241 FXUS63 KFSD 231442 AFDFSD AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA NATIONAL WEATHER SERVICE SIOUX FALLS SD 945 AM CDT FRI MAR 23 2012 .DISCUSSION... DENSE FOG VERY SLOWLY DECREASING THIS MORNING. WILL LET THE ADVISORY EXPIRE AT 1000 AM...BUT STILL PLAN ON SOME DENSE FOG IN MAINLY NW IA AND SW MN FOR AN HOUR OR TWO LONGER. OTHERWISE...ANOTHER UNSEASONABLY WARM DAY AHEAD WITH VERY LITTLE WIND...SO VERY PLEASANT. HIGHS FROM THE UPPER 60S TO LOWER 70S STILL LOOKS ON TARGET. /08 && .PREV DISCUSSION.../ISSUED AT 416 AM CDT/ INITIAL CONCERN IS DENSE FOG WHICH HAS DEVELOPED ACROSS EASTERN PORTIONS OF THE FORECAST AREA. HRRR HAD A DECENT HANDLE ON THE DEVELOPMENT AREA THROUGH THE EARLY MORNING AND TRIES TO EXPAND THE WESTERN EDGE OF THE FOG WESTWARD TO THE JAMES VALLEY THROUGH 13Z BEFORE RAPID IMPROVEMENT IN VISIBILITY BETWEEN 15Z AND 17Z. NOT YET SEEING ANY INDICATION OF MUCH WESTWARD PUSH INTO SOMEWHAT DRIER LOW LEVEL AIR THOUGH...SO WILL LET DENSE FOG ADVISORY ISSUED EARLIER FOR AREAS ALONG/EAST OF HURON-SALEM-AKRON LINE STAND. FOG EXPECTED TO LIFT DURING THE LATE MORNING...THOUGH MAY SEE SOME LINGERING STRATUS IN EASTERN AREAS INTO THE EARLY AFTERNOON. BETWEEN THAT AND MID-UPPER LEVEL CLOUDS CONTINUING TO WRAP AROUND SLOWLY DEPARTING UPPER LOW... EASTERN COUNTIES SHOULD BE PARTLY SUNNY THROUGH THE AFTERNOON WITH MORE SUNSHINE EXPECTED ALONG/WEST OF I-29. LIGHT WIND FLOW TODAY WILL LIKELY NOT SCOUR OUT RESIDUAL LOW LEVEL MOISTURE IN THE EAST THOUGH AND COULD SEE SOME REDEVELOPMENT OF FOG LATE TONIGHT/EARLY SATURDAY AND HAVE ADDED PATCHY FOG TO AREAS ALONG/EAST OF THE BUFFALO RIDGE AND IOWA GREAT LAKES. OTHERWISE BUILDING UPPER RIDGE STILL ON TRACK TO BRING DRY WEATHER FOR THE WEEKEND. GFS/NAM NOW STRONGER THAN THEIR RUNS FROM 24 HOURS AGO WITH NORTHERN STREAM WAVE TRACKING ACROSS CANADA THIS WEEKEND. THIS ALLOWS FRONT AND TRAILING RELATIVELY COOLER AIRMASS TO PUSH FARTHER SOUTH THROUGH THE UPPER MIDWEST...SIMILAR TO WHAT ECMWF/GEM WERE SHOWING YESTERDAY MORNING. SATURDAY SHOULD BE QUITE MILD OVER THE ENTIRE CWA WITH HIGHS IN THE 70S...POSSIBLY HITTING 80 IN PARTS OF THE MISSOURI VALLEY. FRONTAL BOUNDARY THEN SETS UP NORTHWEST TO SOUTHEAST ACROSS THE CWA...MAKING TEMPERATURES A LITTLE MORE TRICKY FOR SUNDAY INTO MONDAY. BOUNDARY WILL SEPARATE 50S AND LOWER 60S IN MINNESOTA...FROM UPPER 70S-LOWER 80S IN NEBRASKA AND SOUTHWEST SOUTH DAKOTA...AND THE GRADIENT LIKELY TO BE SOMEWHERE IN OUR CWA. FOR NOW HAVE COOLED NORTHEAST AREAS FROM PREVIOUS FORECAST FOR SUNDAY/MONDAY BUT GRADIENT MAY HAVE TO BE TIGHTENED FURTHER IF 925MB