SWOD48
SPC AC 220859
DAY 4-8 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0359 AM CDT WED APR 22 2009
VALID 251200Z - 301200Z
...SEVERE STORMS POSSIBLE FROM PARTS OF THE CENTRAL PLAINS THRU THE
UPPER MISSISSIPPI VALLEY THIS WEEKEND...
CONSIDERABLE SPREAD PERSISTS AMONG THE MEDIUM RANGE MODELS...
INCLUDING THE MREF...REGARDING THE SHORT WAVE DEVELOPMENTS WITHIN
THE PREVAILING SPLIT UPPER FLOW PATTERN THROUGH THIS FORECAST
PERIOD. BUT...REASONABLE ENOUGH AGREEMENT APPEARS TO EXIST
CONCERNING THE EMERGENCE OF ONE SIGNIFICANT SHORT WAVE IMPULSE FROM
THE AMPLIFIED SOUTHERN BRANCH TROUGH OVER THE WESTERN STATES THIS
WEEKEND. THIS IS EXPECTED TO BE ACCOMPANIED BY THE NORTHEASTWARD
MIGRATION OF A SIGNIFICANT FRONTAL WAVE ACROSS THE CENTRAL PLAINS
LATE SATURDAY/SATURDAY NIGHT...INTO AND THROUGH THE UPPER
MISSISSIPPI VALLEY/UPPER GREAT LAKES REGION SUNDAY/SUNDAY NIGHT.
GIVEN THAT THIS APPEARS LIKELY TO BE PRECEDED BY A SUBSTANTIVE GULF
RETURN FLOW BENEATH VERY STEEP MID-LEVEL LAPSE RATES...ASSOCIATED
MODERATE INSTABILITY IN THE PRESENCE OF STRONG SHEAR AND LARGE-SCALE
FORCING FOR UPWARD VERTICAL MOTION WILL BE QUITE FAVORABLE FOR
SEVERE STORMS...INCLUDING DISCRETE SUPERCELL DEVELOPMENT...AND THE
POSSIBLE EVOLUTION OF ONE OR MORE SEVERE MESOSCALE CONVECTIVE
SYSTEMS.
..KERR.. 04/22/2009
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