River Flood Watch 13, NWS Sioux Falls, SD

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
833 AM CDT WED SEP 22 2010

.DISCUSSION...
WENT AHEAD AND RAISED POPS ACROSS THE CWA FOR TODAY. CONVECTION IN
CENTRAL SD AND NORTHERN NEBRASKA CONTINUES TO BECOME MORE ORGANIZED
AND THE 6Z NAM/GFS AND HRRR OUTPUT INDICATE THIS TREND WILL CONTINUE
DURING THE AFTERNOON. LOOKS LIKE A FAIRLY STRONG MID LEVEL WAVE
DRIVING THIS ACTIVITY WITH THE 800-700MB WINDS RAPIDLY INCREASING
ALONG WITH VERY GOOD MOISTURE CONVERGENCE AND INCREASING
INSTABILITY. DID DECREASE HIGHS A BIT AND MAY NEED TO DO SO FURTHER
IF THIS TREND FOR MORE THUNDERSTORM ACTIVITY CONTINUES. /08

&&

.PREV DISCUSSION...
MAJOR SYSTEM APPEARS DUE TO BRING TWO BIG SHOTS OF CONVECTION TO OUR
FORECAST AREA. ONE WILL INVOLVE RAPID DEVELOPMENT OF TSTMS THIS
MORNING AND IN FACT THROUGH THE DAY AS SRLY/SERLY LOW LEVEL FLOW
INCREASES AHEAD OF LOW DEVELOPING TO THE SW. MOIST AND VERY UNSTABLE
AIR WILL FLOW TOWARD AND INTO AREA AS SHEAR BECOMES GREATER OVER
AREA NEAR AND N OF APPROACHING WARM FRONT. BY AFTERNOON THESE
PARAMETERS SHOULD BE RIPE ENOUGH FOR A SIGNIFICANT SEVERE RISK OVER
MUCH OF OUR AREA ESPECIALLY THE SOUTHEAST HALF. SPC DAY 1 OUTLOOK
AND DISCUSSION GENERALLY LOOKS GOOD INCLUDING ENHANCED VERY LARGE
HAIL AND POSSIBLY EVEN TORNADO POTENTIAL. IF INITIAL ACTIVITY IS NOT
SO WIDESPREAD IT DAMPENS POTENTIAL...AND I DONT THINK IT WILL BE...I
WOULD THINK AN UPGRADE TO EVEN STRONGER POTENTIAL MIGHT COME LATER
THIS MORNING. WILL HIT THE SEVERE POTENTIAL FAIRLY HARD IN HAZARDOUS
WX OUTLOOK...AND PERHAPS MENTION SEVERE IN GRIDS/PUBLIC FORECASTS AT
LEAST SOUTHEAST PART OF AREA. FOR RAINFALL POTENTIAL WITH THIS FIRST
SHOT...THE NORTHEASTERN PART OF AREA...SWRN MN AND NEARBY...SEEMS
TARGETED. CONDITIONS NOT RIPE ENUF FOR FLASH FLOOD WATCH BUT THIS
COULD SET THINGS UP FOR THE NEXT ROUND WHICH I WILL GET TO SHORTLY.

TEMPERATURES EXPECTED TO WARM PRETTY MUCH ACCORDING TO GUIDANCE.
MAJOR INCREASE IN SHOWER/TSTM COVERAGE NOT COMING TILL LATER TODAY
IN THE NORTHEASTERN FORECAST AREA SHOULD ALLOW WARMING TO GUIDANCE
VALUES THERE...WHILE STRONG WARMING PUSHES TEMPS UP TO 80 PLUS FAR
S/SE DESPITE INCREASING CLOUDS. WINDS WILL INCREASE AS SYSTEM
DEVELOPS AND IT SHOULD BECOME BREEZY TO WINDY SOUTHEAST CORNER THIS
AFTERNOON.

FIRST ROUND SHOULD PULL OUT TO THE NORTHEAST THIS EVENING WITH A
SEVERAL HOUR LULL LIKELY EARLY TONIGHT BEFORE NEXT ROUND SPREADS IN
QUICKLY FROM THE SW LATE TONIGHT. THIS ACTIVITY SHOULD BE MORE
WIDESPREAD THAN THE FIRST SHOT WITH LESS SEVERE POTENTIAL...AND THAT
MAINLY OUR SOUTHEAST CORNER ON THU. HOWEVER HEAVY RAINFALL COULD
BECOME FAIRLY WIDESPREAD. WILL HOLD OFF ON FLASH FLOOD WATCH FOR NOW
BUT THE POTENTIAL SEEMS THERE FOR AT LEAST OUR NORTHEAST GIVEN A
THREAT OF TWO HEAVY RAIN SHOTS. IN ANY EVENT...HIGH UNQUALIFIED POPS
ARE IN ORDER FOR LATE TONIGHT AND THU. TEMPS WILL BE COOLER MOST
AREAS FRIDAY BUT COULD BE SLIGHTLY WARMER IN OUR NORTHEAST DEPENDING
ON RAINFALL COVERAGE. LOWS TONIGHT OF COURSE WILL BE QUITE MILD/WARM.

SHOWERS SHOULD MOVE OUT QUICKLY THURSDAY NIGHT AND LEAVE A MOSTLY
DRY FORECAST MOST OF THE WAY THROUGH NEXT TUE. WILL KEEP SOME LITTLE
CHANCE OF SHOWERS FRI NIGHT INTO EARLY SAT. THIS COULD BE GREATER
IF DEVELOPING UPPER LOW DIGS AS FAR W AS GFS SHOWS...WILL ACCEPT
SLIGHTLY MORE MODERATE ECMWF FOR NOW. THIS SEEMS TO BE THE ONLY
THREAT OF RAIN AFTER THU EVENING. TEMPS WILL GENERALLY BE MILD WITH
THE SAT UPPER LOW POSSIBLY MAKING IT A LITTLE COOLER THAN WHAT WE
HAVE IF THE UPPER LOW DIGS FURTHER W AS THE GFS DEPICTS. / WILLIAMS

&&

.AVIATION...
VFR THROUGH MID MORNING THEN SCATTERED THUNDERSTORMS DEVELOPING.
WARM FRONT LIFTING NORTHWARD TOWARD CWA DURING THE AFTERNOON IN
RESPONSE TO DEVELOPING LOW PRESSURE OVER SOUTHWEST NEBRASKA. LOW
LEVEL MOISTURE ASSOCIATED WITH FRONTAL BOUNDARY TO PRODUCE
WIDESPREAD MVFR CIGS ACROSS CWA DURING THE AFTERNOON WITH LCL IFR
CONDITIONS VICINITY OF THUNDERSTORMS. GENERAL MVFR TO IFR CONDITIONS
TONIGHT AS WIDESPREAD RAIN AND ISOLATED THUNDERSTORMS CONTINUE
ACROSS CWA.  SOUTHEAST SURFACE WINDS INCREASING TO 15 TO 25 KTS
DURING THE AFTERNOON.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...

SD...NONE.
MN...NONE.
IA...NONE.
NE...NONE.

&&

WILLIAMS

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