SPC AC 090555
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1255 AM CDT FRI MAY 09 2008
VALID 091200Z - 101200Z
...THERE IS A SLGT RISK OF SVR TSTMS TN/MID MS VALLEYS AND
VICINITY...
...THERE IS A SLGT RISK OF SVR TSTMS COASTAL CAROLINAS...
...SYNOPSIS...
RELATIVELY LOW-AMPLITUDE FLOW FIELD ALOFT IS FORECAST THIS
PERIOD...WITH A SERIES OF SHORT-WAVE FEATURES MOVING ACROSS THE ERN
HALF OF THE CONUS...WHILE ANOTHER FEATURE MOVES ACROSS THE
INTERMOUNTAIN WEST TOWARD THE PLAINS THROUGH THE PERIOD.
AT THE SURFACE...A LOW IS FORECAST TO MOVE OFF THE MID-ATLANTIC
COAST THROUGH THE DAY...WHILE A BOUNDARY TRAILS WWD ACROSS THE
MID-SOUTH AND INTO TX. MEANWHILE...LEE TROUGHING IS EXPECTED OVER
THE HIGH PLAINS BENEATH WLY/WSWLY FLOW ALOFT AHEAD OF THE WRN
TROUGH.
...TN VALLEY REGION AND VICINITY...
EVOLVING SCENARIO REMAINS DIFFICULT TO FORECAST ATTM...WITH WEAK
BAROCLINIC ZONE/CONVERGENCE AXIS FORECAST TO EXTEND WWD FROM THE LOW
MOVING ACROSS THE MID-ATLANTIC COASTAL REGION INTO THE TN/MID MS
VALLEY REGION. THIS BOUNDARY IS EXPECTED TO LINGER ACROSS THIS
REGION THROUGH THE DAY...BENEATH STRONG BUT WEAKLY-ANTICYCLONIC FLOW
ALOFT. THIS WEAK SHORT-WAVE RIDGING SHOULD HINDER WIDESPREAD
CONVECTIVE DEVELOPMENT INTO THE AFTERNOON...WHEN COMBINATION OF
HEATING AND PRESENCE OF LOW-LEVEL BOUNDARY FINALLY ALLOW ISOLATED
STORM DEVELOPMENT TO OCCUR.
ANY STORMS WHICH DEVELOP WILL POSE A SEVERE THREAT...AS 60 KT WSWLYS
AT MID-LEVELS WILL SUPPORT RAPID STORM INTENSIFICATION. ATTM...IT
APPEARS THAT COVERAGE OF STORMS MAY REMAIN SCATTERED AT BEST...WITH
DEVELOPMENT AIDED BY WEAK LOW-LEVEL WARM ADVECTION. GIVEN A
GENERALLY CELLULAR STORM MODE...EXPECT HAIL AND LOCALLY-DAMAGING
WINDS ALONG WITH THE POTENTIAL FOR A TORNADO OR TWO. HOWEVER...IF A
POCKET OR TWO OF GREATER STORM COVERAGE COULD OCCUR THUS ALLOWING
STORMS TO MERGE AND ORGANIZE ON THE MESOSCALE...ROUGHLY
UNIDIRECTIONAL FLOW ABOVE THE BOUNDARY LAYER WOULD SUPPORT A
RAPIDLY-MOVING/BOWING-TYPE SYSTEM WITH MORE WIDESPREAD DAMAGING WIND
POTENTIAL.
ATTM...DEGREE OF STORM COVERAGE -- AS WELL AS LOCATION OF GREATEST
SEVERE POTENTIAL -- REMAIN DIFFICULT TO DISCERN GIVEN LACK OF A
CLEAR FOCUSING MECHANISM. THUS...WILL MAINTAIN SLIGHT RISK/30%
THREAT CENTERED OVER THE TN VALLEY REGION.
THOUGH STORMS MAY CONTINUE WELL INTO THE EVENING AS THEY SPREAD
EWD/ESEWD -- PARTICULARLY IF AN MCS EVOLVES...THREAT SHOULD DIMINISH
SLOWLY WITH TIME OVERNIGHT/THROUGH THE END OF THE PERIOD.
...COASTAL CAROLINAS...
