SPC AC 270530

   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   1230 AM CDT FRI SEP 27 2013

   VALID 271200Z - 281200Z

   ...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON INTO TONIGHT
   ACROSS MUCH OF THE CENTRAL INTO SOUTHERN PLAINS...

   ...SYNOPSIS...
   LITTLE CHANGE TO THE LARGE-SCALE MID/UPPER PATTERN IS EXPECTED
   THROUGH THIS FORECAST PERIOD ACROSS THE EASTERN U.S...WITH WEAK
   TROUGHING LINGERING EAST OF THE ATLANTIC COAST...AND A SHARP UPPER
   RIDGE REMAINING PROMINENT NORTH OF A WESTERN GULF OF MEXICO
   SUBTROPICAL HIGH CENTER THROUGH THE GREAT LAKES REGION. 
   HOWEVER...LARGE SCALE MID/UPPER TROUGHING OVER THE WESTERN STATES
   APPEARS LIKELY TO BECOME LESS AMPLIFIED...AS A SIGNIFICANT EMBEDDED
   SHORT WAVE IMPULSE SHIFTS NORTHEASTWARD OUT OF THE GREAT BASIN/
   SOUTHERN PLATEAU REGION...THROUGH THE NORTHERN AND CENTRAL PLAINS BY
   12Z SATURDAY.  MODELS SUGGEST THAT THIS LATTER DEVELOPMENT WILL
   OCCUR AS A STRONG UPSTREAM IMPULSE AND ASSOCIATED JET STREAK DIG
   THROUGH THE NORTHEAST PACIFIC.

   IN RESPONSE TO THE IMPULSE EMERGING FROM THE INTERMOUNTAIN
   REGION...MODELS SUGGEST THAT A SURFACE CYCLONE MAY SLOWLY DEEPEN
   OVER THE CENTRAL HIGH PLAINS DURING THE DAY TODAY...BEFORE A SURFACE
   FRONT...EXTENDING NORTHEASTWARD THROUGH THE MID MISSOURI AND UPPER
   MISSISSIPPI VALLEYS...STRENGTHENS AND BEGINS TO SURGE EASTWARD
   ACROSS THE CENTRAL PLAINS TONIGHT.  WHILE A RETURN FLOW OF GULF
   MOISTURE IS IN THE PROCESS OF DEVELOPING ACROSS THE LOWER RIO GRANDE
   VALLEY...SUBSTANTIVE BOUNDARY LAYER MOISTENING FROM THIS SOURCE
   REGION MAY NOT REACH THE CENTRAL PLAINS UNTIL LATE THIS AFTERNOON OR
   EVENING.  HOWEVER...MOISTENING ACROSS THE SOUTHERN INTO CENTRAL HIGH
   PLAINS DOES APPEAR LIKELY TO BE AUGMENTED BY AN INFLUX OF MOISTURE
   OF LOWER LATITUDE EASTERN PACIFIC ORIGINS...ON SOUTHWESTERLY FLOW AT
   LOWER/MID-LEVELS...AHEAD OF THE UPPER TROUGH.

   ...CENTRAL AND SOUTHERN PLAINS...
   WHILE IT DOES APPEAR LIKELY THAT CONSIDERABLE LOW/MID-LEVEL
   MOISTENING WILL OCCUR TODAY ALONG A PRE-FRONTAL SURFACE TROUGH
   ACROSS THE SOUTHERN INTO CENTRAL HIGH PLAINS...AND ALONG THE FRONT
   ACROSS NORTHEAST COLORADO THROUGH CENTRAL NEBRASKA...INITIALLY
   MODESTLY STEEP MID-LEVEL LAPSE RATES MAY NOT BE MAINTAINED.  AND
   MODELS SUGGEST THAT SURFACE HEATING TODAY WILL BE MORE MODEST THAN
   YESTERDAY...SO UNCERTAINTY EXISTS CONCERNING HOW FAVORABLE
   THERMODYNAMIC PROFILES WILL BE FOR SEVERE WEATHER POTENTIAL. 
   STILL...GIVEN THE STRENGTHENING OF THE DEEP LAYER MEAN FLOW FIELDS
   AND SHEAR AS THE UPPER IMPULSE PROGRESSES ACROSS THE ROCKIES LATE
   THIS AFTERNOON AND EVENING... FAIRLY DEEP BOUNDARY LAYER
   MIXING...AND THE EVOLUTION OF SIZABLE SUB-CLOUD TEMPERATURE/DEW
   POINT SPREADS...SHOULD BE SUFFICIENT TO SUPPORT SEVERE WIND GUST
   POTENTIAL.

   ACROSS THE TEXAS PANHANDLE AND PARTS OF WESTERN OKLAHOMA INTO
   WESTERN KANSAS...CLOCKWISE CURVED LOW-LEVEL HODOGRAPHS ARE FORECAST
   TO BE LARGE ENOUGH TO SUPPORT POTENTIAL FOR TORNADOES...
   PARTICULARLY IN DISCRETE STORM DEVELOPMENT.  SOUTH OF THE COLD
   FRONT...THIS COULD BE THE PRIMARY STORM MODE INITIALLY. 
   HOWEVER...AS FRONTOGENETIC FORCING INCREASES ALONG THE EASTWARD AND
   SOUTHEASTWARD SURGING COLD FRONT...AN ORGANIZING LINE OF STORMS
   APPEARS POSSIBLE JUST AHEAD OF IT...FROM PARTS OF CENTRAL AND
   SOUTHERN NEBRASKA SOUTHWARD THROUGH THE TEXAS PANHANDLE THIS
   EVENING.  

   CONVECTION MAY PERSIST AHEAD OF THE FRONT ADVANCING TOWARD THE LOWER
   PLAINS OVERNIGHT.  HOWEVER...AS IT PROGRESSES THROUGH THE RELATIVELY
   NARROW CORRIDOR OF PEAK LATE AFTERNOON BOUNDARY LAYER
   DESTABILIZATION /CHARACTERIZED BY CAPE ON THE ORDER OF 1000 J PER
   KG/...SEVERE WIND POTENTIAL IS EXPECTED TO DIMINISH EAST OF THE HIGH
   PLAINS.

   ..KERR/PETERS.. 09/27/2013

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