The Palm Sunday, March 28, 2010, Severe Thunderstorm Outbreak

SPC AC 270555
   
DAY 2 CONVECTIVE OUTLOOK  
NWS STORM PREDICTION CENTER NORMAN OK
1255 AM CDT SAT MAR 27 2010
   
VALID 281200Z - 291200Z
   
...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS AL/GA/CAROLINAS...
   
...SYNOPSIS...
PROGRESSIVE AND AMPLIFIED SHORTWAVE PATTERN IS FCST TO CONTINUE
THROUGH PERIOD.  STG TROUGH -- NOW EVIDENT IN MOISTURE CHANNEL
IMAGERY INVOF TX/NM BORDER NWD TO SERN CO -- IS FCST TO DEVELOP INTO
CLOSED 500 MB CYCLONE DAY-1...REACHING VICINITY CGI BY START OF THIS
PERIOD.  ALTHOUGH THIS FEATURE MAY DEVOLVE SLIGHTLY BACK INTO
OPEN-WAVE TROUGH...EMBEDDED CIRCULATION REMNANT/VORTICITY LOBE WILL
REMAIN WELL-DEFINED AND SHOULD REACH OH OR ERN KY BY 29/12Z.  TIMING
OF THIS EVOLUTION HAS BECOME MUCH MORE CONSISTENT IN LATEST
OPERATIONAL AND ENSEMBLE RUNS THAN PRIOR MODELS.  BY THAT
TIME...TRAILING/POSITIVELY-TILTED TROUGH WILL MOVE EWD ACROSS S
TX...MS DELTA REGION AND PORTIONS TN/AL.
   
AT SFC...CYCLONE NOW ANALYZED OVER NRN TX PANHANDLE WILL BECOME
ESSENTIALLY COLLOCATED WITH MID-UPPER LEVEL LOW BY 28/12Z...AND
REMAIN SO THROUGH PERIOD.  TRAILING COLD FRONT IS FCST TO MOVE EWD
FROM WRN TN/MS POSITION AT 28/12Z TO SRN APPALACHIANS...GA AND FL
PANHANDLE AROUND 29/00Z...THEN BY 29/12Z...FROM MID-ATLANTIC REGION
SWD NEAR OUTER BANKS AND SWWD ACROSS CENTRAL/SWRN FL.
   
...SERN CONUS EXCEPT FL...
GREATEST UNCONDITIONAL SVR POTENTIAL APPEARS TO BE IN CORRIDOR FROM
PORTIONS AL EWD ACROSS GA DURING AFTERNOON...TO CAROLINAS AFTER
DARK.  MOISTURE RETURN WILL BE MRGL..WITH SFC DEW POINTS LIKELY TO
REACH ONLY MID-UPPER 50S WITH POCKETS OF LOW 60S. 
HOWEVER...MODIFIED FCST SOUNDINGS SUGGEST BOUNDARY LAYER THETAE WILL
BE SUFFICIENT FOR WEAK CINH WITH EFFECTIVE INFLOW PARCELS THAT ARE
SFC-BASED OR VERY NEARLY SO.  LAPSE RATES ALOFT WILL BE SMALL --
JUST SLIGHTLY ABOVE MOIST-ADIABATIC -- BUT SHOULD SUPPORT MLCAPE IN
200-800 J/KG RANGE.  DEEP-LAYER WIND FIELDS AND ALMOST ANY MEASURE
OF VERTICAL SHEAR WILL BE STG OVER THIS REGION.  45-60 KT OF
EFFECTIVE SHEAR MAGNITUDE AND 0-1 KM SRH ABOVE 200 J/KG MAY BE
COMMON FROM AL/GA BORDER AREA IN AFTERNOON EWD ACROSS OUTLOOK AREA
AFTER DARK.  MAIN THREAT APPEARS ATTM TO BE DAMAGING WIND...THOUGH
TORNADOES CANNOT BE RULED OUT. SVR THREAT SHOULD DIMINISH SWD TOWARD
GULF COAST AND SRN GA WITH INCREASING CINH AND WEAKER LOW-LEVEL
CONVERGENCE...REACHING RELATIVE MIN BEFORE INCREASING AGAIN OVER FL
PENINSULA.
   
