April 27-29 Severe Threat — Sunday Update

The severe thunderstorms threat is increasing across Middle Tennessee and southern Kentucky — the greatest threat looks like it will develop Monday afternoon and evening, but other time-frames will bring potentially damaging storms as well.  It’s not often you see phrases like this being used by the Storm Prediction Center:

“Widespread severe storms including strong tornadoes, damaging winds and very large hail appear likely Monday and Monday night across much of Mississippi, northern Alabama, Tennessee, southern Kentucky, and northeastern Louisiana.”

We’ll get to Monday’s threat down the line — but first, let’s set the stage with what’s happening this afternoon…then we’ll progress through the next 3 days in order.

SUNDAY AFTERNOON:Showers and thunderstorms have been moving toward and into Middle Tennessee and southern Kentucky all day.  This activity is not expected to become severe, but it is producing cloud-to-ground lightning, along with heavy rains that will saturate the ground and help to increase our flooding threat over the next few days.  This afternoon’s rain developed along a warm front that’s surging to the north — the air behind that front is warm and humid, and will help to fuel the development of stronger and stronger storms.

SUNDAY NIGHT/EARLY MONDAY: Severe thunderstorms to our west are going to head this way late tonight.  Those storms are responsible for the heightened severe weather threat in Arkansas today, and they’ll march eastward as the night progresses.  At this point, I expect these storms to move into the Tennessee River valley around 3-5am, progressing to I-65 by 5-7am and the Cumberland Plateau by around 7-9am.  Our two Futurecast computer models have some differences regarding the specific timing, but overall depict a similar pattern (a reminder — these are two different computers simulating what the atmosphere will do, which is why the two images don’t always depict exactly the same thing):

As they move east, the storms will be gradually weakening — the greatest severe threat will be along the Tennessee River, which is why you’re included in the Storm Prediction Center’s “Slight Risk” area for late tonight:

The main severe threat with these storms will be strong straight-line winds and large hail.  The tornado threat isn’t zero, but it should remain limited.

MONDAY AFTERNOON/EVENING: Once the morning storms move east, partial clearing will allow the atmosphere to heat up quickly and reload for the most significant weather threat that we’re facing.  Here’s the wild-card, though: if Monday’s rain moves more slowly, if clouds stick around and block that midday sunshine, it will help to reduce our severe weather potential later in the day.  That’s really our best-case scenario…I don’t think it’s very likely that things will play out that way, but I haven’t abandoned hope just yet.

Unfortunately, the other scenario is more likely: storms will develop in West Tennessee and Mississippi by early to mid-afternoon, then begin trekking our way.  The environment overhead on Monday will comprise an almost-ideal mixture of the ingredients necessary for supercell thunderstorms: lots of moisture to fuel the energy necessary for storms to explosively grow, plenty of deep wind shear to produce rotating storms, and lots of low-level shear for that rotation to reach the ground.

At this point, I expect the first “discrete” (meaning separate from one another) supercells to move into Middle Tennessee by late afternoon.  These storms will move to the east-northeast at a rapid pace, which is an especially dangerous set of conditions.  As the storms move E/NE, the separate cells will likely merge into a squall line.  ALL TYPES OF SEVERE WEATHER WILL BE POSSIBLE WITH THESE STORMS: TORNADOES, LARGE HAIL, DAMAGING STRAIGHT-LINE WINDS, AND FLASH FLOODING.  Here are the Futurecast images for Monday afternoon and evening…again, some differences between the two, but both paint an ominous picture:

I really can’t stress enough how dangerous the weather could become Monday afternoon and evening.  There are still plenty of factors that could develop to help reduce our severe weather potential, but most of the severe-weather parameters I’ve been looking at today (the SPC’s SREF-based Supercell Composite parameter, Significant Tornado parameter, and the Craven-Brooks Significant Severe parameter, for those of you who are serious weather geeks) paint a consistently grim picture, especially for areas south of I-40.

The Storm Prediction Center’s “Day 2” outlook includes much of Middle Tennessee south of I-40 in their “Moderate Risk” category, with the remainder of the Midstate in the “Slight Risk” area:

The term “Slight Risk” is misleading — it means an elevated threat of severe thunderstorms, up to a 30% chance of severe weather happening within 25 miles of a given point.  I’m honestly surprised that the “Moderate Risk” doesn’t include more of Middle Tennessee, and I’m also surprised that a “High Risk” region hasn’t been outlined at least for northern Mississippi and parts of West Tennessee…but we’ll see how the areas are drawn when tomorrow morning’s outlook is issued (that comes out at 1am, and I’ll have it for you on-air tomorrow morning beginning at 4am).

MONDAY NIGHT:The evening storms should be off the Cumberland Plateau by midnight or shortly thereafter…the rest of the night will give us a bit of a break to get ready for Tuesday’s threat.

