Severe Thunderstorm Warning 1, NWS Aberdeen, SD

AREA FORECAST DISCUSSION FOR CENTRAL/N-EAST SD AND W-CENTRAL MN
NATIONAL WEATHER SERVICE ABERDEEN SD
325 PM CDT SAT APR 13 2002

GREATEST CONCERN FOR THIS FORECAST THROUGH DAY 3 WILL BE THE
HANDLING OF SEVERAL PRECIPITATION CHANCES ACROSS CWA.  ETA AND AVN
NOT SHOWING A GREAT DEAL OF DIFFERENCES IN THE SHORT TERM...ALTHOUGH
DEVIATE SOMEWHAT IN COMPACT PACIFIC SHORT WAVE MOVING ONTO SOUTHWEST
CANADIAN AND NORTHWEST US COAST BY DAY 2.  DECIDED TO LEAN TOWARD
ETA A BIT MORE GIVEN THE ETA HAVING RESOLVED WEAK UPPER WAVE OVER
NORTHERN PLAINS A LITTLE BETTER IN TERMS OF UPPER LEVEL HEIGHT
FIELD.

SURFACE REFLECTION OF WEAK UPPER WAVE SEEN ON 19Z SURFACE ANALYSIS
WITH TROUGH EXTENDING FROM CENTRAL NORTH DAKOTA INTO SOUTHEAST SOUTH
DAKOTA.  SOME MOSTLY MID LEVEL RETURNS HAVE BEEN ASSOCIATED WITH
THIS BOUNDARY AS IT HAS MOVED ACROSS CWA THIS AFTERNOON...WITH
BETTER CONVECTION NOTED ON WSR 88-D JUST SOUTHEAST OF CWA.  IN
WESTERN CWA BEHIND BOUNDARY SKIES HAVE CLEARED IN DEEP LAYER
SUBSIDENCE.  PARTLY CLOUDY SHOULD COVER TONIGHT AS CIRRUS BEGINNING
TO ENTER WESTERN CWA AS NEXT WEAK UPPER WAVE APPROACHES CWA IN
NORTHWEST FLOW DOWNSTREAM OF DEEPER PACIFIC WAVE.  WHILE 925 HPA
SPECIFIC HUMIDITIES STAY UP IN THE 4-5 G/KG RANGE TONIGHT...MUCH
DRIER MID LEVEL AIR IS ADVECTED INTO CWA.  THUS DESPITE FORCING FROM
UPPER WAVE AND LOW LEVEL WARM ADVECTION...WILL NOT EVEN MENTION
SMALL POP.

FOR SUNDAY...NEXT SLP MINIMUM FORMS OVER SOUTHWEST CANADA IN UPPER
DIVERGENCE REGION DOWNSTREAM OF MORE IMPRESSIVE UPPER VORT MAX.
FURTHER SOUTH...LOW LEVEL TROUGHING NOTED IN STRONG ZONAL FLOW
COMPONENT OVER CENTRAL ROCKIES.  THIS WILL RESULT IN GOOD SHOT OF
WARM ADVECTION TOMORROW AFTERNOON ESPECIALLY ACROSS WESTERN CWA
WHERE AFTERNOON HIGHS COULD REACH INTO MID 80S.  LIFTED INDICES WITH
850 HPA BASED SURFACE PARCEL NEAR ZERO ACROSS MUCH OF CWA.  MESOETA
FORECAST SOUNDINGS SHOW DEEP MIXED LAYER TOMORROW AFTERNOON.  GIVEN
DEEP MIXED LAYER AND FAIRLY DRY MID LEVELS REMAINING INTO TOMORROW
AFTERNOON FEEL THAT MOST OF CWA WILL REMAIN DRY.  WILL INCLUDE SMALL
POP IN WESTERN CWA WITH SLIGHTLY HIGHER LOW LEVEL SPECIFIC
HUMIDITIES BY LATE TOMORROW AFTERNOON.  STEEP MID LEVEL LAPSE RATES
PERSIST INTO TOMORROW NIGHT AND COMBINED WITH DECENT LOW LEVEL
FRONTOGENESIS AND APPROACHING FRONTAL ZONE SHOWN BY ETA...FEEL
CONVECTIVE POTENTIAL WILL BE ENOUGH FOR MENTION OF SMALL POP FOR
SHOWERS AND THUNDERSTORMS ACROSS MOST OF CWA SUNDAY NIGHT.

