039 
FXUS63 KFSD 102153
AFDFSD

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
453 PM CDT MON SEP 10 2012

.DISCUSSION.../ISSUED AT 315 PM CDT/
MAIN FOCUS FOR TONIGHT AND TOMORROW WILL BE THE FRONTAL PASSAGE AND 
HOW WARM AND WINDY IT GETS. FOR TONIGHT...WINDS WILL DROP OFF SOME 
WITH SUNSET ACROSS THE AREA ALTHOUGH STILL GUSTING UP TO 20 MPH. 
WITH RAPID COOLING EARLY THIS EVENING A STRONG INVERSION SHOULD SET 
UP OVER THE BUFFALO RIDGE AND WITH A 40 TO 50 KT LOW LEVEL 
JET...THIS WILL SET UP CONDITIONS FOR A DOWNSLOPE WIND ENHANCEMENT. 
EXPECT THAT MUCH OF THE TIME AFTER MIDNIGHT WILL SEE WINDS OF 20 TO 
30 MPH WITH A FEW GUSTS APPROACHING 35 MPH. THIS WILL ALSO KEEP LOWS 
IN THE UPPER 60S AROUND MARSHALL. ELSEWHERE WINDS AND A FEW CIRRUS 
WILL ALSO KEEP LOWS IN THE 60S WITH LOWS NEAR 70 POSSIBLE IN SOUTH 
CENTRAL SOUTH DAKOTA.

ON TUESDAY...THE FRONT WILL MOVE ACROSS EASTERN SD. THE QUESTION IS 
HOW FAST IT WILL DO IT. GIVEN THE INITIAL WAVE IS LIFTING NORTHEAST 
INTO MANITOBA AND ONTARIO...THE UPPER LEVEL PUSH WILL DIMINISH AS 
THE FRONT HEAD SOUTHEAST. AND DIURNALLY...THESE TYPE OF FRONTS CAN 
SOMETIMES SLOW AS MIXING WASHES OUT THE WIND SHIFT. SO EXPECT THAT 
THE FRONT WILL APPROACH I-29 TOWARD 00Z WEDNESDAY KEEPING MUCH OF 
THE AREA IN THE WARM SECTOR. EXPECT WINDS ONCE AGAIN GUSTING BETWEEN 
25 AND 35 MPH EAST OF THE JAMES VALLEY MUCH OF THE DAY. WITH 925 MB 
TEMPS AROUND 30S...EXPECT HIGHS IN THE 90S IN THE WARM SECTOR. 
BEHIND THE FRONT...NORTHWEST WINDS COULD BRIEFLY GUST TO 30 MPH 
BEFORE SLOWLY DECREASING. ENOUGH MIXING RIGHT BEHIND THE FRONT TO 
WARM INTO THE MID TO UPPER 80S PLACES WEST OF THE JAMES MAY ACTUALLY 
SEE THEIR HIGH AROUND 18Z AND THEN STEADY TEMPS AFTER THAT AS COOLER 
AIR TRIES TO MOVE IN. SEE BELOW FOR IMPACT ON FIRE WEATHER CONCERNS.

NORTHEAST FLOW IS EXPECTED TO DEVELOP ACROSS THE AREA TUESDAY NIGHT 
NORTH OF THE FRONT. BAND OF 700 MB FRONTOGENESIS DEVELOPS ACROSS 
CENTRAL SOUTH DAKOTA TUESDAY EVENING AND GRADUALLY SAGS SOUTH AND 
EAST THROUGH THE DAY ON WEDNESDAY. DYNAMICS WILL LIKELY BE 
SACRIFICED INITIALLY TO SATURATION. FRONTOGENESIS AND UPPER LEVEL 
DIV Q MAXIMIZES DURING THE DAYTIME HOURS ON WEDNESDAY.  HAVE 
CONTINUED CATEGORICAL POPS FOR FRONTAL BAND. LIKE OVERNIGHT SHIFT 
MENTIONED...LITTLE IN THE WAY OF INSTABILITY SO FOCUS IS MAINLY ON 
SHOWER ACTIVITY BUT STILL CAN NOT RULE OUT A RUMBLE OF THUNDER OR 
TWO. 925 HPA TEMPS COOL DURING THE DAY DUE TO EVAPORATIVE 
COOLING...WHICH SHOULD REALLY LIMIT WARMING POTENTIAL.  HAVE FAIRLY 
STEADY TEMPERATURES THROUGHOUT THE DAY...WITH HIGHS IN THE CENTER OF 
THE FRONTAL BAND NEARLY 10 DEGREES BELOW NORMAL.  

CONTINUED THE POP MENTION IN THE SOUTHEAST WED NIGHT THOUGH THE 
FORCING CONTINUES TO WEAKEN THROUGHOUT THE NIGHT.  

LIGHT NORTHERLY FLOW WILL KEEP TEMPERATURES SEASONABLY COOL THURSDAY 
THROUGH SATURDAY.  THE DEVELOPING LOW PRESSURE SYSTEM OVER EASTERN 
IOWA/WESTERN ILLINOIS SHOULD KEEP THE PATTERN FROM EVOLVING TOO 
QUICKLY.  AS RETURN FLOW SETS UP...EXPECT A GRADUAL WARMING TREND 
NEXT WEEKEND INTO EARLY NEXT WEEK. 

