Thursday, March 23, 2023

Severe Weather Outlook For Friday Night


  The severe weather probabilities have been taken up a notch for Friday's event.

The main tornado threat is expected to be over much of Mississippi back into Louisiana and Arkansas, where supercell thunderstorms will initially form and perhaps produce tornadoes with a fairly long track that are capable of significant damage. That's where you see the hatched area. Even around here though, that 5% tornado risk is the basic risk for isolated tornadoes within the Tennessee Valley region. Still worth respecting, even though the higher probabilities, especially of a longer-lived and more intense storm (that could produce a strong tornado, E/F-2 strength or greater), are to our West. 

I don't have a link to the article at hand, but hats off to Jim Stefkovich, who used to be MIC in Birmingham, for picking up on this sooner than anyone else I saw, the upgrade and eastward expansion of the risk being so likely. He wrote that for Alabama EMA yesterday morning. 


The threat for large hail is looking marginal around here, but our threat for damaging thunderstorm winds is a little enhanced over Northwest Alabama. Of course the really widespread wind damage is again expected to be focused mainly to our West. Our probabilities here are still well worth respecting. All it takes is one damaging storm, and we've got problems if there is not awareness and a good solid safety plan.



What we expect is for supercell thunderstorms to fire up within mainly that red area, the Level 4 out of 5 Moderate Risk. These have the highest threat for producing tornado damage, could be significant damage in some places. And then later in the evening and night, the storms will try to form at least one line, we call a squall line. Sometimes more than one line forms. Sometimes it even gets messy where you still have a supercell or two along with one or more lines of storms. But for the most part, by the time the storms get into Alabama and Southern Middle Tennessee, they are expected to be more in a line, which means the primary threat will shift to a risk for damaging thunderstorm winds, with the threat. 

That risk is enhanced generally West of I-65, or if you want to get nitpicky down to county by county, I posted the zoomed-in maps above. But we all have a risk of both damaging thunderstorm winds and isolated tornadoes as this line moves through. If we were to see a supercell try to form ahead of the line or in any breaks in a line or lines of storms, then that individual storm would still carry a higher threat for doing significant damage, could be tornado damage. It will probably just be a squall line, but even a squall line (or Quasi-Linear-Convective-System . . . modern term for it) is dangerous if precautions are not taken. 

So if you get a severe thunderstorm warning or a tornado warning tomorrow night, remember:

* Be in a sturdy house that is anchored to the ground, not in a mobile home.

* Stay away from windows. 

* Get to the lowest floor.

* Get in a small room such as a bathroom, closet, or hallway.

* Make that room as near the center of the building as you can, as much barrier between you and the storm outside as possible.

* If you have time, get up under something sturdy or use something to shield your body, especially your head and neck, from any falling or flying debris that might happen if the storm were to produce a tornado. Or if, for example, a tree fell on the house, or a window broke under the force of the winds. 

And do not rely on an outdoor siren as your primary source of getting the warning. A weather radio is best. If you don't have that, at least make sure you'll get the Wireless Emergency Alert on your phone. Or leave the radio or television on a station that will give the alert. Something that will wake you up, since this looks like a late-night event that will last into the pre-dawn hours of Saturday morning, for us. 

You've got friends, family, let them know what is going on. There is no need to get overly excited or nervous, but you do need to make plans for yourself and loved ones, including pets, to the best of your ability, ahead of time. Once the warning comes out, need to be able to get to your shelter within five minutes. 

And I know that takes some planning if you live in a trailer or other weak structure, or are in something like a top-floor apartment. You have to plan ahead of time to use a public shelter or ideally go stay with a trusted friend or family member who has proper shelter. Of course, if you have the option to go to an actual storm shelter or a basement, I would never discourage that. Most people have to shelter in place in a regular house, or else have to try to get to a site-built house instead of being a sitting duck in a mobile home. So do the best you can. The focus of this potential severe weather outbreak looks to stay to our West, but hey, sometimes the leftovers can get pretty nasty too. So I really encourage everyone to take this seriously. We can be glad we're not in the highest threat areas and still do the right things for whatever bad weather we do get. Our risk here is worth respecting. And these risk levels are only basic guidelines anyway. They give you an idea of what to expect, but there have been times when some of the worst storms of an event happen outside the areas where they are most likely to occur. So a Level 2 Slight Risk and a Level 3 Enhanced Risk of severe weather deserves respect even if it is not AS dangerous as the Level 4 Moderate Risk. 

Let's all play it safe. 

Sharing this wonderful resource from Craig Ceecee that I saw tonight, where he has tried to include all the public tornado shelters in the country. Please use one of these if you think you need it. If you live in a mobile home and are not sure you can get to a safer location before the storms get to you, then please plan to go to one of these, allowing enough time to drive there safely, and not to get caught in the stormy weather. 

For people who are very weather-savvy, I don't mean to condescend or insult your intelligence by explaining things slowly and simply, in some detail. But this is a situation where I'm trying to make it simple enough for basically anybody to get the message. When it comes to an overnight event, safety has to come first, not sounding scientific or fancy. Which I may not be that good at anyway. 

If you have any questions, leave me a comment on here before the event gets going. Or ask your local National Weather Service office (or favorite broadcast meteorologist) on social media. This is a deal where if people know what's going on and have preparations, I think we'll be all right around here. But the timing of it is unfortunate. Especially on a Friday night, well, I guess more people will be awake, but they sure won't be wanting to think about the weather. So it is important to have a way to get the warning if one comes out for your location. And to have a clear safety plan that you can put into action right away, get to a safe place in less than five minutes of hearing the warning. 

