SWODY1
SPC AC 170558
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1258 AM CDT MON SEP 17 2007
VALID 171200Z - 181200Z
..THERE IS A SLGT RISK OF SVR TSTMS FROM THE CENTRAL PLAINS TO THE
UPPER MS VALLEY...
..SYNOPSIS...
PROGRESSIVE MID/UPPER LEVEL FLOW PATTERN IS EXPECTED THROUGH DAY 1.
HEIGHT FALLS ARE FORECAST ACROSS THE CENTRAL/NRN PLAINS TO UPPER MS
VALLEY ON MONDAY AS A TROUGH...CURRENTLY LOCATED OVER NV/SRN CA...
TRACKS NEWD INTO THE CENTRAL PLAINS/MID MO VALLEY...AND AN UPSTREAM
TROUGH ALONG PAC NW COAST DIGS SEWD INTO THE GREAT BASIN/ROCKIES.
AT THE SURFACE...SSWLY LOW LEVEL FLOW WILL ALLOW MOISTURE TO ADVECT
NNEWD WITH A WARM FRONT MOVING NWD ACROSS THE UPPER MS VALLEY...WITH
A PRE-FRONTAL TROUGH/DRY LINE EXPECTED TO EXTEND FROM CENTRAL MN
SWWD THROUGH ERN NEB TO WRN KS BY LATE THIS AFTERNOON. A COLD FRONT
IS EXPECTED TO SPREAD ESEWD ACROSS THE NRN PLAINS...AND MAY MERGE
WITH OR OVERTAKE NRN PART OF PRE-FRONTAL TROUGH BY 12Z TUESDAY.
..CENTRAL PLAINS TO UPPER MS VALLEY...
MODELS SUGGEST MID LEVEL IMPULSES EMBEDDED WITHIN MOISTURE PLUME
EXTENDING FROM MEXICO NWD INTO CENTRAL ROCKIES WILL TRANSLATE NEWD
ACROSS THE CENTRAL PLAINS TO MID MO/UPPER MS VALLEYS IN ADVANCE OF
LARGE SCALE TROUGH EVOLVING OVER THE WRN HALF OF THE CONUS. HEIGHT
FALLS IN ADVANCE OF THIS UPPER TROUGH ARE EXPECTED TO MAINTAIN SSWLY
LLJ FROM THE SRN PLAINS INTO THE MID MO VALLEY WITH LOW LEVEL WINDS
ALSO INCREASING ACROSS THE UPPER MS VALLEY/WRN GREAT LAKES REGION
THROUGH THE PERIOD. AS A RESULT...MOISTURE WILL ADVECT NEWD ALONG
AND E OF PRE-FRONTAL TROUGH WITH MID-UPPER 60S SURFACE DEWPOINTS
REACHING ERN MN/NWRN-WRN WI BY THIS AFTERNOON.
AT 12Z TODAY...ELEVATED TSTMS MAY BE ONGOING FROM PARTS OF THE ERN
DAKOTAS INTO NERN NEB/IA WITHIN LOW LEVEL WAA REGIME. THIS ACTIVITY
IS EXPECTED TO SPREAD NE TOWARD THE UPPER GREAT LAKES REGION WITH
ATTENDANT CLOUDINESS TENDING TO LIMIT DESTABILIZATION ACROSS NRN
MN/NRN WI. STRONGER SURFACE HEATING IS FORECAST TO OCCUR ALONG AND
E OF PRE-FRONTAL TROUGH FROM CENTRAL MN SWWD INTO THE CENTRAL PLAINS
BENEATH ERN EXTENT OF ELEVATED MIXED LAYER /STEEP MID LEVEL LAPSE
RATES/. THIS WILL CONTRIBUTE TO MODERATE INSTABILITY /MLCAPE
1500-2000 J/KG/ DEVELOPING BY MID-LATE AFTERNOON ACROSS THIS REGION.
ASCENT SPREADING NEWD WITH LEAD IMPULSES AND REACHING CENTRAL MN
SWWD TO NRN/WRN KS BY PEAK HEATING IS EXPECTED TO SUPPORT TSTM
DEVELOPMENT BY LATE AFTERNOON ALONG THE PRE-FRONTAL TROUGH IN THESE
AREAS. INCREASING DEEP LAYER WIND FIELDS WILL SUPPORT BOTH
INCREASING DEEP LAYER AND LOW LEVEL SHEAR VALUES FOR ORGANIZED
STORMS WITH SUPERCELLS LIKELY. STRENGTHENING LLJ FROM AFTERNOON
INTO THE EVENING WILL ENHANCE HODOGRAPH CURVATURE AND AN ATTENDANT
TORNADO THREAT EXTENDING FROM ERN NEB INTO SRN/CENTRAL MN AND WEST
CENTRAL WI. GIVEN STRENGTH OF WIND FIELDS...DAMAGING WIND THREAT
SHOULD BE ENHANCED FROM NERN NEB INTO CENTRAL MN.
..FOUR CORNERS/CENTRAL ROCKIES...
TERRAIN FORCING AND INCREASED LARGE-SCALE FORCING FOR ASCENT
ASSOCIATED WITH A COUPLE OF MID LEVEL TROUGHS APPROACHING THIS
REGION FROM THE GREAT BASIN THIS AFTERNOON AND EVENING ARE EXPECTED
TO CONTRIBUTE TO FAIRLY WIDESPREAD TSTM DEVELOPMENT. STEEP LOW TO
MID-LEVEL LAPSE RATES AND STRENGTHENING VERTICAL SHEAR THROUGH THE
PERIOD SUGGESTS SOME POTENTIAL WILL EXIST FOR A FEW ORGANIZED STORMS
WITH A THREAT OF LOCALLY STRONG WIND GUSTS AND SOME MARGINALLY
SEVERE HAIL.
.PETERS/CROSBIE.. 09/17/2007
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