TEMPS...WHICH RANGE FROM 7-9C IN SOUTHWEST MN TO THE LOWER 20S IN SOUTH CENTRAL SD...PAN OUT FOR MONDAY AFTERNOON. BOUNDARY COULD ALSO BE FOCUS FOR A FEW SHOWERS OR THUNDERSTORMS LATE SUNDAY NIGHT AND MONDAY AS LOW LEVEL JET INCREASES AHEAD OF THE NEXT SYSTEM. INSTABILITY ON THE WEAK SIDE ALONG/NORTHEAST OF THE BOUNDARY WITH MORE FAVORABLE CAPE BUILDING INTO WARM SECTOR ACROSS SOUTHWEST CWA MONDAY AFTERNOON...THOUGH PRETTY STOUT CAP REMAINS IN PLACE THROUGH THE AFTERNOON SO KEEPING POPS ON THE LOW SIDE FOR DAYTIME HOURS. BETTER CHANCE LOOKS TO BE MONDAY NIGHT AS BOTH GFS/ECMWF SHOW POTENT MID-UPPER LEVEL WAVE CRASHING INTO THE RIDGE OVER THE EASTERN DAKOTAS. WITH STRONG DYNAMIC LIFT AND VERY STRONG LOW-MID LEVEL WINDS AROUND SOUTH SIDE OF DEEP LOW...MAY NOT TAKE MUCH TO PRODUCE DAMAGING WIND GUSTS WITH ANY STORMS THAT DEVELOP/MOVE INTO THE MORE UNSTABLE AIR ACROSS THE SOUTHWEST CWA MONDAY NIGHT. QUICK MOVING COMPACT SYSTEM WILL SHIFT EAST OF THE AREA TUESDAY... THOUGH COULD SEE SOME SHOWERS WRAP AROUND INTO MAINLY NORTHERN PARTS OF THE AREA TUESDAY BEFORE DRIER AIR BUILDS IN AND PRECIP THREAT SHIFTS EAST OF THE AREA TUESDAY NIGHT. WITH STRONG SURFACE LOW PASSING TO OUR NORTH TUESDAY...LIKELY TO SEE STRONG SYNOPTIC SCALE FLOW DEVELOP IN WELL-MIXED DEEP WESTERLY FLOW BEHIND THE FRONT. UPPER RIDGING WILL BRING DRY WEATHER BACK TO THE AREA FOR WEDNESDAY AND WEDNESDAY NIGHT...WITH UPPER TROUGH SET TO MOVE INTO THE NORTHERN PLAINS LATE IN THE WEEK. DIFFERENCES IN TIMING AMONG THE MODELS SO WILL HAVE JUST A SLIGHT CHANCE OF THUNDERSTORMS IN THE WESTERN CWA THURSDAY...WITH BETTER CHANCE HOLDING OFF UNTIL THURSDAY NIGHT OR FRIDAY...DEPENDING ON EVENTUAL TIMING. TEMPERATURES WILL REMAIN ON THE MILD SIDE OF NORMAL...THOUGH LATTER PART OF THE FORECAST PERIOD SHOULD BE COOLER THAN THE NEXT FEW DAYS. && .AVIATION.../FOR THE 12Z TAF ISSUANCE/ DENSE VLIFR FOG WILL CONTINUE AT KFSD UNTIL AT LEAST 15Z...THEN SHOULD GRADUALLY LIFT BETWEEN 15Z-17Z. KHON AND KSUX HAVE REMAINED JUST WEST OF THE DENSE FOG...AND SOME IFR VISIBILITIES AND CEILINGS MAY BE BRIEFLY POSSIBLE AT THESE LOCATIONS BEFORE 15Z. BY 18Z...ALL TAF SITES SHOULD IMPROVE TO VFR...HOWEVER SOME LIGHT FOG MAY BE POSSIBLE AGAIN AFTER MIDNIGHT SATURDAY AT KFSD AND KSUX. /LAFLIN && .FSD WATCHES/WARNINGS/ADVISORIES... IA...DENSE FOG ADVISORY UNTIL 10 AM CDT THIS MORNING FOR IAZ001>003- 012>014-020>022. MN...DENSE FOG ADVISORY UNTIL 10 AM CDT THIS MORNING FOR MNZ071-072- 080-081-089-090-097-098. NE...NONE. SD...DENSE FOG ADVISORY UNTIL 10 AM CDT THIS MORNING FOR SDZ038>040- 054>056-061-062-067. && $$