SPC AC 222000
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0300 PM CDT SAT MAY 22 2010
VALID 222000Z - 231200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE NRN AND
CNTRL PLAINS...
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE
CAROLINAS...
SEVERAL CHANGES HAVE BEEN MADE FOR THE 20Z OUTLOOK. THE FIRST CHANGE
IS TO MOVE THE 10 PERCENT TORNADO A BIT FURTHER NORTHEAST TO COVER
MOST OF CNTRL AND NE SD. A SIGNIFICANT TORNADO PROBABILITY CONTOUR
HAS BEEN ADDED ACROSS CNTRL AND NE SD DUE TO THE IMPRESSIVE
ENVIRONMENT CURRENTLY IN PLACE ACROSS SD. OBJECTIVE ANALYSIS SHOWS
MLCAPE VALUES APPROACHING 5000 J/KG IN NCNTRL SD WITH 50-60 KT OF
0-6 KM SHEAR. SUPERCELLS THAT TRACK ENEWD ACROSS NE SD MAY PRODUCE
AN ISOLATED SIGNIFICANT TORNADO ESPECIALLY AS LOW-LEVEL FLOW
STRENGTHENS DURING THE EARLY TO MID-EVENING. THE SECOND CHANGE IS TO
EXTEND THE SLIGHT RISK AREA FURTHER WEST ACROSS FAR WRN SD WHERE
MODERATE INSTABILITY IS CURRENTLY ANALYZED BY OBJECTIVE GUIDANCE.
THE THIRD CHANGE IS TO BRING THE SRN EDGE OF THE 30 PERCENT HAIL AND
SIGNIFICANT HAIL PROBABILITY CONTOURS FURTHER NORTH INTO FAR SRN SD
WHERE THE GREATEST INSTABILITY IS LOCATED. THE FOURTH CHANGE TO THE
OUTLOOK IS TO MAKE THE SLIGHT RISK AREA IN THE CAROLINAS MUCH
SMALLER AND CO-LOCATED WITH ONGOING THUNDERSTORMS NEAR FLORENCE SC.
ALSO...REMOVED THE 5 PERCENT HAIL AND WIND PROBABILITIES FROM MUCH
OF THE GULF COAST STATES WHERE STORMS HAVE YET TO INITIATE.
..BROYLES.. 05/22/2010
.PREV DISCUSSION... /ISSUED 1125 AM CDT SAT MAY 22 2010/
...SYNOPSIS...
LARGE-SCALE TROUGH OVER THE WRN CONUS WILL BECOME INCREASINGLY
ELONGATED IN NE-SW FASHION AS A STRONG SHORT WAVE TROUGH LIFTS
NEWD...FORMING A CLOSED CIRCULATION OVER SASKATCHEWAN.
MEANWHILE...AN UPSTREAM JET STREAK WILL DIG SWD OVER THE ERN
PACIFIC...SUPPORTING THE FORMATION OF A MORE SUBSTANTIAL UPPER LOW
OVER CA/NV. BETWEEN THESE EVOLVING FEATURES...SMALLER-AMPLITUDE
PERTURBATIONS WILL TRANSLATE NEWD FROM THE GREAT BASIN INTO NRN
PLAINS. IN THE EAST...A MUCH WEAKER MIDLEVEL TROUGH WILL DRIFT EWD
THROUGH THE LOWER GREAT LAKES AND MID ATLANTIC STATES.
...NRN PLAINS LATE THIS AFTERNOON INTO TONIGHT...