...FL PENINSULA...
TWO PRIMARY CONVECTIVE EPISODES ARE POSSIBLE ACROSS THIS AREA...WITH
AT LEAST MRGL UNCONDITIONAL SVR THREAT.
1. DIURNAL TSTMS FORMING IN DIABATICALLY HEATED AND WEAKLY CAPPED
AIR MASS...IN ENVIRONMENT WHERE MOISTURE ADVECTION COULD OFFSET
DIURNAL MIXING ENOUGH TO MAINTAIN OR EVEN INCREASE BOUNDARY-LAYER
PW.  STG UPPER-LEVEL WINDS...E.G. 80-100 KT AT 250 MB...WILL AID
STORM VENTILATION ALOFT.  POTENTIAL EXISTS FOR ORGANIZED
MULTICELLS...OR PERHAPS SUPERCELL STRUCTURES WITH SOME HELP FROM
MESOSCALE BOUNDARIES.  ALTHOUGH VEERING FLOW WITH HEIGHT YIELDS
FAVORABLE HODOGRAPH SHAPE...MAIN LIMITING FACTOR WILL BE WEAKNESS OF
LOW-LEVEL SPEEDS...AND THEREFORE OF SHEAR VALUES.
2. ONE OR TWO BANDS OR CLUSTERS OF TSTMS MOVING ACROSS PENINSULA
AFTER 00Z...AND ALONG/AHEAD OF SFC COLD FRONT.  BRIEF/3-4 HOUR
WINDOW MAY DEVELOP WITHIN WHICH SWRN FRINGES OF ATLANTIC LLJ
INCREASES HODOGRAPH SIZE...WHILE PARCELS ARE STILL NEAR
SFC-BASED...AND BEFORE THAT LLJ WEAKENS AND SHIFTS AWAY FROM
PENINSULA.  ALTHOUGH MID-UPPER FLOW WILL REMAIN STG...SFC WINDS
SHOULD VEER WITH TIME OVERNIGHT...REDUCING BULK SHEAR WITH TIME. 
HOWEVER...MOISTURE CONTENT SHOULD INCREASE THROUGH MUCH OF LATTER
HALF OF PERIOD AHEAD OF FRONT...PERHAPS RE-INTRODUCING CONVECTIVE
POTENTIAL LATE FEW HOURS OF PERIOD IN SRN FL.  THESE OFFSETTING
TRENDS AND UNCERTAINTIES PRECLUDE CATEGORICAL OUTLOOK ATTM.
 
....TN/OH VALLEYS...
MRGL SVR POTENTIAL MAY BE ONGOING AT BEGINNING OF PERIOD OVER
PORTIONS WRN TN/WRN KY AREA...CARRYING OVER FROM DAY-1 OUTLOOK. 
COMBINATION OF SFC THETAE ADVECTION AND DIABATIC HEATING...IN MRGLLY
MOIST BOUNDARY LAYER AIR MASS...SHOULD CONTRIBUTE TO DEVELOPMENT OF
SCATTERED TSTMS NEAR COLD FRONT AND SFC LOW DURING MIDDAY THROUGH
LATE AFTERNOON FROM SRN PORTIONS INDIANA/OH SWD ACROSS MIDDLE-ERN
PORTIONS TN AND KY.  MID-UPPER LAPSE RATES WILL BE ENHANCED BY
COOLING ALOFT RELATED TO DPVA...SOMEWHAT OFFSETTING LACK OF MORE
ROBUST BOUNDARY LAYER MOISTURE...IN RENDERING SFC-BASED EFFECTIVE
PARCELS.  RELATIVE CONCENTRATIONS OF SVR POTENTIAL WITHIN BROAD 5%
UNCONDITIONAL PROBABILITY AREA WILL DEPEND ON MESOSCALE EFFECTS.
 
..EDWARDS.. 03/27/2010
   
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NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 1730Z