FXUS63 KFSD 082129
AFDFSD
AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
330 PM CST TUE DEC 8 2009
.DISCUSSION...
WV IMAGERY NOW INDICATING DEEP TROUGH STARTING TO TAKE ON A CYCLONIC
WRAP THRU CNTRL PLAINS. LEADING WAVE PUSHING INTO SWRN IA WITH MAIN
WAVE ROUNDING THE BASE THRU THE PANHANDLES OF TX/OK. INSTABILITY TO
THE SOUTH OF THE AREA...BUT VERY LITTLE NORTHWARD ABOVE STRONG MID
LVL FRONTAL BAND. WIDESPREAD WAA SNOWFALL CONTINUES LATE THIS AFTN
OVER THE ENTIRE AREA. SOME OF THE SNOWFALL VARIATIONS THIS AFTN
APPEAR TO BE A RESULT OF THE PORTION OF THE THERMODYNAMIC PROFILE
WITHIN THE DENDRITIC LAYER...WHICH HAS GREATEST DEPTH THRU THE EARLY
TO MID AFTN ALG A YAKNTON TO SIOUX FALLS TO SLAYTON LINE. THIS AREA
SHOWN TO SETTLE SOUTHEASTWARD INTO THE EVENING. EXTREME SERN SD/NERN
NEBRASKA/NWRN IA/SWRN MN WL BE IN A FAVORABLE LOCATION FOR NEXT
SEVERAL HOURS FOR HIGHER SNOWFALL RATES...ON NRN EDGE OF INSTABILITY
GRADIENT/PV FORCING. EVENTUALLY BY 03Z-06Z...THE SECONDARY PV
ADVECTIVE FORCING/DIV Q SHUD REFOCUS THE MORE SIGNIFICANT SNOWFALL
TOWARD THE SE...TAPERING OFF THE MORE BROAD ADVECTIVE FORCING AS
FLOW IS INDUCED LESS CROSS FRONTAL THROUGH THE LATTER EVENING.
OTHERWISE...STRONG CYCLOGENESIS WL TAKE PLACE SOUTHEAST OF THE
REGION TONIGHT...AND SFC GRADIENT WL CONTINUE TO INCREASE. MOST OF
THE WIND IS IN THE LOWER 1 KM OR SO...AND LOWER LVL LAPSE RATES BLO
0.5 KM ARE SUFFICIENT TO TRANSFER MOMEMTUM DOWN...AND THE WIND GUSTS
ALREADY ARND 25 KTS WL BOOST TO A HEALTHY 35+ KT THRU ERN CWA TOWARD
MIDNIGHT...LINGERING THRU THE MORNING HOURS. THE WIND WILL INTERACT
EVEN WITH LESSER FALLING SNOW TO PRODUCE WIDESPREAD VSBYS NEAR ZERO
AT TIMES THRU MUCH OF NW IA AND NEIGHBORING AREAS. WITH ANY
SIGNIFICANT SNOWFALL TAPERED OFF BY LATER TONIGHT...TOTAL SNOWFALL
AMOUNTS ARE EXPECTED TO VARY FROM 3-4 INCHES ARND KHON...TO 8 TO 11
INCHES ACROSS MUCH OF NW IA...EXTREME SERN SD...AND NERN NEBRASKA.
CURRENT HIGHLIGHT REMAIN UNCHANGED FROM THE EARLY AFTERNOON UPGRADE
TO BLIZZARD WARNING FOR FORMER WINTER STORM WARNING AREA. ANTICIPATE
THAT WL END UP WITH SOME WIND CHILL HIGHLIGHTS AFTER EXPIRATION OF
THE SNOW/VSBY INDUCED HIGHLIGHTS. /CHAPMAN
FOR WED THRU FRIDAY...A VERY COLD UPPER TROF GETS SITUATED OVER
CENTRAL CANADA DRAINING ARCTIC AIR OVER FRESH SNOW COVER IN OUR
AREA. THE STRONG SFC LOW EXITS OUR REGION ON WED VIA THE CENTRAL
GRTLKS. DECIDED TO RAISE POPS IN OUR FAR ERN SECTIONS TO THE LIKELY
CATEGORY AS MID AND UPPER QG FORCING REMAINS STUBBORNLY STRONG. SHUT
OFF THE POPS ALONG AND WEST OF A TYNDALL SD TO DE SMET SD LINE EARLY
IN THE MORNING USING THE LIMITING H85 STREAMLINE. BUT THERE WL
LIKELY BE AREAS OF BLSN ALONG AND WEST OF THE JAMES. AT ANY RATE...
