USA Weather — Severe Weather Division
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS EXTENDING ACROSS FAR EAST-CENTRAL IOWA...NORTHERN ILLINOIS...AND NORTHWEST INDIANA...
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS THE SOUTHERN PLAINS...
...SUMMARY...
WIDESPREAD SEVERE THUNDERSTORMS ARE EXPECTED TODAY INTO TONIGHT EXTENDING FROM THE GREAT LAKES AND OHIO VALLEY SOUTHWESTWARD INTO THE SOUTHERN PLAINS. STRONG TO INTENSE (EF2-EF3) TORNADOES, WIDESPREAD VERY LARGE TO POTENTIALLY GIANT HAIL, AND SEVERE WIND GUSTS ARE EXPECTED ACROSS THE MIDWEST, GREAT LAKES, AND OHIO VALLEY. A COUPLE OF TORNADOES WHICH COULD BE STRONG, IN ADDITION TO GIANT HAIL AND POTENTIALLY WIDESPREAD SIGNIFICANT DAMAGING WINDS ARE ALSO EXPECTED ACROSS THE SOUTHERN PLAINS.
...SYNOPSIS...
A COMPLEX SPLIT-FLOW UPPER-LEVEL PATTERN WILL PERSIST ACROSS THE CONUS TODAY. A POLAR JET STREAM EXTENDS ACROSS THE NORTHERN QUARTER OF THE US, WITH A FAST-MOVING EMBEDDED SHORTWAVE TROUGH NOTED ACROSS THE PACIFIC NORTHWEST INTO THE NORTHERN HIGH PLAINS. A CUTOFF LOW LOCATED OVER THE DESERT SOUTHWEST INTO NORTHERN MEXICO IS EMBEDDED WITHIN THE SOUTHERN STREAM/SUBTROPICAL JET. BOTH THE CUTOFF LOW AND THE NORTHERN SHORTWAVE TROUGH WILL BOTH MOVE IN TANDEM EASTWARD INTO THE GREAT PLAINS THROUGHOUT THE PERIOD. THE CUTOFF LOW WILL BEGIN TO BECOME ELONGATED WHILE THE NORTHERN SHORTWAVE BEGINS TO AMPLIFY THIS EVENING AS THEY BEGIN TO PHASE. AHEAD OF BOTH OF THESE FEATURES, STRONG SOUTHWESTERLY 500MB FLOW IN EXCESS OF 80KT OVERSPREADS MUCH OF THE PLAINS, OHIO VALLEY, AND GREAT LAKES REGION, WITH THESE FLOWS GRADUALLY INTENSIFYING WITH TIME, LIKELY EXCEEDING 100KT TONIGHT.
AT THE SURFACE, A STATIONARY FRONT DRAPED FROM ONTARIO INTO THE GREAT LAKES AND INTO A BROAD SURFACE LOW OVER NORTHERN KANSAS COULD SLOWLY BEGIN TO DRIFT NORTHWARD THIS AFTERNOON AND EVENING AS A WARM FRONT, ESPECIALLY ACROSS SEGMENTS IN THE MIDWEST AND GREAT LAKES. A DRYLINE IS ALSO NOTED TO BE EXTENDING FROM THIS SURFACE LOW SOUTHWARD INTO THE TEXAS BIG BEND AND NORTHERN MEXICO. AT LEAST THE NORTHERN HALF OF THE DRYLINE SHOULD MIX EASTWARD POTENTIALLY INTO THE LOWER MISSOURI VALLEY THROUGH EARLY EVENING. ANOTHER BROAD SURFACE LOW IS ALSO BEGINNING TO DEVELOP OVER NORTHEAST NEW MEXICO THIS MORNING IN ASSOCIATION WITH THE RIGHT EXIT REGION OF THE JET STREAK AHEAD OF THE CUTOFF LOW, AIDED BY MODERATE UPPER-LEVEL DIVERGENCE. A WEAK PACIFIC COLD FRONT WILL DEVELOP AND GRADUALLY SHIFT EASTWARD WITH THE LOW, OVERTAKING THE SOUTHERN PORTIONS OF THE DRYLINE THIS AFTERNOON AND EVENTUALLY REACHING EAST-CENTRAL TEXAS BY 12Z WEDNESDAY. FINALLY, A STRONG COLD FRONT WILL BEGIN TO SURGE SOUTHWARD THROUGH THE PLAINS TONIGHT BEHIND THE NORTHERN SURFACE LOW AS IT DEEPENS WHILE QUICKLY SHIFTING NORTHEASTWARD ALONG THE STATIONARY/WARM FRONT.
...MIDWEST/GREAT LAKES/OHIO VALLEY...
