Mike Wilhelm's Alabama Weather Blog

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Bamawx.com

   DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1259 AM CDT FRI APR 10 2009

VALID 101200Z - 111200Z

...THERE IS A MDT RISK OF SVR TSTMS THIS AFTN/EARLY EVE ACROSS PARTS
OF MIDDLE/ERN TN...NRN AL...NR GA INTO PARTS OF WRN NC/SC....

...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MDT RISK
AREA...FROM PARTS OF THE MID SOUTH/NRN GULF STATES INTO THE LEE OF
THE SRN APPALACHIANS....

...SYNOPSIS...
A CLOSED MID-LEVEL CIRCULATION IS NOW IN THE PROCESS OF FORMING
WITHIN AN AMPLIFYING TROUGH IN THE SOUTHERN BRANCH OF SPLIT POLAR
FLOW...OFF THE SOUTHERN CALIFORNIA COAST.  AND...MUCH OF THE MODEL
GUIDANCE IS SIMILAR...INDICATING THAT THIS FEATURE WILL TURN
EASTWARD ACROSS SOUTHERN CALIFORNIA INTO THE LOWER COLORADO VALLEY
LATER TODAY/TONIGHT.  AS THIS OCCURS...UPPER RIDGING APPEARS LIKELY
TO BUILD THROUGH MUCH OF THE PLAINS/LOWER MISSISSIPPI VALLEY AND
CENTRAL GULF COASTAL AREAS...IN THE WAKE OF THE WEAKENING REMNANTS
OF THE COMPACT AND VIGOROUS CLOSED LOW NOW PROGRESSING EASTWARD
INTO/THROUGH THE OZARK PLATEAU.  JUST HOW FAST THIS LATTER SYSTEM
WEAKENS...TO THE SOUTH OF A NORTHERN STREAM TROUGH PROGGED TO DIG
ACROSS THE EASTERN CANADIAN PROVINCES AND UPPER GREAT LAKES
REGION...IS A BIT UNCERTAIN.  BUT...IT WILL ENCOUNTER AT LEAST A
SOMEWHAT MORE CONFLUENT REGIME BETWEEN THE NORTHERN AND SOUTHERN
STREAMS...ACROSS THE LOWER OHIO/TENNESSEE VALLEYS INTO THE MID
ATLANTIC STATES.  REGARDLESS...LOW AND MID-LEVEL JET STREAKS
ACCOMPANYING THIS FEATURE ARE EXPECTED TO REMAIN STRONG AND
SUFFICIENTLY SHEARED TO SUPPORT CONTINUING SEVERE POTENTIAL...AS A
MOIST AND POTENTIALLY UNSTABLE GULF RETURN FLOW DEVELOPS THROUGH THE
MID SOUTH AND NORTHERN GULF STATES INTO THE LEE OF THE SOUTHERN
APPALACHIANS.

...MID SOUTH/NRN GULF STATES INTO LEE OF THE SRN APPALACHIANS...
THE BOUNDARY LAYER OVER THE GULF OF MEXICO STILL IS IN THE PROCESS
OF MODIFYING IN THE WAKE OF A PRIOR FRONTAL PASSAGE.  BUT...NCEP
SREF SUGGESTS A HIGH LIKELIHOOD OF SURFACE DEW POINTS NEAR OR JUST
ABOVE 60F RETURNING NORTHWARD INTO THE TENNESSEE VALLEY AND SOUTHERN
APPALACHIANS...AND MID/UPPER 50S DEW POINTS FARTHER NORTH INTO THE
LOWER OHIO VALLEY/CENTRAL APPALACHIANS TODAY.

THE LEADING EDGE OF THE HIGHER MOISTURE CONTENT AIR WILL PROBABLY BE
COINCIDENT WITH A ZONE OF ENHANCED LOWER/MID TROPOSPHERIC WARM
ADVECTION ON THE EDGE OF SUBTROPICAL RIDGING BUILDING NORTHEASTWARD
THROUGH THE CENTRAL/EASTERN GULF STATES.  AND...THIS
FORCING...DOWNSTREAM OF THE BASE OF THE APPROACHING MID-LEVEL SHORT
WAVE IMPULSE...IS EXPECTED TO PROVIDE THE FOCUS FOR INTENSE
CONVECTIVE DEVELOPMENT BY THE AFTERNOON HOURS...ACROSS MIDDLE AND
EASTERN TENNESSEE...MUCH OF NORTHERN ALABAMA AND GEORGIA...INTO THE
WESTERN CAROLINAS.  LOW-LEVEL HODOGRAPHS ACROSS THIS REGION ARE
EXPECTED TO BE LARGE AND CLOCKWISE CURVED NEAR A 30-50 KT
SOUTHWESTERLY 850 MB JET AXIS.  AND...50-70 KT WESTERLY MID-LEVEL
FLOW NEAR THE 500 MB JET AXIS WILL CONTRIBUTE TO STRONG DEEP LAYER
SHEAR.  SHEAR PROFILES APPEAR QUITE FAVORABLE FOR THE EVOLUTION OF
CLASSIC LONG LIVED SUPERCELLS...CAPABLE OF PRODUCING LARGE DAMAGING
HAIL AND A FEW TORNADOES...IN THE PRESENCE OF THERMODYNAMIC PROFILES
CHARACTERIZED BY STEEP LAPSE RATES WITH MIXED LAYER CAPE OF 1000-
2000 J/KG.

AT THE SAME TIME...A BAND OF STRONGER FORCING ASSOCIATED WITH THE
MID-LEVEL CYCLONIC VORTICITY CENTER IS EXPECTED TO CONTRIBUTE TO AT
LEAST A BROKEN LINE OF STORMS AS IT SPREADS ACROSS KENTUCKY/
TENNESSEE DURING THE AFTERNOON HOURS...JUST AHEAD OF THE WEAKENING
SURFACE LOW.  DEEP LAYER SHEAR ACROSS KENTUCKY...WELL NORTH OF THE
POLAR JET AXIS...IS EXPECTED TO BE MORE MODEST THAN AREAS TO THE
SOUTH...WITH INSTABILITY ALSO A BIT WEAKER.  BUT...LOW-LEVEL
HODOGRAPHS WILL STILL PROBABLY BE CLOCKWISE CURVED AND
SIZABLE...SUPPORTIVE OF A FEW EMBEDDED SUPERCELLS WITH THE ENHANCED
RISK FOR DAMAGING WINDS/LARGE HAIL...POSSIBLY ISOLATED TORNADOES.

LARGE-SCALE FORCING IN ADVANCE OF THE WEAKENING UPPER IMPULSE IS
EXPECTED TO STRENGTHEN EAST OF THE SOUTHERN APPALACHIANS THROUGH
REMAINDER OF THE CAROLINAS BY THIS EVENING.  BUT...WEAKER
DESTABILIZATION...ASSOCIATED WITH A MORE LIMITED LOW-LEVEL MOISTURE
RETURN AND THE ONSET OF DIURNAL COOLING...MAY MITIGATE SEVERE
POTENTIAL...OR AT LEAST RESULT IN A GRADUAL DIMINISHING TREND TO
CONVECTIVE ACTIVITY LIKELY TO SPREAD ACROSS THE REGION.

..KERR/GRAMS.. 04/10/2009

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