FXUS63 KFSD 241556
AFDFSD
AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
953 AM CST THU DEC 24 2009
DISCUSSION...
ALTHOUGH PORTIONS OF THE AREA ARE SEEING A LULL IN THE SNOWFALL THIS
MORNING...SNOW WILL AGAIN BEGIN TO SPREAD ACROSS THE AREA LATER
TODAY AND TONIGHT. MAIN UPPER LOW CURRENTLY OVER THE ARK-LA-TEX
REGION WILL BEGIN TO TRACK NWRD...AS NRN STREAM WAVE WRAPS INTO THE
BACKSIDE. AS THE UPPER LOW SHIFTS N...COPIOUS AMOUNT OF GULF
MOISTURE WILL STREAM NWRD AHEAD OF THE UPPER LOW. ALREADY SEEING
SOME OF THIS MOISTURE SPREADING ACROSS IA IN THE FORM OF CONVECTIVE
SHOWERS. THIS WILL KEEP THE SNOW FALLING ON AND OFF OVER PORTIONS OF
SW MN AND NW THROUGHOUT THE DAY TODAY. UPPER LEVEL ENERGY WILL WRAP
BACK WEST ACROSS THE AREA TONIGHT...PUSHING THE PRECIPITATION BACK
INTO CNTRL SD. SNOW IS EXPECTED TO BE HEAVY AT TIMES AND WHEN
COMBINED WITH GUSTY NRLY WINDS WILL PRODUCE DIFFICULT DRIVING
CONDITIONS. THESE CONDITIONS WILL CONTINUE THROUGH FRIDAY NIGHT AND
INTO SATURDAY. AT THIS TIME...NO CHANGES ARE PLANNED TO THE
HEADLINES. MADE SOME MINOR ADJUSTMENTS TO THE GRIDS. NEW GRIDS OUT
SHORTLY. 97
&&
.AVIATION...
TODAY...THIS AREA WILL BE GENERALLY IN BETWEEN TWO STORM SYSTEMS.
HOWEVER RESIDUAL MOISTURE AND ASCENT WILL STILL GIVE PLENTY OF IFR
TO MVFR CONDS IN PERIODS OF LIGHT SNOW...AND WINDS AVERAGING 20 TO
30 MPH MOVG FM W TO E ACROSS THIS FORECAST AREA THIS MORNING. AS THE
BULK OF THE LARGE STORM LIFTS NWD TONIGHT...LOOK FOR MORE LIFR TO
IFR CONDS IN SNOW AND BLOWING SNOW...PROBABLY VLIFR AT TIMES IN THE
HEAVIER SNOW BANDS. THIS MESS WILL THEN LAST THROUGH CHRISTMAS AND
CHRISTMAS NIGHT...LIKELY INTO SATURDAY MORNING. /MJF
&&
.PREV DISCUSSION...
COMPLEX AND IMPRESSIVE WINTER STORM CONTINUES TO EVOLVE ACROSS THE
CNTRL CONUS THIS MORNING. THE LARGER PICTURE SHOWING SEVERAL PLAYERS
IN THIS LITTLE WINTER GAME...A IMPRESSIVE UPR TROUGH IN NRN STREAM
DIVING THRU ERN MT...WITH STRENGTH MARKED BY TROP DOWN TO ARND 800
HPA. FIRST PIECE IN THE SRN STREAM MOVED THROUGH THE AREA WITH
GENERALLY 3 TO 6 INCHES SNOWFALL DURING THE AFTERNOON AND OVERNIGHT
HOURS. CURRENTLY...IN A RELATIVE LULL IN THE OVERALL SCHEME...WITH
WK WAVE LIFTING NWRD THRU NW IA. EARLIER FRONTAL BAND OF SNOWFALL
WORKED WWRD AND HAS BEEN SLOWLY DISSIPATING THRU THE JAMES VALLEY AS
NET ISENTROPIC LIFT WANES. OTHER AREA OF SNOWFALL LIFTING NWRD THRU
ERN CWA. IS A BIT OF INSTABILITY ALFT THRU ERN CWA AND SHOWERY
LOOKING PRECIP ON RADAR...COINCIDENT WITH A WK AREA OF ISENTROPIC
LIFT MOVING FM SERN NEBRASKA TO NW IA...AND INDICATED NICELY ON 00Z
GFS. NOT OUT OF THE QUESTION THAT A COUPLE SPOTS OF FZDZ ARE INDUCED
BETWEEN ICE BEARING CLDS BEHIND THE EXITING WAVE EARLY THIS
MORNING...AND SOME SLIGHTLY WARMER TMPS ALFT TOUCHING A THIN
FREEZING TEMP COULD EVEN BRING A BRIEF THREAT FOR FZRA/IP IN THE FAR
SERN CWA EARLY ON...BUT COOLER TMPS PUSHING EWRD SHUD ALLOW SNOW TO
BE THE PREDOMINANT TYPE. MOST OF THE SNOW TDA WL BE VERY LIGHT...
