SPC AC 291729
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1229 PM CDT WED APR 29 2009
VALID 301200Z - 011200Z
...THERE IS A SLGT RISK OF SVR TSTMS THU ACROSS MUCH OF THE OH
VALLEY....
...THERE IS A SLGT RISK OF SVR TSTMS LATE THU AFTN/THU NIGHT ACROSS
THE CENTRAL PLAINS INTO PARTS OF THE OZARK PLATEAU....
...SYNOPSIS...
EAST OF THE BLOCKING UPPER HIGH CENTER NEAR THE YUKON TERRITORY...
MODELS CONTINUE TO INDICATE THAT A NORTHERN BRANCH TROUGH WILL
AMPLIFY THROUGH THIS FORECAST PERIOD ACROSS THE CENTRAL CANADIAN
PROVINCES. AS THIS OCCURS...A COUPLE OF IMPULSES...COMPRISING THE
ELONGATED CLOSED LOW PORTION OF THE UPPER BLOCKING PATTERN...ARE
PROGGED TO ACCELERATE EASTWARD OUT OF THE NORTHWESTERN STATES. THIS
IS EXPECTED TO MAINTAIN MODERATELY STRONG...BROADLY CYCLONIC
MID/UPPER FLOW FROM THE CENTRAL ROCKIES THROUGH THE OHIO VALLEY AND
GREAT LAKES REGION...WHILE SUPPORTING A DEEPENING SURFACE CYCLONE
ACROSS THE UPPER GREAT LAKES REGION INTO THE EASTERN CANADIAN
PROVINCES. AN ASSOCIATED COLD FRONT APPEARS LIKELY TO SURGE THROUGH
MUCH OF THE NORTHERN AND CENTRAL PLAINS...AND THE UPPER HALF OF THE
MISSISSIPPI VALLEY.
ON THE SOUTHERN PERIPHERY...OR JUST TO THE SOUTH AND EAST...OF THIS
REGIME...SHORT RANGE MODEL GUIDANCE MAINTAINS A SIGNAL SUGGESTING
THAT A SIGNIFICANT IMPULSE WILL EVOLVE FROM ONE OF SEVERAL LOWER
AMPLITUDE IMPULSES EMANATING FROM BROAD SOUTHERN STREAM TROUGHING
ACROSS THE SOUTHERN PLATEAU/ROCKIES. THE SPREAD AMONG THE DATA
CONCERNING THE STRENGTH/TRACK OF THIS FEATURE STILL SEEMS
CONSIDERABLE...BUT IT CURRENTLY APPEARS MOST PROBABLE TO BE SHIFTING
EAST OF THE MIDDLE MISSISSIPPI VALLEY EARLY THURSDAY...BEFORE
CONTINUING EAST NORTHEASTWARD THROUGH THE OHIO VALLEY
THURSDAY/THURSDAY NIGHT. THIS SECONDARY WAVE IS PROGGED TO BE
ACCOMPANIED BY STRENGTHENING LOWER/MID TROPOSPHERIC WIND FIELDS...ON
THE ORDER OF 40-50+ KTS.
...MID MS/OH VALLEYS AND GREAT LAKES REGION...
