SPC AC 230734
DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0234 AM CDT THU APR 23 2009
VALID 251200Z - 261200Z
...THERE IS A SLGT RISK OF SVR TSTMS SAT/SAT NIGHT FROM PARTS OF THE
S CNTRL PLAINS INTO UPPER PORTIONS OF THE MIDDLE MISSISSIPPI
VALLEY....
...SYNOPSIS...
SHORT WAVE DEVELOPMENTS WITHIN THE PREVAILING SPLIT UPPER FLOW
REGIME REMAIN A SOURCE OF CONSIDERABLE SPREAD AND DISCREPANCIES
AMONG MODEL DATA FOR THIS FORECAST PERIOD AND BEYOND. HOWEVER...
GUIDANCE IS IN REASONABLE AGREEMENT CONCERNING THE GENERAL PATTERN
EVOLUTION. LARGE-SCALE TROUGHING LIKELY WILL ENCOMPASS MUCH OF
CANADA AND THE WESTERN/NORTH CENTRAL U.S. BY LATE SATURDAY
NIGHT...WHILE UPPER RIDGING PERSISTS ACROSS THE GULF COAST AND
EASTERN STATES. WITHIN THE NORTHERN BRANCH OF SPLIT FLOW EMANATING
FROM THE NORTHEAST PACIFIC/NORTHWESTERN NORTH AMERICA...A VIGOROUS
SHORT WAVE TROUGH IS PROGGED TO RAPIDLY LIFT NORTHEAST OF THE UPPER
GREAT LAKES REGION DURING THE DAY SATURDAY...WHILE AN UPSTREAM
IMPULSE DIGS SOUTH OF THE CANADIAN ROCKIES. THIS LATTER FEATURE
APPEARS LIKELY TO CONTRIBUTE TO THE NORTHEASTWARD ACCELERATION OF
ANOTHER STRONG SHORT WAVE IMPULSE OUT OF THE BASE OF THE SOUTHERN
BRANCH TROUGH OVER THE FOUR CORNERS REGION. JUST HOW FAST THIS WILL
OCCUR IS UNCERTAIN...BUT THIS SYSTEM IS EXPECTED TO HAVE ENOUGH OF
AN IMPACT ACROSS THE SOUTH CENTRAL PLAINS INTO PARTS OF THE MIDDLE
MISSISSIPPI VALLEY TO SUPPORT CONSIDERABLE SEVERE POTENTIAL.
...SOUTHERN PLAINS INTO THE GREAT LAKES REGION...
STEEP MID-LEVEL LAPSE RATES ARE EXPECTED TO PERSIST DOWNSTREAM OF
THE UPPER TROUGH ACROSS MUCH OF THE CENTRAL/SOUTHERN PLAINS THROUGH
UPPER PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY INTO SATURDAY. IN
THE WAKE OF THE DEPARTING NORTHERN BRANCH IMPULSE...WITH
CONSIDERABLE CONVECTIVE DEVELOPMENT PERHAPS ONGOING EARLY IN THE
DAY...THIS IS MORE UNCERTAIN EAST NORTHEASTWARD INTO THE GREAT LAKES
REGION. BUT...GULF RETURN FLOW...AROUND A LARGE LOW-LEVEL RIDGE
CENTERED OVER THE SOUTHEAST...COULD BE SUFFICIENT IN THE PRESENCE OF
FAVORABLE DEEP LAYER SHEAR ON THE SOUTHERN FRINGE OF THE
WESTERLIES...TO SUPPORT A CONTINUATION OF STRONG/SEVERE STORMS
THROUGH THE DAY.
MORE PROBABLE...A PRE-FRONTAL LOW-LEVEL CONFLUENCE ZONE WILL BECOME
THE FOCUS FOR INTENSE CONVECTIVE DEVELOPMENT AS INHIBITION WEAKENS
BY/SHORTLY AFTER PEAK HEATING...FROM PARTS OF EASTERN/ SOUTHERN IOWA
THROUGH EASTERN KANSAS. MIXED LAYER CAPE UP TO 2000 J/KG WILL
SUPPORT VIGOROUS UPDRAFTS IN THE PRESENCE OF A FAVORABLY SHEARED
SOUTHWESTERLY DEEP LAYER MEAN FLOW ON THE ORDER OF 40 KTS...WHICH
WILL SUPPORT THE RISK FOR SEVERE STORMS CAPABLE MAINLY OF PRODUCING
LARGER HAIL AND DAMAGING WINDS.
THE LIKELIHOOD OF A COLD FRONTAL PASSAGE THROUGH MUCH OF WESTERN/
CENTRAL KANSAS ADDS UNCERTAINTY TO THE FORECAST. BUT...THE DRY LINE
AND DRY LINE/COLD FRONTAL INTERSECTION ACROSS THE TEXAS/ OKLAHOMA
PANHANDLE AREA INTO THE VICINITY OF THE WESTERN OKLAHOMA/KANSAS
BORDER SHOULD PROVIDE THE FOCUS FOR ADDITIONAL DISCRETE STORM
DEVELOPMENT. IN THE PRESENCE OF MODERATE TO STRONG SHEAR AND
INSTABILITY...A FEW SUPERCELLS ARE LIKELY...CAPABLE OF PRODUCING
LARGE HAIL/DAMAGING WINDS...PERHAPS A FEW TORNADOES. THIS ACTIVITY
IS EXPECTED TO PERSIST INTO THE EVENING HOURS...BEFORE PRIMARY
DEVELOPMENT SHIFTS TO THE STRENGTHENING WARM ADVECTION REGIME ABOVE
THE SURFACE FRONT ACROSS KANSAS...WHERE THE RISK FOR AT LEAST LARGE
HAIL MAY PERSIST THROUGH THE NIGHT.
...CENTRAL APPALACHIANS...
MODELS SUGGEST MOISTENING/DESTABILIZATION BENEATH UPPER RIDGE WILL
BE SUPPORTIVE OF SCATTERED STORM DEVELOPMENT TO THE LEE OF THE
CENTRAL APPALACHIANS. STEEP ENOUGH LAPSE RATES WILL EXIST TO
SUPPORT LOW PROBABILITIES FOR HAIL/GUSTY WINDS...DESPITE VERY WEAK
SHEAR.
..KERR.. 04/23/2009
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