SWODY2
SPC AC 230543
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1243 AM CDT THU APR 23 2009
VALID 241200Z - 251200Z
...THERE IS A SLGT RISK OF SVR TSTMS FRI AFTN/FRI NIGHT ACROSS PARTS
OF THE MID MISSOURI AND UPPER MISSISSIPPI VALLEYS....
...SYNOPSIS...
MODEL GUIDANCE APPEARS TO BE COMING INTO BETTER AGREEMENT CONCERNING
SOME OF THE SMALLER SCALE DEVELOPMENTS WITHIN THE PREVAILING SPLIT
UPPER FLOW PATTERN THROUGH AT LEAST THIS FORECAST PERIOD. IN
GENERAL...AN UPPER TROUGH IS PROGGED TO CONTINUE EVOLVING ACROSS
MUCH OF THE WESTERN STATES...AS A VIGOROUS SHORT WAVE
TROUGH...EMBEDDED WITHIN THE SOUTHERN BRANCH OF SPLIT BELTS
EMANATING FROM THE NORTHEAST PACIFIC...DIGS INTO THE INTERMOUNTAIN
WEST. AT THE SAME TIME...A VIGOROUS NORTHERN BRANCH IMPULSE IS
PROGGED TO TURN EAST OF THE CANADIAN ROCKIES...BEFORE LIFTING
THROUGH THE CENTRAL CANADIAN PROVINCES...WHILE ANOTHER
IMPULSE...EMBEDDED WITHIN A STREAM EMANATING FROM THE CENTRAL
PACIFIC...MIGRATES ACROSS THE SOUTH CENTRAL STATES. RIDGING IN THE
WAKE OF THIS LATTER FEATURE AT UPPER LEVELS APPEARS LIKELY TO BUILD
NORTHEAST OF THE SOUTHERN ROCKIES THROUGH THE CENTRAL/SOUTHERN
PLAINS. DOWNSTREAM... MID-LEVEL RIDGING IS EXPECTED TO CONTINUE
BUILDING EAST OF THE MISSISSIPPI VALLEY THROUGH THE ATLANTIC
SEABOARD.
THIS EVOLVING PATTERN WILL MAINTAIN THE NORTHEASTWARD ADVECTION OF
ELEVATED MIXED LAYER AIR OUT OF THE SOUTHERN U.S./NORTHERN MEXICAN
PLATEAU AND SOUTHERN ROCKIES REGION...AS AN INCREASINGLY MOIST
RETURN FLOW DEVELOPS INTO THE PLAINS...OFF THE WESTERN GULF OF
MEXICO. MEANWHILE...A SIGNIFICANT COLD FRONT ASSOCIATED WITH THE
POLAR TROUGHS WILL ADVANCE THROUGH THE GREAT BASIN...AND TO THE LEE
OF THE ROCKIES...MORE RAPIDLY ACROSS THE NORTHERN HIGH PLAINS...THAN
THE UPPER MISSISSIPPI VALLEY.
...MID MO VALLEY INTO THE UPPER MISSISSIPPI VALLEY...