SNOW AMOUNTS WED MORNING WL BE LIGHT...BUT A LOT OF BLSN WL BE FOUND
E OF THE JAMES RIVER VALLEY IN RURAL AREAS. FOR TEMPS...WENT CLOSER
TO THE ALREADY FRIGID MAV GUIDANCE AS THERE REALLY WL NOT BE HARDLY
ANY MOVEMENT IN THE TEMPS. SOME SELF DESTRUCTIVE CLEARING IS EVIDENT
ON THE MODEL SDNGS. AND IF THE SUN DOES SHINE...THE ALBEDO WL
TAKEOVER AND WL LIKELY GIVE COOLER READINGS THEN EVEN WHAT I HAVE.
BUT I ENVISION THAT CLEARING SKIES WL AT LEAST TEMPORARILY FILL BACK
PARTIALLY IN WITH COLD AIR STRATOCU...BEFORE POSSIBLY ERODING AGAIN
AS THE AFTERNOON PROGRESSES. WITH THE WIND...WE ARE LOOKING AT A LOT
OF -20 TO -30F WIND CHILLS IN THE MORNING. IF THE WIDESPREAD BLSN
DOES NOT MATERIALIZE IN OUR ERN HALF...THEN OUR HEADLINES MAY BE
CHANGED TO A WIND CHILL ADVY IN THE FUTURE FOR WED.
TEMPERATURES ARE TRICKY WED NIGHT IN HOW FAR WE WL PLUMMET. WE DO
KEEP A WLY FETCH OF SFC AIR...BUT GOING CROSS GRADIENT...THE SPEEDS
COULD FALL OFF MORE THEN WE HAVE. IF THAT HAPPENS...THE FSD CWFA
WITH MOSTLY CLEAR SKIES WL LIKELY SEE A LOT OF DOUBLE DIGITS BLO
ZERO. BUT THIS FCST IS BANKING ON A WEST 5 TO 15 MPH TO HELP
MODERATE MINS SLIGHTLY. OF COURSE...THAT DOES NOT DO THE WIND CHILL
ANY FAVORS. ON THURSDAY...THIS WLY FETCH OF SFC AIR CONTS ALBEIT
MORE BRISK THEN WED NIGHT. MIXING TEMPS TO ABOUT H925 GIVES OUR FCST
HIGH TEMPS. THE WIND LOOKS LIGHTER THUR NIGHT WHEN COMPARED TO WED
NIGHT SO WE WL HAVE TO WATCH FOR PLUMMETING LOWS DESPITE AN OVERALL
WARMER AIR MASS AT H925. AND ON FRIDAY...CONTINUED THE TREND OF
GOING COLDER THEN THE LATEST MEX VALUES AS A SOUTH WIND BEGINS
PRODUCING TERRIBLE MIXING. FRIDAY WL LIKELY NOT EVEN MIX TO H925.
/MJF
IN THE EXTENDED FRI NIGHT THROUGH TUESDAY...OVERALL PATTERN IS
CHARACTERIZED BY A CONTINUED COLD AIR MASS SITUATED OVER THE NRN
HALF OF THE CONUS. THERE ARE DIFFERENCES OF COURSE IN THE S/W
DETAILS PASSING THRU THE FAST FLOW ALOFT. BUT IN GENERAL...ANOTHER
ARCTIC SURGE OF AIR MAY BE COMING DOWN ON SATURDAY. THE MEDR MODELS
WERE IN PRETTY GOOD AGREEMENT IN BRINGING DOWN THIS COLD SURGE IN
THE FSD FA ON SATURDAY. HOWEVER...THE 12Z ECMWF AND CANADIAN GLOBAL
HAVE THROWN A MONKEY WRENCH INTO THE TIMING...SLOWING DOWN THE FROPA
BY NEARLY 12 HOURS. IF THIS HAPPENS...OUR MAX TEMPS FOR SATURDAY WL
BE TOO COLD PROBABLY BY ABOUT 4 TO 7 DEGS. ON THE OTHER HAND...NOT
WILLING TO JUMP ALL OVER THE SLOWER SOLUTIONS YET AS MANY TIMES...