&&

.AVIATION.../FOR THE 00Z TAF ISSUANCE/
CIGS/VSBYS EXPECTED TO BE VFR THROUGH 00Z WEDNESDAY. STRONG GUSTY 
WINDS WILL PREVAIL THROUGH THE EARLY EVENING WITH GUSTS OVER 30 KTS 
IN MANY LOCATIONS. WINDS WILL GRADUALLY DECREASE TOWARD SUNSET. 
HOWEVER...AFTER SUNSET A LOW LEVEL JET OF 35 TO 45 KTS WILL SET UP 
JUST ABOVE THE SURFACE RESULTING LLWS AT ALL LOCATIONS. FOR 
KHON...ONLY HAD LLWS THROUGH 10Z AS THE LOW LEVEL JET WILL SHIFT 
WEST TOWARD DAWN. FOR KSUX AND KFSD...EXPECT LOW LEVEL WIND SHEAR BY 
LATE EVENING AND CONTINUE THROUGH SUNRISE ON TUESDAY. THEN STRONGER 
WINDS SHOULD SURFACE DURING THE MORNING...EXCEPT NEAR KHON WHICH 
WILL SEE A FROPA SHORTLY AFTER 18Z WHILE KFSD WILL SEE THE WINDS 
GRADUALLY LIGHTEN AND TURN TO THE WEST AND SOUTHWEST AFTER 21Z. /08 

&&

.FIRE WEATHER...
AS NOTED ABOVE THE FRONT IS EXPECTED TO MOVE FROM CENTRAL SD EARLY 
TUESDAY MORNING TO INTERSTATE 29 BY 00Z WEDNESDAY. WHAT IS CERTAIN 
IS THAT THE ENTIRE AREA WILL SEE HIGHS IN THE 80S AND 90S RESULTING 
IN AFTERNOON RELATIVE HUMIDITY AROUND 20 PERCENT. WHAT IS LESS 
CERTAIN IS THE WIND ACROSS SOUTH DAKOTA. IN SW MINNESOTA AND NW 
IOWA...FROPA IS EXPECTED TO BE AFTER 00Z AND WINDS WILL REMAIN 
STRONG THROUGHOUT THE DAY...PERHAPS BEGINNING TO DECREASE IN SW MN 
LATE AFTERNOON. FREQUENT GUSTS OVER 30 MPH ARE EXPECTED AND WITH 
SUCH DRY VEGETATION WILL CONTINUE THE RED FLAG WARNING. 

IN SOUTH DAKOTA...IT ALL WILL DEPEND ON THE TIMING. IF THE FRONT 
SLOWS ITS EASTWARD PROGRESSION MORE...IT IS LIKELY THE ENTIRE I-29 
CORRIDOR WILL SEE FREQUENT WIND GUSTS TO 30 MPH. EVEN WITH THE 
CURRENT FORECAST EXPECT THAT WINDS WILL GUSTS TO 30 MPH MUCH OF THE 
DAY IN THE LOWER BIG SIOUX VALLEY...INCLUDING CANTON AND 
VERMILLION...AND HAVE ISSUED A RED FLAG WARNING IN FIRE WEATHER ZONE 
258 EVEN THOUGH AREAS ON THE WESTERN EDGE OF THIS ZONE...SUCH AS 
TYNDALL AND TRIPP MAY NOT SEE WINDS GUST OVER 25 MPH. 

ELSEWHERE  HAVE DECIDED TO ISSUE A FIRE WEATHER WATCH FOR LATE 
TOMORROW MORNING AND TOMORROW AFTERNOON. IN SDZ257...INCLUDING SIOUX 
FALLS AND BROOKINGS...CURRENT TIMING WOULD RESULTS IN WINDS OF 25 TO 
30 MPH THROUGH MID AFTERNOON. BUT IF THE FRONT IS 2 OR 3 HOURS 
FASTER...WINDS WILL BEGIN DROPPING OFF BY EARLY AFTERNOON KEEPING 
WINDS CLOSER TO 20 MPH. 

FARTHER WEST...IT IS BOTH THE UNCERTAINTY ON FRONTAL TIMING AND 
STRENGTH OF THE NORTHWEST WINDS. RIGHT BEHIND THE FRONT...WHEN 
MIXING IS MAXIMIZED...WINDS COULD GUST TO 30 MPH FOR A FEW 
HOURS...ESPECIALLY IN SDZ255. BUT IF THE FRONT SLOWS DOWN OR MIXING 
IS LESS EFFICIENT THEN THESE WINDS WILL NOT BE REALIZED.

CONDITIONS SHOULD IMPROVE TUESDAY NIGHT AS COLDER AIR MOVES BACK 
INTO THE AREA.
&&

.FSD WATCHES/WARNINGS/ADVISORIES...
IA...RED FLAG WARNING UNTIL 9 PM CDT TUESDAY FOR IAZ300-301.

MN...RED FLAG WARNING UNTIL 9 PM CDT TUESDAY FOR MNZ900.

NE...RED FLAG WARNING UNTIL 9 PM CDT TUESDAY FOR NEZ249.

SD...RED FLAG WARNING UNTIL 10 PM CDT THIS EVENING FOR SDZ255>258.

     RED FLAG WARNING FROM 10 AM TO 9 PM CDT TUESDAY FOR SDZ258.

     FIRE WEATHER WATCH FROM TUESDAY MORNING THROUGH TUESDAY EVENING 
     FOR SDZ255>257.

&&

$$