this:
Third, the squall line. This is a little more clear. The Euro model still has it coming in around midnight. So does the GFS. I think we may see it earlier than that, but keep in mind we tend to be too early with these ETAs. For now, we are going with midnight, but there’s not much certainty in that ETA. It might come earlier than midnight, so planners beware.
Severe thunderstorm winds are the main concern. A thunderstorm wind is severe if it blows 58 MPH or more. Wind dynamics aloft will be very strong, and those winds will be transported to the surface in the downdraft part of the thunderstorms. Begin making plans how to secure loose outdoor objects, including lightweight family members.
Tornadoes remain possible. This threat exists if we get a supercell out ahead of the line (this seems possible, but unlikely) and inside the evening squall line itself. Speed shear and directional shear both favor tornado development; however, there remains disagreement in the models as to whether there will be enough “stuff” (CAPE) to lift so it can start rotating. Notably, the GFS model doesn’t think there will be any CAPE (got to have CAPE to get bad stuff to happen).
It’s hard to believe zero CAPE for this event. GFS may be NAM’s weed-smoking friend. The SREF (usually conservative) injects enough CAPE Tuesday night (around 500 j/kg for you weather nerds) to make some tornadoes. Even the Euro model has a plenty of CAPE for these storms to work with. This does not look like one of those universally-agreed-upon tornado days, but the threat is real and shouldn’t be ignored.
As you can tell, we don’t have a good handle on specifics, but generally, the hazards are severe winds and tornadoes, most likely arriving Tuesday night.
More to follow here and especially on Twitter throughout the weekend and the event, whenever it arrives.
This website supplements @NashSevereWx on Twitter, which you can find here.
Categories: Forecast Blogs