MID-LEVEL LAPSE RATES ARE LIKELY TO REMAIN WEAK ACROSS MUCH OF THE
REGION...AND CONSIDERABLE CLOUD COVER MAY EXIST AT THE OUTSET OF THE
PERIOD FROM ONGOING/WEAKENING CONVECTION OR CONVECTIVE DEBRIS. THIS
WILL LIMIT POTENTIAL FOR SIGNIFICANT DESTABILIZATION...EVEN WITH
SOME INSOLATION. HOWEVER...THE ENVIRONMENT WILL BE MOIST... LIKELY
CHARACTERIZED BY SURFACE DEW POINTS IN THE LOWER 60S...AND
ANOMALOUSLY HIGH PRECIPITABLE WATER OF 1.25 TO 1.5 INCHES. GIVEN AT
LEAST WEAK DESTABILIZATION...AND THE ANTICIPATED STRENGTH OF THE
LOW/MID-LEVEL WIND FIELDS...FAVORABLE LARGE-SCALE FORCING APPEARS TO
EXIST FOR THE DEVELOPMENT OF AN ORGANIZED CONVECTIVE SYSTEM CAPABLE
OF PRODUCING DAMAGING SURFACE WINDS. THIS APPEARS MOST PROBABLE
ACROSS PORTIONS OF SOUTHERN ILLINOIS/CENTRAL AND SOUTHERN INDIANA BY
EARLY AFTERNOON...EASTWARD ACROSS OHIO AND NORTHERN KENTUCKY THROUGH
THE REMAINDER OF THE AFTERNOON. ACTIVITY MAY ALSO BE ACCOMPANIED BY
DISCRETE AND EMBEDDED SUPERCELL DEVELOPMENT WITH THE RISK FOR
ISOLATED TORNADOES.
ALONG THE MAIN SURFACE FRONT TRAILING FROM THE PRIMARY SURFACE
CYCLONE...DESTABILIZATION COULD BECOME SUFFICIENT FOR AFTERNOON/
EVENING STORM DEVELOPMENT...PARTICULARLY ACROSS PARTS OF NORTHERN
ILLINOIS AND SOUTHEAST IOWA INTO NORTHERN MISSOURI. IF THIS
OCCURS...DEEP LAYER SHEAR BENEATH MODERATE WESTERLY MID-LEVEL FLOW
MAY BE SUFFICIENT FOR AT LEAST MID-LEVEL UPDRAFT ROTATION...WITH AN
ATTENDANT RISK FOR HAIL/LOCALIZED DAMAGING WIND GUSTS. BUT...
PARTLY DUE TO WEAKER FORCING...THIS RISK PROBABLY WILL BE MORE
SPARSE IN COVERAGE AND SHORTER-LIVED.
...PARTS OF CNTRL PLAINS INTO OZARK PLATEAU...
THE SURFACE BOUNDARY LEFT IN THE WAKE OF THE OHIO VALLEY IMPULSE IS
EXPECTED TO BECOME A FOCUS FOR THE PRIMARY CONVECTIVE DEVELOPMENT
DURING THIS FORECAST PERIOD. THIS MAY NOT OCCUR UNTIL THE MID TO
LATE EVENING HOURS THURSDAY...ON THE NOSE OF A NOCTURNALLY
STRENGTHENING SOUTHERLY LOW-LEVEL JET...NEAR THE KANSAS/OKLAHOMA
BORDER AREA. BUT...IT APPEARS POSSIBLE THAT WEAK LOW-LEVEL WARM
ADVECTION AND CONVERGENCE ALONG THE FRONT... ESPECIALLY THE
FRONT/DRY LINE INTERSECTION ACROSS NORTHWEST OKLAHOMA...COULD WEAKEN
INHIBITION SUFFICIENTLY...AS CONVECTIVE TEMPERATURES ARE APPROACHED
LATE IN THE AFTERNOON...TO SUPPORT SCATTERED STORM DEVELOPMENT. IF
THIS OCCURS...MODELS INDICATE A MODERATE TO STRONGLY UNSTABLE
ENVIRONMENT BENEATH STEEP MID-LEVEL LAPSE RATES...CHARACTERIZED BY
MIXED LAYER CAPE OF 2000-3000 J/KG
...WILL BE SUPPORTIVE OF SEVERE STORMS. MODERATE WESTERLY MID-LEVEL
FLOW ON THE SOUTHERN FRINGE OF THE WESTERLIES WILL BE SUFFICIENT FOR
SUPERCELLS...CONTRIBUTING TO THE RISK FOR VERY LARGE HAIL/DAMAGING
WIND GUSTS...PERHAPS ISOLATED TORNADOES.
..KERR.. 04/29/2009
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