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(T. F. “Storm” Walsh)

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The forecast center is closed on Sunday’s

CURRENT DAY 1 SEVERE WEATHER OUTLOOK
The Storm Prediction Center (SPC) has issued a SLIGHT risk for severe thunderstorms in the current DAY 1 Outlook: FOR THE ARKLAMISS EAST ACROSS NORTH-CENTRAL MISSISSIPPI…

…SPC SUMMARY…
Scattered severe thunderstorms capable of large hail will be possible from the ArkLaMiss into north-central Alabama, mostly from mid-afternoon through early/mid-evening.
SPC DAY 1 SEVERE THUNDERSTORM OUTLOOK (LINKED FOR OUTLOOK TEXT)

TORNADO PROBABILITY

Probability of a tornado within 25 miles of a point.
HAIL PROBABILITY

Probability of one inch diameter hail or larger within 25 miles of a point.
DAMAGING THUNDERSTORM WIND PROBABILITY

Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point.
CSU – MLP DAY 1 FORECAST (CLICK FOR LARGER IMAGE)

NSSL ML DAY 1 PROBABILITY

DAY1 PROBABILITY CONVERSION TABLE

SPC DISCUSSION EXCERPT:
…Deep South including ArkLaMiss to north-central Alabama…
A mid-level disturbance over the central High Plains is forecast to quickly move southeastward into the ArkLaMiss by late tonight. As the upper system approaches, a cold front draped from southeast OK eastward along the MS-TN border is forecast to accelerate southeastward this evening and through much of the central Gulf Coast states through tonight.

Modest mid-level cold-air advection will continue through the afternoon and steepen 700-500 mb lapse rates across MS/AL. The advection of the mid-level lapse rate plume and additional heating will combine to increase buoyancy (ranging from 1500+ J/kg SBCAPE west, to less than 500 J/kg east). Guidance is consistent in showing weak low-level flow, but strong mid/high-level winds will yield 40+ kt effective shear, supporting the potential for severe storms including some supercells as storms develop and intensify through mid/late afternoon. Both the prospects for appreciable destabilization and probable widely scattered coverage of an initial supercell storm mode, lend confidence for upgrading severe hail probabilities (Slight Risk). A gradual waning in storm intensity is expected through the evening as instability lessens and a transition to less organized storm modes occur.

The following maps are from NADOCAST for tornado and hail probabilities.  Click images for larger view.  
00Z NADOCAST TORNADO PROBABILITY (CLICK FOR LARGER IMAGE)

00Z NADOCAST SIGTOR PROBABILITY (CLICK FOR LARGER IMAGE)

00Z NADOCAST HAIL PROBABILITY (CLICK FOR LARGER IMAGE)

00Z NADOCAST SIGHAIL PROBABILITY (CLICK FOR LARGER IMAGE)

Based on my analysis of the severe weather indices, the forecast seems t0 call for a moderately unstable atmosphere at the moment.  Indices and current SPC outlook indicate severe thunderstorms may be possible from mid afternoon  the early evening, into the early and mid evening hours. The main severe risk as of analysis this morning appears to be isolated hail, some possibly reaching severe levels based on lifted indices, bulk shear and steep mid level lapse rates within the 15% HAIL outline.  CAPE values are in the moderately unstable range.  The tornado risk  is currently low at the moment.  It was noticed in my analysis that maximum MLCAPE is forecast to be over the Mississippi severe outline.  However, this is the area with the minimum 0 – 3 km SRH.  Nadocast indicates any tornado activity should it occur, will be mainly over the Alabama portion of the outline, and even though MLCAPE is lower over the area, 0 – 3 km SRH is where the maximum should be located, hence the Nadocast forecast.  While tornado activity is at a low risk, any tornadoes that may occur should remain weak (EF0 – EF1).  A small 5% damaging thunderstorm winds / gusts outline has been introduced in the SPC update.

Based on analysis of model animations, the current outlook, and analysis of indices, the strongest convection and indices should occur approximately between 3:00 P.M. CST – 9:00 P.M. Rain and general thunderstorms could initiate by 1:00 P.M CST, possibly lasting well into the evening.  Please visit the SPC homepage via the link provided for any changes to the forecast today.
CAPE VALUES FORECAST

DEW POINT FORECAST

Indices were analyzed from the NAM 3KM, HRRR 3KM, CIPS DETERMINISTIC, and SPC SREF model guidance.

The following were the forecast parameters and indices analyzed this morning, and lie within the SLIGHT risk 15% HAIL outline.  Bear in mind, MAXIMUM indices recorded below are for the time of peak intensity.  Some indices meanings are posted below the indices themselves, and the NWS page containing a more extensive explanation can be accessed further on in the synopsis:
SBCAPE: 500- 1500 j/kg-1                  
MLCAPE: 500 – 1500 j/kg-1                   
MUCAPE: 500 – 1500 j/kg-1                
SRH 0 -1 km: 25 – 250 m2/s2               
SRH 0 -3 km: 150 – 350 m2/s2        
SRH EFFECTIVE: 50 – 200 m2/s2          
L. I.: -3 to -6                                                  
SCP: 1.0 – 4.0                                          
STP: 0.0 – 0.4 
0 -6 km SHEAR: 40 kts – 60 kts                  
EFF. SHEAR: 30 kts – 40 kts                        
MID LEVEL LAPSE RATE: 7.5C – 8.0C   
DEWPOINT: 60F – 65F                                    
EHI: 0.3- 1.3                                     
TOTAL TOTALS INDEX: 52C                     
K INDEX: 21C to 34C                                          
SWEAT INDEX: 285 – 350                         
THOMPSON INDEX: 24 to 40      
CRAVEN – BROOKS INDEX: 10,000 – 25,000    