12Z OBSERVED SOUNDINGS INDICATE THE PRESENCE OF VERY STEEP MIDLEVEL
LAPSE RATES AND A SUBSTANTIAL CAP IN ASSOCIATION WITH AN EML WHICH
HAS OVERSPREAD THE GREAT PLAINS. MEANWHILE...A 50+ KT LLJ CONTINUES
TO TRANSPORT AN INCREASINGLY MOIST BOUNDARY LAYER NWD INTO THE
REGION WITH DEW POINTS RISING THROUGH THE 60S ACROSS SYSTEM WARM
SECTOR. THIS MOISTURE INCREASE...COUPLED WITH DAYTIME HEATING AND
THE STEEP LAPSE RATES WILL CONTRIBUTE TO AN EXTREMELY UNSTABLE AIR
MASS BY AFTERNOON OVER CNTRL/ERN PARTS OF SD/NEB WHERE MLCAPE WILL
APPROACH 4000 J/KG
12Z MODEL GUIDANCE REMAINS CONSISTENT IN INDICATING THAT A COLD
FRONT NOW EXTENDING FROM LOW PRESSURE OVER SERN MT SWWD INTO S-CNTRL
WY WILL PROGRESS EWD INTO THE WRN DAKOTAS BY AFTERNOON. ALONG THIS
BOUNDARY...A SURFACE WAVE IS FORECAST TO DEVELOP OVER CNTRL SD WITH
A DRY LINE EXTENDING SWD FROM THE TRIPLE POINT THROUGH THE NEB
SANDHILLS INTO WRN KS. THESE DATA SUGGEST THAT STORM INITIATION
WILL BECOME INCREASINGLY LIKELY BY LATE AFTERNOON OR EARLY EVENING
ALONG THE COLD FRONT AS THE WRN EDGE OF THE CAP IS ERODED BY A WEAK
IMPULSE LIFTING NEWD THROUGH THE NRN HIGH PLAINS.
FORECAST SOUNDINGS VALID 22/21Z-23/03Z AHEAD OF THE COLD FRONT OVER
CNTRL SD SUGGEST THAT STORMS SHOULD BECOME RAPIDLY SEVERE WITH THE
STRONG INSTABILITY...40-50 KT OF DEEP-LAYER SHEAR AND EFFECTIVE SRH
OF 100-200 M2/S2. VERY LARGE...DESTRUCTIVE HAIL APPEARS LIKELY AND
TORNADOES WILL BE POSSIBLE WILL ANY SUPERCELLS THAT CAN MATURE AND
REMAIN MORE DISCRETE. STORMS SHOULD ORGANIZE INTO AN MCS WITH THE
THREAT FOR MAINLY DAMAGING WINDS AND HAIL CONTINUING OVER PARTS OF
THE ERN DAKOTAS INTO NWRN MN OVERNIGHT.
...CAROLINAS INTO THIS EVENING...
SCATTERED DEEP CONVECTION IS ONGOING LATE THIS MORNING/EARLY
AFTERNOON FROM PARTS OF THE CAROLINA PIEMONT SWWD INTO E-CNTRL GA IN
CONJUNCTION WITH PASSAGE OF WEAK MIDLEVEL TROUGH AXIS. AMBIENT AIR
MASS IS RELATIVELY MOIST WITH DEW POINTS IN THE 60S. AND WHILE
MIDLEVEL LAPSE RATES ARE FORECAST TO REMAIN RATHER WEAK...STRONGER
DAYTIME HEATING WITHIN CLOUD BREAKS SHOULD YIELD POCKETS OF MODERATE
INSTABILITY WITH MLCAPE OF 1000-1500 J/KG.
CURRENTLY IT/S NOT CLEAR WHETHER ONGOING CONVECTION WILL FURTHER
DEEPEN INTO ROBUST TSTMS...OR WHETHER ADDITIONAL STORMS WILL FORM TO
THE W OF THIS INITIAL ACTIVITY. IN EITHER CASE...INCREASING DEEP
NWLY SHEAR TO 30 KT AND THE MODERATE INSTABILITY SUGGEST SOME
POTENTIAL FOR STORMS TO GROW UPSCALE INTO CLUSTERS WITH THE THREAT
FOR DAMAGING WINDS AND HAIL INTO THIS EVENING.
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z
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