Friday, March 24, 2023

Public Severe Weather Outlook

We may have some rough storms here too, but things look especially dangerous for some of our neighbors to the West later today. 

And there is some scuttlebutt going around about whether or not SPC will upgrade this to the rare High Level 5 out of 5 Risk or not. I don't really care if they do or don't. The message is essentially the same: This looks like a potential tornado outbreak for the Mississippi River Valley, and one where we could have some particularly damaging (and thus dangerous) tornadoes. Back in 2011, a lot of people were upset that April 15th was only a Moderate Risk (and back then, that was a Level 2 out of 3 - things have changed as far as jargon and categories and flashy colors over the years). It was a significant tornado outbreak for Central and South Alabama, also several tornadoes back in Mississippi if I recall correctly, I think North Alabama and into Tennessee mainly had flooding problems that day, the air was stabler. Let's not get hung up on technicalities, can always fight about those later. On a day like today, the important thing is to protect life. With or without the absolute highest risk level being forecast, we know that this is serious business. 

ZCZC SPCPWOSPC ALL

   WOUS40 KWNS 240622

   ARZ000-LAZ000-MOZ000-MSZ000-TNZ000-241800-


   PUBLIC SEVERE WEATHER OUTLOOK  

   NWS STORM PREDICTION CENTER NORMAN OK

   0122 AM CDT FRI MAR 24 2023


   ...Severe thunderstorms expected over parts of the lower Mississippi

   Valley this afternoon and tonight...


   * LOCATIONS...

     Northern and central Mississippi

     Central and eastern Arkansas

     Northern Louisiana

     Western Tennessee

     Missouri Bootheel


   * HAZARDS...

     Several tornadoes, a few intense

     Scattered damaging winds, some hurricane force

     Isolated large hail


   * SUMMARY...

     An outbreak of severe weather is expected across the Lower

     Mississippi Valley this afternoon and evening. Tornadoes, some

     strong to potentially intense, as well as damaging winds and

     hail are expected.


   Preparedness actions...


   Review your severe weather safety procedures for the possibility

   of dangerous weather today. Stay tuned to NOAA Weather Radio, 

   weather.gov, or other media for watches and warnings. A tornado

   watch means that conditions are favorable for tornadoes to form

   during the next several hours. If a tornado warning is issued for

   your area, move to a place of safety, ideally in a basement or

   interior room on the lowest floor of a sturdy building.


   &&


   ..Grams.. 03/24/2023


   $$        

SPC AC 240556

                       


   

Day 1 Convective Outlook  

   NWS Storm Prediction Center Norman OK

   1256 AM CDT Fri Mar 24 2023


   Valid 241200Z - 251200Z


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

   OF THE MID-MISSISSIPPI VALLEY...


   ...SUMMARY...

   An outbreak of severe weather is expected from the Lower Mississippi

   Valley toward the lower Ohio Valley Friday afternoon and evening.

   Tornadoes, strong to potentially intense, as well as damaging winds

   and hail are expected.


   ***Tornado Outbreak Possible Across Portions of the Mid Mississippi

   Valley Friday Evening***


   ...Synopsis...

   A mid-level trough can be seen on water vapor east of the northern

   Baja Peninsula this morning. This trough will move quickly across

   the southern Plains through the day and into the Mid

   Mississippi/Lower Ohio Valley by Saturday morning. A very strong

   mid-level jet (90-100 knots) will develop as this wave impinges on a

   strong upper-level High across the Southeast. 


   Broad warm air advection is expected across the Mid-Mississippi

   Valley during the day Friday with a strengthening low-level jet

   through the day. Significant mass response is expected across this

   area by early evening as the mid-level trough approaches the area.

   As a result, the surface low will deepen rapidly between 00Z to 06Z

   to around 992-994mb in the southern Illinois/Indiana vicinity.

   During this period of rapid deepening, a warm front which is

   forecast to be mostly stationary from northeast Arkansas to central

   Tennessee during most of the day, will start to move quickly north

   during the late afternoon with the northern extent of the warm

   sector depicted by the approximate path of the surface low.