VERY STRONG LOW-LEVEL MASS RESPONSE IS WELL UNDERWAY ACROSS THE MIDWEST INTO PARTS OF THE OHIO VALLEY THIS MORNING, WITH VISIBLE IMAGERY SHOWING A LARGE AREA OF CLEAR SKIES ACROSS MUCH OF ILLINOIS TO THE SOUTH OF THE WARM FRONT. SURFACE DEWPOINTS INTO THE MID-60S ALREADY EXTEND INTO THIS FRONT AS FAR NORTH AS SOUTHEAST IOWA AND NORTHERN ILLINOIS. CONTINUED MOISTURE ADVECTION WILL RESULT IN DEWPOINTS CONTINUING TO INCREASE POTENTIALLY TO NEAR 70 ACROSS PARTS OF NORTHERN OR CENTRAL ILLINOIS INTO PARTS OF IOWA AND MISSOURI BY MID TO LATE AFTERNOON IN THE VICINITY OF THE SURFACE LOW. CONSIDERABLE DESTABILIZATION IS EXPECTED TODAY, ESPECIALLY INTO THE AFTERNOON HOURS AS DOWNSTREAM MID-LEVEL LAPSE RATES IN EXCESS OF 8.0-8.5 C/KM PER 12Z RAOBS ADVECT NORTHEASTWARD INTO THE REGION. COMBINED WITH STRONG DIURNAL HEATING INTO THE UPPER 70S TO LOW 80S WILL SUPPORT STRONG TO LOCALLY EXTREME INSTABILITY WITH WIDESPREAD MLCAPE OF 2500-3300 J/KG, PARTICULARLY ACROSS NORTHERN ILLINOIS INTO SOUTHERN IOWA AND NORTHERN MISSOURI.
INTENSE CONVECTIVE INITIATION IS EXPECTED TO OCCUR SOMETIME LATER THIS AFTERNOON OR EARLY EVENING BETWEEN 21Z-00Z AS INCREASING HEIGHT FALLS COMBINED WITH STRENGTHENING CONVERGENCE ALONG THE FRONT AND PERHAPS JUST SOUTH OF THE FRONT ALONG ANY DIFFERENTIAL HEATING ZONES THAT OCCUR. VERY IMPRESSIVE KINEMATIC PROFILES WITH RATHER LARGE CURVED LOW-LEVEL HODOGRAPHS AND STRAIGHTER IN THE MID TO UPPER LEVELS WILL INITIALLY FAVOR SEVERAL INTENSE SUPERCELLS SPREADING QUICKLY EASTWARD IN THE VICINITY OF THE FRONT. AFOREMENTIONED THERMODYNAMIC PROFILES WILL SUPPORT QUICK INTENSIFICATION ABOVE SEVERE LIMITS. MANY OF THESE SUPERCELLS WILL PRODUCE FAIRLY LENGTHY SWATHS OF LARGE TO VERY LARGE HAIL (1.5-2.5 IN) WITH INSTANCES OF GIANT HAIL POTENTIALLY TO 4” LIKELY. MDT RISK HAIL PROBABILITIES ARE IN EFFECT AS A RESULT, WITH A 60% REGION DELINEATED OVER NORTHERN ILLINOIS INTO FAR NORTHWEST INDIANA, INCLUDING MUCH OF THE CHICAGO METRO AREA. AT LEAST A FEW TORNADOES ARE LIKELY TO OCCUR DUE TO 250-350 M2/S2 0-1KM SRH LATE THIS AFTERNOON, INCREASING TO OVER 400 M2/S2 AS THE LOW-LEVEL JET OVERHEAD STRENGTHENS TO OVER 60KT RATHER QUICKLY. WITH SUCH INCREASE EXPECTED TO OCCUR DURING THE TIMEFRAME WHERE SEVERAL DISCRETE SUPERCELLS SHOULD ALREADY BE WELL-ESTABLISHED AND SURFACE BASED, POTENTIAL FOR STRONG TO INTENSE TORNADOES (EF2-EF3) APPEARS HIGH ENOUGH TO WARRANT MDT RISK LEVEL TORNADO PROBABILITIES.
INCREASINGLY WIDESPREAD STORMS ACROSS A LARGE AREA WILL EVENTUALLY SUPPORT UPSCALE GROWTH INTO ONE OR MORE ORGANIZED, FORWARD PROPAGATING CLUSTERS ASSOCIATED WITH ALL SEVERE HAZARDS. A MORE INTENSE BOWING MCS INITIATING OVER NORTHEAST MISSOURI IS NOT OUT OF THE QUESTION, WHICH WOULD BE ACCOMPANIED BY A MORE WIDESPREAD AND INTENSE WIND DAMAGE TREAT, AS WELL AS LINE-EMBEDDED TORNADOES. INTO THE LATE EVENING AND OVERNIGHT HOURS, EVOLUTION INTO A LARGE QLCS ALONG THE COLD FRONT APPEARS PROBABLE, WHICH WILL GRADUALLY PROPAGATE EASTWARD, WITH DAMAGING WINDS, POTENTIALLY TO 75MPH, AND A COUPLE EMBEDDED TORNADOES THROUGH THE END OF THE PERIOD.
