May 21, 2011 2000 UTC Day 1 Convective Outlook
Updated: Sat May 21 20:02:11 UTC 2011
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20110521 2000 UTC Day 1 Outlook Graphic
Probabilistic Tornado Graphic
20110521 2000 UTC Day 1 Tornado Probabilities Graphic
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.
Probabilistic Damaging Wind Graphic
20110521 2000 UTC Day 1 Damaging Wind Probabilities Graphic
Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point.
Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Probabilistic Large Hail Graphic
20110521 2000 UTC Day 1 Large Hail Probabilities Graphic
Probability of hail 1" or larger within 25 miles of a point.
Hatched Area: 10% or greater probability of hail 2" or larger within 25 miles of a point.
   SPC AC 211958
   
   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0258 PM CDT SAT MAY 21 2011
   
   VALID 212000Z - 221200Z
   
   ...THERE IS A SLGT RISK OF SVR TSTMS FROM THE MID MO VALLEY INTO 
   PARTS OF THE LOWER MS VALLEY AND SRN TX...
   
   ...SRN PLAINS INTO MID MS VALLEY...
   ONLY MINOR CHANGES MADE TO 1630Z OUTLOOK. EXTENDED SLIGHT RISK SWWD
   INTO SRN TX...REFERENCE WW 316...AS EXIT REGION OF 60 KT JET MAX
   MOVING NEWD INTO THE ARKLATEX HAS INCREASED CONVERGENCE ALONG
   DRYLINE. VERY UNSTABLE AIR MASS...MLCAPES NEAR 3500 J/KG... AND DEEP
   LAYER SHEAR AROUND 40 KT WILL SUPPORT SUPERCELLS WITH SEVERE HAIL
   THE PRIMARY THREAT. ALSO SHIFTED THE SRN PORTION OF SLIGHT RISK IN
   TX SWD...SINCE SOME OF THE HIGH RESOLUTION MODELS INDICATE A FEW OF
   THE STORMS DEVELOPING EAST OF THE DRYLINE IN CENTRAL TX WILL SHIFT
   EWD ALONG AND NORTH OF INTERSTATE 10.
   
   RELATIVELY THICK LOW CLOUDS ACROSS NERN TX/SERN OK SHOULD DECREASE
   THROUGH THE AFTERNOON AS JET MAX MOVES NEWD AND LARGE SCALE LIFT
   INCREASES ACROSS THE REGION...WITH A THREAT FOR SEVERE STORMS LATE
   AFTERNOON/EVENING. THE STORMS MAY CONTINUE NEWD FROM CENTRAL AR INTO
   WRN TN OVERNIGHT...BUT STORMS SHOULD WEAKEN IN INTENSITY AS THEY
   MOVE INTO A LESS UNSTABLE ENVIRONMENT.  
   
   ....ERN SD/SRN MN INTO CENTRAL PLAINS...
   HAVE RAISED TORNADO PROBABILITIES TO 5% IN FAR SERN NEB AND WRN IA
   WHERE LOW LEVEL WIND PROFILES MAY IMPROVE ENOUGH FOR A BRIEF TORNADO
   OR TWO. SLIGHT RISK WAS EXTENDED WWD ACROSS FAR SRN NEB AND NRN KS
   AS VISIBLE SATELLITE SHOWS CONVECTION DEVELOPING ALONG SURFACE
   TROUGH MOVING EWD INTO THE AREA. ALSO INCREASED WIND PROBABILITIES
   ACROSS NRN MO AND IA WHERE STORMS ARE EXPECTED TO CONGEAL INTO A
   SMALL LINEAR MCS THIS EVENING...WITH GREATER COVERAGE OF DAMAGING
   WINDS POSSIBLE.
   
   ..IMY.. 05/21/2011
   
   .PREV DISCUSSION... /ISSUED 1117 AM CDT SAT MAY 21 2011/
   
   ...ERN SD/SRN MN INTO CNTRL PLAINS...
   
   IN THE WAKE OF INITIAL VORTICITY MAXIMUM LIFTING NWD THROUGH THE MID
   MO VALLEY...MORNING WATER VAPOR IMAGERY SHOWS THE NEXT UPSTREAM
   IMPULSE OVER WRN PARTS OF NEB/KS.  THIS LATTER FEATURE IS FORECAST
   TO PIVOT NEWD THROUGH THE MID MO VALLEY TODAY PROVIDING FAVORABLE
   LARGE-SCALE FORCING FOR ASCENT FOR STRONG-SEVERE TSTM DEVELOPMENT. 
   WEAK COLD FRONT/PRESSURE TROUGH CURRENTLY EXTENDING FROM SURFACE LOW
   S OF BIS SEWD INTO ERN NEB WILL LIKELY SERVE AS THE FOCUS FOR THIS
   STORM DEVELOPMENT AS IT SLOWLY PROGRESSES EWD ACROSS THE REGION.
   
   12Z OAX SOUNDING SHOWED THAT THE PRE-FRONTAL AIR MASS IS ONLY WEAKLY
   CAPPED WITH STEEP MIDLEVEL LAPSE RATES APPROACHING 8 C/KM.  DAYTIME
   HEATING...COUPLED WITH BOUNDARY LAYER DEWPOINTS IN THE LOWER 60S AND
   THE STEEP MIDLEVEL LAPSE RATES WILL YIELD A MODERATELY UNSTABLE AIR
   MASS BY AFTERNOON WITH MLCAPE VALUES APPROACHING 1500-2500 J/KG. 
   VERTICAL SHEAR PROFILES WILL REMAIN SOMEWHAT CHAOTIC...EXHIBITING A
   VEERING-BACKING PATTERN.  NONETHELESS...THE PRESENCE OF THE STEEP
   LAPSE RATES AND SEASONABLY COOL MIDLEVEL TEMPERATURES WILL BE QUITE
   SUPPORTIVE OF LARGE HAIL WITH THE MOST INTENSE STORMS.
   