   ...Mid Mississippi Valley...

   Thunderstorms will be ongoing at the beginning of the period across

   eastern Oklahoma and western Arkansas. Some embedded supercells are

   possible with the threat for a few weak tornadoes. Most guidance is

   consistent with the convectively enhanced cold front drifting south

   into north-central/northeast Arkansas in the morning. Therefore,

   this early activity will likely wane as it interacts with this

   southward moving front by late morning. 


   A pocket of drier air can be seen on water vapor moving north in the

   west-central Gulf early this morning. This is associated with a

   relative minimum in PWAT which will overspread eastern Louisiana and

   much of Mississippi during the late morning and through the

   afternoon. This seems to be responsible for the significant mixing

   and surface dewpoint reductions seen my much of the guidance across

   Mississippi in the afternoon where temperatures warm into low 80s.

   However, despite this drier air further east, deep moisture will

   remain across the western Gulf and will advect northward into

   Louisiana during the afternoon as low-level mass response increases.

   By mid to late afternoon, upper 60s to potentially low 70s dewpoints

   are expected across northern Louisiana and eastern Arkansas,

   spreading into northern Mississippi by the evening. This will lead

   to an uncapped warm sector featuring MLCAPE of 500-1000 J/kg up the

   Mississippi River to near Memphis and 1500-2000 J/kg farther south

   across northern Louisiana and west-central Mississippi. 


   Expect storms to strengthen during the afternoon as the better

   moisture advects northward and destabilizes the airmass ahead of

   ongoing activity. CAM guidance is in agreement for a strong QLCS to

   develop from central to northern Arkansas during the afternoon. This

   line of storms will pose a threat for damaging wind and QLCS

   tornadoes given the long, curved low-level hodographs with the best

   overlap of favorable shear and instability in the vicinity of the

   Mississippi River. This line of storms will eventually outrun the

   better instability as it moves toward Middle Tennessee/southern

   Kentucky, but the strong low-level jet (~70 kts), and strong forcing

   with the deepening surface cyclone will help to maintain some severe

   threat well into the overnight despite progressively more meager

   instability. 


   Across southeast Arkansas, northern Louisiana, and northwest

   Mississippi, a more volatile environment will develop Friday

   evening/early overnight. More discrete convection is anticipated on

   the southern periphery of the aforementioned QLCS. The more discrete

   mode, combined with greater instability and strong shear should

   allow for multiple supercells to develop across northern Louisiana

   and southern Arkansas and move northeastward. Low-level hodographs

   are very favorable in this region with 0-500m SRH around 200 m2/s2

   and 0-1km SRH 300+ m2/s2. Therefore, any sustained supercells will

   be capable of producing strong to intense (EF3+) tornadoes, with

   long-track tornadoes possible with any longer-lived, undisturbed

   supercells.


   00Z HREF members showed a variety of solutions which cast some

   uncertainty on the forecast. WRF members are notably less bullish

   with warm sector supercell development from northeast Louisiana into

   northern Mississippi while the HRRR was most aggressive with

   convective coverage and environment. After further investigation it

   appears the more aggressive HRRR solution can be attributed to a

   more robust mass response during the afternoon/early evening hours.

   This results in a pronounced shortwave trough which can be seen at

   700 and 850mb and reflected as a significant confluence zone at the

   surface. Not only does this act as a forcing for storm development,

   but it also acts as moisture convergence with a more broad region of

   70+F dewpoints. In this scenario, numerous strong tornadoes would be

   likely, with the potential for several intense tornadoes. Despite

   being the most aggressive, this solution does not seem unreasonable

   as similar low-level confluence features can been on both the 00Z

   GFS and the 18Z ECMWF. 


   As is often the case, the severity of the tornado threat across the

   moderate risk will be modulated by mesoscale influences in the

   region. As the event approaches, these mesoscale effects may become

   more clear and allow the greatest risk corridor to become better

   defined.


   ..Bentley/Weinman.. 03/24/2023

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