SWODY3
SPC AC 220729
DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0229 AM CDT WED APR 22 2009
VALID 241200Z - 251200Z
...THERE IS A SLGT RISK OF SVR TSTMS FRI AFTN/FRI NIGHT ACROSS PARTS
OF THE MID MISSOURI/UPR MS VALLEYS....
...SYNOPSIS...
THE SPREAD AMONG MODEL DATA REMAINS LARGE CONCERNING SHORT WAVE
DEVELOPMENTS WITHIN THE PREVAILING SPLIT UPPER FLOW. BUT...
GUIDANCE...IN GENERAL...SUGGESTS THAT THE AMPLIFICATION OF A TROUGH
WITHIN THE SOUTHERN BRANCH...ACROSS THE WESTERN U.S. WILL CONTINUE
THROUGH THIS FORECAST PERIOD. AT THE SAME TIME...ANOTHER VIGOROUS
SHORT WAVE IMPULSE WITHIN THE NORTHERN STREAM IS PROGGED TO TURN
EAST OF THE CANADIAN ROCKIES...THEN LIFT THROUGH THE CENTRAL
CANADIAN PROVINCES BY LATE FRIDAY/FRIDAY NIGHT. IT APPEARS THAT
COUPLED UPPER JET STREAKS ASSOCIATED WITH THE TWO FEATURES WILL
CONTRIBUTE TO AN INCREASINGLY DIVERGENT UPPER FLOW FIELD ACROSS
PARTS OF THE NORTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY
DURING THE LATTER HALF OF THE PERIOD. THIS COULD CONTRIBUTE TO
STRENGTHENING FORCING FOR UPWARD VERTICAL MOTION AS FAR SOUTH AS A
STALLING OR SLOW MOVING COLD FRONT ACROSS THE MID MISSOURI VALLEY.
...CNTRL PLAINS INTO THE UPPER MISSISSIPPI VALLEY...
THE SLOW MOVING FRONTAL ZONE ACROSS THE UPPER MISSISSIPPI VALLEY
THROUGH THE MID MISSOURI VALLEY AND NORTH CENTRAL HIGH PLAINS... AND
PERHAPS A DRY LINE EXTENDING SOUTH OF THE FRONT THROUGH THE CENTRAL
PLAINS...APPEAR TO PROVIDE A FOCUS FOR INTENSE CONVECTIVE
DEVELOPMENT. MID-LEVEL LAPSE RATES ASSOCIATED WITH A CAPPING
ELEVATED MIXED LAYER EMANATING FROM THE PLATEAU REGION ARE PROGGED
TO BE VERY STEEP...ON THE ORDER OF 8-9 DEG C...ACROSS MUCH OF THE
PLAINS INTO THE UPPER MISSISSIPPI VALLEY. AND...THIS WILL LIKELY
CONTRIBUTE TO AT LEAST MODERATE POTENTIAL INSTABILITY AS A
SUBSTANTIVE GULF MOISTURE RETURN ADVECTS IN A TONGUE EAST OF THE DRY
LINE INTO THE VICINITY OF THE FRONT. MIXED LAYER CAPE MAY EXCEED
2000 J/KG SHORTLY AFTER PEAK HEATING...IN THE PRESENCE OF SIZABLE
CLOCKWISE CURVED LOW-LEVEL HODOGRAPHS AND AT LEAST MODERATE DEEP
LAYER SHEAR.
FORCING TO BREAK CAP DURING THE LATE AFTERNOON IS STILL
UNCERTAIN...BUT CONVECTIVE TEMPERATURES MAY BE APPROACHED OR
EXCEEDED ON THE NOSE OF A PRE-FRONTAL THERMAL RIDGE EMANATING FROM
THE HIGH PLAINS...ACROSS SOUTHERN/EASTERN NEBRASKA. THIS... COUPLED
WITH A CONFLUENT LOW-LEVEL FLOW INTO THE VICINITY OF THE
FRONT...ACROSS EASTERN NEBRASKA/WESTERN IOWA...MAY BE SUFFICIENT TO
SUPPORT THE INITIATION OF DISCRETE STORMS IN AN ENVIRONMENT
FAVORABLE FOR SUPERCELLS. GIVEN THE WIND PROFILES...IF THIS
OCCURS...THE RISK FOR VERY LARGE HAIL AND LOCALIZED DAMAGING WIND
GUSTS PROBABLY WILL BE THE PRIMARY THREATS...BUT ISOLATED TORNADOES
WILL BE POSSIBLE.
INTO THE EVENING HOURS...ON THE NOSE OF A SOUTHERLY LOW-LEVEL JET
STRENGTHENING FROM 30-50 KTS...TO IN EXCESS OF 50 KTS...FORCING MAY
SUPPORT THE UPSCALE GROWTH OF A STRONG/SEVERE MESOSCALE CONVECTIVE
SYSTEM...PERHAPS AIDED BY STRENGTHENING DIVERGENCE ALOFT. AS THIS
OCCURS...THE POTENTIAL FOR DAMAGING WIND GUSTS WILL INCREASE...WITH
THE THREAT FOR LARGE HAIL ALSO CONTINUING... AS THE ENVIRONMENT
FAVORS BOTH A FORWARD PROPAGATING AND BACK BUILDING COMPONENT.
..KERR.. 04/22/2009
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