FXUS63 KFSD 102125
AFDFSD
AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
325 PM CST THU DEC 10 2009
.DISCUSSION...
TEMPERATURES WARMED MORE THAN ANTICIPATED LATE THIS AFTERNOON IN A
SOUTHWESTERLY TO WESTERLY FLOW...THOUGH WITH THE VERY DRY AIR AND
SNOW COVER ACROSS THE AREA EXPECT TEMPERATURES TO FALL PRETTY
QUICKLY DURING THE EVENING. WHILE IT WILL DEFINITELY BE COLD
TONIGHT...IN THE SINGLE DIGITS BELOW ZERO...WITH A MODERATING
AIRMASS MINS SHOULD BE A COUPLE OF DEGREES WARMER THAN THIS MORNING.
WE DO HAVE A WEAK SHORTWAVE TRACKING OVER OUR NORTHEAST
TONIGHT...BUT SOUNDINGS LOOK WAY TOO DRY TO PRODUCE ANYTHING IN THE
WAY OF PRECIPITATION...SO REMOVED THE FLURRIES FROM THE FORECAST.
/JM
IN THE INTERMEDIATE TERM...WE ARE STILL DEALING WITH A COLD UPPER
LOW SITUATED IN CENTRAL CANADA...WHICH BECOMES MORE WEST TO EAST
ORIENTATED BY FRIDAY NIGHT. THIS HELPS TO ALLOW OUR MID AND UPPER
FLOW TO FLATTEN OVER THE CONUS FRI NIGHT AND CONTINUING THRU THE
WEEKEND...WHICH WL MODERATE OUR AIR MASS. THE SFC WINDS TURN TO A
SLY DIRECTION ON FRIDAY AS HIGH PRES PASSES TO THE EAST...AND CONTS
FRI NIGHT. THE MIXING IS ABSOLUTELY DEAD ON FRIDAY...AND THERMAL
PROFILES ARE INVERTED FM THE SFC ALL THE WAY UP TO THE MID LEVELS.
SO DESPITE A LOW LEVEL AIR MASS WHICH IS A COUPLE OF DEGS C WARMER
THEN TODAY...MAX TEMPS ON FRIDAY MAY BE A TOUCH COOLER MOST
LOCATIONS DUE TO A LACK OF MIXING AND UNFAVORABLE WIND DIRECTION
OVER SNOW COVER. THE FLIP SIDE...IS THAT TEMPERATURES MAY NOT FALL
OFF ALL THAT MUCH FRI NIGHT AS THE SLY FLOW MAY INCREASE SOME. SO
FOR MAX TEMPS ON FRIDAY...STAYED COOLER THEN THE MET...WHICH WAS
COOLER THAN THE MAV. BUT THE MET MINS FRIDAY NIGHT DID NOT LOOK TOO
BAD.
THE MID AND UPPER FLOW BECOMES MORE WAVY THIS WEEKEND. THE NAM AND
GFS ARE VERY CLOSE IN SHOWING A S/W MOVG RAPIDLY EWD SAT AND SAT
NIGHT THRU THE CENTRAL PLAINS. BOTH MODELS ARE VERY BULLISH IN
RETURNING LOW LEVEL MOISTURE AND STRATUS TO THE ERN SECTIONS OF OUR
CWFA LATE SAT AFTERNOON...CONTINUING THRU SAT NIGHT. THE DIFFERENCE
LIES IN THE MID LEVELS...WHERE THE NAM IS SATURATED AND THE GFS IS
NOT. THIS MAKES A HUGE DIFFERENCE ON PCPN TYPE IF PCPN WERE TO
DEVELOP. FOR NOW...LEFT SATURDAY DAY DRY ALTHOUGH LIGHT PCPN MAY BE
CLOSE TO THE SIOUX CITY AREA BY LATE SATURDAY AFTERNOON WITH DECENT
WAA AND MODEST ISENTROPIC LIFT. BUT EVEN ON THE GFS...THE MOISTURE
PROFILE INCREASES IN DEPTH JUST ENOUGH TO BE BOTHERSOME SATURDAY
EVENING IN OUR FAR ERN AREAS. THEREFORE ADDED A SLGT CHC OF -SN WITH
AREAS OF -FZDZ FROM WINDOM MN AND POINTS SWD. IF THE NAM WERE TO
VERIFY...WITH A SATURATED AND COLD PROFILE...IT WOULD BE ALL SNOW.
