SPC AC 170551

   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   1251 AM CDT SUN MAY 17 2015

   VALID 171200Z - 181200Z

   ...THERE IS AN ENH RISK OF SVR TSTMS ACROSS THE UPPER MS VALLEY...

   ...THERE IS A SLGT RISK OF SVR TSTMS FROM NRN MN TO AR...

   ...THERE IS A MRGL RISK OF SVR TSTMS FROM SWRN TX TO WRN LOWER MI...

   ...SUMMARY...
   SCATTERED SEVERE THUNDERSTORMS ARE EXPECTED TO DEVELOP FROM THE
   UPPER MISSISSIPPI VALLEY SOUTHWARD ACROSS ILLINOIS INTO ARKANSAS. 
   MORE ISOLATED ACTIVITY SHOULD EVOLVE ACROSS TEXAS.  LARGE
   HAIL...DAMAGING WINDS...AND PERHAPS A FEW TORNADOES ARE
   POSSIBLE...ESPECIALLY FROM SOUTHEASTERN MINNESOTA INTO NORTHERN
   ILLINOIS.

   ...UPPER MS VALLEY/GREAT LAKES...

   90M 12HR HEIGHT FALLS WILL SPREAD ACROSS THE UPPER MS VALLEY DURING
   THE DAY SUNDAY AS STRONG MID-LEVEL SPEED MAX OVER OK LIFTS NNEWD
   INTO SERN MN BY MID AFTERNOON.  THIS FEATURE IS EXPECTED TO FOCUS
   SFC LOW OVER THE ERN DAKOTAS...WITH SLIGHT DEEPENING OVER SERN ND BY
   PEAK HEATING.  17/00Z SHORT-RANGE MODEL GUIDANCE IS IN FAIRLY GOOD
   AGREEMENT ALLOWING A SUBSTANTIAL MID-LEVEL DRY SLOT TO ADVECT INTO
   IA BY MID DAY AND SIGNIFICANT BOUNDARY LAYER WARMING SHOULD BE NOTED
   ACROSS IA INTO SERN MN WHERE SFC-3KM LAPSE RATES SHOULD STEEPEN
   PRIOR TO CONVECTIVE DEVELOPMENT.  CURRENT THINKING IS NAM GUIDANCE
   GENERATES TOO MUCH INSTABILITY ACROSS THE WARM SECTOR AS BOTH THE
   GFS AND PARALLEL RUN OF THE NAM ARE CONSIDERABLY LESS BUOYANT WITH
   AIR MASS ACROSS MN/IA/WI.  EVEN SO...SCT THUNDERSTORMS ARE EXPECTED
   TO DEVELOP BY EARLY AFTERNOON WITHIN A STRONGLY SHEARED ENVIRONMENT
   THAT WOULD SEEMINGLY FAVOR SUPERCELL DEVELOPMENT...AND PERHAPS EVEN
   A FEW TORNADOES.  HOWEVER...LARGE HAIL AND DAMAGING WIND SHOULD BE
   THE PRIMARY THREATS WITH ORGANIZED CONVECTION THAT EVOLVES ACROSS
   THE UPPER MS VALLEY REGION.

   ...SRN IL TO TX...

   FATHER SOUTH...CONSIDERABLY WEAKER LAPSE RATE ENVIRONMENT WILL
   EXTEND ACROSS IL INTO AR ALONG TRAILING PRE-FRONTAL CONFLUENT ZONE. 
   MODEST-STRONGLY SHEARED AND MOIST AIR MASS SHOULD SUPPORT ISOLATED
   THUNDERSTORM DEVELOPMENT...ESPECIALLY ALONG REMNANT CONVECTIVE
   BOUNDARIES WHERE POCKETS OF HEATING ARE OBSERVED.

   EARLY THIS MORNING A WELL-DEFINED SQUALL LINE HAD DEVELOPED FROM WRN
   MO...SWWD INTO SWRN TX.  THIS MCS SHOULD ADVANCE EWD TOWARD THE
   LOWER OH VALLEY/ARKLATEX REGION WHERE WEAKENING IS EXPECTED.  STRONG
   HEATING IN ITS WAKE ACROSS WCNTL TX MAY WEAKEN CAP SUFFICIENTLY FOR
   PARCELS TO REACH THEIR CONVECTIVE TEMPERATURES.  HOWEVER...WEAK
   CONVERGENCE APPEARS INADEQUATE FOR MORE THAN A FEW STORMS WITHIN A
   MODESTLY SHEARED ENVIRONMENT THAT WOULD FAVOR STORM ROTATION AND
   HAIL DEVELOPMENT.  FOR THIS REASON WILL MAINTAIN MRGL SEVERE THREAT
   INTO SWRN TX DUE TO EXPECTED LIMITED STORM COVERAGE.

   ..DARROW.. 05/17/2015

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