BUT MOST AREAS LKLY WL FIND MORE FLAKY TIMES THAN NOT...AS ISENTROPIC
LIFT CONTINUES WK...BATTLING IN THE SHORT TERM THIS MORNING THE
SUBSIDENCE BEHIND EXITING WAVE.
INVERTED TROUGH STARTING TO FOCUS FROM MN/SD BORDER INTO NW IA...
AND INCREASE IN NW WINDS AS FAR E WITH GUSTS TO 35 MPH THRU THE
JAMES VALLEY. THIS SHALLOW ARCTIC PUSH IS A BIT UNDERDONE ON RAW
MODELS...AND POSSIBLE THAT STRONGER LOW LVL DRYING MOVING INTO WRN
CWA WL CUT OFF SNOWFALL A BIT MORE THAN EXPECTED...BUT STILL A LOT
OF LOW END RETURNS BACK THRU WRN/CNTRL SD WHERE MID LVL BNDRY
WORKING EWRD. THESE WINDS THOUGH WL CONTINUE TO PUSH EAST AND
ACTUALLY INCREASE IN MAGNITUDE TOWARD EVENING AS INVERTED TROUGH
SLOWS AND SHARPENS THRU SW MN AND NW IA. BLOWING SNOW THRU WRN CWA
WL BECOME A COMMON FEATURE THRU THE AFTN...AND EVEN THOUGH SNOWFALL
IS DOWN MUCH OF THE DAY...VSBY ISSUES SHOULD INCREASE WITH WINDS.
WL ALSO FIND FALLING TMPS TO W OF INVERTED TROUGH...WITH WIND CHILLS
DROPPING WELL BLO ZERO W OF I29.
WL BE A TREMENDOUS DEGRADATION IN CONDITIONS THIS EVENING AS
PRECIP ASSOCIATED WITH MAIN WAVE LIFTS INTO THE AREA. INTERAL PV
ANOMALY WITH SRN PLAINS CONVECTION SEEMS TO LEND A LITTLE MORE
SUPPORT TO STRONGER CROSS FRONT FLOW INDICATED TONIGHT AND INTO
FRIDAY IN GFS VS NAM...AND SIDED WITH A MORE MAJORITY SLN WHEN
COMPARING FAVORABLY WITH GEM/00Z ECMWF/CANADIAN GLOBAL. THIS SHOULD
YIELD A BROAD AREA OF SNOWFALL TONIGHT...TAPPING SLIGHT INSTABILITY
TO E TO CONTINUE TO REGENERATE AS SHIFTS INTO MID LVL FRONTAL BAND
WITH DEEP ESE FLOW ALFT. EXPECT MAINLY MDT SNOWFALL...BUT SHUD BE
HEAVY AT TIMES INTO CHRISTMAS DAY. THERMAL PROFILES GET A LOT MORE
DENDRITIC FRIDAY...ESPLY THRU WRN CWA...SO IF NOT FOR THE WINDS
WHICH WOULD BREAK THE DENDRITES TO A LESS IMPRESSIVE RATIO. TROWAL
REALLY LOOKS TO BACK WWRD WITH SLOW OCCLUSION PROCESS THRU IA FRI
NIGHT INTO SAT...WITH FRONTOGENESIS ON THE WWRD SIDE EVENTUALLY
GIVING WAY TO THE SERN FLANK BY LTR FRI NIGHT AND SAT...LEAVING WWRD
BAND TO SLOWLY WEAKEN AND REFOCUSED TO SNOW THRU THE ERN CWA.
OVERALL...STILL EXPECTING STORM TOTAL SNOWFALL IN EXCESS OF A
FOOT...AND MAX OF 18-20 INCHES THRU NERN CWA. COUPLED WITH WINDS...
SEEMS INEVITABLE TO GET BLIZZARD CONDITIONS...AND FULLY EXPECT THAT
DAY SHIFT WL BE EVOLVING TO THIS OVER A GOOD PORTION OF THE AREA.
ONE AREA THAT MAY ESCAPE THE WORST WOULD BE FAR ERN CWA FOR A TIME
FRIDAY INTO FRIDAY NIGHT...AS LONG AS WAVE RETROGRADES ENOUGH.
SLOWER YET ON MOST SLNS...SO HAD TO EXTEND WARNING INTO MIDDAY
SATURDAY TO COVER THE WINDS...WHICH SHOULD CONTINUE STRONGEST THRU
SD. REST OF FCST BEYOND SAT NIGHT RELATIVELY MINOR ON CHANGES.
/CHAPMAN
&&
.FSD WATCHES/WARNINGS/ADVISORIES...
IA...WINTER STORM WARNING UNTIL NOON CST SATURDAY FOR IAZ001>003-
012>014-020>022-031-032.
MN...WINTER STORM WARNING UNTIL NOON CST SATURDAY FOR MNZ071-072-080-
081-089-090-097-098.
NE...WINTER STORM WARNING UNTIL NOON CST SATURDAY FOR NEZ013-014.
SD...WINTER STORM WARNING UNTIL NOON CST SATURDAY FOR SDZ038>040-
050-052>071.
&&
$$
