FXUS63 KFSD 050258
AFDFSD

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
855 PM CST THU DEC 4 2008

.DISCUSSION...
STILL A PRETTY SOLID STRATUS DECK OVER THE AREA...THOUGH DOES APPEAR
TO BE SLOWLY ERODING FROM THE WEST. HAVE BUMPED UP SKY COVER IN THE
GRIDS FOR THE REMAINDER OF THE EVENING...BUT BOTH NAM AND RUC
SHOWING RH IN CLOUD LAYER DECREASING AFTER 06Z...SO WILL STICK WITH
DECREASING CLOUD IDEA AFTER MIDNIGHT. CLOUDS MAY HOLD TEMPS UP A
LITTLE MORE THAN FORECAST IN HOURLY GRIDS THIS EVENING...BUT WITH AT
LEAST PARTIAL CLEARING STILL EXPECTED LATER TONIGHT...WILL NOT MAKE
ANY SIGNIFICANT CHANGES TO OVERNIGHT LOWS.

&&

.AVIATION...
WIDESPREAD STRATUS DECK WILL REMAIN ABOVE 3KFT AT KHON/KFSD/KSUX...
WITH GRADUAL CLEARING OF THE STRATUS EXPECTED FROM THE WEST AND
NORTH AFTER 06Z. VFR CONDITIONS WILL CONTINUE THROUGH 06/00Z. UPPER
WAVE DROPPING SOUTHEAST THROUGH MINNESOTA FRIDAY EVENING. PERIOD OF
MVFR CIGS/VSBYS POSSIBLE BETWEEN 06/00Z AND 06/06Z AT KHON/KFSD...
WITH GREATER CHANCE OF MVFR CONDITIONS FARTHEAST EAST INTO SWRN MN.

&&

.PREV DISCUSSION...
COLD HIGH PRESSURE OVER SOUTHERN MONTANA AT 20Z WILL BUILD SOUTH
INTO KANSAS AND SOUTHERN NEBRASKA BY FRIDAY MORNING. WINDS SHOULD BE
BREEZY NORTHWEST LATE THIS AFTERNOON...THEN SLOWLY TURN TO THE WEST
AND SOUTHWEST LATE. THE EXCEPTION WILL LIKELY BE THE BUFFALO RIDGE
WHERE A MORE WESTERLY COMPONENT AND SPEEDS CLOSER TO 10 MPH ARE
MORE LIKELY. A LARGE AREA OF LOW END VFR CEILINGS TO THE WEST WILL
CONTINUE TO ADVECT EAST...BUT EXPECT SUBSIDENCE AND DRY AIR TO AID
IN BREAKING APART CLOUD DECK WHICH WILL ALLOW TEMPERATURES TO FALL
OFF. WILL AIM FOR LOWS IN THE LOWER SINGLE DIGITS FOR MOST
LOCATIONS. IF CLOUDS HOLD ON...LOWS WOULD LIKELY BE 5 DEGREES OR SO
LESS COLD.

WILL STILL MAINTAIN THE CHANCE FOR SOME PATCHY VERY LIGHT SNOW OR
FLURRIES FOR THE REMAINDER OF THE AFTERNOON.

FULL LINK UP TO STRONGER WAA ALFT IS NOT LKLY TO OCCUR FRIDAY...
BUT WL HAVE ENOUGH OF A S TO SW GRADIENT TO ALLOW SOME WARMING.
WAA CI WL SURGE SWRD FAIRLY EARLY IN THE DAY TO FURTHER DIMINISH
WARMING POTENTIAL. WITH AS MUCH DRY AIR AROUND AT LOW LVLS...
EXPECT TAHT APPROACH OF MAIN DIV Q AND PV ADVECTION IN AFTN WL BE
MET WITH RESISTANCE TO PCPN...AND KEPT THREAT OUT UNTIL EVNG...WHICH
LOOKS TO BE MOST FAVORED ACRS NERN 1/3 OF CWA. BULK OF PCPN LKLY TO
OCCUR DURING EVNG HOUR AHEAD OF STRONGER CAA AND SUBSIDENCE IN WAKE
OF WAVE MOVING THRU MN. LKLY TO BE SOME WIGGLES IN TMPS TRENDS WITH
THE INITIAL STRONGER SW GRADIENT AHEAD OF BNDRY...THEN WIND SHIFT
AROUND TO NW AND N LATE BEHIND FNT. GFS MORE AGGRESSIVE...AND SEEMS
TO HAVE SUPPORT OF MOST SLNS.

