Wednesday, May 8, 2024

Slight Upgrade in Severe Thunderstorm Potential

The Level 3/5 Enhanced Risk for severe thunderstorms has been expanded farther South to include a good chunk of North Alabama for this event. 

You can see that places like Jasper and Blountsville, Albertville are still under the Level 2/5 Slight Risk. Cullman is technically just South of the Enhanced Risk area, but with this kind of a messy setup, no need to get too hung up on it, just use these as basic guidelines. 

This expansion was made because a much larger area is now thought to be at risk for especially damaging thunderstorm winds today and (much more likely) tonight. That is a 30% chance of damaging winds that could reach hurricane force, like 75-80 mph or perhaps even greater. 

So like I said earlier today, definitely take shelter if you get a Severe Thunderstorm Warning tonight. For this event, I would treat it with about as much caution as you would a Tornado Warning. But definitely do not just ignore it. 

The threat for the extra-large hail is thought to stay farther into Middle Tennessee, not so much a problem in the Southern Middle Tennessee counties that border Alabama, but it is a close call. At least Wayne and Lawrence Counties are clipped by that hatched area for golfball-sized hail or larger. And again, it's better not to think of these threat levels as etched in stone. They are basic guidelines of what to expect and where. 

Basically all of North Alabama and Southern Middle Tennessee only has the standard isolated tornado threat. The hatched 10% area North of Shelbyville is where it is most likely for somebody to see a tornado that comes out of a long-tracked supercell and might do fairly significant damage. Again, don't get too hung up on this, like especially if you are in the 2% risk area South of Cullman, please do not assume that means you're in the clear for this event. Any of us could get a tornado from this, even though the higher threats are for very damaging straight-line winds and large hail - in some areas, at least across the TN line, maybe very large hail. 

Remember that you can't stay in a mobile home with a tornado coming at you. They would even be vulnerable to some of these winds, if we do see some that are extra-damaging, up to 70-80 mph. Actually even 50-60 mph winds can damage a mobile home, especially if there are trees around. So please try to plan tonight to stay at a sturdy house or be able to go to a stronger building, even if it's something like a gas station or restaurant that stays open all night. Hopefully you have a friend or family member who will help you out. 

If driving tonight, plan places you could pull over and get inside if you have to. 

The absolute last resort for sheltering from a tornado is to get into an unflooded culvert or ditch, or even flat on the ground, covering your head and neck with your hands. 

Best case scenario . . . well actually the ideal scenario is if you have a storm shelter. But most people don't. So for most people, best case scenario, in a sturdy house properly anchored to the ground, you need to:

* Stay away from windows. 

* Get to the lowest floor. 

* Get into a smaller room like a bathroom, closet, or hallway. 

* Put some walls between you and the outside - make that room near the center of the building.

* Cover your body in case of falling or flying debris, especially your head. 

If you can do that, you should be all right. 

Definitely have a reliable source of warnings tonight. That means something better than outdoor sirens. 

Best is a NOAA Weather Radio with battery backup. A service like Weathercall is also great. 

At least enable Wireless Emergency Alerts on your cell phone and make sure it is charged and has the volume on so that it'll wake you up if it needs to. 

Those are the two main problems I see out of this event around here: If people sleep through it or if people can't get out of mobile homes fast enough. 

They had a rare violent E/F-4 tornado (and by the way, they are still looking at the damage in case that needs to be upgraded to the highest rating, a 5, which you hardly ever see) in a town called Barnsdall, Oklahoma the other night. And out of a town of 1,000 or so, only one person was killed, think another may still be missing. Now it's sad if we lost one or two people, I don't mean to downplay it. But just thinking in cold scientific terms, that is an excellent outcome, compared to what we are used to seeing with a lot of tornadoes. Even a tornado that ends up with a rating as strong as E/F-2 often produces serious injuries and deaths, and a lot of it is because people are in mobile homes. 

So I just thought I'd throw that out there, that even in a setup that was worse than ours today/tonight, the warning system worked pretty well. They had a rare Level 5/5 High Risk out that day, Monday. 

That's just a reminder that things can work. We don't have to have mass casualties with every organized tornado event. The organized tornado stuff may stay to our North for this one. It's a close call. But even the damaging wind/hail threat here is significant and is well worth respecting. 

Now, as far as timing:

We could see something isolated, a supercell thunderstorm, somewhere in the TN Valley this afternoon or evening. The atmosphere is probably going to stay capped and prevent that, but there is a very low risk we could see something isolated. 

The much more likely scenario is what we expect tonight. I am not sure I agree with the forecasts staying it will all be after Midnight, but I think most of it will be. I'd say for Northwest Alabama and places like Waynesboro or Lawrenceburg, TN, between 10 PM and Midnight is a good estimate for the onset time. 