TUESDAY: The severe weather threat Tuesday is very dependent on what happens Monday evening.  If Monday’s storms sufficiently “work over” the atmosphere overhead, that will help to limit our severe weather potential.  At this point, the SPC does NOT have us included for anything more than “general thunderstorms” in their Day 3 outlook (except right along the TN/AL border):

However, as good a sign as that is, I’m still concerned that the atmosphere will be able to recharge once again and give us one more round strong to severe storms late Tuesday.  Here are the Futurecast simulations for Tuesday late afternoon and evening, and they (at this point) support that concern:

These storms are going to develop along the cold front that will finally sweep this active weather pattern out of here.  While I don’t think the amount of energy (buoyancy, specifically) will be as favorable for severe storms, the lift produced by the front itself might be sufficient to overcome that factor.  The wind field will still be very favorable for severe storms…in fact, maybe even more so than in Monday’s scenario.

The problem here is that before Tuesday’s storms develop we have to make it through today’s rain, tonight’s marginally-severe storms, and then Monday’s severe storms.  Every time one batch of storms moves through, it leaves a changed environment behind…that factor confuses the computer models, so our data becomes a little more suspect the farther into the future we look.  At this point, I’d say that we should worry about Tuesday once we’re done with Monday’s threat…just be aware that Monday isn’t the last of our concerns.

OVERALL: This is the most dangerous setup for severe weather that we’ve seen in a while.  So, let me give you a list of DOs and DON’Ts.

DO: Review your personal severe-weather plan — if your “safe place” doubles as a storage area, make sure it’s cleared out.
DON’T: Go to your safe place now and hide there until Tuesday evening.  This would be an overreaction.
DO: Check the battery levels in your flashlights and weather radio, and make sure your cell phone is fully charged.  Pick up some bottled water, just in case.
DON’T: Clear out the grocery store shelves of non-perishable food items.
DO: Make sure your kids know where to go in the event of a warning.
DON’T: Pull your kids out of school, or drive out in dangerous weather to pick them up from school.  Schools are strongly-built structures (they’re built to withstand children!) and every teacher I’ve ever talked to knows exactly where to take their students in the event of a warning.
MOST IMPORTANTLY, DO: plan on staying weather-aware the next couple of days.  We’ll be on the air with our regularly scheduled newscasts to give you updates on the forecast, and when warnings are issued we’ll be on immediately to let you know.  We’ll be on social media as well — Twitter is better since Facebook will “filter” what appears in your timeline.  (If you’re a Facebook-only person, just have a tab with our 4WARN Weather page open, and refresh it to get updates.)  Links to our Twitter/Facebook pages are at the bottom of this post, along with a link to the NWS-Nashville accounts.

And now, the obligatory disclaimer: undoubtedly, this severe weather event will unfold with at least a few differences compared to what I’ve outlined above.  That’s always the case.  If we’re very lucky, we may even be able to dodge this particular bullet — like I said earlier, I haven’t abandoned hope.  But as of right now, this represents our best interpretation of a complicated and potentially life-threatening situation.

One last important point: don’t freak out.  Yes, I know it can be scary when we’re talking about the potential for tornadoes and damaging storms, and I literally just used the phrase “life-threatening”…but there’s nothing that worrying can do to improve your situation.  Stay calm, and make sure you’ve arranged everything to be able to react quickly if you need to.  The National Weather Service office in Nashville has been putting it this way: “Don’t be scared, be prepared.”

It ain’t Shakespeare, but it’s good advice.


Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @WSMVNancyVanC, @NWSNashville

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Nancy Van Camp WSMV, NWS Nashville

About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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4 Responses to April 27-29 Severe Threat — Sunday Update

  1. Kat says:

    My main concern is getting home from work tomorrow. I live 40 minutes from my home, and it sounds like afternoon/evening is when the worst is going to hit.

  2. Amii Stone says:

    I have a flight from Houston to Nashville. Leaving Houston at 4:45 pm arriving Nashville 6:45 pm. What are your thoughts?? Should I try to reschedule now?

  3. Amii Stone says:

    My flight leaving tomorrow (Monday)

  4. ac says:

    Where’s Jurassic Park Jello girl when you need her? O_o

    Ditto what Kat said. “Monday afternoon and evening” is an awfully big window. Are we talking leave-work-early scenario and be prepared to hunker down for hours? Eating dinner in the safe place? Putting kids to bed in the safe place? Really wish we had a basement. Darn Tennessee limestone.

    Thank you, Paul, for these fantastic posts. We rarely watch TV news (except for during severe weather) so I check here when storms are coming because I know you’ll keep us informed! (And thank you for the note about not freaking out…)

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