And I think that's true even if we mainly see damaging thunderstorm winds around here instead of a tornado or two in the mix. It's dangerous for people to sleep through those without taking any shelter. Remember we did have some deaths early this month from damaging straight-line winds. The tornadoes that day actually did not kill anyone. And I think there were three in North Alabama. So as a better-safe-than-sorry approach, I would take a severe thunderstorm warning seriously tomorrow night. Sometimes a tornado can form quickly from a severe thunderstorm anyway, without additional warning. And the winds are dangerous either way. If you take the trouble to read my ramblings, you deserve some safety and peace of mind for your time. 

If you want to try to understand all the scientific jargon, it is below. 

SPC AC 230600


   Day 2 Convective Outlook  

   NWS Storm Prediction Center Norman OK

   0100 AM CDT Thu Mar 23 2023


   Valid 241200Z - 251200Z


   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS NORTHEAST

   LOUISIANA...SOUTHEAST ARKANSAS...AND WESTERN MISSISSIPPI...


   ...SUMMARY...

   Supercell thunderstorms capable of all severe hazards, including

   strong tornadoes, are possible across the Lower Mississippi Valley

   on Friday.


   ...Synopsis...

   Upper troughing is expected to be in place over the western CONUS

   early Friday morning. Strong mid-level flow will extend throughout

   the periphery of this trough into the more confluent flow north of

   the subtropical ridging across the eastern CONUS. A shortwave trough

   is forecast to move through this enhanced mid-level flow, tracking

   quickly eastward across the southern Plains during the day, and more

   northeastward into the Mid MS Valley overnight. Mid-level flow is

   expected to strengthen as the shortwave moves eastward, with 100+ kt

   at 500 mb spreading across TX into the Mid-South.


   At the surface, a low initially over north TX is forecast to move

   northeastward ahead of the approaching shortwave, moving into

   northern AR by Friday evening and through the Lower OH Valley

   overnight. This low is expected to deepen throughout the day, and

   this cyclogenesis will result in a broad area of moderate southerly

   flow across the Lower MS Valley/Mid-South into the Southeast and TN

   Valley. Environmental conditions appear favorable for numerous

   severe thunderstorms from the Lower MS Valley through the Mid-South

   and into the Mid MS and Lower OH Valleys.


   ...Lower MS Valley into the Southeast...

   A broad warm sector, characterized by dewpoints in the upper 60s, is

   forecast to be in place from east TX across much of the Lower MS

   Valley and Mid-South early Friday morning. Thunderstorms may be

   ongoing early Friday morning along a cold front moving eastward

   across east TX. This cold front is expected to make gradual eastward

   progress, as its parent surface low deepens while moving from

   eastern OK into AR. This overall evolution will contribute to a

   continued mass response across the warm sector, with low-level

   moisture increasing throughout the day amid strengthen southerly

   flow. This increase in low-level moisture coupled with modest

   heating is expected to result in airmass destabilization during the

   late afternoon. This destabilization coupled with large-scale ascent

   attendant to the approaching shortwave (perhaps augmented by

   low-level confluence) will likely result in discrete thunderstorm

   development within the warm sector ahead of the front. 


   Current thinking is that this initial development is most likely to

   occur in the TX/LA border vicinity. The downstream air mass will be

   moderately buoyant, with guidance suggesting MLCAPE around 1000-1500

   J/kg and max 2-6 km AGL lapse rates around 8 deg C per km. Robust

   vertical shear is also expected, with a strong low-level jet (i.e.

   50-60 kt at 850 mb) developing during the evening beneath

   strengthening mid-level flow. Forecast hodographs depict 

   substantial low-level speeds and veering, with 0-1 km storm-relative

   helicity from 200 to 300 m2/s2. A discrete supercell mode is

   anticipated initially, with all severe hazards possible, including

   strong tornadoes. With storms expected to develop in the LA/TX

   border vicinity, discrete storm maturation is anticipated across

   northeast LA, southeast AR, and western MS.


   Upscale growth into a convective line is anticipated after this

   initially discrete mode, with the line pushing eastward across MS

   and AL overnight. Robust kinematic fields are expected to persist,

   support a continued threat for strong gusts and line-embedded

   tornadoes.


   ...Mid-South into the TN and Lower OH Valleys...

   A more convoluted convective evolution is anticipated from

   central/northern AR northeastward into the Lower OH Valley on

   Friday. Storms will likely be ongoing along and north of stationary

   boundary extending from east-central OK northeastward into northern

   KY. A low-probability threat for hail is expected throughout the

   morning and into the early afternoon as warm-air advection promotes

   continued thunderstorm development along this boundary. 


   A gradual increase in storm intensity is then expected during the

   afternoon as the surface low begins to deepen across AR and

   large-scale forcing for ascent attendant to the shortwave increases.

   Buoyancy will be more modest than areas farther south, but the

   strong ascent and increasing shear is still expected to support

   intense updrafts. Given the presence of the stationary front and

   stronger forcing, a linear mode is anticipated, with this line then

   progressing quickly east-northeastward across the Mid-South during

   the evening and into more of the Lower OH and TN Valleys overnight.

   Strong wind gusts will be primary hazard within this line, but

   line-embedded tornadoes will be possible as well.


   ..Mosier.. 03/23/2023

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