But we will have a threat for severe thunderstorms capable of significantly damaging winds, large hail (maybe isolated extra-large hail), and isolated tornadoes between then and about Daybreak in North Alabama and Southern Middle Tennessee. 

The storm mode may be mixed and messy. I'm thinking we'll see one or two isolated cells, but mostly clusters and maybe a line segment or two, like mini-squall lines. 

Stay safe. 

1:43 PM - Going to go ahead and take a look at tomorrow's very low severe weather potential. This is actually in case any severe thunderstorms can get organized again tomorrow evening, after the overnight/morning round, if we see breaks in the rain and such. 

And the chances of that really look low until you get down to about Birmingham. I'm putting some of my own opinion into this, but you can see the formal SPC outlook above. Most of North Alabama only has a Marginal Risk - Level 1/5. And up toward Athens, the Shoals, into Tennessee, just general thunderstorms expected. 

Even if things do come together, it is just a low-end threat for some strong winds up to about 60 mph that could do some damage or hail up to about quarter-size. 

And the tornado threat tomorrow is looking just about nil. But some minimal probabilities are in effect for places like Jasper, Oneonta, Gadsden, or parts of Cullman County like Bremen and Hanceville. 

The main event is going to be tonight anyway, probably starting close to midnight, between about 10 PM-Midnight in the Shoals area. 

We could see an isolated supercell thunderstorm sooner than that, this evening, but the chance of that is low. 

Already quite a few reports of large hail and wind damage today across Missouri, Kentucky, and Tennessee. 

I saw where one guy who is with Ryan Hall's team tweeted that there had been a death in East Tennessee from a storm this morning that carried a tornado warning. I considered that an unconfirmed report, but usually those Hall boys have a pretty good handle on things. And that's just a reminder that it doesn't take a more significant tornado to seriously hurt somebody. And it doesn't always take a tornado. People have been injured and killed from severe thunderstorms that did not produce a tornado. A lot of times it is from simple things like falling trees or a window blowing out. 

So this doesn't look like a big outbreak for us, but it is just a notch below that, to where I would show a lot of respect for the potential. 

And if you want to keep in the back of your mind that something else could develop again tomorrow evening, no harm in that. I don't think it's likely, but it never hurts to be prepared just in case. 

It is nearly 2 PM now. So I'm going to see if anything else is worth adding to this post. 

And then I'm going to take a break from weather for a few hours. Because otherwise I'm going to be burnt out before this event begins for us. And I've heard very experienced meteorologists say they've done that to themselves. The flood of information these days is nonstop. Sometimes you have to recharge your batteries. 

1:57 PM - All right, so they have a Tornado Watch that includes much of Northern and Central Tennessee now, includes Nashville. 

Numerous severe thunderstorm warnings in Kentucky, some flash flood warnings already in TN as well. There have been some reports of baseball sized hail already, as a lot of these storms are mature supercells. 

And that general area from now to about 4-5 PM this afternoon will have to watch for supercell thunderstorms that can produce hail that large, or destructive thunderstorm winds, or tornadoes. An isolated stronger tornado or two is a real possibility with this system. That means it ends up rated at least E/F-2, which means it can totally take the roof off a well-built house and can throw a mobile home across the road. 

By the time the storms get down here, we're expecting a threat for mainly wind damage similar to what they could have up that way, could see an isolated instance or two of the really large hail, but most our hail will probably be measured in coin sizes. And we could see isolated tornadoes around here too. But the highest tornado threat today is focused in these areas well to our North. 

Can't rule out an isolated supercell down this way (especially around Shoals or nearby in TN) this evening, but I'm doubting more and more we'll see anything like that. 

Main idea is that between 10 PM and Midnight, the window opens up for Northwest Alabama. And then things will likely wind down around Daybreak in Northeast Alabama. That means an awful lot of us will probably get whatever severe weather we're going to see overnight, after Midnight, in those wee hours. 

And not all of us will see severe weather. But you've gotta' prepare in case you do. 

SPC AC 081645

   Day 1 Convective Outlook  

   NWS Storm Prediction Center Norman OK

   1145 AM CDT Wed May 08 2024

   Valid 081630Z - 091200Z





   Severe thunderstorms appear likely from parts of the mid

   Mississippi, Ohio, and Tennessee Valleys into the southern Plains. 

   All severe hazards, including tornadoes, very large to giant hail,

   and potentially significant damaging winds are possible.  Some

   tornadoes may be strong.

   ...Ozarks to Mid Mississippi, lower Ohio and Tennessee Valleys...