DESPITE SOME DIFFERENCES IN TIME...MODELS SHOW A WEAKENING FRONT
MOVING SLOWLY OFF THE ATLANTIC COAST THROUGH THE FIRST HALF OF THE
PERIOD. AHEAD OF THE FRONT HOWEVER...VERY MOIST BOUNDARY LAYER WILL
CONTRIBUTE TO MARGINAL TO MODERATE INSTABILITY.
ALONG WITH AMPLE INSTABILITY...PRE-FRONTAL ENVIRONMENT WILL FEATURE
STRONG DEEP-LAYER WIND FIELD...INCLUDING 50 KT WLYS AT MID-LEVELS.
THOUGH LOWER-TROPOSPHERIC FLOW SHOULD WEAKEN THROUGH THE DAY AS THE
LOW-LEVEL JET SHIFTS OFFSHORE WITH TIME...SHEAR WILL REMAIN
SUPPORTIVE OF SUPERCELLS PRIOR TO FRONTAL PASSAGE.
CONVECTION ONGOING AT THE START OF THE PERIOD WILL COMPLICATE THE
SCENARIO...BUT ATTM IT APPEARS THAT STORMS SHOULD SHIFT ACROSS THE
ERN CAROLINAS THROUGH THE MORNING AND INTO THE AFTERNOON BEFORE
MOVING OFFSHORE. ALONG WITH THE POTENTIAL FOR ISOLATED
TORNADOES...HAIL AND LOCALLY-DAMAGING WINDS WILL BE POSSIBLE.
THREAT SHOULD WANE THROUGH THE AFTERNOON AS FRONT MOVES OFFSHORE.
...CENTRAL HIGH PLAINS REGION...
SELY/ESELY LOW-LEVEL FLOW IS FORECAST TO DEVELOP ACROSS THE CENTRAL
HIGH PLAINS REGION...WHICH WILL PROMOTE BOTH A RETURN OF MARGINAL
MOISTURE NWWD AS WELL AS UPSLOPE-INDUCED UVV. THOUGH ONLY MODEST
DESTABILIZATION IS EXPECTED DUE TO MARGINAL BOUNDARY-LAYER
MOISTURE...EXPECT ISOLATED HIGH-BASED STORMS TO DEVELOP OVER THE
HIGHER TERRAIN BY LATE AFTERNOON...AND THEN SHIFT SEWD AND EXPAND
SOMEWHAT IN COVERAGE INTO THE EVENING AS SELY LOW-LEVEL JET
INCREASES.
WITH FLOW VEERING FROM ESELY/SELY IN THE BOUNDARY LAYER TO WLY AT 50
KT AT H5...SHEAR WILL SUPPORT ROTATION WITHIN A FEW STRONGER
UPDRAFTS -- AND AN ASSOCIATED POTENTIAL FOR HAIL AND DAMAGING WINDS
WITH THESE CELLS. IT APPEARS ATTM THAT WEAK INSTABILITY SHOULD
LIMIT THE OVERALL SEVERE POTENTIAL...AND THUS WILL MAINTAIN ONLY LOW
SEVERE PROBABILITY.
...NERN TX SWWD TOWARD THE HILL COUNTRY...
MOIST BOUNDARY-LAYER AIRMASS IS ANTICIPATED ALONG AND E OF
WEAK/LINGERING FRONT FORECAST TO EXTEND NE-SW FROM E TX SWWD INTO
THE HILL COUNTRY. HOWEVER...CAPPING IS FORECAST TO BE A LIMITING
FACTOR FOR CONVECTIVE DEVELOPMENT ACROSS THIS REGION.
WHILE WEAK SHORT-WAVE RIDGING SHOULD ALLOW CAP TO
PERSIST...HEATING/MIXING COMBINED WITH THE FORECAST APPROACH OF A
WEAK FEATURE ALOFT LATE IN THE DAY MAY BE SUFFICIENT FOR THE
DEVELOPMENT OF A FEW STORMS ACROSS THIS REGION. SHOULD STORMS
OCCUR...MODERATE INSTABILITY AND 35-45 KT WLY MID-LEVEL FLOW WOULD
PROMOTE VIGOROUS UPDRAFTS. WITH RELATIVELY WEAK LOW-LEVEL WIND
FIELD EXPECTED...PRIMARY SEVERE THREATS WOULD LIKELY BE LARGE HAIL
AND DAMAGING GUSTS. THREAT SHOULD DECREASE AFTER SUNSET WITH THE
ONSET OF DIURNAL COOLING.
..GOSS/GRAMS.. 05/09/2008
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z
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