Sep 8, 2013 1630 UTC Day 1 Convective Outlook
Updated: Sun Sep 8 16:12:36 UTC 2013 (20130908 1630Z Day 1 shapefile | 20130908 1630Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20130908 1630 UTC Day 1 Outlook Graphic
Probabilistic Tornado Graphic
20130908 1630 UTC Day 1 Tornado Probabilities Graphic
Probability of a tornado within 25 miles of a point.
Hatched Area: 10% or greater probability of EF2 - EF5 tornadoes within 25 miles of a point.
Probabilistic Damaging Wind Graphic
20130908 1630 UTC Day 1 Damaging Wind Probabilities Graphic
Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point.
Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Probabilistic Large Hail Graphic
20130908 1630 UTC Day 1 Large Hail Probabilities Graphic
Probability of hail 1" or larger within 25 miles of a point.
Hatched Area: 10% or greater probability of hail 2" or larger within 25 miles of a point.
Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SLIGHT 155,625 1,132,688 Billings, MT...Rapid City, SD...Great Falls, MT...Bismarck, ND...Aberdeen, SD...
   SPC AC 081608

   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   1108 AM CDT SUN SEP 08 2013

   VALID 081630Z - 091200Z

   ...THERE IS A SLGT RISK OF SVR TSTMS FROM PARTS OF MT/WY EWD INTO
   PORTIONS OF THE DAKOTAS...

   ...SYNOPSIS...

   AN UPPER HIGH CENTERED OVER THE CNTRL PLAINS WILL SHIFT EWD INTO THE
   OZARKS IN RESPONSE TO THE EWD PROGRESSION OF AN UPPER LOW FROM THE
   NRN INTERMOUNTAIN REGION INTO THE NRN HIGH PLAINS.  THIS PATTERN
   EVOLUTION WILL RESULT IN STRENGTHENING LARGE-SCALE FORCING FOR
   ASCENT AND MID AND UPPER-LEVEL FLOW FROM THE NRN HIGH PLAINS TOWARD
   UPPER MS VALLEY.

   IN THE LOW LEVELS...A LEE CYCLONE OVER NWRN NEB WILL DEEPEN WHILE
   DEVELOPING INTO NERN SD BY 9/12Z.  AN ASSOCIATED SURFACE FRONT
   CURRENTLY EXTENDING FROM THE LOW ESEWD THROUGH ERN NEB...SRN IA INTO
   THE OH VALLEY WILL LIFT NWD IN RESPONSE TO A STRENGTHENING LOW-LEVEL
   WAA REGIME DOWNSTREAM FROM THE MIGRATORY CYCLONE.  MEANWHILE...A
   DEVELOPING PACIFIC FRONT FROM THE NRN ROCKIES INTO GREAT BASIN WILL
   ADVANCE EWD THROUGH WRN PARTS OF THE DAKOTAS AND NEB BY THE END OF
   THE FORECAST PERIOD.

   ...PARTS OF MT/WY INTO THE DAKOTAS THIS AFTERNOON INTO TONIGHT...

   12Z SOUNDINGS AND SATELLITE-DERIVED PW IMAGERY SHOW THE CONTINUED
   PRESENCE OF A SEASONABLY MOIST AIR MASS INVOF OF STALLED SYNOPTIC
   FRONT WITH BOUNDARY LAYER DEWPOINTS IN THE 60S TO LOWER 70S AND PW
   VALUES OF 1.2-1.8 INCHES.  DECREASING MORNING CLOUDINESS COUPLED
   WITH THIS HIGH MOISTURE CONTENT AND STEEP MIDLEVEL LAPSE RATES WILL
   YIELD A MODERATELY UNSTABLE ENVIRONMENT BY AFTERNOON WITH MLCAPE
   VALUES RANGING FROM 1000-1500 J/KG OVER MT TO AS HIGH AS 2000-3000
   J/KG ACROSS SD.

   CONVECTIVE EVOLUTION WILL BE DICTATED TO SOME DEGREE BY THE AREAL
   COVERAGE AND EXTENT OF LINGERING CLOUDS.  BUT IN GENERAL...IT
   APPEARS THAT HEATING ALONG THE ELEVATED TERRAIN OF MT...WY...AND WRN
   SD...ALONG WITH INCREASING CONVERGENCE INVOF THE SYNOPTIC LOW AND
   NWD-DEVELOPING WARM FRONT WILL BE THE PRIMARY FOCI FOR STRONG TO
   SEVERE TSTM DEVELOPMENT THIS AFTERNOON INTO EVENING.  

   GIVEN THE PRESENCE OF A VERTICALLY VEERING WIND PROFILE AND
   STRENGTHENING MID TO UPPER-LEVEL FLOW --AND RESULTANT VERTICAL
   SHEAR-- INITIAL STORM MODE WILL LIKELY BE SUPERCELLULAR WITH A RISK
   FOR LARGE HAIL...DAMAGING WINDS...AND PERHAPS A BRIEF TORNADO. 
   AGGREGATING STORM-SCALE COLD POOLS AND THE DEVELOPMENT OF A
   NOCTURNAL LOW-LEVEL WIND MAXIMUM --WHICH WILL BE STRONGER AND
   BROADER THAN PAST NIGHTS-- WILL SUPPORT UPSCALE GROWTH OF STORMS
   INTO ONE OR MORE MCSS WITH THE RISK FOR DAMAGING WINDS AND HAIL
   CONTINUING INTO TONIGHT...MAINLY ACROSS NRN SD/SRN ND.

   ...OZARKS THIS AFTERNOON INTO EARLY EVENING...

   WEAK LOW-LEVEL WAA IS EXPECTED TO PERSIST TODAY...ENHANCING
   INFLOW/ASCENT ALONG THE WRN FLANK OF NOCTURNAL MCS ONGOING ACROSS
   PARTS OF CNTRL MO.  THIS SHOULD SUPPORT REGENERATIVE STORM
   DEVELOPMENT ON THE WRN END OF THE RESIDUAL MCS WHERE DAYTIME HEATING
   AND BOUNDARY LAYER DEWPOINTS OF 65-70 F WILL PROMOTE MLCAPE
   APPROACHING 1500-2500+ J/KG.  VERTICAL SHEAR WILL REMAIN RATHER
   WEAK...LIMITING THE POTENTIAL FOR STORM ORGANIZATION.
   NONETHELESS...LOCALLY DAMAGING WIND GUSTS WILL BE POSSIBLE OWING TO
   THE DEGREE OF INSTABILITY AND HIGH PW ENVIRONMENT.

   ..MEAD/LEITMAN.. 09/08/2013

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