THESE ARCTIC AIR SURGES OVER SNOW COVER MOVE SOUTH MORE RAPIDLY THEN
ANTICIPATED. IF THE ARCTIC FRONT DOES BARREL THROUGH...TEMPS WL SEE
A MINIMAL RISE ON SATURDAY IF NOT FALL ALTOGETHER. SO FAR NOW...DID
THE PERVERBIAL COMPROMISE UNTIL WE CAN GET A BETTER BEAD ON THE
TIMING OF THIS FEATURE. ONE THING IS MORE CERTAIN...AHEAD OF THE
FRONT FRIDAY NIGHT...THERE WL BE A STRENGTHENING SLY FLOW. THIS
SHOULD ACT TO MODERATE OUR MINS FRI NIGHT. ALTHOUGH ONE ALWAYS
WORRIES ABOUT AN EVENING PLUNGE IN TEMPS...ESP THRU OUR ERN HALF
WHERE INITIALLY THE SFC WINDS SHOULD BE LIGHT. AFTERWORDS...WENT BLO
GUIDANCE READINGS SAT NIGHT AS THE FROPA SHOULD BE THRU BY THEN NO
MATTER WHAT SOLUTION YOU GO WITH...COUPLED WITH SUBSIDENCE GIVING
WHAT SHOULD BE FAIRLY CLEAR SKIES. ON SUNDAY AND SUNDAY NIGHT...
IT APPEARS THAT S/W ENERGY WL MOVE RAPIDLY EWD ACROSS THE PLAINS
THRU A QUASI ZONAL FLOW ALOFT. THE NEW 12Z CANADIAN GLOBAL WAS THE
STRONGEST WITH THE DIGGING OF THIS SYSTEM IN THE WRN CONUS BY 12Z
MONDAY. AND IF IT VERIFIED...WOULD BRING ANOTHER BONAFIDE WAA
SNOWFALL TO THIS FA BY SUNDAY NIGHT...PROBABLY LINGERING INTO
MONDAY. BUT THE ECMWF AND GFS WERE MUCH LESS DIGGY AND MORE
PROGRESSIVE AND WERE THE CONSENSUS FOR NOW. IT IS CERTAINLY WORTH
AT LEAST SMALL POPS ATTM SUNDAY AND SUNDAY NIGHT. MONDAY AND TUESDAY
LOOK MAINLY DRY ATTM AS ARCTIC AIR SETTLES OVER THE REGION. GIVEN
H925 TEMPS...THEY WL NOT MODERATE MUCH AT ALL ON DAYS 6 AND 7. /MJF
&&
.AVIATION...
WIDESPREAD IFR/LIFR VSBYS WITH IFR TO LOWER MVFR CIGS WL PREVAIL WELL
INTO THE OVERNIGHT HOURS. SNOW WILL START TO DROP OFF FIRST FOR THE
KHON AREA AFTER MIDNIGHT...BUT WL LINGER INTO THE LATE NIGHT/EARLY
MORNING HOURS ARND KFSD AND KSUX. HOWEVER...NE WINDS WL CONTINUE TO
INCREASE WITH FREQUENT GUSTS ALONG AND EAST OF THE INTERSTATE 29
CORRIDOR 30-35 KTS WL CREATE THE ADDTIONAL THREAT OF BLOWING SNOW.
IFR VSBYS IN BLOWING SNOW LKLY TO LINGER INTO WED MORNING FOR KSUX.
/CHAPMAN
&&
.FSD WATCHES/WARNINGS/ADVISORIES...
IA...BLIZZARD WARNING UNTIL NOON CST WEDNESDAY FOR IAZ001>003-
012>014-020>022-031-032.
MN...BLIZZARD WARNING UNTIL NOON CST WEDNESDAY FOR MNZ089-090.
WINTER WEATHER ADVISORY UNTIL NOON CST WEDNESDAY FOR
MNZ072-080-081-097-098.
WINTER WEATHER ADVISORY UNTIL 6AM CST WEDNESDAY FOR
MNZ071.
NE...BLIZZARD WARNING UNTIL NOON CST WEDNESDAY FOR NEZ013-014.
SD...BLIZZARD WARNING UNTIL NOON CST WEDNESDAY FOR SDZ070-071.
WINTER WEATHER ADVISORY UNTIL NOON CST WEDNESDAY FOR SDZ056-
062-066-067-069.
WINTER WEATHER ADVISORY UNTIL 6AM CST WEDNESDAY FOR SDZ038>
040-050-052>055-057>061-063>065-068.
&&
$$