COLD ADVECTION IN LOW LEVELS LATE SUNDAY NIGHT INTO EARLY MONDAY
SHOULD RESULT IN AFTERNOON HIGHS NEAR SEASONAL LEVELS FOR MONDAY.
FAIRLY SIGNIFICANT RAIN EVENT LOOKS POSSIBLE LATE MONDAY INTO
TUESDAY AS EMBEDDED SHORT WAVE IN LONG WAVE TROUGH PATTERN EJECTS
NORTHEAST INTO CWA.  AVN/ETA/CANADIAN ALL FAIRLY CONSISTENT IN
SHOWING SLP MINIMUM JUST SOUTH OF CWA WITH SUBSTANTIAL QPF FIELDS
MAINLY ACROSS NORTHERN CWA ASSOCIATED WITH STRONGER ISENTROPIC LIFT
REGION.  BOUNDARY LAYER VERY MOIST BY 18Z TUESDAY WITH SURFACE DEW
POINTS FROM AVN IN UPPER 50S TO NEAR 60 ACROSS EASTERN CWA.  WILL
INCLUDE CHANCE OF THUNDERSTORMS GIVEN THE FRONTAL NATURE OF FORCING
AND MOIST LOW LEVELS.

.EXTENDED DISCUSSION...
EXTENDED MDLS ALL SHOW GOOD 50H TROUGHING DEVELOPING ACROSS THE
WESTERN UNITED STATES EARLY THROUGH MID WEEK WITH S/WS EJECTING OUT
OF THIS TROUGH ACROSS THE CENTRAL AND NORTHERN PLAINS. AFTER THE
FIRST S/W MOVES AWAY FROM SD TUESDAY NIGHT...SFC HIGH PRES WILL
BUILD IN TUESDAY NIGHT AND MOVE SLOWLY EAST THROUGH WED. AS ANOTHER
50H S/W AND ITS ASSOCIATED SFC LOW PRES MOVES EAST ACROSS THE
ROCKIES AND ACROSS THE C AND N PLAINS WED AFTN THROUGH
THURS...A WARM FRONT WILL LIFT NORTHWARD WED NIGHT WHILE A COLD
FRONT MOVES IN FROM THE NW LATER WED NIGHT THROUGH THURSDAY. IT
SHOULD BE CLEAR TO P/C...DRY...WITH NEAR TO ABOVE NORMAL TEMPS
ACROSS THE CWA ON WED. THE SFC LOW PRES ALONG WITH THE WARM AND COLD
FRONTS WILL BRING A CHC OF SHRAS AND TSRAS TO THE CWA WED NIGHT AND
THURS. OTHERWISE...AFTER +4 TO +9 85H TEMPS ACROSS THE CWA ON
WED...85H TEMPS COOL TO -2 TO +2 ON THURS. IT SHOULD BE MOSTLY CLOUDY
ON THURS WITH COOLER SFC TEMPS ACROSS THE CWA. SKIES SHOULD GO P/C
THURS NIGHT. WITH -6 TO -8 85H TEMPS ACROSS THE CWA ON FRI...IT WILL
BE A COOL DAY WITH ALOT OF COLD AIR CU FORMING. ON SAT...THE COOLER
AIR BEGINS MOVING EAST WHILE A WARM FRONT MOVES IN FROM THE WEST AND
WINDS BECOME SW. THEREFORE...IT SHOULD BE DRY ON SAT WITH P/C SKIES
AND WARMER TEMPS.

.ABR...NONE.
MARSILI/MOHR