Thursday, March 26, 2009

New SPC Convective Outlook



Here is the text of the new "Day Two" Outlook from the SPC, that was issued at 12:30 p.m.:

   DAY 2 CONVECTIVE OUTLOOK  
NWS STORM PREDICTION CENTER NORMAN OK
1229 PM CDT THU MAR 26 2009

VALID 271200Z - 281200Z

...THERE IS A MDT RISK OF SVR TSTMS FRI NIGHT ACROSS PARTS OF SE
ARKANSAS...NE LOUISIANA....W CNTRL AND NW MISSISSIPPI...AND WRN
TN....

...THERE IS A SLGT RISK OF SVR TSTMS FRI/FRI NIGHT...FROM PARTS OF
THE SERN PLAINS THROUGH THE LWR MS/MID SOUTH AND GULF STATES....

...SYNOPSIS...
WITHIN A LINGERING LARGE-SCALE TROUGH THAT ENCOMPASSES MUCH OF THE
INTERIOR U.S...A STRONG...COLD SHORT WAVE TROUGH IS NOW DIGGING
THROUGH THE GREAT BASIN AND CENTRAL/SOUTHERN ROCKIES REGION. MODELS
CONTINUE TO INDICATE THAT...AS THIS SYSTEM PROGRESSES INTO AND
THROUGH THE BASE OF THE LARGER SCALE TROUGH LATER TONIGHT INTO
FRIDAY...IT WILL COME INCREASINGLY IN PHASE WITH A BRANCH OF
STRONGER UPPER WESTERLIES EMANATING FROM THE SUBTROPICAL PACIFIC.
AS THIS OCCURS...MODELS GENERALLY INDICATE THAT AN EMBEDDED LOWER/
MID TROPOSPHERIC CYCLONE WILL DEEPEN SUBSTANTIALLY...PARTICULARLY
FRIDAY NIGHT...PROBABLY ACCOMPANIED BY SIGNIFICANT SECONDARY SURFACE
CYCLOGENESIS FROM PARTS OF THE ARKLATEX THROUGH UPPER PORTIONS OF
THE LOWER MISSISSIPPI VALLEY.

THIS EVOLVING SYSTEM WILL BE PRECEDED BY COUPLED POLAR AND
SUBTROPICAL IMPULSES THROUGH PARTS OF THE MID SOUTH AND
SOUTHEASTERN STATES EARLY IN THE PERIOD. WHILE THE INFLUENCE OF
THESE FEATURES ON CONVECTIVE POTENTIAL IS STILL A SOURCE OF FORECAST
UNCERTAINTY...THE PRESENCE OF CONSIDERABLE POTENTIAL INSTABILITY
ACROSS MUCH OF THE GULF OF MEXICO AND GULF STATES SUPPORTS AN
ELEVATED RISK FOR SEVERE STORMS. AND...POTENTIAL EXISTS FOR AN
OUTBREAK OF SEVERE STORMS...INCLUDING THE RISK FOR A FEW STRONG
TORNADOES AND DAMAGING WINDS...IN ADDITION TO LARGE HAIL.

...EASTERN GULF STATES...
A STRONG/SEVERE CLUSTER OF STORMS MAY BE ONGOING ACROSS PARTS OF
ALABAMA/WESTERN FLORIDA AND GEORGIA EARLY FRIDAY. BEFORE LARGE-
SCALE FORCING ASSOCIATED WITH SUPPORTING LEAD UPPER IMPULSES
WEAKENS/SHIFTS NORTHEASTWARD AND EASTWARD INTO A PROGRESSIVELY MORE
STABLE ENVIRONMENT...A RISK FOR DAMAGING WINDS...PERHAPS ISOLATED
TORNADOES...MAY LINGER INTO MIDDAY.

