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(T. F. “Storm” Walsh)

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The forecast office is closed on Sunday

The Storm Prediction Center (SPC) has issued a SLIGHT risk for severe thunderstorms in the current DAY 2 Outlook: ACROSS PORTIONS OF THE MIDWEST AND GREAT LAKES…

…SPC SUMMARY DAY 2…
Scattered strong to severe thunderstorms are expected late Thursday afternoon and evening across parts of the Midwest and Ohio River Valley. Very large hail, a few tornadoes, and severe winds will be possible.

SPC DAY 2 SEVERE THUNDERSTORM OUTLOOK (LINKED FOR OUTLOOK TEXT)

TORNADO PROBABILITY

Probability of a tornado within 25 miles of a point.  Hatched Area: 10% or greater probability of EF2 – EF5 tornadoes within 25 miles of a point.
HAIL PROBABILITY

Probability of one inch diameter hail or larger within 25 miles of a point.  Hatched Area: a 10% or greater probability of 2-inch diameter or larger hail 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.
The following is the current CSU – MLP severe weather forecast map and NSSL ML probability forecast:
CSU – MLP DAY 2 FORECAST (CLICK FOR LARGER IMAGE)

NSSL ML DAY 2 SEVERE PROBABILITY 

DAY 2 PROBABILITY CONVERSION TABLE

SPC DISCUSSION EXCERPT DAY 2:
…Midwest/Great Lakes…
Thunderstorm development is expected by late afternoon across the southern Lake Michigan region as the cold front begins to impinge on a plume of returning moisture. Stout capping at the base of an EML will likely limit initial storm coverage, and elongated hodographs (featuring 40-50 knot effective bulk shear values) will promote organization of discrete/semi-discrete supercells for at least an hour or two. Given the very favorable convective environment (characterized by SCP values upwards of 8-12), large to very large (2-3 inches in diameter) hail appears possible. However, it remains unclear how long storm modes will remain discrete with latest HRRR, RRFS, and MPAS solutions all suggesting relatively quick upscale growth along the front by early evening. These solutions appear reasonable given strong frontal ascent and deep-layer flow predominantly along the boundary. This may modulate the duration/coverage of the significant hail threat, and would favor a scattered severe wind threat downstream into IN and OH.

Regardless, 200-400 m2/s2 effective SRH immediately ahead of the front will support some threat for tornadoes, including the potential for a significant tornado if supercells can be maintained just ahead of the front.

The following maps are PRELIMINARY DAY 2 probability maps from NADOCAST for tornado and hail probabilities.  
12Z NADOCAST TORNADO PROBABILITY

12Z NADOCAST SIGTOR PROBABILITY 

12Z NADOCAST HAIL PROBABILITY 

12Z NADOCAST SIGHAIL PROBABILITY 

Based on my analysis of the preliminary severe weather indices, the forecast calls for a moderately unstable to borderline very unstable atmosphere at the moment.  Indices and current outlook maps indicate ALL severe weather threats are possible.  Large to significant hail is in the forecast, and should be located mainly within the current 15% HATCHED HAIL outline.  Based on analysis of deep layer shear, veering winds with height, 0 – 3km SRH, and EHI values, tornadoes are likely.  Based on the maximum EHI value and maximum 0 – 3km SRH, the possibility of some strong tornadoes, (EF2+) probably isolated, could be experienced.  Currently, strongest severe weather indices should be within the SLIGHT risk, 5% TORNADO outline, and darkest area of the NSSL ML probability outlines, mainly over Illinois and Indiana.  Based on analysis, I would not rule out at the moment of the SPC upgrading a small area to an ENHANCED risk.  Some indices will most likely change between issuance of this synopsis, and by the onset of the DAY1 outlook, so this forecast should not be taken as absolute.  
CAPE VALUES FORECAST

DEW POINT FORECAST

Based on analysis of model animations, the current outlook, and analysis of indices, the strongest convection and indices should occur by mid – late afternoon on Thursday, through early evening, with current animations indicating the strongest between 4:00 P.M. – 9:00 P.M. CDT.  

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

The following were the forecast parameters and indices analyzed this morning.  Bear in mind, 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 – 2500 j/kg-1                  
MLCAPE: 500 – 2000 j/kg-1                   
MUCAPE: 750 – 2500 j/kg-1                
SRH 0 -1 km: 100 – 250 m2/s2               
SRH 0 -3 km: 200 – 400 m2/s2        
SRH EFFECTIVE: 200 – 300 m2/s2          
L. I.: -3 to -8                                                  
SCP: 2 – 16                                            
STP: 0.2 – 1.9 
0 -6 km SHEAR: 50 kts – 60 kts                  
EFF. SHEAR: 40 kts – 50 kts                        
MID LEVEL LAPSE RATE: 7.5C – 8.5C   
DEWPOINT: 62F – 66F                                    
EHI: 0.6 – 3.9                                        
TOTAL TOTALS INDEX: 53C                       
K INDEX: 21C – 28C                                          
SWEAT INDEX: 485 – 565                             
THOMPSON INDEX: 24– 36                     
CRAVEN – BROOKS INDEX: 10,000 – 50,000 

The following are some severe weather parameters and indices explanations:
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 2:00 P.M. CDT MAR. 26 – 10:00 P.M. CDT MAR. 26

NAM 3KM STP FORECAST 2:00 P.M. CDT MAR. 26 – 10:00 P.M. CDT MAR. 26

NAM 3KM FORECAST RADAR ANIMATION 2:00 P.M. CDT MAR. 26 – 10:00 P.M. CDT MAR. 26

I have provided the SPC homepage link below, so you may get the updated information regarding any changes to the outlook:
SPC HOMEPAGE
https://www.spc.noaa.gov/classic.html

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 any forecasts, 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