   FARTHER S INTO ERN KS AND MO...12Z TOP SOUNDING INDICATED A STRONGER
   CAP RELATIVE TO OAX WITH STORM INITIATION LIKELY BEING DELAYED UNTIL
   LATER THIS AFTERNOON INTO EVENING.  LATEST MODEL GUIDANCE SUGGESTS
   THAT AREAS GENERALLY N OF I-70 WILL SEE THE GREATEST COVERAGE OF
   TSTMS...WITH AREAL COVERAGE DECREASING WITH SWD EXTENT.  HERE
   TOO...THERMODYNAMIC ENVIRONMENT WILL BE CHARACTERIZED BY STEEP LAPSE
   RATES AND MLCAPE OF 2000-2500 J/KG.  WHEN COUPLED WITH 30-35 KT OF
   DEEP WLY SHEAR...SETUP WILL SUPPORT THE DEVELOPMENT OF ORGANIZED
   MULTICELL AND SUPERCELL STRUCTURES WITH LARGE HAIL BEING THE PRIMARY
   HAZARD.  STORMS MAY GROW UPSCALE INTO AN MCS WITH SOME THREAT FOR
   GUSTY WINDS AND HAIL CONTINUING ACROSS THE LOWER MO VALLEY
   OVERNIGHT.
   
   ...SRN PLAINS INTO LOWER MS VALLEY...
   
   MID MORNING SURFACE ANALYSIS SHOWS THAT THE MARITIME TROPICAL AIR
   MASS --SEE 12Z BRO/CRP SOUNDINGS-- EXTENDS FROM THE TX HILL COUNTRY
   INTO SRN LA...ALONG AND S OF OUTFLOW BOUNDARY ASSOCIATED WITH A
   NOCTURNAL MCS...THE REMAINS OF WHICH ARE STILL IN PROGRESS OVER
   W-CNTRL LA.  IT DOES APPEAR THAT THIS HIGH THETAE AIR WILL BE
   ADVECTED NWD INTO NERN TX/SERN OK TODAY AHEAD OF DRYLINE THAT IS
   FORECAST TO MIX EWD TO NEAR OR E OF THE I-35 CORRIDOR.  THIS
   MOISTURE COMBINED WITH THE NRN FRINGE OF STEEP MIDLEVEL LAPSE RATE
   PLUME WILL CONTRIBUTE TO A MODERATELY TO STRONGLY UNSTABLE WARM
   SECTOR BY LATE AFTERNOON WITH MLCAPE OF 2000-3500 J/KG.
   
   WEAK MIDLEVEL IMPULSE OVER WRN TX /EVIDENT IN WATER VAPOR IMAGERY/
   WILL TRANSLATE EWD AND PROVIDE FAVORABLE BACKGROUND FORCING FOR
   DRYLINE TSTM DEVELOPMENT LATE THIS AFTERNOON INTO EVENING.  REGIONAL
   PROFILER/VAD DATA INDICATE THE PRESENCE OF A 50-60 KT MIDLEVEL JET
   STREAK ACROSS REGION TODAY WHICH WILL RESULT IN MORE THAN SUFFICIENT
   VERTICAL SHEAR FOR SUPERCELLS GIVEN THE DEGREE OF INSTABILITY. 
   INITIALLY...LARGE HAIL WILL BE THE PRIMARY THREAT.  HOWEVER...A RISK
   FOR TORNADOES WILL INCREASE THIS EVENING AS LOW-LEVEL SHEAR
   STRENGTHENS IN RESPONSE TO THE DEVELOPMENT OF A SLY LLJ. 
   THEREAFTER...THERE IS SOME INDICATION THAT STORMS MAY GROW UPSCALE
   INTO AN MCS WITH A WIND/HAIL THREAT CONTINUING THROUGH TONIGHT INTO
   PORTIONS OF THE LOWER MS VALLEY/MID SOUTH.
   
   ...OH VALLEY...
   
   CONSIDERABLE UNCERTAINTY EXISTS WITH RESPECT TO DIURNAL TSTM
   DEVELOPMENT GIVEN THE ABSENCE OF ANY WELL-DEFINED FORCING
   MECHANISMS.  DIFFUSE WARM FRONT EXTENDING FROM NRN IL INTO CNTRL IND
   AS OF LATE MORNING MAY SERVE AS THE FOCUS FOR ISOLATED TSTM
   DEVELOPMENT THIS AFTERNOON.  AMBIENT ENVIRONMENT IS CHARACTERIZED BY
   VERTICALLY VEERING WIND PROFILES INVOF WARM FRONT WITH A RELATIVELY
   MOIST BOUNDARY LAYER WITH DEWPOINTS IN THE LOW/MID 60S.  THE PRIMARY
   FACTORS LIMITING A MORE ROBUST SEVERE WEATHER THREAT APPEAR TO BE
   THE MARGINAL DEEP-LAYER SHEAR AND WEAK MIDLEVEL LAPSE RATES WHICH
   WILL LIMIT THE DEGREE OF DESTABILIZATION.  STILL...ISOLATED
   STRONG-SEVERE STORMS WILL BE POSSIBLE WITH A RISK FOR GUSTY
   WINDS...SOME HAIL AND PERHAPS A BRIEF TORNADO.
   
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