Severe Thunderstorm Warning 177, NWS Aberdeen, SD

AREA FORECAST DISCUSSION FOR CENTRAL/NORTHCENTRAL AND NORTHEAST
SOUTH DAKOTA AND WEST CENTRAL MINNESOTA
NATIONAL WEATHER SERVICE ABERDEEN SD
320 PM CDT WED AUG 8 2001

MAIN FORECAST CHALLENGE IS PRECIP CHANCES TONIGHT WITH THE PASSAGE
OF THE COLD FRONT. 19Z SURFACE ANALYSIS SHOWED THE COLD FRONT
EXTENDING FROM NORTH CENTRAL MINNESOTA TO NORTHWESTERN SOUTH DAKOTA
TO NORTHERN UTAH. HIGH PRESSURE WAS CENTERED OVER SOUTHWESTERN
CANADA. THERE WERE NO MAJOR DIFFERENCES NOTED IN THE MODELS WITH
THEIR HANDLING OF THESE SYSTEMS OVER THE NEXT FEW DAYS.

THE COLD FRONT WILL CONTINUE AT A FAIRLY DECENT PACE ACROSS SOUTH
DAKOTA TONIGHT. IT WILL BE JUST THROUGH THE CWA AROUND MIDNIGHT AND
OUT OF THE STATE BY MORNING. GOOD MOISTURE HAS BEEN POOLING IN THE
AREA SOUTH OF THE FRONT WITH DEWPOINTS IN THE UPPER 60S TO LOWER 70S
FROM ABR TO MITCHELL ON EASTWARD. TEMPERATURES HAVE RISEN INTO THE
90S IN THOSE AREAS AS WELL. AS THE FRONT GETS CLOSER TO THIS VERY
UNSTABLE PART OF THE STATE...EXPECT SHOWERS AND THUNDERSTORMS TO
DEVELOP. WILL GO WITH SLIGHTLY HIGHER POPS IN THE EAST...VS THE WEST
WHERE THE DRIER AIR IS ALREADY MOVING IN. AN UPPER LEVEL TROUGH WILL
SWING THROUGH SOUTHERN MANITOBA AND NORTH DAKOTA TONIGHT AS
WELL...WHICH SHOULD AID IN THE INSTABILITY.

ON THURSDAY...STRONG HIGH PRESSURE WILL BEGIN TO MOVE INTO THE
REGION BEHIND THE EXITING FRONT. THE EASTERN CWA WILL LIKELY SEE
SOME BREEZY CONDITIONS AS THE TIGHTEST PART OF THE GRADIENT BETWEEN
THE SYSTEMS SETS UP THERE DURING THE DAY. TEMPERATURES BECOME TRICKY
AS MUCH DRIER AND COOLER AIR ACCOMPANIES THE HIGH. H85 TEMPS FALL
FROM THE MID 20S TODAY INTO THE 9 TO 14 DEGREE RANGE BY THURSDAY
EVENING. MOST OF THE GUIDANCE TRIES TO KEEP TEMPS DOWN AROUND 70 FOR
HIGHS...BUT THAT SEEMS UNLIKELY AFTER SO MANY DAYS OF 90+...SO WILL
LEAN A BIT MORE TOWARD THE WARMER FWC GUIDANCE...MORE LIKE UPPER 70S
TO AROUND 80. THE SURFACE HIGH BECOMES THE DOMINANT FEATURE THROUGH
THE REST OF THE PERIOD WITH THE UPPER LEVEL FLOW REMAINING
NORTHWESTERLY. WILL KEEP CONDITIONS DRY AND TEMPS IN THE 50S AT
NIGHT AND 70S TO LOWER 80S DURING THE DAY.

.EXTENDED DISCUSSION...
ENSEMBLES TODAY SHOW MODERATE AGREEMENT IN THE MID/UPPER LEVELS
THROUGH THE EARLY PORTION OF THE EXTENDED PERIOD. UPPER TROF BECOMES
ESTABLISHED ACROSS SRN CANADA AND INTO THE ERN U.S. WITH A MEAN
RIDGE THROUGH THE INTERMOUNTAIN WEST. IN BETWEEN NW FLOW ALOFT WILL
REMAIN THE DOMINANT FEATURE WITH BELOW NORMAL TEMPERATURES EXTENDING
TO THE SURFACE. ENSEMBLES SEEM TO HAVE A HARDER TIME RESOLVING MAIN
SURFACE FEATURES WITH SPREADS SHOWING QUITE A RANGE OF SOLUTIONS.
THINK THIS MAY BE MORE A RESULT OF WEAK SURFACE BOUNDARY MOVING SE
ACROSS THE NORTHERN AND CENTRAL PLAINS SATURDAY SINCE AGREEMENT
BECOMES BETTER BY DAY 5 AS SURFACE HIGH DVLPS TO THE EAST. BY THIS
TIME SRLY FLOW IS DVLPG INTO THE MID LEVELS WILL UPPER LEVELS SHOW A
TRANSITION FOR ERN TROF/WRN RIDGE TO A TROF DIGGING OFF THE WEST
COAST WITH THE MEAN RIDGE MIGRATING E INTO THE PLAINS BY TUESDAY.
SOME INDICATIONS OF MINOR SHORT WAVES PROGRESSING NE THROUGH THE
DVLPG SW FLOW THEN AS WELL WITH LOW LEVEL LEE SIDE TROFFING DVLPG.
WILL OPT TO KEEP DAYS 3 THROUGH 5 DRY BUT WILL INTRODUCE WARMING
TEMPS AND A CHANCE OF THUNDERSTORMS DAYS 6 AND 7. DO NOT HAVE GREAT
CONFIDENCE IN ANY SOLUTION THAT FAR OUT GIVEN LITTLE AGREEMENT IN
THE ENSEMBLES BUT WITH LOW LVLS WARMING AND UPPER LVLS NOT SEEMING
TO WARM AS FAST WE SHOULD BE UNSTABLE ENOUGH FOR AT LEAST ISOLATED
CONVECTION.

.ABR...NONE.

PARKIN/FAUCETTE