SPC AC 181730
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1230 PM CDT SAT APR 18 2009
VALID 191200Z - 201200Z
...THERE IS A SLGT RISK OF SVR TSTMS SRN KY/TN VALLEY SWD TO
CENTRAL/ERN GULF COAST STATES...
...SYNOPSIS...
CLOSED MID/UPPER LEVEL LOW...VERTICALLY STACKED WITH H85/SURFACE
LOWS...AND EMBEDDED WITHIN A CENTRAL/SRN PLAINS TROUGH...IS EXPECTED
TO EVOLVE INTO AN OPEN WAVE REACHING ERN KS/OK AND THE OZARKS REGION
BY 12Z SUNDAY. MEANWHILE...MODELS CONTINUE TO INDICATE AN
AMPLIFICATION OF THE POLAR WESTERLIES WITH AN UPPER RIDGE BUILDING
ACROSS THE WRN U.S. INTO THE CANADIAN ROCKIES...AND DOWNSTREAM
TROUGH DEVELOPING ALONG AND E OF MS RIVER VALLEY ON SUNDAY. THE
LATTER FEATURE WILL BE THE RESULT OF THE PHASING OF A SHORTWAVE
TROUGH CURRENTLY LOCATED OVER CENTRAL CANADA WITH THE AFOREMENTIONED
CENTRAL/SRN PLAINS TROUGH. A SHORT WAVE TROUGH CONCURRENT WITH THE
CLOSED LOW IS EXPECTED TO ACCELERATE ENEWD ACROSS AR/TN VALLEY AND
REACH THE UPPER OH VALLEY BY 12Z MONDAY.
IN THE LOW LEVELS...A SURFACE LOW IS EXPECTED TO DEEPEN AS IT TRACKS
NNEWD FROM THE UPPER PART OF THE LOWER MS VALLEY TO WRN OH BY SUNDAY
EVENING.
...LWR OH/TN VALLEYS...CNTRL AND ERN GULF STATES...
MODELS CONTINUE TO INDICATE THAT FORCING ASSOCIATED WITH THE PRIMARY
MID-LEVEL IMPULSE...EMBEDDED WITHIN THE REMNANT SRN PLAINS
TROUGH...WILL GENERALLY TURN EAST OF THE OZARK PLATEAU THROUGH THE
TN VALLEY/NORTHERN GULF STATES REGION FROM MID DAY SUNDAY THROUGH
EARLY SUNDAY EVENING. HOWEVER...MODELS CONTINUE TO INDICATE THAT
THIS WILL BE RATHER CLOSELY PRECEDED BY FORCING ASSOCIATED WITH
ANOTHER IMPULSE...WHICH SHOULD SUPPORT CONSIDERABLE CONVECTIVE
DEVELOPMENT AND CLOUD COVER...AS LOW-LEVEL GULF MOISTURE RETURNS
NEWD. THE BULK OF THIS INITIAL ACTIVITY PROBABLY WILL BE BASED IN
AN ELEVATED WARM ADVECTION REGIME...FROM THE NORTHERN GULF STATES
THROUGH THE LOWER OH VALLEY. HOWEVER...MODELS CONTINUE TO INDICATE
LARGE CLOCKWISE CURVED LOW-LEVEL HODOGRAPHS NEAR A 30-50 KT SLY 850
MB JET COULD CONTRIBUTE TO TORNADO/WIND DAMAGING POTENTIAL WITH
STORMS FORMING NEAR THE LEADING EDGE OF BOUNDARY LAYER MOISTURE
RETURN...ACROSS PARTS OF CENTRAL AND SRN MS/AL. THIS THREAT APPEARS
MOST LIKELY TO OCCUR AT THE START OF DAY 2...WITH SOME POTENTIAL
THAT THIS ACTIVITY MAY BE ONGOING FROM THE END OF THE DAY 1 PERIOD.
THIS MORNING THREAT SHOULD DIMINISH AS THE UPPER IMPULSE PIVOTS
NORTHEASTWARD TOWARD THE OHIO VALLEY...AND THE LOW-LEVEL JET CORE
BEGINS TO SHIFT OFF TO THE NORTH/EAST BY EARLY AFTERNOON.
HOWEVER...THE SRN EXTENT OF THE LLJ MAY LINGER FOR A PERIOD ALONG
THE GULF COAST...AND MAINTAIN A THREAT FOR SEVERE STORMS INTO THE
AFTERNOON.
THE INCREASING LIKELIHOOD OF WEAKENING AND VEERING OF LOW-LEVEL FLOW
FIELDS IN THE WAKE OF THIS INITIAL IMPULSE CONTRIBUTES TO UNCERTAIN
SEVERE POTENTIAL WITH THE STRONGER MAIN IMPULSE THROUGH THE
REMAINDER OF THE PERIOD. THE WINDOW OF OPPORTUNITY FOR SUBSTANTIAL
SURFACE HEATING MAY BE LIMITED...BUT IT STILL SEEMS PROBABLE THAT
BOUNDARY LAYER MOISTENING COUPLED WITH STEEPENING LAPSE
RATES...ASSOCIATED WITH STRONG MID-LEVEL COOLING...WILL CONTRIBUTE
TO WEAK TO MODERATE DESTABILIZATION. THIS INSTABILITY ...AND COOL
THERMODYNAMIC PROFILES...IN THE PRESENCE OF STRONG DEEP LAYER SHEAR
SHOULD BE SUFFICIENT TO SUPPORT STORMS CAPABLE AT LEAST OF PRODUCING
LARGE HAIL/DAMAGING WINDS...AS FAVORABLE MID-LEVEL FORCING
OVERSPREADS THE TN VALLEY/NRN GULF STATES REGION SUNDAY AFTERNOON
AND EVENING.
..PETERS/KERR.. 04/18/2009
CLICK TO GET WUUS02 PTSDY2 PRODUCT
NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 0600Z
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