...SOUTHERN PLAINS...
VERY MOIST SURFACE AIRMASS WITH DEWPOINTS IN THE UPPER 60S TO LOW 70S WITH AT LEAST FILTERED SURFACE HEATING UNDERNEATH VERY STEEP MID-LEVEL LAPSE RATES IN EXCESS OF 8.5 C/KM WILL CONTRIBUTE TO STRONG MLCAPE OF 2500-3500 J/KG ACROSS MUCH OF CENTRAL TEXAS INTO CENTRAL OKLAHOMA, WITH VALUES OVER 4000 J/KG EXPECTED OVER PARTS OF RIO GRANDE VALLEY REGION. STRENGTHENING CONVERGENCE ALONG THE DRYLINE AND POSSIBLY A WEAK SURFACE TROUGH AXIS FURTHER EAST WILL SUPPORT RAPID CONVECTIVE INITIATION AROUND 21-23Z AS SUBSTANTIAL HEIGHT FALLS SPREAD ACROSS THE REGION, WITH INITIATION EXTENDING FROM WEST-CENTRAL OKLAHOMA INTO THE TEXAS BIG BEND. INTENSE DEEP-LAYER SHEAR OVER 65-70KT FROM LARGE AND ELONGATED HODOGRAPHS WITHIN THIS HIGHLY UNSTABLE AIRMASS WILL INITIALLY FOSTER SUPERCELL ACTIVITY PRODUCING INSTANCES OF GIANT HAIL AND PERHAPS A COUPLE TORNADOES. RAPID UPSCALE GROWTH INTO A NORTHEASTWARD PROPAGATING MCS IS EXPECTED BY EARLY EVENING, WITH SWATHS OF DAMAGING WINDS INCREASINGLY LIKELY, ESPECIALLY OVER NORTH-CENTRAL TEXAS INTO CENTRAL OKLAHOMA DUE TO IMPRESSIVE KINEMATICS AND EXTREMELY STEEP LOW-LEVEL LAPSE RATES AROUND 9.0 C/KM CONTRIBUTING TO DCAPE WELL OVER 1100 J/KG. CONSIDERATION FOR MDT RISK WIND PROBABILITIES WAS MADE FOR THIS OUTLOOK, THOUGH CONFIDENCE IS A BIT LOW FOR NOW IN WIDESPREAD INTENSE (>75MPH) GUSTS OR DEVELOPMENT OF >84MPH GUSTS TO WARRANT SUCH AN UPGRADE, THOUGH TRENDS WILL BE MONITORED CLOSELY AND FUTURE UPGRADES ARE POSSIBLE.
DEVELOPMENT FURTHER SOUTH COULD REMAIN SUPERCELLULAR FOR A BIT LONGER INTO THE EVENING, WHERE INCREASING LOW-LEVEL JET WINDS SHOULD CONTRIBUTE TO 0-1KM SRH EXCEEDING 300 M2/S2, SUPPORTIVE FOR A STRONG TORNADO THREAT WITH ANY DISCRETE SUPERCELL – IN ADDITION TO VERY LARGE TO GIANT HAIL. EVENTUALLY THESE STORMS SHOULD GROW UPSCALE LATER IN THE EVENING WITH DAMAGING WINDS LIKELY AS THEY SPREAD NORTHEASTWARD.
...CENTRAL PLAINS INTO LOWER MISSOURI VALLEY...
AN AREA FROM EASTERN KANSAS INTO SOUTHERN MISSOURI SHOULD BE SITUATED BETWEEN THE TWO MAIN FEATURES RESULTING IN SYNOPTIC SCALE FORCING, LIKELY WARRANTING CONVECTION TO BE MORE ISOLATED. DESPITE THIS, VERY STRONG INSTABILITY AND SUFFICIENT SHEAR WILL BE IN PLACE FOR ISOLATED DEVELOPMENT OF BOTH SUPERCELLS AND FORWARD-PROPAGATING CLUSTERS TO PRODUCE A THREAT FOR LARGE TO VERY LARGE HAIL AND DAMAGING WINDS. HIGHER LCLS WILL ALSO RENDER THE TORNADO THREAT RATHER ISOLATED, THOUGH A STRONG TORNADO CANNOT BE RULED OUT IF A DISCRETE SUPERCELL CAN BECOME TORNADIC DUE TO FAIRLY STRONG LOW-LEVEL HELICITY ESPECIALLY IN THE EVENING.