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