SWODY1
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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