Flash Flooding in South Dakota, August 20 to 21, 2002

AREA FORECAST DISCUSSION FOR CENTRAL/N-EAST SD AND W-CENTRAL MN
NATIONAL WEATHER SERVICE ABERDEEN SD
315 PM CDT TUE AUG 20 2002

OBVIOUSLY CONVECTION IS THE MAIN FORECAST ISSUE THROUGH THE SHORT
RANGE. INTENSE ELEVATED WAA NORTH OF WARM FRONT SITUATED ACROSS
SOUTHERN SOUTH DAKOTA HAS ALLOWED FOR ELEVATED HAILERS TO DEVELOP
OVER CWA.  MODELS CONTINUE TO ADVERTISE THAT THIS WILL LAST INTO THE
EVENING AS WE WAIT FOR MAIN S/W OVER COLORADO TO LIFT NE INTO THE
PLAINS.  THIS ENTIRE CONVECTIVE EVENT WILL LIKELY TRANSITION INTO A
HEAVY RAINFALL EVENT.  BUT EXACTLY WHERE?  MODELS ARE ALL OVER THE
PLACE FROM RUN TO RUN WITH SIGNFICIANT CONVECTIVE FEEDBACK ISSUES
LITERALLY MAKING IT TOUGH TO PIN DOWN THE AREAL THREAT.  FOR NOW
WILL STICK WITH A HEAVIEST RAIN AXIS JUST SOUTH OF CWA.  BUT THIS
WILL HAVE TO BE MONITORED.  GOOD NEWS IS THAT MOST OF CWA
HAS RELATIVELY HIGH FFG VALUES BECAUSE OF A LACK OF WIDESPREAD HEAVY
RAIN IN THE LAST COUPLE WEEKS.  REST OF FORECAST RATHER MUDDY AS
BOUNDARY AGAIN SINKS FURTHER SOUTH AND BOUNDARY LAYER WINDS SHIFT TO
EASTERLY...WHICH IN ITSELF WILL PROBABLY MAINTAIN COOLER TEMPS AND
MORE CLOUDS THAN PREVIOUSLY THOUGHT.  FLOW REMAINS WEAKLY SOUTHWEST
ALOFT AND THEREFORE WILL BE HARD TO TIME PCPN EVENTS.  FOR NOW WILL
GO MORNING TSRA EAST WED WITH AFTERNOON POSSIBILITY WEST.  KEPT LOW
POPS ACROSS ENTIRE CWA WED NIGHT BUT BEST EVENT LIKELY OVER FAR
WEST/SW CWA. ONLY MADE MINOR MODIFICATIONS TO THE REST OF THE
FORECAST.  ALL MODELS HAVE TRENDED TO MORE RIDGING OVER THE CENTRAL
PLAINS BY LATE WEEK VS THE EARLIER SOLUTIONS OF TROF DROPPING INTO
EASTERN PLAINS/LAKES WITH RIDGING OVER ROCKIES.

TDK

.EXTENDED DISCUSSION...

EXPECTING A FAIRLY QUIET EXTENDED PERIOD WITH HIGH PRESSURE
DOMINATING AT BOTH THE SURFACE AND ALOFT DURING THE FIRST THREE
DAYS. THE RIDGE STARTS TO SHIFT TOWARD THE GREAT LAKES ON
TUESDAY...WITH A WEAK TROUGH SETTING UP OVER THE ROCKIES...AND AN
UPPER TROUGH DIGGING DOWN THE WEST SIDE OF THE ROCKIES. WILL KEEP
CONDITIONS DRY AND TEMPERATURES NEAR NORMAL...AS H85 TEMPS REMAIN IN
THE +10 TO +16 DEGREE RANGE.

AP

.ABR...NONE.

MNZ039-046-SDZ004>011-016>023-210030-

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ABERDEEN SD
700 PM CDT TUE AUG 20 2002

...SEVERE THUNDERSTORM WATCH 624 HAS BEEN CANCELLED ACROSS SECTIONS
OF NORTHCENTRAL AND NORTHEASTERN SOUTH DAKOTA...

THUNDERSTORMS ARE DIMINISHING ACROSS NORTHCENTRAL AND NORTHEAST
SOUTH DAKOTA...AND THE SEVERE THREAT HAS DIMINISHED.

IT IS POSSIBLE THAT THUNDERSTORMS MAY REDEVELOP LATER THIS EVENING
TOWARD THE OVERNIGHT HOURS.

HINTZ