SPC AC 271955
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0255 PM CDT TUE MAY 27 2008
VALID 272000Z - 281200Z
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTN/EARLY EVE ACROSS
PARTS OF THE NRN MID ATLANTIC COAST STATES INTO SRN NEW ENGLAND....
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTN AND EVE ACROSS
PARTS OF THE SRN PLAINS THRU THE LWR MS VLY AND NRN GULF STATES....
...SYNOPSIS...
A LARGE COLD SURFACE RIDGE CONTINUES TO DEVELOP SOUTHEASTWARD ACROSS
THE CENTRAL CANADIAN/U.S. BORDER AREA...ASSOCIATED WITH A
SIGNIFICANT SHORT WAVE TROUGH IN THE NORTHERN BRANCH OF POLAR
WESTERLIES...NOW TURNING EAST OF NORTHERN/CENTRAL QUEBEC AND THE
UPPER GREAT LAKES REGION. THE FRONTAL ZONE ON THE LEADING EDGE OF
THIS AIR MASS IS EXPECTED TO ACCELERATE SOUTHWARD THROUGH MUCH OF
NORTHERN/MID ATLANTIC COASTAL AREAS AND THE LOWER OHIO/TENNESSEE
VALLEYS BY LATE TONIGHT. THE FRONT HAS ALREADY NOSED SOUTHWARD
THROUGH MUCH OF THE SOUTH CENTRAL HIGH PLAINS...BUT SHOULD STALL
BENEATH A BUILDING SUBTROPICAL RIDGE...TO THE EAST OF THE LARGE
SOUTHERN BRANCH TROUGH OVER THE EASTERN PACIFIC/PACIFIC COAST
STATES. THE MID/UPPER CIRCULATION CENTER WITHIN THIS LATTER FEATURE
HAS REFORMED OFF THE CALIFORNIA COAST...BUT THE PRIMARY JET STREAK
IS ALREADY IN THE PROCESS OF NOSING AROUND ITS SOUTHERN/EASTERN
PERIPHERY...INTO THE SOUTHERN GREAT BASIN.
...HIGH PLAINS INTO THE APPALACHIANS...
COMPOSITE CONVECTIVELY GENERATED OUTFLOW BOUNDARY ASSOCIATED WITH A
COUPLE OF ONGOING CONVECTIVE CLUSTERS HAS NOW SURGED WELL AHEAD OF
THE COLD FRONT...STABILIZING THE LOW-LEVEL ENVIRONMENT ACROSS MUCH
OF THE OZARK PLATEAU AND LOWER MISSOURI/LOWER OHIO VALLEYS.
HOWEVER...INSTABILITY REMAINS SUFFICIENT NEAR THE FRONT TO SUPPORT
THUNDERSTORMS ACROSS THE LOWER OHIO VALLEY. FRONTAL FORCING...AND
LIFT ASSOCIATED WITH A WEAKENING MID-LEVEL CIRCULATION...ARE
CONTRIBUTING TO ONGOING ACTIVITY NEAR AND JUST NORTH OF THE OHIO
RIVER. THIS PROBABLY WILL PERSIST THROUGH THE REMAINDER OF THE
AFTERNOON...GRADUALLY SPREADING EASTWARD. MODEST SHEAR AND WEAK
INSTABILITY ARE STILL EXPECTED TO MARGINALIZE THE SEVERE THREAT...
BUT SOME HAIL AND GUSTY WINDS ARE POSSIBLE IN STRONGEST CELLS.
AND...AMBIENT VORTICITY...PERHAPS SOMEWHAT ENHANCED VERTICAL SHEAR
PROFILES NEAR THE FRONT...COULD SUPPORT AN ADDITIONAL BRIEF TORNADO.
THE PRIMARY SEVERE THREAT THROUGH THE REMAINDER OF THE AFTERNOON/
EVENING SEEMS TO EXIST SOUTHEAST THROUGH SOUTH AND WEST OF THE
SIGNIFICANT COLD POOL NOW SURGING SOUTHWARD THROUGH EASTERN TEXAS.
THIS IS GENERALLY WITHIN A WEAK SHEAR REGIME BENEATH THE BUILDING
RIDGE. BUT...BOUNDARY LAYER MOISTURE CHARACTERIZED BY SURFACE DEW
POINTS NEAR 70F ARE SUPPORTING MIXED LAYER CAPE OF 2000-3000 J/KG.
STEEPEST MID-LEVEL LAPSE RATES HAVE BECOME CONFINED TO THE VICINITY
OF THE OUTFLOW AND COLD FRONT INTERSECTION ACROSS SOUTHWESTERN
OKLAHOMA...SOUTHWARD ALONG THE COLD FRONT INTO THE TEXAS SOUTH
PLAINS. AND...THE BOUNDARY INTERSECTION REMAINS IN THE PRESENCE OF
MODERATE DEEP LAYER SHEAR...WHERE THE MOST PRONOUNCED VEERING OF
WINDS WITH HEIGHT EXISTS BENEATH MODEST WESTERLY 500 MB FLOW.
THUS...IT APPEARS AN ISOLATED SUPERCELL OR TWO MAY FORM AS
INHIBITION WEAKENS IN THE PEAK AFTERNOON HEATING ...ACCOMPANIED BY
THE RISK OF LARGE HAIL...POSSIBLY A TORNADO... BEFORE DAMAGING WIND
GUSTS BECOME THE MORE PROMINENT THREAT LATER THIS AFTERNOON OR EARLY
EVENING.
...NRN MID ATLANTIC INTO PARTS OF SRN NEW ENGLAND...
LARGE-SCALE FORCING ALONG OR JUST AHEAD OF SURFACE COLD FRONT WILL
MAINTAIN A BROKEN LINE OF STORMS THROUGH THE REMAINDER OF THE
AFTERNOON HOURS. UNTIL ACTIVITY SPREADS SOUTHEAST OF COASTAL AREAS
DURING THE 28/00-02Z TIME FRAME...MIXED LAYER CAPE AROUND 1000 J/KG
IN THE PRESENCE OF STRONG DEEP LAYER SHEAR AND 40+ KT MEAN FLOW ON
THE SOUTHERN FRINGE OF THE STRONGER WESTERLIES WILL PROVIDE A THREAT
FOR SEVERE STORMS...MAINLY WITH DAMAGING WINDS AND SOME HAIL.
...NORTHERN GREAT BASIN...
FORCING/MID-LEVEL DESTABILIZATION IN THE EXIT REGION OF APPROACHING
MID/UPPER JET STREAK HAS ALREADY SUPPORTED CONSIDERABLE THUNDERSTORM
ACTIVITY...WHICH SHOULD PERSIST/DEVELOP NORTHWARD THROUGH THE
REMAINDER OF THE AFTERNOON AND EVENING. GENERALLY WEAK CAPE
PROBABLY WILL LIMIT THE VIGOR OF THE UPDRAFTS. BUT...ISOLATED
MICROBURSTS MAY BECOME MORE LIKELY AS THE BOUNDARY LAYER BECOMES
INCREASINGLY WELL MIXED THROUGH THE PEAK AFTERNOON HEATING HOURS.
..KERR.. 05/27/2008
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