   A complex, partially mesoscale-driven forecast is apparent for the

   Ozarks eastward into the lower OH/TN Valleys.  A morning

   thunderstorm complex over TN has draped an outflow boundary

   --western portion of it advancing northward-- across the lower TN/OH

   Valleys.  A very moist/unstable airmass is located along and south

   of the boundary across the MS Valley, with surface dewpoints in the

   upper 60s to lower 70s contributing to a very unstable airmass

   (2500-4000 J/kg MLCAPE).  A strong belt of southwesterly 500-mb flow

   extending through the base of a mid-upper level low over the

   north-central High Plains, will remain overhead from the southern

   Great Plains east-northeast into the Mid South/OH Valley.  

   Ongoing severe storm cluster late this morning will likely expand in

   convective coverage over eastern MO into western TN/KY through the

   mid afternoon.  Supercells potentially capable of tornadoes, a few

   of which may be strong, and large to very large hail and significant

   severe gusts are possible with this activity.  Have adjusted

   (lowered) severe probabilities on the northern periphery of the risk

   area given expected concentration of thunderstorms and associated

   severe to favor the corridor in the vicinity of the boundary. 

   Eventual upscale growth is expected later today into tonight as this

   activity moves east-southeast with an accompanying swath of wind


   ...Eastern OK to AR, Mid-South and central TX...

   Thunderstorms should develop by mid-late afternoon near the front

   and dryline, as a combination of lift along those boundaries and

   strong surface heating combine with very rich low-level moisture to

   erode the cap.  Once storms initiate, rapid evolution to supercells

   is expected.  Forecast soundings in the OK/AR/MO border area show

   very large CAPE and elongated hodographs.  Large to giant hail and

   tornadoes will be the primary hazards early in the convective life

   cycle.  Have extended the 10-percent significant tornado

   probabilities into far southwest MO, northwest AR, and far eastern

   OK.  There is some signal for upscale growth to occur this evening

   across AR with an MCS moving east across the Mid South.  Have

   adjusted severe-wind probabilities farther south to account for this

   potential scenario.  

   Farther southwest, lower storm coverage is expected across the

   TX/Arklatex.  However, steep lapse rates and 70s dewpoints will

   contribute to extreme MLCAPE.  Although near-surface flow generally

   will be 10 kt or less, limiting lowest-km hodographs/shear,

   effective-shear magnitudes in the 40-55-kt range indicate supercells

   will be possible.  These will be capable of very large/destructive

   hail exceeding 3 inches in diameter.  Clusters or upscale mergers of

   convection also may offer deep, precip-loaded, hail-cooled

   downdrafts with locally severe gusts.  Cell mergers and interactions

   with boundaries will factor into tornado potential on the storm

   scale, since the environmental low-level shear appears on the


   ...Southern Appalachians/Cumberland Plateau into Carolinas...

   Ongoing strong to severe thunderstorm cluster over the eastern TN

   will likely continue to develop to the east-southeast during the

   afternoon.  The airmass will continue to destabilize downstream

   across the western Carolinas.  Scattered strong to severe gusts

   (55-70 mph) will be capable of wind damage.  A few of the stronger

   cells may also pose a large hail threat.  Additional storms are

   possible east over the Piedmont and into portions of the coastal

   plain this afternoon into the early evening.  An attendant severe

   risk may accompany the stronger storms.  Later tonight, an MCS or

   several smaller-scale bows are forecast to move east-southeast along

   the instability gradient forecast to remain draped across the Mid

   South.  Moist/unstable conditions will support a continued risk for

   damaging gusts moving into northern AL/southern Middle TN into

   northern GA late tonight.  

   ...Northeastern CONUS...

   Widely scattered to scattered thunderstorms may develop this

   afternoon over parts of eastern NY and New England.  A diurnally

   destabilizing airmass will become weakly unstable (500-1000 J/kg

   MLCAPE) along/ahead of a prefrontal surface trough.  Ample

   deep-layer shear attendant to a Northeast U.S. shortwave trough,

   will act to organize updrafts.  A mix of multicell and modest

   supercell characteristics are forecast before activity moves over

   more-stable air and/or offshore, and weakens by early evening. 

   Isolated hail/damaging gusts are the primary expected hazards.

   ...Eastern SD to eastern IA and vicinity...

   Widely scattered to scattered thunderstorms are possible from early

   afternoon in western parts to evening closer to the Mississippi

   River, with isolated damaging gusts, large hail and possibly a

   tornado or two.  Activity should form in an area of deep-layer lift

   (including low-level convergence/mass response) related to a

   vorticity lobe in the southeastern part of the mid/upper-level

   cyclone.  Associated cooling aloft will combine with residual

   low-level moisture (dewpoints mid 40s to mid 50s F) to yield pockets

   of around 500-1000 J/kg MLCAPE.  Backed low-level winds will aid

   convergence and storm-relative low-level flow, and may contribute to

   locally enlarged hodographs.

   ..Smith/Jewell.. 05/08/2024

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