SPC AC 280600
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0100 AM CDT TUE APR 28 2009
VALID 281200Z - 291200Z
...THERE IS A SLGT RISK OF SVR TSTMS FROM THE SRN HIGH PLAINS INTO
THE TX BIG BEND REGION/HILL COUNTRY...
...SYNOPSIS...
A LARGE CLOSED MID/UPPER LEVEL LOW DEVELOPING OVER THE NWRN STATES
EARLY TODAY IS EXPECTED TO REMAIN CENTERED OVER WA/ORE THIS FORECAST
PERIOD. FARTHER NE...A SHORT WAVE TROUGH IS EXPECTED TO TRACK FROM
NRN ONTARIO/UPPER GREAT LAKES REGION EWD THROUGH QUEBEC AND NY/NEW
ENGLAND. MEANWHILE...A BROAD ZONE OF WSWLY FLOW ALOFT...WITH
SEVERAL EMBEDDED SHORT WAVE TROUGHS AND/OR WEAKER IMPULSES...WILL
EXTEND FROM THE SWRN STATES ACROSS THE CENTRAL/SRN PLAINS TO THE
NERN STATES/MID ATLANTIC STATES TODAY.
A COLD FRONT WILL SPREAD E/SEWD THROUGH THE NERN STATES AND OH
VALLEY REGION TODAY. THE WRN EXTENT OF THIS BOUNDARY EXTENDING FROM
NRN TO WEST CENTRAL TX AT 12Z TODAY WILL SHIFT SLOWLY NWD REACHING
SRN OK TO THE SRN TX PANHANDLE LATE TUESDAY NIGHT.
...SRN HIGH PLAINS/N TX TO PARTS OF CENTRAL PLAINS...
WATER VAPOR IMAGERY INDICATED A SHORT WAVE TROUGH TRACKING ENEWD
ACROSS ERN AZ AT 28/06Z. THIS FEATURE IS EXPECTED TO TRACK ACROSS
THE SRN HIGH PLAINS THIS MORNING...AND MAY BE THE IMPETUS FOR SOME
ONGOING TSTMS...SOME POSSIBLY PRODUCING HAIL...AT THE START OF THE
FORECAST PERIOD OVER FAR NERN NM INTO THE TX PANHANDLE. SUBSIDENCE
IN THE WAKE OF THIS TROUGH/MID LEVEL WARMING SUGGESTS NEW CONVECTIVE
DEVELOPMENT MAY BE INHIBITED UNTIL LATE AFTERNOON OR EVEN EARLY
EVENING AS THE NEXT SHORT WAVE TROUGH APPROACHES THE SRN HIGH PLAINS
TOWARD 29/00Z.
SSELY LOW LEVEL WINDS WILL INCREASE THROUGH THE DAY FROM THE LOWER
RIO GRANDE VALLEY INTO THE SRN HIGH PLAINS WITH LOW LEVEL MOISTURE
/SURFACE DEWPOINTS IN THE 60S/ RETURNING NWWD INTO THE SRN HALF OF
THE SLIGHT RISK AREA BY AFTERNOON. STEEP LAPSE RATES ATOP THE LOW
LEVEL MOISTURE RETURN COMBINED WITH SURFACE HEATING ARE EXPECTED TO
RESULT IN MODERATE INSTABILITY ALONG AND S OF THE NWD MOVING SURFACE
FRONT. STRONGEST INSTABILITY IS PROGGED FROM THE TX HILL COUNTRY
WWD TO THE PECOS RIVER VALLEY.
SELY LOW LEVEL WINDS VEERING TO 30-40 KT WSWLY MID LEVEL WINDS WILL
RESULT IN STRONG EFFECTIVE BULK SHEAR /AROUND 50 KT/ IN THE SAME
REGION WITH THE STRONGEST INSTABILITY SUPPORTING SUPERCELLS WITH ANY
TSTM DEVELOPMENT THAT OCCURS BY LATE AFTERNOON. GIVEN THE STRENGTH
OF THE SHEAR/INSTABILITY...VERY LARGE HAIL AND ISOLATED TORNADOES
WILL BE POSSIBLE ACROSS SW TX THIS AFTERNOON INTO THE EVENING.
A NOCTURNAL SSELY LLJ VEERING OVERNIGHT TO MORE SLY IS EXPECTED TO
DEVELOP ALONG THE FULL EXTENT OF THE HIGH PLAINS AFTER DARK. MODELS
SUGGEST AN MCS SHOULD DEVELOP ACROSS ERN NM/TX PANHANDLE THIS
EVENING WITHIN ZONE OF INCREASING LOW LEVEL WAA...WITH HAIL AND
STRONG WIND GUSTS BEING THE INITIAL THREATS. HAIL SHOULD BECOME THE
PRIMARY THREAT LATER TUESDAY NIGHT AS THE LLJ VEERS AND THE ACTIVITY
BECOMES MORE ELEVATED AND POSSIBLY REACHES WRN/SWRN OK AFTER 29/06Z.
ADDITIONAL...MAINLY ELEVATED TSTMS ARE EXPECTED TO DEVELOP INTO PART
OF THE CENTRAL PLAINS LATE TONIGHT ALONG LLJ/WAA REGIME WITH AN
ISOLATED THREAT FOR HAIL.
...LOWER TN AND OH VALLEYS...
A SHORT WAVE TROUGH IS EXPECTED TO WEAKEN AS IT TRACKS NEWD THROUGH
THE OH VALLEY TODAY...WHILE AN MCV ATTENDANT TO AN MCS OVER LA
MONDAY NIGHT IS EXPECTED TO TRACK TOWARD THE LOWER TN VALLEY TODAY.
ALTHOUGH SUFFICIENT SURFACE HEATING IS EXPECTED IN THE WARM SECTOR
TODAY...PRODUCING STEEP LOW LEVEL LAPSE RATES...WEAK MID LEVEL LAPSE
RATES SHOULD LIMIT THE DEVELOPMENT OF STRONG INSTABILITY. ASCENT
WITH EACH OF THE MID LEVEL FEATURES SHOULD SUPPORT CONVECTIVE
DEVELOPMENT...BUT OVERALL SEVERE WEATHER THREAT IS EXPECTED TO
REMAIN LOW GIVEN THE WEAK INSTABILITY.
...NERN STATES...
A NARROW CORRIDOR OF SURFACE DEWPOINTS IN THE LOWER-MID 50S WILL
PRECEDE THE COLD FRONT SPREADING ESEWD ACROSS NEW ENGLAND TODAY.
ALTHOUGH SUFFICIENT SURFACE HEATING IS EXPECTED PRODUCING STEEP LOW
LEVEL LAPSE RATES...WEAK MID LEVEL LAPSE RATES SHOULD LIMIT THE
DEVELOPMENT OF STRONG INSTABILITY AND OVERALL TSTM DEVELOPMENT.
HOWEVER...GIVEN THE STEEP LOW LEVEL LAPSE RATES AND 40-50 KT WSWLY
MID LEVEL FLOW...ISOLATED STRONG WIND GUSTS WILL BE POSSIBLE INTO
THE AFTERNOON.
..PETERS/GARNER.. 04/28/2009
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z
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