SPC AC 190456

   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   1156 PM CDT THU SEP 18 2014

   VALID 191200Z - 201200Z

   ...NO SVR TSTM AREAS FORECAST...

   ...SUMMARY...
   A FEW THUNDERSTORMS MAY BECOME MARGINALLY SEVERE FROM EASTERN
   PORTIONS OF NEBRASKA AND SOUTH DAKOTA NORTHEASTWARD INTO PORTIONS OF
   MICHIGAN FOR THIS EVENING INTO TONIGHT.

   ...SYNOPSIS...
   THE SRN INFLUENCE OF THE POLAR MID/UPPER-LEVEL STREAM WILL GLANCE
   THE NRN STATES WITH EMBEDDED/PROGRESSIVE LOW-AMPLITUDE PERTURBATIONS
   FROM THE NRN PLAINS TO THE NE STATES. MEANWHILE...A MID-LEVEL
   CYCLONIC CIRCULATION NEAR THE CA COAST WILL BECOME INCREASINGLY
   DISASSOCIATED FROM THE AFOREMENTIONED STREAM...AS A RIDGE BUILDS
   OVER THE PACIFIC NW. AT THE SFC...A MODEST COLD FRONT WILL ADVANCE
   SEWD ACROSS PORTIONS OF THE NRN/CNTRL PLAINS TO THE UPPER MS
   VALLEY...WHILE AN ANTICYCLONE COVERS PORTIONS OF THE NERN STATES.

   ...ERN PORTIONS OF NEB/SD NEWD INTO PORTIONS OF MI...
   A QUASI-ZONAL SPEED MAX AT THE SRN EDGE OF THE POLAR STREAM WILL
   ADVANCE FROM THE NRN PLAINS TO THE ERN GREAT LAKES REGION. SHOWERS
   AND ISOLATED THUNDERSTORMS WILL LIKELY BE SPREADING EWD AT THE
   BEGINNING OF THE PERIOD FROM PORTIONS OF ND INTO MN...IN RESPONSE TO
   ASCENT ACCOMPANYING THE SPEED MAX AND WAA RELATED TO A SWLY LLJ. THE
   LLJ WILL PERSIST THROUGH THE PERIOD WHILE GRADUALLY SHIFTING
   EWD...ROUGHLY EXTENDING FROM THE CNTRL PLAINS TO THE GREAT LAKES
   REGION. THE COMBINATION OF WARMING ALOFT ASSOCIATED WITH
   LLJ-ENHANCED WAA AND THE INFLUX OF AN EML FROM THE WRN-CONUS PLATEAU
   WILL ENCOURAGE STRONG CAPPING AMIDST A PRE-FRONTAL WARM SECTOR.
   DESPITE RICH BOUNDARY-LAYER MOISTURE AHEAD OF THE FRONT CONTRIBUTING
   TO STRONG CONDITIONAL INSTABILITY FROM PORTIONS OF ERN NEB/SD INTO
   SRN MN...ASCENT ALONG/AHEAD OF THE COLD FRONT IS NOT EXPECTED TO
   INCREASE SUFFICIENTLY FOR MORE ROBUST CONVECTIVE DEVELOPMENT UNTIL
   THE EVENING HOURS.

   BY EARLY EVENING...MODEST MID-LEVEL HEIGHT FALLS SHOULD BECOME MORE
   PREVALENT ACROSS THE N-CNTRL STATES DOWNSTREAM OF A SHORTWAVE
   IMPULSE CROSSING THE CANADIAN PRAIRIE PROVINCES. RELATED COOLING
   ALOFT MAY BE SUFFICIENT FOR LOW-LEVEL PARCELS TO BREACH A WEAKENING
   CAP AS FRONT/LLJ-ENCOURAGED ASCENT CONTINUES. THIS SHOULD SUPPORT
   TSTM DEVELOPMENT/INTENSIFICATION FROM PORTIONS OF ERN NEB/SD NEWD TO
   PORTIONS OF MI INTO THE OVERNIGHT HOURS. CLOUD-BEARING SHEAR AND
   MODERATELY STEEP MID-LEVEL LAPSE RATES MAY SUPPORT ISOLATED
   INSTANCES OF MARGINALLY SVR HAIL WITH CONVECTION...WHICH IS
   GENERALLY EXPECTED TO BE ROOTED ATOP A NOCTURNALLY STABILIZING
   BOUNDARY LAYER. THERE MAY ALSO EXIST A BRIEF WINDOW OF OPPORTUNITY
   FOR NEAR-SFC-BASED CONVECTION TO EVOLVE FROM ERN NEB/SERN SD INTO
   SRN MN/NRN IA THROUGH MID-EVENING. THIS REGIME COULD FEATURE
   RELATIVELY WARMER SFC CONDITIONS WELL S OF EARLIER
   PRECIPITATION...TO LOCALLY ENHANCE REDUCTIONS TO SFC-BASED
   CINH...AND SUPPORT ISOLATED DMGG WIND GUSTS. CONFIDENCE IN
   GREATER-COVERAGE SVR-TSTM POTENTIAL OWING TO ANTECEDENT CAPPING
   PRECLUDES HIGHER PROBABILITIES AT THIS TIME.

   ...NERN ND AND NRN MN...
   A FEW STRONG TO PERHAPS LOCALLY/MARGINALLY SVR STORMS CANNOT BE
   RULED OUT ACROSS THE AREA IN THE WAKE OF EARLIER-DAY
   PRECIPITATION...AND AHEAD OF A MID-LEVEL VORT MAX CROSSING THE
   INTERNATIONAL BORDER REGION. MODERATE DEEP SHEAR WILL PERSIST.
   UNCERTAINTY IN THE EVOLUTION OF THE PRECEDING PRECIPITATION EXTENDS
   TO THE PROPENSITY FOR LATER-DAY DESTABILIZATION TO SUPPORT A
   SVR-TSTM RISK. AS SUCH...SVR-TSTM PROBABILITIES HAVE NOT BEEN
   INCLUDED AT THIS TIME.

   ..COHEN/DARROW.. 09/19/2014

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