Very large unstable air-mass on Friday
Posted by JAC on 6/18/2009, 7:48 am





DAY 2 CONVECTIVE OUTLOOK 
  NWS STORM PREDICTION CENTER NORMAN OK
  1240 AM CDT THU JUN 18 2009
 
  VALID 191200Z - 201200Z
 
  ...THERE IS A MDT RISK OF SVR TSTMS FROM PORTIONS OF IA/MO...EWD
  INTO OH...
 
  ...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MODERATE RISK
  FROM THE SRN PLAINS TO WV/WRN PA...
 
  ...MID MS/OH VALLEY...
 
  A POTENTIALLY SIGNIFICANT SEVERE WEATHER EVENT COULD EVOLVE DURING
  THE DAY2 PERIOD FROM PORTIONS OF THE MID MS VALLEY ACROSS THE OH
  VALLEY REGION AS STRENGTHENING FLOW ALOFT SPREADS ATOP A VERY
  UNSTABLE AIRMASS.  A POTPOURRI OF HAZARDOUS WEATHER CAN BE EXPECTED
  WITH LARGE HAIL...DAMAGING WINDS AND PERHAPS TORNADOES.
 
  LATE EVENING MODEL GUIDANCE CONTINUES EARLIER TRENDS OF A
  SIGNIFICANT SHORTWAVE TROUGH EJECTING ACROSS MN/IA/MO DURING THE
  DAYLIGHT HOURS.  NAM IS A BIT MORE AMPLIFIED WITH THIS FEATURE...AND
  500MB FLOW IS STRONGER...ON THE ORDER OF 70KT...THAN THE GFS.  EVEN
  SO BOTH MODELS SUGGEST THUNDERSTORMS WILL EASILY DEVELOP IN ADVANCE
  OF SHORTWAVE...INITIALLY OVER MN/IA...POSSIBLY BY 18Z...THEN SWD
  INTO MO AS BOUNDARY LAYER WARMS.  THESE UPDRAFTS WILL QUICKLY BECOME
  SEVERE AS BOTH LARGE SCALE ASCENT/SHEAR AND INSTABILITY WILL BE VERY
  SUPPORTIVE OF ORGANIZED UPDRAFTS.  FORECAST SOUNDINGS BY EARLY
  AFTERNOON SUGGEST MLCAPE VALUES AOA 3500 J/KG WITH DEEP LAYER SHEAR
  IN EXCESS OF 50KT AHEAD OF ADVANCING WIND SHIFT.  ALTHOUGH WARM
  ADVECTION WILL LIKELY DRIVE AN MCS EARLY IN THE PERIOD ALONG/NORTH
  OF WARM FRONT ACROSS WI/MI...THE PRIMARY SEVERE THREAT WILL OCCUR
  WITH ACTIVITY THAT EVOLVES WITHIN THE WARM SECTOR DURING THE
  AFTERNOON/EVENING...AND PERHAPS THE OVERNIGHT HOURS.  SUPERCELL
  CLUSTERS SHOULD DEVELOP BY EARLY AFTERNOON WITHIN RAPIDLY WARMING
  BOUNDARY LAYER THEN MATURE INTO A POTENTIAL DAMAGING SQUALL LINE
  WITH SEVERE WIND GUSTS...OR MULTIPLE MCS/S...AS SPEED MAX EJECTS
  ACROSS THE OH VALLEY TOWARD WRN PA/WV BY 20/12Z.  GIVEN THE
  STRENGTHENING...DEEPENING WLY FLOW ACROSS THE OH VALLEY...IT SEEMS
  REASONABLE THAT DAMAGING SQUALL LINE/S COULD RACE ACROSS IND/OH AND
  POTENTIALLY SPREAD INTO/ACROSS THE HIGHER TERRAIN OF WRN PA/WV
  BEFORE SUNRISE.  LARGE HAIL AND EVEN A FEW TORNADOES MAY ALSO BE
  NOTED WITH THESE SEVERE STORMS...ESPECIALLY GIVEN THE HIGH
  INSTABILITY AND VERY MOIST AIRMASS IN PLACE.
 
  DOWNSTREAM...NAM/GFS DIFFER CONSIDERABLY REGARDING THE INSTABILITY
  EXPECTED DURING THE DAYLIGHT HOURS ACROSS THE MIDDLE ATLANTIC REGION
  FROM MD INTO VA.  GFS DESTABILIZES WRN PORTIONS OF VA CONSIDERABLY
  BY LATE AFTERNOON WHILE NAM MAINTAINS WEAKER LAPSE RATES...POSSIBLY
  DUE TO MORE CLOUDS AND OVERTURNING FROM EARLY MORNING PRECIPITATION.
  FOR THIS REASON WILL MAINTAIN LOW PROBABILITIES ACROSS THIS REGION.
  HOWEVER...VERTICAL SHEAR WILL CERTAINLY SUPPORT ORGANIZED UPDRAFTS
  IF THEY DEVELOP DURING THE DAY...WHILE THERE IS A SMALL CHANCE FOR
  OH VALLEY MCS TO APPROACH THIS REGION LATE.
 
  ...PLAINS...
 
  FARTHER SW...STRONG BOUNDARY LAYER HEATING WILL REMOVE INHIBITION
  ALONG ADVANCING FRONTAL ZONE ACROSS SRN KS/NRN OK FRIDAY.  SFC
  TEMPERATURES WILL SOAR WELL INTO THE 90S...AND WITH PWAT VALUES
  POSSIBLY ON THE ORDER OF 1.5-1.75 INCHES...MODEST SWLY FLOW SHOULD
  SUPPORT A FEW SEVERE THUNDERSTORMS INTO THE EVENING HOURS.
 
  ..DARROW.. 06/18/2009
 
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Possible Severe Weather this Weekend for Great-Lakes & Ontario - JAC, 6/15/2009, 7:29 am
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