SPC Discussion
Posted by JAC on 8/20/2009, 7:48 am
DAY 1 CONVECTIVE OUTLOOK  
  NWS STORM PREDICTION CENTER NORMAN OK
  1257 AM CDT THU AUG 20 2009
 
  VALID 201200Z - 211200Z
 
  ...THERE IS A SLGT RISK OF SVR TSTMS TODAY FROM PARTS OF THE
  SOUTHERN PLAINS NORTHEASTWARD INTO THE GREAT LAKES REGION...
 
  ...SYNOPSIS...
  MODELS SUGGEST THAT THE LARGE-SCALE UPPER TROUGH...CURRENTLY STILL
  EVOLVING OVER THE CENTRAL U.S...WILL REMAIN AMPLIFIED THROUGH THIS
  FORECAST PERIOD.  ONLY A SLOW PROGRESSION EAST OF THE MISSISSIPPI
  VALLEY IS ANTICIPATED...WITH THE NEXT SIGNIFICANT UPSTREAM TROUGH
  LIKELY TO REMAIN WEST OF PACIFIC NORTHWEST/BRITISH COLUMBIA COASTAL
  AREAS THROUGH 12Z FRIDAY.  A WEAKER IMPULSE IS PROGGED TO MIGRATE
  INLAND WITHIN SOUTHWESTERLY FLOW ACROSS THE PACIFIC NORTHWEST IN
  ADVANCE OF THE LATTER FEATURE.  AS THIS PERTURBATION LIFTS THROUGH
  THE CREST OF THE WESTERN U.S. UPPER RIDGE...SOME WEAKENING OF THE
  RIDGE MAY ENSUE...BUT A RELATIVELY DRY...GENERALLY SUBSIDENT REGIME
  APPEARS LIKELY TO CONTINUE TO MINIMIZE CONVECTIVE POTENTIAL ACROSS
  MUCH OF THE PACIFIC COAST STATES/INTERMOUNTAIN WEST AND ROCKIES.  AT
  THE SAME TIME...HIGH MOISTURE LEVELS FROM PARTS OF THE SOUTHERN
  PLAINS THROUGH MUCH OF THE EAST WILL CONTRIBUTE TO WEAK TO MODERATE
  INSTABILITY SUPPORTIVE OF WIDESPREAD THUNDERSTORM ACTIVITY...IN THE
  PRESENCE OF FAVORABLE LARGE-SCALE ASCENT AHEAD OF THE UPPER TROUGH
  AXIS.
 
  ...SRN PLAINS THROUGH THE GREAT LAKES REGION...
  WITHIN THE LARGER-SCALE UPPER TROUGH...MODELS INDICATE THAT ONE
  SIGNIFICANT SHORT WAVE IMPULSE WILL PIVOT AROUND AN EMBEDDED CLOSED
  LOW OVER THE UPPER MISSISSIPPI VALLEY.  SOME VARIABILITY EXISTS
  AMONG THE GUIDANCE...BUT THIS FEATURE IS PROGGED TO LIFT ACROSS THE
  GREAT LAKES REGION LATE THIS AFTERNOON AND EVENING...EVENTUALLY
  CONTRIBUTING TO AN EASTWARD/NORTHEASTWARD ACCELERATION OF THE CLOSED
  LOW AS IT CONTINUES INTO EASTERN ONTARIO.  MEANWHILE...ADDITIONAL
  PERTURBATIONS PROGRESSING INTO AND THROUGH THE BASE OF THE TROUGH
  ARE PROGGED TO PERIODICALLY ENHANCE DIVERGENCE ALOFT ACROSS PARTS OF
  THE OZARK PLATEAU AND RED RIVER VALLEY/ARKLATEX REGION.
 
  IN BETWEEN THE TWO AREAS OF RELATIVELY STRONGER MID/UPPER FORCING
  FOR UPWARD VERTICAL MOTION...A MOIST AND STRONGLY SHEARED
  PRE-FRONTAL ENVIRONMENT ACROSS PARTS OF THE MIDDLE MISSISSIPPI AND
  LOWER OHIO VALLEYS WILL BE SUPPORTIVE OF POTENTIAL FOR SEVERE
  STORMS.  HOWEVER...DESTABILIZATION MAY BE CONSIDERABLY INHIBITED BY
  CLOUD COVER/PRECIPITATION ASSOCIATED WITH WEAKENING OVERNIGHT
  CONVECTION.  IN THE WAKE...AND TO THE WEST/NORTHWEST...OF THIS
  ACTIVITY...THERE ARE INDICATIONS THAT CLEARING WITHIN A DRY SLOT
  ACROSS NORTHERN INDIANA INTO LOWER MICHIGAN COULD ALLOW SUFFICIENT
  HEATING FOR MODERATELY STRONG DESTABILIZATION.  WITHIN THE WARM
  SECTOR OF A DEEP SURFACE CYCLONE...THE BOUNDARY LAYER IS EXPECTED TO
  REMAIN VERY MOIST...AND LOW-LEVEL HODOGRAPHS ARE LIKELY TO BE
  CLOCKWISE CURVED AND LARGE BENEATH 30-40 KT SOUTHWESTERLY 850 MB
  FLOW.  THE ENVIRONMENT IS EXPECTED TO BECOME FAVORABLE FOR THE
  DEVELOPMENT OF DISCRETE SUPERCELLS...INCLUDING THE RISK FOR
  TORNADOES...BEFORE POSSIBLE CONSOLIDATION INTO A GROWING STORM
  CLUSTER OCCURS...AIDED BY FORCING ON THE TAIL END OF THE SHORT WAVE
  IMPULSE.
 
  LOW-LEVEL FLOW FIELDS ARE EXPECTED TO BE CONSIDERABLY WEAKER BENEATH
  THE DIVERGENT UPPER FLOW NEAR THE BASE OF THE UPPER TROUGH ACROSS
  PARTS OF NORTHERN TEXAS/SOUTHEAST OKLAHOMA/NORTHERN LOUISIANA AND
  ARKANSAS.  HOWEVER...MODERATE TO STRONG POTENTIAL INSTABILITY
  BENEATH STEEPER MID-LEVEL LAPSE RATES...COUPLED WITH FAVORABLE SHEAR
  ASSOCIATED WITH SEASONABLY STRONG NORTHWESTERLY MID-LEVEL
  FLOW...LIKELY WILL BE CONDUCIVE TO THE DEVELOPMENT OF ORGANIZED
  STORM CLUSTERS CAPABLE OF PRODUCING DAMAGING WIND GUSTS AND LARGE
  HAIL.
 
  ...NORTHERN MID ATLANTIC COAST INTO THE LOWER HUDSON VALLEY...
  DESTABILIZATION...ASSOCIATED WITH STRONG HEATING OF A MOIST BOUNDARY
  LAYER CHARACTERIZED BY SURFACE DEW POINTS NEAR 70F...IS EXPECTED TO
  BECOME SUFFICIENT FOR SCATTERED AFTERNOON STORMS... WITHIN BROAD
  SURFACE TROUGHING BENEATH DIFLUENT UPPER FLOW.  MID-LEVEL LAPSE
  RATES AND FLOW LIKELY WILL BE WEAK...BUT MODEST SOUTHERLY LOW-LEVEL
  FLOW COULD CONTRIBUTE TO THE RISK FOR POTENTIAL DAMAGING SURFACE
  GUSTS IN LOCALIZED DOWNBURSTS.
 
  ..KERR.. 08/20/2009
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06Z GFS 500MB Major Change - Cape_Fear_NC, 8/20/2009, 6:30 am
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