STRONG COLD HIGH BUILDS SWRD SAT INTO SAT NIGHT...SPREADING INTO MN
AND IA WITH DRYING INFLUENCE. DWPTS LKLY TO BE TOO HIGH IN ALL
GUIDANCE AS HAS BEEN THE CASE WITH LAST COUPLE OF COOLER PUSHES.
THIS AIRMASS IS SUGGESTED TO BE A COUPLE OR SO DEGREES C MORE
MODERATE THAN CURRENT ONE...SO TMPS LKLY TO BE SOMEWHAT WARMER AS
WELL. SFC RIDGE WL BE OVER ERN CWA BY LTR EVNG...AND WINDOW BEFORE
INCREASE IN CLOUDS FROM W AND FLOW TURNING SE STARTS TMPS IN THE
OTHER DIRECTION.

INCREASE IN CLDS IS SIGN OF RETURNING BAROCLINIC ZONE WHICH FEATURES
STRONG FRONTOGENESIS IN 800-600 HPA LYR...AS WELL AS NICE COUPLET OF
FRONTOLYSIS ABOVE WHICH SHUD MEAN A VIGOROUS CIRCULATION. WAVE WORKS
IT WAY DOWN THE BNDRY FROM NW TO SE BY SUN MORNING AND EXITS SERN
AREAS BY MID AFTN. THIS SHUD TEMPORALLY ENHANCE COVERAGE AS THE
BNDRY SHIFTS EWRD...AND CONSIDERED DROPPING PCPN CHC IN ALL BUT FAR
E/SE BY MID AFTN. GIVEN STRENGTH OF FORCING...CHOSE TO DRASTICALLY
INCREASE THE POPS TO LKLY WORKING INTO SCNTRL AREAS AS EARLY AS LATE
SAT NIGHT...MARCHING STEADILY EWRD THRU ERLY AFTN ACRS CWA. LOOKS
LIKE ENOUGH LIFT AND MOISTURE FOR A BROAD BAND OF LIGHT SNOWFALL...
PROBABLY AN INCH OR TWO ON THE AVG. MAIN DIV Q WL SLIDE SE FROM ND
TO MN OVERNIGHT SUNDAY NIGHT...WITH MAIN PCPN THREAT OVR NERN HALF
OF AREA...BUT FAR LESS WIDESPREAD COVERAGE OR CERTAINTY THAN PRIOR
WAA EVENT. WARMER PLUME WORKING ACRS AREA ALFT...PLENTY OF CLOUDS...
AND SLY COMP TO WINDS WL AGAIN MAKE LOW TMPS CHALLENGING.

THE EXTENDED RANGE /MON THRU THU/ WL FEATURE ANOTHER CDFNT WORKING
SWRD EARLY IN THE PERIOD...AND NOT MUCH FOR SIGNIFICANT WARMING
THEREAFTER...AS OVERALL ENERGY TENDING TO DIG INTO NRN PLAINS/GTLKS
BEFORE REINFORCING TENDENCY FOR ERN U.S. TROUGHINESS. THIS COLD AIR
MASS WL BE OVER THE AREA TUE INTO TUE EVNG. 00Z GFS ENSEMBLE SET
WAS QUITE A BIT SKEWED TOWARD THE EC/CANADIAN SLN BY 12Z WED WITH
STRONGER CLIPPER WAVE FARTHER TO NE...ALLOWING A NARROW WINDOW FOR
WARMING BEFORE NEXT ARCTIC FRONT PUSHES SWRD LTR WED THRU THU. FACT
IS THAT IF THIS HAPPENS...MOST OF THE WARMING WL LKLY TRY TO OCCUR
AT NIGHT...THEREFORE GENERALLY ALFT. SKEWED SLNS TO ALLOW FOR WAVE
TO PASS A LITTLE NEWRD OF OPERATIONAL GFS TRACK...BUT FORCING REALLY
FAVORS BEST PCPN THREAT TO N AND E OF AREA WED. WITH A LITTLE BIT OF
ENERGY TRACKING DOWN POTENTIAL WRN PLAINS BAROCLINC BNDRY THU...KEPT
A LOW CHC FOR LIGHT SNOWFALL IN FAR SWRN CWA.

IN GENERAL...CONSIDERING SNOWCOVER WHICH HAS VERY LITTLE CHANCE OF
ENDING UP LESS THAN CURRENT...AND AIRMASS SOURCE WITH BIAS FOR POOR
NEAR SFC FCST VARIABLES...TRENDED COLDER...DRIER...AND CLEARER WITH
EACH COLD PUSH.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
MN...NONE.
SD...NONE.
NE...NONE.

&&

$$
JH


Back to Main Forecast Discussion Page