Severe Thunderstorm Warning 88, NWS Aberdeen, SD

AREA FORECAST DISCUSSION FOR CENTRAL/N-EAST SD AND W-CENTRAL MN
NATIONAL WEATHER SERVICE ABERDEEN SD
330 PM CDT SUN JUL 7 2002

IMMEDIATE FORECAST FOCUS WILL BE ON CONVECTIVE CHANCES TONIGHT...AND
THEN AGAIN TUESDAY/WEDNESDAY.  AVN AND ETA HANDLING MID LEVEL WAVE
THROUGH 06Z WITH ETA TENDING TOWARD MORE POSITIVE TILT...AND BASED
ON TRENDS IN WATER VAPOR IMAGERY OVER PAST 3-5 HOURS THE ETA SEEMS
TO HAVE A BETTER HANDLE ON THIS EVOLUTION..BUT SURFACE FEATURES NOT
SIGNIFICANTLY DIFFERENT.  AVN AND ETA DIVERGE TO A GREATER DEGREE
FROM 54 TO 80 HOURS INTO THIS FORECAST WITH NEXT UPPER WAVE RIDING
OVER PERSISTENT WESTERN US RIDGE.

SURFACE ANALYSIS FROM 18Z SHOWING SLP MINIMUM ACROSS CENTRAL SOUTH
DAKOTA WITH SLP TROUGH EXTENDING NORTHEAST INTO SOUTHEAST NORTH
DAKOTA.  LINGERING -SHRA ACROSS FAR EASTERN CWA...WITH MORE
SIGNIFICANT RETURNS IN SOUTHEAST NORTH DAKOTA AT THIS TIME.  WATER
VAPOR IMAGERY SHOWING UPPER SHORT WAVE APPROACHING WESTERN SOUTH
DAKOTA.  LAPS DATA SHOWING ONLY MODERATE AMOUNTS OF INSTABILITY
ACROSS CWA DUE TO LACK OF SURFACE HEATING THIS AFTERNOON.  EXCEPTION
TO THIS IS ACROSS SOUTHWEST CWA AND IN WEST CENTRAL MINNESOTA WHICH
HAVE SEEN A BIT MORE IN THE WAY OF SURFACE HEATING.  HAVE ALREADY
SEEN SOME CONVECTION REDEVELOP EXTENDING FROM JUST EAST OF KMBG TO
AROUND KPIR.  AS 700 HPA WAVE MOVES INTO SOUTH CENTRAL ND THIS
EVENING EXPECT LOW LEVEL CONVERGENCE TO BECOME MORE SUBSTANTIAL WITH
INCREASED CONVECTIVE COVERAGE...ESPECIALLY IN VICINITY OF THE
AFOREMENTIONED BOUNDARY.  ALTHOUGH SURFACE BASED INSTABILITY WILL
ONLY BE MODEST AT AROUND 1000-1500 J/KG...SHEAR PROFILES ACROSS
EASTERN CWA BECOME MORE IMPRESSIVE BY 00Z ASSOCIATED WITH A STRONGER
SOUTHWEST 700 HPA WIND IN ADVANCE OF SHORT WAVE.  THUS...FEEL THAT
EASTERN CWA WILL SEE GREATEST COVERAGE TONIGHT.  HEAVY RAINFALL
STILL A CONCERN DESPITE STRONGER STEERING FLOW...WITH PWATS ON THE
ORDER OF 1.5 TO 1.8 INCHES.

BEST LOW LEVEL WARM ADVECTION MOVES EAST OF CWA BY MONDAY MORNING
AND WILL CARRY ONLY A SMALL POP FOR FAR EASTERN CWA FOR THE EARLY
MORNING.  DRIER MID LEVEL AIR MOVES INTO CWA FOR MONDAY AFTERNOON AS
700 HPA RIDGE APPROACHES CWA IN ADVANCE OF NEXT SHORT WAVE OF
CONCERN.  MONDAY WILL BE DRY WITH DEWPOINTS FALLING BACK INTO LOW
AND MID 60S.  WILL CARRY CHANCE POPS FOR TUESDAY AFTERNOON THROUGH
WEDNESDAY EVENING AS NEXT UPPER SHORT WAVE IS SLOW TO MOVE THROUGH
NORTHERN PLAINS.  BOTH AVN AND ETA SHOWING STRONG MID LEVEL
FRONTOGENESIS ACROSS EXTREME WESTERN SOUTH DAKOTA WHICH WEAKENS AS
700 HPA WAVE MOVES EASTWARD.  FEEL THAT AVN IS TOO FAR SOUTH AND TOO
AGGRESSIVE IN BREAKING DOWN WESTERN US RIDGE AND WILL LEAN MORE
TOWARD TIMING OF THE CANADIAN MODEL. MARSILI

.EXTENDED DISCUSSION...
EXTENDED MODEL GUIDANCE CONTINUES TREND OF DEEP TROUGH TO THE EAST
WITH A RIDGE TO THE WEST. ONLY HINT OF ANY PCPN IS ACROSS THE FAR
EAST EARLY DAY 4 AND THEN AGAIN ACROSS THE NORTH LATE IN DAY 7 AS
THE RIDGE IS FLATTENED BY A FAIRLY VIGOROUS SHORT WAVE RIDING OVER
IT. ENSEMBLE IN VERY GOOD AGREEMENT WITH PLACEMENT OF THE RIDGE
THROUGH THE EXTENDED. ONLY DISAGREEMENT SEEMS TO BE HOW SHARP THE
EASTERN TROUGH WILL BE. ETAX THROUGH 84 HOURS IS MORE PROGRESSIVE
THAN AVN WITH SHORT WAVE MOVING OUT OF THE CWA BY EARLY THURSDAY.
WITH SHORT RANGE FORECAST MORE IN LINE WITH THE ETAX WILL ONLY
MENTION CLOUDS EARLY THURSDAY ACROSS THE FAR EASTERN CWA. THE
RIDGING THROUGH EARLY SUNDAY. AS NEXT SHORT WAVE MOVES JUST NORTH OF
THE CWA SUNDAY WOULD EXPECT AN INCREASE IN CLOUDS. WILL NOT MENTION
PCPN FOR NOW AS ENSEMBLES POPS STILL QUITE LOW. WITH RIDGING TO THE
WEST WILL GO A BIT ABOVE MODEL GUIDANCE TEMPS FOR THE PERIOD.
FAUCETTE

.ABR...NONE.