...SOUTHERN PLAINS THROUGH THE LWR MS VALLEY AND MID SOUTH...
A CONTINUING RETURN FLOW OFF THE GULF OF MEXICO...BENEATH STEEP
MID-LEVEL LAPSE RATES ASSOCIATED WITH ELEVATED MIXED LAYER AIR
ADVECTING EAST NORTHEASTWARD OFF THE MEXICAN PLATEAU...IS EXPECTED
TO MAINTAIN MODERATE INSTABILITY INTO THIS PERIOD. CAPE OF
1000-2000 J/KG IS EXPECTED OVER A FAIRLY BROAD AREA FROM THE
SOUTHEASTERN PLAINS THROUGH THE LOWER MISSISSIPPI VALLEY/CENTRAL
GULF COAST REGION. HOWEVER...MUCH OF THIS INSTABILITY ALONG AND
EAST OF THE LOWER MISSISSIPPI VALLEY MAY BE BASED ABOVE A SHALLOW
STABLE NEAR SURFACE LAYER...NORTH OF A CONVECTIVELY ENHANCED SURFACE
BOUNDARY LINGERING NEAR THE CENTRAL/EASTERN GULF COAST.
WITH THE MAIN UPPER FORCING LAGGING TO THE WEST...CONVECTIVE
POTENTIAL THROUGH MUCH OF THE DAY MAY BE FAIRLY LOW.

AN EXCEPTION MAY BE ALONG A DRY LINE OR PRE-FRONTAL WIND SHIFT
ACROSS EASTERN TEXAS INTO SOUTHEAST OKLAHOMA...WHERE GUIDANCE
SUGGESTS STORMS MAY INITIATE AS EARLY AS MIDDAY...IN AN ENVIRONMENT
FAVORABLE FOR SUPERCELLS. DESPITE SOMEWHAT MODEST SOUTHERLY
LOW-LEVEL FLOW...STEEP LAPSE RATES ARE EXPECTED TO BE FAVORABLE FOR
THE PRODUCTION OF LARGE HAIL AND DAMAGING WIND GUSTS. AS
LARGE-SCALE FORCING INCREASES WITHIN THE EXIT REGION OF A STRONG
MID/UPPER POLAR JET STREAK...AND AHEAD OF AN EASTWARD SURGING COLD
FRONT...ONE OR MORE BANDS AND CLUSTERS OF STORMS ARE EXPECTED TO
FORM AND SPREAD NORTHEASTWARD THROUGH THE OZARK PLATEAU. ACTIVITY
ALSO SHOULD DEVELOP SOUTHWARD THROUGH THE LOWER MISSISSIPPI VALLEY
DURING THE LATE AFTERNOON AND EVENING HOURS.

OF PRIMARY CONCERN IS THE DEVELOPMENT AND INTENSIFICATION OF A
SOUTHERLY LOW-LEVEL JET NEAR THE SECONDARY CYCLONE DURING THE MID/
LATE EVENING HOURS...WHICH WILL YIELD ENLARGING CLOCKWISE CURVED
LOW-LEVEL HODOGRAPHS. THIS IS EXPECTED TO CONTRIBUTE TO INCREASING
TORNADIC POTENTIAL...INCLUDING THE RISK FOR A FEW STRONG
TORNADOES...ACROSS PARTS OF SOUTHEASTERN ARKANSAS/NORTHEAST
LOUISIANA/WEST CENTRAL AND NORTHWEST MISSISSIPPI AND WESTERN
TENNESSEE BY THE 28/03-06Z TIME FRAME.

RAPID NOCTURNAL WARM SECTOR BOUNDARY LAYER DESTABILIZATION APPEARS
POSSIBLE...AS STRONG UPPER FORCING...AHEAD OF AN INCREASINGLY
NEGATIVELY TILTED UPPER TROUGH PIVOTING AROUND THE MID-LEVEL
CYCLONE. AND...SEVERAL LINES/CLUSTER OF STRONG/SEVERE STORMS
CAPABLE OF PRODUCING DAMAGING WINDS/HAIL AND ISOLATED TORNADOES
APPEAR LIKELY TO SPREAD ACROSS THE LOWER MISSISSIPPI VALLEY/CENTRAL
GULF COAST AND MID SOUTH FRIDAY NIGHT INTO EARLY SATURDAY.

..KERR.. 03/26/2009

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