CAPE

ENERGY HELICITY INDEX

K INDEX

TOTAL TOTALS INDEX

STORM RELATIVE HELICITY

LIFTED INDEX

SWEAT VALUES

THOMPSON INDEX

Craven SigSvr Parameter:
The simple product of 100mb MLCAPE and 0-6km magnitude of the vector difference (m/s; often referred to as “deep layer shear”) accounts for the compensation between instability and shear magnitude. Using a database of about 60,000 soundings, the majority of significant severe events (2+ inch hail, 65+ knot winds, F2+ tornadoes) occur when the product exceeds 20,000 m3/s3.

A little fact on SRH values and tornadoes from NOAA / NWS
Storm Relative Helicity (m² s^-2)
SRH (Storm Relative Helicity) is a measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1-km and 3-km layers above ground level. There is no clear threshold value for SRH when forecasting supercells, since the formation of supercells appears to be related more strongly to the deeper layer vertical shear. Larger values of 0-3-km SRH (greater than 250 m² s^-2) and 0-1-km SRH (greater than 100 m² s^-2), however, do suggest an increased threat of tornadoes with supercells. For SRH, larger values are generally better, but there are no clear thresholds between non-tornadic and significant tornadic supercells.

STP ( Significant Tornado Parameter) EXPLAINED:
A majority of significant tornadoes (EF2 or greater damage) have been associated with STP values greater than 1, while most non-tornadic supercells have been associated with values less than 1 in a large sample of RAP analysis proximity soundings.

SCP (Supercell Composite Parameter) EXPLAINED:
A multiple ingredient, composite index that includes effective storm-relative helicity (ESRH, based on Bunkers right supercell motion), most unstable parcel CAPE (muCAPE) and convective inhibition (muCIN), and effective bulk wind difference (EBWD). Each ingredient is normalized to supercell “threshold” values, and larger values of SCP denote greater “overlap” in the three supercell ingredients. Only positive values of SCP are displayed, which correspond to environments favoring right-moving (cyclonic) supercells.

The following are the SCP (Supercell Composite Parameter) and STP (Significant Tornado Parameter) forecast maps from the NAM 3KM model.  Generally, the higher the values and brighter the color, indicates a greater probability of strong thunderstorm and / or tornadic activity over an area:
NAM 3KM SCP FORECAST 1:00 P.M. CST FEB. 26 – 11:00 P.M. CST FEB. 26

NAM 3KM STP FORECAST 1:00 P.M. CST FEB. 26 – 11:00 P.M. CST FEB. 26

NAM 3KM RADAR SIMULATION 1:00 P.M. CST FEB. 26 – 11:00 P.M. CST FEB. 26

Please use the following maps, which should update automatically, for Mesoscale Discussions and Convective Watches.  You may have to refresh your browser, or click on the graphics.  I have provided the SPC homepage link below, so you may get the updated information regarding any changes to the outlook:
https://www.spc.noaa.gov/classic.html
SPC MESOSCALE DISCUSSIONS (CLICK IMAGE FOR UPDATES)

SPC CONVECTIVE WATCHES (CLICK IMAGE FOR UPDATES)

IF A TORNADO WARNING IS ISSUED FOR YOUR AREA, TAKE IMMEDIATE SAFE AND STURDY SHELTER!
The following sites will explain most of the severe weather and tornado values listed above, and will give you an idea of what to expect:
ENVIRONMENTAL INDICES AND PARAMETERS NWS
https://www.weather.gov/lmk/indices

THE WEATHER PREDICTION
http://www.theweatherprediction.com/severe/indices/

The following links will connect you to the Excessive Rainfall probabilities and River Flood Outlook:
EXCESSIVE RAINFALL
https://www.wpc.ncep.noaa.gov/qpf/excessive_rainfall_outlook_ero.php

SIGNIFICANT RIVER FLOOD OUTLOOK
https://www.wpc.ncep.noaa.gov/nationalfloodoutlook/index.html

The following NWS Watch / Warning map will provide local NWS information for your area.  Click the image, then once it refreshes, click on your area of interest to view forecasts, any special weather statements, hazards or advisories for your area.
NWS WATCH / WARNING DISPLAY (LINKED…CLICK MAP, THEN YOUR AREA)

NWS DOPPLER RADAR LOOP (LINKED, CLICK RADAR MAP)

RAP RADAR (CLICK IMAGE THEN GO TO LOOP DURATION AND PICK LENGTH OF LOOP, THEN CLICK RADAR SITE)
WUNDERGROUND RADAR SUMMARY MAP (CLICK FOR UPDATES)

PRECIPITATION TYPE (CLICK TO ACCESS ANIMATION FEATURE)

CARIBBEAN RADAR (CLICK IMAGE TO ACCESS ANIMATION)

You may direct any questions by contacting me personally, ANYTIME, at: [email protected]

Have a blessed day!

T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST