FXUS63 KFSD 251541
AFDFSD
AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
940 AM CST FRI DEC 25 2009
.DISCUSSION...
BLIZZARD AND WINTER STORM WARNINGS TO CONTINUE ACROSS THE AREA TODAY
INTO TOMORROW. NO TRAVEL IS ADVISED ACROSS THE AREA AS SOME ROADS
ARE ALREADY IMPASSABLE. DRIFTS UP TO 5 FT HIGH HAVE BEEN REPORTED BY
LAW ENFORCEMENT IN PORTIONS OF SW MN.
LIGHT TO MODERATE SNOWFALL WILL CONTINUE THROUGH THE DAY WITH
PERIODS OF HEAVY SNOW FALL. STILL A LOT OF MOISTURE TO WRAP W INTO
THIS AREA...AS UPPER LEVEL LOW PRESSURE RETROGRADES FROM ERN IA TO
FAR NW IA. DEEP THETAE ADVECTION SLOWLY WEAKENS THROUGH THE
DAY...AND MORE SO TONIGHT. THIS WILL ALLOW FOR GRADUAL DECREASED
INTENSITY OF FALLING SNOW...HOWEVER...SNOW WILL CONTINUE TO FALL
OVER A LARGE AREA WELL INTO SAT.
WINDS CONTINUE TO BLOW VERY HARD INTO S CNTRL SD AND DOWN THE MO
RIVER VALLEY THIS MORNING...WITH GUSTS TO 40 MPH...PRODUCING WHITE
OUT CONDITIONS IN OPEN AREAS. FORTUNATELY...COLD AIR ADVECTION IS
LACKING...HAMPERING BETTER MIXING...OR WINDS WOULD BE EVEN STRONGER
IN THESES AREAS...AS A 50 TO 60 KT NWRLY JET RESIDES OVER THIS AREA.
WINDS WILL DIMINISH SOME THIS AFTERNOON OVER FAR ERN SD AND OVER NW
IA AND SW MN AS WARMER AIR WRAPS WWRD. WINDS IN THESE AREA WILL DROP
TO 10 TO 20 MPH TODAY BUT WILL RE-ACCELERATE TO 20 TO 35 MPH TONIGHT
AS ARCTIC AIR SHIFTS E.
&&
.AVIATION...
TODAY AND TONIGHT WILL BE UGLY. WIDESPREAD LIFR TO IFR CONDS WL
OCCUR WITH SN AND BLSN. NW WINDS WL BE STRONG...AVERAGING 25 TO 45
MPH ALONG AND W OF I 29. LIGHTER WINDS WILL BE FOUND IN SW MN AND NW
IA...BUT PERIODS OF HEAVY SNOW WILL GREATLY REDUCE VISIBILITIES IN
THOSE AREAS ALSO. IN THE BLIZZARD WARNING AREA WHICH BASICALLY
ENCOMPASSES THE I 29 CORRIDOR AND POINTS WWD...VLIFR VIS WL OCCUR AT
TIMES IN THE HEAVIER SNOW BANDS. PILOTS NEED TO STAY ABREAST OF
AIRPORT CLOSURES IN THIS AREA. /MJF
&&
.PREV DISCUSSION...
UPR WAVES DOIN THE OL CHRISTMAS DO-SI-DO THIS MORNING ON WV...WITH
SRN STREAM WAVE LIFTING TOWARS SERN IA...AND THE FORMER NRN STREAM
WAVE COMING IN BEHIND/BENEATH THRU KS. EACH CONSIDERABLY STRONG...
WITH TROP UNDULATIONS DOWN TO 600 HPA OR LOWER...AND IMPRESSIVE
THERMAL FIELDS SFC TO ALFT. MASSIVE MERGING SYSTEM GRABBING A WEALTH
OF MOISTURE FROM VARIOUS SOURCES...AND CAN TRACE SOME TRAJECTORIES
TO BOTH THE GULF AND THE ATLANTIC...50 DWPTS INTO FAR SERN IA NOT
TOO SHABBY.
WHAT COULD VERY WELL BE A ONCE IN A CAREER LATE DECEMBER SYSTEM
REALLY SHOWING ITS STUFF THIS MORNING. INTENSE SFC LOW AT LEAST DOWN
TO 985 HPA S OF KOTM...AND NOT TOO OFTEN THAT ONE CAN GET RAPIDLY
FALLING PRESSURE WELL TO THE NW OF THE SFC WAVE...AND MAX PRESSURE
FALLS EXTEND FROM NWRN IA INTO NERN SD. INVERTED TROUGH EXTENDS TO
THE NW FROM SFC LOW...AND SHOWING SIGNS OF BACKING SWWRD. FORECAST
ISSUE IS THE VERY STRONG THERMAL GRADIENT ACRS BNDRY...WITH 15 TO 20
DEGREES DIFFERENCE IN ABOUT 30 MILES ACRS BNDRY...IN THE LOWER 30S
JUST NE OF THE MINNESOTA RIVER...AND LOWER TO MID TEENS IN NERN CWA.
NAM/RUC ARE FAIRLY CLOSE WITH HANDLING OF THE LOW LVL THERMAL FIELDS
AND PROGRESSION OF INVERTED TROUGH...WHILE GFS SEEMINGLY A BIT TOO
AGGRESSIVE WITH SWRD WARMING. TMPS THIS MORNING A GOOD FIT TO THE
925 HPA +/- A LITTLE MIXING...AND GENERALLY USED THIS TO ALIGN
TRENDS THRU DAY. DO NOT THINK WL WARM ENOUGH TO CREATE ANY PTYPE
ISSUES EVEN THRU NERN CWA...WITH ONLY HIGHS NEAR 30 ANTICIPATED.
FOR THE MOST PART...NOT EXPECT MUCH TO CHANGE WEATHERWISE THROUGH A
BULK OF THE DAY AND EVEN THE EVENING. ALWAYS WARY THAT MODEL
HANDING OF DRY SLOTS ALFT IS OFTEN TENUOUS AT BEST...WL HAVE TO
WATCH FOR A MORE AGGRESSIVE PUSH. RATHER...CONTINUED WAA WITH
DEVELOPING TROWAL...PRECIP SHOULD BE CONSTANT...NOT UNSTABLE ENOUGH
FOR THE TIME TO GET TOO HEAVY...BUT CHANGES AS MAIN PV WRAPS BACK
WWRD MIDDAY AND INTO THE AFTN. SOME SLIGHT INSTABILITY ALFT WORKS
BACK INTO NWRN IA AND FEEDS INTO STRONG MID LVL FRONTAL BAND...AND
WITH DECENT LIFT THRU DENDRITIC ZONE...COULD GET SOME PERIODS OF
HEAVIER SNOW LATER MORNING AND AFTN IN ADVANCE OF STRONG UPR
FORCING. RATIOS BASED ON THE SHALLOW COLD TMPS WOULD SUGGEST A LOT
HIGHER THAN ACTUALITY...AND LOOKING MORE TOWARD 850-700 HPA TMPS TO
OBTAIN 14-18/1 VALUES USED OVER CWA...EVEN DROPPING BACK TOWARD 12/1
IN THE FAR E. PCPN WL CONVERGE TOWARD THE MID LVL FRONTOGENETIC AXIS
BACK THRU WRN CWA.
WINDS ON THE UPSWING FOR MOST AREAS AS SFC WAVE STARTS TO BACK
TOWARD THE CWA. EXCEPTION TO THE STRONGEST WNDS WL BE NERN/ERN CWA
WHERE GRADIENT WL SLACKEN A BIT AS CORE OF LOW PUSHES IN DURING
AFTN/EARLY EVNG. EXPECT WIDESPREAD G35 MPH...AND MUCH OF LOWER MO
VLY THRU JAMES VLY SUPPORTIVE OF GUSTS 45 MPH INTO THE EVENING.
WIDESPREAD LIFE THREATENING BLIZZARD CONDITIONS WL CONTINUE...
AND WITH ERN SD INTERSTATES CLOSED...AND MANY ROADS TRENDING TOWARD
IMPASSIBLE...CERTAINLY TIME TO STAY HOME AND ENJOY THE HOLIDAY.
OCCLUSION PROCESS IN FULL FORCE LATER AFTN AND EVENING WL START TO
COMPLICATE THE PRECIP DISTIBUTION...ESPLY BY SAT AS TROWAL TAKES
MUCH OF THE FIRST PCPN BACK WWRD...EVEN W OF THE CWA. SOME
SUGGESTION THAT WL GET ANOTHER RESURGENCE OF BANDING THRU HEART/ERN
CWA MIDDAY SAT INTO THE EVENING AS NELY JET HELPS TO REFOCUS FRONTAL
FORCING ON COLD SIDE OF TROWAL. WIND GRADIENT WL REMAIN STRONG INTO
SAT ESPLY THRU SD CWA INTO LOWER MO VLY...AND NO CHANGES TO BLIZZARD
HEADLINES. WL CONTINUE TO SEE WNDS STRONG ENOUGH FOR CONSIDERABLE
BLOWING AND DRIFTING INTO SAT EVNG...BUT WITH WNDS LESS THAN 35 MPH
EXPECTED WL NOT EXTEND. SLOW DOWNTREND TO WEAKENING WRAPAROUND BAND
HEADING THRU SAT NIGHT...WITH ACCUMS MINIMAL.
OVERALL...NET RESULT IS A BROAD ADDITIONAL 8 TO 12 INCHES THRU SAT
EVENING. ON TOP OF THE 4-8 INCHES ALREADY THRU THE NIGHT...LKLY TO
FIND SOME 16-20 INCH AMOUNTS FOR STORM TOTALS. HOWEVER...EQUALLY
LKLY TO NEVER KNOW HOW MUCH THERE REALLY IS DUE TO THE WIND.
SUNDAY...WL CONTINUE A FAIR AMT OF CLDS AND WL SEE SCT FLURRIES. A
LITTLE LINGERING SNOWFALL IN FAR ERN CWA NOT OUT OF QUESTION AS
ANOTHER LOBE SCRAPES THROUGH TO E EARLY IN DAY.
TMPS BACK WHERE CLEARING SKIES AND LIGHT WINDS IN SFC RIDGE ARE
SCARY COLD IN THE TEENS TO LOWER 20S BELOW ZERO...AND EVEN WITH
MODELS ADVERTISING A MODERATION TO AIRMASS WITH INTENSE SYSTEM
PUTTING THE MIXMASTER INTO THE THERMAL FIELD...WONDER IF WILL NOT
HAVE WINDOW TO BOTTOM OUT EACH OF MON MORNING WEST...AND TUE MORNING
CNTRL/ERN CWA. WL HEDGE TMPS DOWN TO FIT THIS THINKING AND THE NEW
SNOWFIELD. INCREASED ALBEDO EFFECT WL NOT HELP MATTERS MON OR EVEN
TUE WHEN SOME WARMING ALFT WL COMMENCE. /CHAPMAN
&&
.FSD WATCHES/WARNINGS/ADVISORIES...
IA...BLIZZARD WARNING UNTIL NOON CST SATURDAY FOR IAZ001-012-020-031.
WINTER STORM WARNING UNTIL NOON CST SATURDAY FOR IAZ002-003-013-
014-021-022-032.
MN...WINTER STORM WARNING UNTIL NOON CST SATURDAY FOR MNZ071-072-080-
081-089-090-097-098.
NE...BLIZZARD WARNING UNTIL NOON CST SATURDAY FOR NEZ013-014.
SD...BLIZZARD WARNING UNTIL NOON CST SATURDAY FOR SDZ038>040-050-
052>071.
&&
$$
