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. && $$