SWODY1
SPC AC 201615
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1115 AM CDT MON SEP 20 2010
VALID 201630Z - 211200Z
...THERE IS A SLGT RISK OF SVR TSTMS OVER PORTIONS OF THE UPPER MS
VALLEY AND UPPER GREAT LAKES...
...SYNOPSIS...
LARGE-SCALE TROUGH OVER THE NRN INTERMOUNTAIN REGION INTO GREAT
BASIN WILL GRADUALLY DEAMPLIFY THROUGH THE DAY ONE PERIOD WHILE
PROGRESSING EWD THROUGH MEAN RIDGE AXIS IN PLACE OVER THE N-CNTRL
CONUS. HOWEVER...THIS SYSTEM WILL STILL BE ACCOMPANIED BY 60-120
M/12 HR HEIGHT FALLS SPREADING EWD THROUGH THE NRN PLAINS INTO UPPER
GREAT LAKES REGION. IN THE LOW LEVELS...SURFACE CYCLONE CURRENTLY
DEEPENING OVER SERN MT WILL DEVELOP SEWD INTO S-CNTRL SD TODAY
BEFORE ACCELERATING NEWD THROUGH WRN LAKE SUPERIOR OVERNIGHT. A
WARM FRONT WILL PRECEDE THE SURFACE LOW...LIFTING NWD THROUGH THE
UPPER MS VALLEY AND UPPER MIDWEST WHILE A TRAILING COLD FRONT SURGES
SEWD THROUGH THE NRN PLAINS AND UPPER MS VALLEY. THE DEEPENING
SURFACE LOW WILL ALSO CONTRIBUTE TO THE INTENSIFICATION OF A LEE OR
PRE-FRONTAL TROUGH TODAY OVER THE CNTRL HIGH PLAINS.
...NRN PLAINS INTO UPPER MS VALLEY/UPPER GREAT LAKES...
12Z REGIONAL SOUNDINGS INDICATE THE PRESENCE OF A WARM EML AND
ASSOCIATED STRONG CAP WHICH WILL BE ADVECTED NEWD THROUGH THE PERIOD
IN ADVANCE OF UPPER TROUGH. AT THE SAME TIME...THE LOWER
TROPOSPHERIC MASS RESPONSE TO THE APPROACH OF THE UPPER SYSTEM WILL
MAINTAIN A STRONG /40-50+ KT/ SWLY LLJ FROM THE CNTRL PLAINS INTO
UPPER GREAT LAKES...ENHANCING THE NEWD FLUX OF HEAT AND MOISTURE
BENEATH THE EML. THESE DIFFERENTIAL ADVECTIONS WILL CONTRIBUTE TO
THE DEVELOPMENT OF A MODERATE TO STRONGLY UNSTABLE AIR MASS /I.E.
MLCAPE OF 1500-3000+ J PER KG/ BY AFTERNOON FROM VICINITY OF SURFACE
TRIPLE POINT OVER ERN SD SEWD ALONG RETREATING WARM FRONT ACROSS
CNTRL/SRN MN...IA INTO SWRN WI/NWRN IL. HOWEVER...GIVEN THE WARM
EML...LATEST MODEL GUIDANCE REMAINS CONSISTENT IN SUGGESTING THAT
THE WARM SECTOR WILL REMAIN CAPPED TO SURFACE-BASED STORM
DEVELOPMENT THROUGH THE DIURNAL HEATING CYCLE.
STORM FORMATION WILL BECOME INCREASINGLY LIKELY BETWEEN 21/00-03Z
NEAR THE SURFACE TRIPLE POINT OVER NRN/CNTRL MN AS SUSTAINED
LOW-LEVEL CONVERGENCE AND INCREASED LARGE-SCALE FORCING FOR ASCENT
SUFFICIENTLY ERODE REMAINING CONVECTIVE INHIBITION. THERE IS SOME
QUESTION AS TO WHETHER STORMS WILL BE COMPLETELY SURFACE-BASED DUE
TO THE ONSET OF RADIATIONAL COOLING DURING THIS TIME FRAME.
HOWEVER...GIVEN THE RAPIDLY MOISTENING BOUNDARY LAYER AND RELATIVELY
STRONG LOW AND DEEP-LAYER SHEAR...ENVIRONMENT WILL BE FAVORABLE FOR
SUPERCELLS CAPABLE OF LARGE HAIL...DAMAGING WINDS AND PERHAPS A
TORNADO OR TWO. TSTMS /LIKELY BECOMING MORE ELEVATED WITH TIME/
SHOULD INCREASE IN COVERAGE OVERNIGHT NEAR AND N OF WARM FRONT FROM
NERN MN EWD THROUGH NRN WI INTO THE UPPER PENINSULA OF MI WHERE A
CONTINUED RISK FOR HAIL AND STRONG WIND GUSTS WILL EXIST.
ADDITIONAL STORM FORMATION IS ANTICIPATED SWWD ALONG COLD FRONT FROM
CNTRL/SRN MN INTO IA WHERE A THREAT FOR ISOLATED HAIL AND DAMAGING
WIND GUSTS WILL EXIST.
...CNTRL HIGH PLAINS...
A HOT AND DEEPLY-MIXED BOUNDARY LAYER IS EXPECTED TO DEVELOP THIS
AFTERNOON ALONG STRENGTHENING LEE TROUGH FROM WRN NEB INTO ERN
CO/WRN KS. WHILE...MOISTURE CONTENT WILL BE LESS THAN AREAS TO THE
E/NE...THE PRESENCE OF A STEEP LAPSE RATE ENVIRONMENT WILL YIELD
MLCAPE VALUES OF 1000-2000 J/KG. ISOLATED...HIGH-BASED TSTM
DEVELOPMENT IS EXPECTED TO OCCUR BY LATE AFTERNOON/EARLY EVENING
ALONG LEE TROUGH AS THE STRONG HEATING AND INCREASING HEIGHT
FALL/DYNAMIC FORCING REMOVE REMAINING CONVECTIVE INHIBITION.
VERTICAL SHEAR WILL DECREASE WITH SWD EXTENT ALONG LEE TROUGH WITH
ENVIRONMENT SUPPORTIVE OF MULTICELL OR MARGINAL SUPERCELL
STRUCTURES. ISOLATED OCCURRENCES OF HAIL AND/OR STRONG WIND GUSTS
WILL ACCOMPANY THE STRONGEST STORMS INTO TONIGHT.
..MEAD/GRAMS.. 09/20/2010
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