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000
FXUS63 KFSD 101006
AFDFSD

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
405 AM CST WED FEB 10 2010

.DISCUSSION...
SURFACE RIDGE MOVING ACROSS THE AREA TODAY...WITH IMMEDIATE CONCERNS
BEING FOG WHICH HAS DEVELOPED OVER THE WESTERN CWA AND TEMPERATURES
THROUGH THE DAY AS RIDGE AXIS MOVES INTO AREA WITH DEEPER SNOW COVER
IN THE NORTHEAST CWA. SO FAR...DENSE FOG APPEARS TO BE PATCHY ENOUGH
BASED ON WEBCAMS AND SURFACE OBS TO FORGO ANY KIND OF ADVISORY FOR
THE EARLY MORNING...BUT WILL HAVE TO KEEP AN EYE ON THE EXPANSE OF
THAT DURING THE PRE-DAWN HOURS. WITH COLD START THIS MORNING...ALONG
WITH LIMITED MIXING WITHIN THE SURFACE RIDGE AXIS COUNTER-PRODUCTIVE
SUN OVER FRESH SNOW COVER...HAVE STUCK WITH HIGHS COOLER THAN MOST
GUIDANCE VALUES ACROSS THE NORTHEAST HALF OF THE CWA TODAY. EXPECT
MORE CLOUD COVER IN SOUTHWEST AREAS...WHICH ALONG WITH WARMER START
SHOULD HELP TEMPERATURES WARM CLOSER TO GUIDANCE IN THAT AREA.

RIDGE AXIS IN THE EAST WILL ALLOW FOR QUICK EVENING DROP IN TEMPS
THERE...FOLLOWED BY STEADY TO WARMING TEMPS IN THE LATE EVENING AND
OVERNIGHT AS CLOUDS EXPAND INTO THE AREA. BOTH NAM AND GFS SHOWING
STRATUS SPREADING INTO THE AREA BENEATH WARMING TEMPS ALOFT TONIGHT
AND HAVE ADDED AREAS OF FOG TO WESTERN PARTS OF THE AREA AFTER 06Z
TONIGHT. WEAK LIFT WITH THE STRONG WARM ADVECTION ABOVE THE FOG AND
STRATUS COULD PRODUCE SOME VERY LIGHT PRECIP...AND QUESTION IS WHAT
TYPE. NAM FORECAST SOUNDINGS INDICATE MUCH STRONGER RADIATIONAL
COOLING...RESULTING IN FOG/STRATUS LAYER MUCH COOLER THAN ON GFS.
THE COOLER TEMPS ON THE NAM WOULD ALLOW FOR SOME ICE CRYSTALS IN THE
STRATUS LAYER AND RESULT IN FLURRIES...BUT THIS SEEMS OVERDONE AND
PREFER WARMER GFS BOUNDARY LAYER TEMPS LATER TONIGHT. UNFORTUNATELY
THIS MEANS NO ICE IN THE CLOUDS...RESULTING IN FREEZING DRIZZLE
THREAT INSTEAD OF FLURRIES. LIFT IS VERY WEAK SO WAS HESITANT TO
INTRODUCE...BUT GIVEN POTENTIAL IMPACTS OF EVEN TRACE AMOUNTS OF
FREEZING DRIZZLE...THOUGHT IT BEST TO INCLUDE AT THIS TIME. KEPT
PRECIP MENTION TIED TO STRONGEST OF WARM ADVECTION...PRIMARILY IN
THE JAMES VALLEY LATE TONIGHT...AND EXPANDING EAST THROUGH THE I-29
CORRIDOR AND FAR WESTERN PORTIONS OF SWRN MN/NWRN IA THURSDAY
MORNING.

ATTENTION THEN TURNS TO A COUPLE OF NORTHERN STREAM WAVES DROPPING
INTO THE NORTHERN PLAINS/UPPER MIDWEST LATE IN THE WEEK AND INTO
THIS WEEKEND. FIRST IS WEAKER OF THE TWO...COMING THROUGH THURSDAY
NIGHT INTO FRIDAY MORNING. TIMING AMONG THE VARIOUS MODELS IS PRETTY
SIMILAR ON THIS...WITH FOCUS OF LIGHT PRECIP POTENTIAL LARGELY OVER
NORTHEAST HALF OF THE CWA. HAVE BUMPED POPS THERE UP TO LOWER END OF
CHANCE RANGE FOR NOW. STRONGER WAVE AFFECTS THE REGION SATURDAY INTO
SATURDAY NIGHT...AND MUCH LESS CONSISTENCY WITH TIMING OF THIS WAVE.
TRACK SIMILAR ON ECMWF/GFS...WITH GEM QUITE A BIT FARTHER WEST AND
DISCARDED AS THE OUTLIER. ECMWF REMAINS ABOUT 6 HOURS FASTER THAN
GFS...THOUGH 00Z RUN HAS SLOWED DOWN A BIT FROM 09/12Z TIMING. WITH
EITHER RUN...SATURDAY LOOKS TO BE MAIN FOCUS OF TIMING AND HAVE
BUMPED POPS FOR THIS PERIOD AS WELL...WITH TARGET AGAIN THE EASTERN
PORTIONS OF THE FORECAST AREA. NEITHER EVENT LOOKS SIGNIFICANT AT
THIS TIME...THOUGH MODELS INDICATE POTENTIAL FOR A TENTH OF AN INCH
OR TWO LIQUID EQUIVALENT...SO WILL BEAR WATCHING TO SEE IF THIS IS
MAINTAINED IN FUTURE MODEL RUNS. ANY SNOWFALL SATURDAY WOULD BE
FOLLOWED BY STRONGER NORTHWEST WINDS SATURDAY NIGHT/SUNDAY...AND
WILL INCLUDE POTENTIAL FOR LIGHT SNOW ACCUMULATIONS AND POSSIBLE
BLOWING/DRIFTING OVER THE WEEKEND IN THE HWO.     /JH

&&

.AVIATION...
CONCERN FOR THIS PERIOD IS LARGELY CENTERED ON EITHER END OF THE
TAF. FOR EARLY THIS MORNING...HIGH PRESSURE ACRS ERN SD AND FAVORED
RADIATIVE COOLING CONDITIONS WL LEAD TO AREAS OF VLIFR FOG/IC/STRATUS
THROUGH THE JAMES AND MISSOURI RIVER VALLEY AREAS. THIS FOG WL BE
SLOW TO DISSIPATE WITH THE STRONG INVERSION AND WK FLOW EARLY THIS
MORNING. NEXT WL COME ADVECTION OF LARGER STRATUS FIELD CURRENTLY
LOCATED FROM ERN CO THRU SWRN KS. THIS SHUD BE ON THE DIRECT TRACK
TO IMPACT FIRST KHON TOWARD 06Z...THEN KFSD/KSUX AREAS AFT
06Z...SINCE WL HAVE EASY NWRD TRANSPORT WITH ESTABLISHED INVERSION.

/CHAPMAN

&&

.FSD WATCHES/WARNINGS/ADVISORIES...

IA...NONE.
MN...NONE.
NE...NONE.
SD...NONE.
&&

$$
 JH/CHAPMAN