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Severe WX Severe Threat 17-18 March 2021

AL_ham_op said:
That's a lot of cloud cover that still needs to burn off.
Click to expand...

It's not projected to burn off. The warm front and retreating wedge is what will push temps/dews higher west of I-65 by 2-3pm.

Any areas where the sun does peak out will just have additional instability added.
 
Mesoscale Discussion 0200
NWS Storm Prediction Center Norman OK
1056 AM CDT Wed Mar 17 2021

Areas affected...Central MS...Central AL

Concerning...Severe potential...Tornado Watch likely

Valid 171556Z - 171800Z

Probability of Watch Issuance...95 percent

SUMMARY...The environment is becoming increasingly favorable for
tornadic supercells and a PDS tornado watch will likely be needed
within the next hour or two.

DISCUSSION...Regional radar and satellite imagery continues to show
deepening convection within the broad and robust warm-air advection
regime across much of MS and AL. Some modest clearing occurred
briefly ahead of the leading showers over MS and temperatures across
much of central and southern MS are now in the mid 70s. Slightly
cooler surface temperatures exist across central/southern AL.
Dewpoints exhibit a similar trend, with upper 60s/low 70s across
much of central/southern MS and mid to upper 60s across
central/southern AL. These thermodynamic conditions have eroded much
of the convective inhibition across the region, although forecast
soundings do suggests some minimal (i.e. MLCIN of -25 J/kg or more)
likely remains in place at the top of the boundary layer. Continued
moistening of the low-level profile coupled with slight cooling
aloft should result in a removal of all convective inhibition.

In addition to improving thermodynamics, the low-level wind fields
continue to increase. Recent VAD profiles from LIX and DGX show 50
kts within the 1-2 km layer. Latest VAD from DGX also sampled 0-3 km
storm-relative helicity over 350 m2/s2. These strengthening
low-level winds are expected to persist while gradually spreading
northward/northeastward into more of northern AL. Deep-layer
vertical shear is already in place over the region, with 0-6 km bulk
shear currently 50 kt over central MS/AL, increasing to 70 kt over
northern MS/AL.

In all, the overall environment is expected to becoming increasingly
favorable for discrete supercells capable of all severe hazards,
including intense tornadoes. These initial storms are forecast to
develop within the 17-19Z time frame. A prolonged threat for
tornadoes is anticipated across the region, with conditions
remaining favorable well into the evening.

..Mosier/Grams.. 03/17/2021
Click to expand...
 
Evan said:
It's not projected to burn off. The warm front and retreating wedge is what will push temps/dews higher west of I-65 by 2-3pm.

Any areas where the sun does peak out will just have additional instability added.
Click to expand...
Yeah warm air advection will do the trick. No need for sun.
 
andyhb said:


PDS tornado watch coming for MS/AL.
Click to expand...


Yeah that's why I was thinking this morning the High risk would be expanded to the south and maybe nudged to the east a bit. Model trends are reflecting a growing threat south of I-20.
 
Evan said:
On a similar topic, if you rely on broadcast or social media for warnings, I strongly recommend adding an additional warning source on your phone especially if you are driving or commuting today. As is often the case with potential outbreak scenarios, broadcast media can become overwhelmed with the frequency and duration of warnings and on a day like today minutes and seconds matter. Likewise, social media will be extremely active and it can be tough to filter for the most current information for your particular location.

Accordingly, I cannot recommend enough the following warning app for smartphones:

www.weatherradioapp.com

Weather Radio App

The Only Weather App You Need: Gain all the functionality of a Weather Radio right on your mobile device! Download today to acquire unique life-saving information in the form of alerts, audio, real-time streaming video, interactive mapping and more. Download today.
www.weatherradioapp.com www.weatherradioapp.com

Send the link to your less weather aware friends, colleagues, and family. I'm sure there are other similar apps out there but I have been using the Weather Radio App for years and it has never let me down. I believe I initially found it due to a post on Alabamawx.com by James Spann.
Click to expand...
I don't find any of the Smartphone apps adequate for alerting at night while I am sleeping. The alerts are too brief and not loud enough. Someone needs to make an app that blasts a horn like a weather radio and for a long enough duration to actually wake up the dead. Maybe there is one but I'm not aware of it.
 
The warm front has moved north of Anniston now. It's a full sunny sky outside with 65/60. Temps and dewpoints are going up quickly here.
 
1615997419622.png
...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS WEST-CENTRAL
ALABAMA...NORTHERN AND CENTRAL MISSISSIPPI...NORTHEAST
LOUISIANA...FAR SOUTHEAST ARKANSAS...

..SUMMARY

A SIGNIFICANT TORNADO OUTBREAK IS EXPECTED WITH NUMEROUS STRONG AND
A FEW LONG-TRACK, POTENTIALLY VIOLENT TORNADOES. AN INITIAL ROUND IS
EXPECTED TO BEGIN ACROSS EASTERN MISSISSIPPI AND ALABAMA THIS
AFTERNOON. A SEPARATE SWATH SHOULD EMANATE FROM SOUTHEAST ARKANSAS
TO LOUISIANA THIS AFTERNOON AND SPREAD EAST ACROSS MISSISSIPPI AND
ALABAMA THIS EVENING INTO TONIGHT.