MODELS INDICATE THAT VERY STEEP MID-LEVEL LAPSE RATES /AROUND 8-9
DEG C/ WILL OVERSPREAD MOST OF THE REGION BY EARLY FRIDAY. THIS
...COUPLED WITH PRE-FRONTAL MOISTURE ALREADY EXPECTED TO BE IN
PLACE...APPEARS LIKELY TO BE SUPPORTIVE OF MODERATELY STRONG
DESTABILIZATION. FURTHER MOISTENING WITH AN INITIAL SURGE OFF THE
GULF OF MEXICO IS PROGGED TO OVERSPREAD THE MID MISSOURI VALLEY BY
LATE FRIDAY AFTERNOON. A NARROW PRE-FRONTAL THERMAL RIDGE AXIS
EMANATING FROM THE CENTRAL HIGH PLAINS SHOULD BE THE FOCUS FOR
STRONGEST DESTABILIZATION...WITH MIXED LAYER CAPE EXPECTED TO EXCEED
2000 J/KG BY PEAK HEATING.
GUIDANCE IS SUGGESTIVE THAT BOUNDARY LAYER BASED STORMS WILL
INITIATE FIRST...DURING THE LATE AFTERNOON...ACROSS PARTS OF WEST
CENTRAL/NORTH CENTRAL WISCONSIN...NEAR A WEAK FRONTAL WAVE...WHERE
CAPPING MAY BE RELATIVELY WEAKER. LOW-LEVEL HODOGRAPHS WILL BE
CLOCKWISE CURVED...AND SOMEWHAT SIZABLE... BENEATH 30+ KT
SOUTHWESTERLY 850 MB FLOW ACROSS THIS REGION. AND...THIS WILL
CONTRIBUTE TO A RISK FOR ISOLATED TORNADOES IN DEVELOPING
SUPERCELLS...DESPITE A SOMEWHAT RELATIVELY MODEST STRENGTH TO THE
SOUTHWESTERLY MID/UPPER FLOW. LARGE HAIL/DAMAGING WIND GUSTS MAY BE
THE PRIMARY SEVERE THREATS...WITH A CONSOLIDATION INTO A GROWING
STORM CLUSTER POSSIBLE DURING THE EARLY EVENING...AIDED BY A
DIVERGENT UPPER FLOW FIELD IN THE ENTRANCE REGION OF A NORTHERN
STREAM JET STREAK.
ALTHOUGH GREATER INHIBITION MAY RESULT IN SLOWER CONVECTIVE
DEVELOPMENT AHEAD OF THE FRONT ACROSS THE MIDDLE MISSOURI VALLEY...A
MOISTURE SURGE WITH SURFACE DEW POINTS INCREASING INTO THE LOWER/MID
60S MAY CONTRIBUTE TO A WEAKENING CAP BY OR SHORTLY AFTER 25/00Z.
LOW-LEVEL WARM ADVECTION BENEATH DIFLUENT UPPER FLOW SOUTH OF THE
STRONGER WESTERLIES...ON THE NORTHEASTERN PERIPHERY OF THE UPPER
RIDGE...IS EXPECTED TO SUPPORT STORM INITIATION. A FEW SUPERCELLS
ARE POSSIBLE...INCLUDING THE RISK FOR ISOLATED
TORNADOES...PARTICULARLY DURING THE EARLY EVENING ON THE NOSE OF A
CENTRAL PLAINS LOW-LEVEL JET STRENGTHENING IN EXCESS OF 50 KTS.
HOWEVER...GIVEN THE ANTICIPATED SIZABLE TEMPERATURE/DEW POINT
SPREADS WITHIN THE LOWER/MID TROPOSPHERE... IN THE PRESENCE OF VERY
STEEP LAPSE RATES AND LARGE CAPE...HAIL/ WIND POTENTIAL APPEARS MORE
PROMINENT...SOME OF WHICH MIGHT BECOME PARTICULARLY LARGE/STRONG.
DESPITE THE LACK OF MORE SIGNIFICANT MID/UPPER FORCING...IT STILL
APPEARS POSSIBLE THAT FORCING/SHEAR ASSOCIATED WITH THE
STRENGTHENING LOW-LEVEL JET MAY SUPPORT UPSCALE GROWTH OF STORMS
INTO A FAIRLY LARGE MESOSCALE CONVECTIVE SYSTEM DURING EVENING
ACROSS SOUTHERN/EASTERN NEBRASKA INTO WESTERN IOWA. IF THIS
OCCURS...HIGH SURFACE WIND GUST POTENTIAL WILL INCREASE...AND COULD
SPREAD TOWARD THE UPPER MISSISSIPPI VALLEY LATE FRIDAY NIGHT.
..KERR.. 04/23/2009
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