SWODY2
SPC AC 171729
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1129 AM CST TUE FEB 17 2009
VALID 181200Z - 191200Z
...THERE IS A SLGT RISK OF SVR TSTMS WED INTO WED NIGHT ACROSS MUCH
OF THE CNTRL/ERN GULF STATES....
...SYNOPSIS...
MODELS CONTINUE TO INDICATE THAT INTERACTING IMPULSES...WITHIN THE
NORTHERN AND SOUTHERN BRANCHES OF THE PREVAILING SPLIT UPPER FLOW
REGIME...WILL CONTRIBUTE TO AN AMPLIFYING LARGER SCALE TROUGH ACROSS
THE CENTRAL AND EASTERN U.S. BY LATE WEDNESDAY NIGHT/EARLY THURSDAY.
SIGNIFICANT SURFACE CYCLOGENESIS IS ALREADY UNDERWAY TO THE LEE OF
THE CENTRAL ROCKIES...IN ASSOCIATION WITH AN INITIAL SHORT WAVE
TROUGH EMANATING FROM THE INTERMOUNTAIN WEST. THE UPPER FEATURE IS
PROGGED TO ADVANCE EAST NORTHEASTWARD THROUGH THE MIDDLE MISSISSIPPI
VALLEY BY 12Z WEDNESDAY...BEFORE LIFTING THROUGH THE OHIO VALLEY
INTO THE NORTHEAST STATES BY 12Z THURSDAY. AT THE SAME TIME...A
VIGOROUS SHORT WAVE IMPULSE...PIVOTING AROUND A COLD EASTERN
CANADIAN VORTEX...IS PROGGED TO DIG SOUTH OF THE INTERNATIONAL
BORDER...ACROSS THE UPPER MISSISSIPPI VALLEY INTO THE OHIO VALLEY.
IN RESPONSE TO THESE DEVELOPMENTS...THE DEEPENING SURFACE LOW CENTER
IS EXPECTED TO MIGRATE ACROSS THE MIDDLE MISSISSIPPI VALLEY...BEFORE
REDEVELOPING NORTHEASTWARD THROUGH THE LOWER GREAT LAKES REGION BY
LATE WEDNESDAY/WEDNESDAY NIGHT.
A 50-70 KT SOUTHWESTERLY LOW-LEVEL JET ASSOCIATED WITH THE EVOLVING
SYSTEM SHOULD CONTRIBUTE TO A RAPID RETURN FLOW OFF THE WESTERN GULF
OF MEXICO...WHERE THE BOUNDARY LAYER IS WARM AND MOIST...WITH
SURFACE DEW POINTS IN THE 60S TO AROUND 70F. HOWEVER...THE CURRENT
COLD/DRY AIR MASS NOW IN PLACE ACROSS THE EASTERN
STATES...ASSOCIATED WITH A RETREATING SURFACE RIDGE...COMPLICATES
THE POTENTIAL FOR BOUNDARY LAYER DESTABILIZATION WITHIN A POSSIBLE
LARGE EVOLVING WARM SECTOR EAST OF THE MISSISSIPPI VALLEY. MOISTURE
RETURN TONIGHT INTO EARLY WEDNESDAY MAY BE FASTER THAN THE RETREAT
OF THE COLD AIR...RESULTING IN CONSIDERABLE LOW CLOUD COVER AND
PRECIPITATION ASSOCIATED WITH BROAD ISENTROPIC ASCENT. AND...AN
EVAPORATIVELY COOLED BOUNDARY LAYER MAY BE SLOW TO RECOVER DURING
THE DAY WEDNESDAY...PARTICULARLY TO THE LEE OF THE SOUTHERN
APPALACHIANS. THIS PROBABLY WILL ALSO BE THE CASE FROM THE
TENNESSEE VALLEY NORTHWARD THROUGH THE OHIO VALLEY...WHERE STRONGEST
MID-LEVEL FORCING ASSOCIATED WITH AN EASTWARD PROPAGATING 90-100+ KT
500 MB JET STREAK IS FORECAST.
...SOUTHEASTERN STATES...
MODEL DATA...INCLUDING THE SREF...INDICATE THAT LOWER/MID 60S
SURFACE DEW POINTS WILL LIKELY ADVECT INLAND THROUGH MUCH OF THE
CENTRAL/EASTERN GULF STATES WEDNESDAY. THIS WILL PROVIDE SUPPORT
FOR POTENTIAL WEAK TO MODERATE BOUNDARY LAYER BASED DESTABILIZATION
IN THE PRESENCE OF STRONG DEEP LAYER SHEAR. THERE ARE INDICATIONS
THAT A LOW-LEVEL CONFLUENCE ZONE...BENEATH THE SUBTROPICAL JET
AXIS...COULD BE A FOCUS FOR CONVECTIVE DEVELOPMENT THROUGH MUCH OF
THE DAY ACROSS PARTS OF SOUTHERN MISSISSIPPI AND ALABAMA INTO
SOUTHWEST GEORGIA. HOWEVER...A PRE-FRONTAL DRY LINE TYPE STRUCTURE
EXPECTED TO EXTEND FROM NORTHERN ALABAMA THROUGH SOUTHWEST
MISSISSIPPI BY MIDDAY MAY BE THE FOCUS FOR STRONGER CONVECTIVE
INSTABILITY AND POTENTIAL...BENEATH A MID LEVEL DRY SLOT.
INITIATION OF STORMS ALONG THIS BOUNDARY SEEMS MOST LIKELY ACROSS
PARTS OF THE UPPER TENNESSEE VALLEY...AIDED BY MID-LEVEL COOLING AND
FORCING ON THE SOUTHERN FRINGE OF THE POLAR TROUGH... WHICH SHOULD
THEN SPREAD SOUTHEASTWARD TOWARD THE SOUTH ATLANTIC COAST WITH
FURTHER TROUGH AMPLIFICATION.
A CLUSTER OF STORMS COULD EVOLVE WITH THIS FORCING ACROSS NORTHERN
/CENTRAL ALABAMA THROUGH CENTRAL/SOUTHERN GEORGIA DURING THE MID TO
LATE AFTERNOON INTO EVENING HOURS. WHILE THE RISK FOR DAMAGING
WINDS/HAIL MAY BECOME THE MOST PROMINENT SEVERE THREATS...SIZABLE
CLOCKWISE CURVED LOW-LEVEL HODOGRAPHS MAY BE SUPPORTIVE OF TORNADO
POTENTIAL WITH THE INITIAL MORE DISCRETE STORMS. OTHER MORE
DISCRETE STORMS ARE POSSIBLE SOUTHWESTWARD ALONG THE PRE-FRONTAL
BOUNDARY TOWARD THE CENTRAL GULF COAST...IN THE PRESENCE OF WEAKER
LARGE-SCALE FORCING. THIS MAY INCLUDE A COUPLE OF SUPERCELLS...
THOUGH THE EXTENT OF THE TORNADO POTENTIAL IS A BIT UNCLEAR DUE TO
THE ANTICIPATED TENDENCY FOR SHRINKING LOW-LEVEL HODOGRAPHS WITH
VEERING/WEAKENING OF LOW-LEVEL FLOW FIELDS.
..KERR.. 02/17/2009
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