Severe Thunderstorm Warning 133, NWS Aberdeen, SD

AREA FORECAST DISCUSSION FOR CENTRAL/N-EAST SD AND W-CENTRAL MN
NATIONAL WEATHER SERVICE ABERDEEN SD
406 AM CDT THU AUG 8 2002

MODELS CONTINUE TO BE FAIRLY CONSISTENT WITH THE HANDLING OF UPPER
AND SURFACE FEATURES WITH SYSTEM TO AFFECT CWA THROUGH FIRST HALF OF
WEEKEND.  TIMING OF SURFACE FRONTAL PASSAGE IN DECENT AGREEMENT
BETWEEN ETA AND AVN WITH ETA SLIGHTLY FASTER IN TAKING UPPER HEIGHT
MINIMUM EAST OF CWA ON SATURDAY NIGHT.  DIFFERENCES ARE MINOR BUT
WILL PREFER THE MORE CONSISTENT AVN IN THE DETAILS OF THIS UPPER
WAVE LATER IN THE FORECAST PERIOD TOWARD FRIDAY NIGHT AND SATURDAY.

MAIN CONVECTIVE ACTIVITY THIS MORNING HAS NOW MOVED INTO NORTH
DAKOTA IN THE AREA OF STRONGEST LOW LEVEL WARM ADVECTION AND
FRONTOGENESIS FORCING.  WEAK RETURNS ALSO NOTED ACROSS EASTERN CWA
LIKELY ASSOCIATED WITH WEAK 700-500 HPA WAVE AND LOW LEVEL THETA-E
ADVECTION.  EXPECT THIS AREA TO DIMINISH TOWARDS SUNRISE AS MID
LEVEL FORCING WEAKENS AND LOW LEVEL FLOW REDUCES.  07Z SURFACE
ANALYSIS SHOWING SLP MINIMUM LOCATED JUST NORTH OF KBIS WITH SURFACE
BOUNDARY EXTENDING SOUTHWARD INTO SOUTH CENTRAL SOUTH DAKOTA.
RATHER TRICKY TEMPERATURE FORECAST THIS AFTERNOON TO EXACTLY WHERE
THIS BOUNDARY SETS UP...BUT WITH GOOD MIXING AND DEEP MIXED LAYER
INDICATED ON MODEL FORECAST SOUNDINGS WILL EXPECT THE ENTIRE WESTERN
HALF OF CWA TO REACH INTO THE 90S.  LITTLE MOVEMENT IN SURFACE
BOUNDARY EXPECTED THROUGH THE AFTERNOON WITH MID AND UPPER FLOW MORE
PARALLEL TO THE BOUNDARY.  ETA SHOWING INSTABILITY AXIS DEVELOPING
JUST TO THE EAST OF THIS WIND SHIFT LINE THIS AFTERNOON WITH SURFACE
BASED CAPES ON THE ORDER OF 1500 G/KG.  WILL GO WITH SLIGHTLY HIGHER
POPS ACROSS THE WEST THIS AFTERNOON WHERE MID LEVEL TEMPERATURES
WILL BE SLIGHTLY COOLER.  WHERE THIS INSTABILITY AXIS EXISTS THIS
AFTERNOON...LOW LEVEL AND DEEP LAYER SHEAR NOT ALL THAT IMPRESSIVE
AND EXPECT MORE SCATTERED NATURE TO CONVECTION THIS AFTERNOON.

THINGS GET A LITTLE MORE INTERESTING TONIGHT AS STRONG UPPER WAVE
BEGINS TO APPROACH MONTANA/WYOMING.  BROAD AREA OF DIVERGENCE IN
UPPER DIFFULENT FLOW DOWNSTREAM OF TROUGH AXIS SHOULD ENHANCE LOW
LEVEL FRONTOGENETIC LIFT ASSOCIATED WITH LOW LEVEL CONVERGENCE
ENHANCED BY THE REDEVELOPMENT OF 40 KNOT LLJ.  SOMEWHAT WORRIED
ABOUT POTENTIAL FOR HEAVY RAIN.  NOTHING REALLY TO MOVE SURFACE
BOUNDARY SIGNIFICANTLY UNTIL UPPER WAVE PUSHES IN DURING FRIDAY
AFTERNOON WHEN MID AND UPPER FLOW BECOMES MORE PERPENDICULAR TO THE
BOUNDARY.  ALSO PWATS HOVER AROUND 1.5 TO 1.7 THROUGH FRIDAY
AFTERNOON.  AS UPPER LOW MIGRATES INTO CWA SHOULD BE ENOUGH
INSTABILITY TO KICK OFF -SHRA OR -TSRA FRIDAY NIGHT INTO SATURDAY
MORNING.  PREVIOUS FORECAST HAS THIS WELL IN HAND.  EXTENDED LOOKS
TO BE IN FINE SHAPE TOO AND MADE NO CHANGES.

.ABR...NONE

MARSILI