..DEEP SOUTH

MINOR CHANGE MADE TO THE CATEGORICAL HIGH RISK AREA TO EXPAND A BIT.
MAIN CHANGE IS TO THE UNDERLYING PROBABILITIES WITH THE ADDITION OF
A 45 TOR AND 45 WIND. A DANGEROUS, LONG-DURATION TORNADO OUTBREAK
EXPECTED TO COMMENCE THIS AFTERNOON AND PERSIST WELL INTO TONIGHT
WITH MULTIPLE ROUNDS OF HEIGHTENED TORNADO POTENTIAL.

ONGOING CONVECTION IS WITHIN AN ARC WITH A PRE-FRONTAL SQUALL FROM
FAR EAST TX TO NORTHEAST AR AND THEN IN A MORE WEST/EAST-ORIENTATION
FROM NORTHEAST AR TO THE AL/TN BORDER AREA. THE NORTHERN ACTIVITY
WILL TEND TO REINFORCE THE SURFACE WARM FRONT APPROACHING THE TN
BORDER WITH MS/AL THIS AFTERNOON. THE SOUTHWEST BAND WILL LIKELY
PERSIST EAST AND SHOULD BREAKUP INTO A BROKEN BAND OF SUPERCELLS AT
SOME POINT THIS AFTERNOON AS LOW-LEVEL SHEAR FURTHER STRENGTHENS
ACROSS THE ARK-LA-MISS REGION. FARTHER EAST, WARM-ADVECTION SHOWERS
IN SOUTHEAST MS TO WEST-CENTRAL AL WILL LIKELY DEEPEN BY EARLY
AFTERNOON INTO A SEPARATE SWATH OF SEVERAL DISCRETE SUPERCELLS IN AN
ENVIRONMENT OF STRENGTHENING LOW TO DEEP-LAYER SHEAR. BETWEEN THESE
TWO AREAS, THE DESTABILIZING WARM SECTOR WITH WARMING BOUNDARY-LAYER
TEMPERATURES AND MID 60S TO LOW 70S DEW POINTS, WILL SUPPORT AN
EXPANSIVE PLUME OF 1500-2500 J/KG MLCAPE.

BY LATE AFTERNOON AND INTO EARLY TONIGHT, A LOW-LEVEL JET SEGMENT
WILL STRENGTHEN TO AT LEAST 50-60 KT ACROSS MS/AL AS THE MIDLEVEL
TROUGH APPROACHES FROM THE WEST, CONTRIBUTING TO VERY STRONG
LOW-LEVEL SHEAR (0-1 KM SRH IN EXCESS OF 300 M2/S2). BUOYANCY WILL
BE SLOW TO DECREASE AFTER SUNSET AND WITH EASTWARD EXTENT BASED ON
THE PREVALENCE OF RATHER RICH BOUNDARY-LAYER DEW POINTS, VERY
FAVORABLE WIND PROFILES (WITH 700-MB WINDS REACHING 70-80 KT) WILL
MAINTAIN THE THREAT FOR LONG-TRACK, INTENSE TORNADOES WITH BOTH
WARM-SECTOR SUPERCELLS, AS WELL AS SUPERCELLS WITHIN THE BROKEN BAND
NEAR AND AHEAD OF THE SURFACE WIND SHIFT PROGRESSING EAST ACROSS MS
BY EARLY TONIGHT. THE STRONG TORNADO AND SIGNIFICANT DAMAGING WIND
THREAT WHILE BECOMING MORE SPATIALLY CONFINED WITH TIME, WILL LIKELY
PERSIST ACROSS PARTS OF AL OVERNIGHT AND SHOULD SPREAD INTO WESTERN
GA BEFORE 12Z.

..GRAMS/BENTLEY.. 03/17/2021
 
David in SW Blount said:
I don't find any of the Smartphone apps adequate for alerting at night while I am sleeping. The alerts are too brief and not loud enough. Someone needs to make an app that blasts a horn like a weather radio and for a long enough duration to actually wake up the dead. Maybe there is one but I'm not aware of it.
Click to expand...
I had an app that played a tornado siren, can't remember the name. It actually came in use once when KILN's radar was down and a rotating Supercell was moving towards Columbus. It alerted me even before the outdoor sirens went off.
 
I think the I-20 corridor between Meridian, MS, and Tuscaloosa, AL, is going to be the most likely focus of a violent tornado, given the temperature differential in the area juxtaposed with outflow boundaries and low-level convergence, along with strong instability. This roughly aligns with recent mesoscale guidance. Any discrete supercell that manages to become surface-based in this area has great potential to produce a violent tornado. Edit: SPC just added 45% hatched TOR right over this area.
 
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