BUT IF THE GFS VERIFIES...IT WOULD BE -FZDZ WITH THE DRY MID LEVELS.
HARD TO TELL AT THIS POINT WHICH ONE IS RIGHT...BUT THE GLOBAL
MODELS INCLUDING THE ECMWF...CANADIAN AND GFS HAVE BEEN CONSISTENT
IN SHOWING THIS FOR DAYS FOR SATURDAY EVENING IN OUR FAR ERN AREA.
CONCERNING TEMPS...CONTINUED TO MODERATE OUR HIGHS ON SATURDAY BUT
STILL WENT BLO GUIDANCE READINGS AS MIXING IS ABSENT AGAIN WITH OUR
SLY SFC FLOW. SAT NIGHT MAY SEE A PRETTY GOOD VARIANCE IN LOWS FM W
TO E ACROSS OUR FA. OUR WRN AREAS MAY BE PRETTY COOL AGAIN UNDER
ONLY PARTLY CLOUDY SKIES WITH HIGH CLOUDS...WHEREAS OUR EAST MAY BE
UNDER STRATUS. SO I HAVE A PRETTY STRONG THERMAL GRADIENT FM NW TO
SE. ANOTHER MID LEVEL WAVE MOVES THRU LATE SUNDAY AND SUNDAY EVENING
WHICH HAS BEEN WELL ADVERTISED...BUT IT IS REALLY BONE DRY. IT DOES
HAVE A LOT OF STRONG ISENTROPIC LIFT WITH IT...THERE IS JUST NO
MOISTURE TO KEY ON. GIVEN H925 TEMPS WHICH ARE SLIGHTLY WARMER THEN
SATURDAY...SUNDAYS HIGHS SHOULD BE A CATEGORY WARMER THEN SATURDAY.
IN THE EXTENDED SUN NIGHT THROUGH THURSDAY...THE BOTTOM DROPS OUT
AGAIN EARLY IN THE EXTENDED ON MONDAY. ALL MODELS HAVE COME AROUND
NOW TO SHOWING ANOTHER ARCTIC PLUNGE COMING DOWN SUNDAY NIGHT AND
EARLY MONDAY IN OUR FA...SO CONFIDENCE IS PRETTY HIGH WITH THIS
FEATURE. ADDED A SLGT CHC OF -SN BEHIND THE CDFNT SUNDAY NIGHT WITH
CAA STRATOCU ANTICIPATED AND UNSTABLE LOW LEVEL AIR. BUT OUR TEMPS
WL TAKE THE BIGGEST HIT. AFTER A FAIRLY MILD WEEKEND...OUR TEMPS WL
LIKELY NOT MOVE ON MONDAY...EXCEPT MAYBE TO GO DOWN. THEREFORE HAVE
MAX TEMPS ON MONDAY WHICH ARE VIRTUALLY THE SAME AS OUR LOWS. AFTER
MONDAY...IT APPEARS OVERALL THAT A NW FLOW ALOFT TAKES SHAPE IN THE
PLAINS WITH UPPER TROFFING ACROSS THE HUDSON BAY AREA. THIS WL KEEP
OUR TEMPS FM WARMING TOO MUCH IF THIS HAPPENS...SO TRENDED WITH THE
COLDER GUIDANCE READINGS WHICHEVER THEY WERE...EITHER THE MEX OR
HPC.
FUTURE SHIFTS WL NEED TO TAKE A CLOSE LOOK AT OUR TUE MORNING
LOWS WHICH OUR PROBABLY TOO WARM...BUT WENT AS COLD AS I COULD FOR
NOW. AFTERWORDS...LITTLE OR NO PCPN IS FORECAST. /MJF
&&
.AVIATION...
VFR CONDITIONS EXPECTED THROUGH 00Z TOMORROW EVENING.
&&
.FSD WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
MN...NONE.
NE...NONE.
SD...NONE.
&&
$$
