FXUS63 KFSD 240304
AFDFSD
AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
905 PM CST WED DEC 23 2009
.DISCUSSION...
HAVE MADE SOME MINOR TWEAKS TO CLARIFY TIMING OF PRECIP. FRONTAL
BAND LIFTING THROUGH THE CWA APPEARS TO LOSE ITS FORCING TOWARDS 06Z
AS ISENTROPIC LIFT WEAKENS AND BECOMES NEUTRAL. EXPECT BAND TO
GRADUALLY FALL APART AFTER 06Z AS IT TRIES TO PIVOT TOWARDS
THE NORTHEAST. UNSTABLE ENVIRONMENT HUGGING THE EASTERN EDGE OF
FORECAST AREA WITH 600-650 MB EQUIVALENT POTENTIAL VORTICITY
REMAINING NEGATIVE THROUGHOUT THE NIGHT ACROSS NORTHWEST IOWA NOSING
INTO SOUTHWEST MINNESOTA. HAVE KEPT POPS FAIRLY HIGH THROUGHOUT MUCH
OF THE NIGHT WITH WEAK CONVECTIVE TYPE ACTIVITY EXPECTED.
PRECIPITATION TYPE MAINLY LOOKS TO BE SNOW ACROSS THE FORECAST
AREA...WITH THE EXCEPTION OF DOWN NEAR STORM LAKE WHERE WE HAVE
RECEIVED REPORTS OF SLEET WITH CONVECTIVE ACTIVITY. AS THIS SYSTEM
CONTINUES TO DEEPEN...EXPECT WINDS TO INCREASE FROM THE WEST
RESULTING IN AREAS OF BLOWING SNOW. MINOR UPDATE ON THE WAY. /BT
&&
.AVIATION...
STILL EXPECTING MAINLY IFR TO LOW END MVFR CEILINGS AND VISIBILITY
THROUGH THE NIGHT IN EITHER SNOW OR FOG. BY TOMORROW MORNING...MUCH
OF THE SNOW WILL EITHER BE VERY LIGHT OR END FOR MUCH OF THE DAY
BEFORE THE NEXT BAND OF SNOW WRAPS IN FROM THE EAST. THE WIND WILL
SLOWLY INCREASE OUT OF THE NORTH OR NORTHWEST THROUGH THE DAY AND BY
THURSDAY NIGHT INTO FRIDAY NORTH WINDS GUSTING TO AROUND 35 MPH WILL
BE POSSIBLE WITH IFR OR LIFR CEILINGS AND VISIBILITIES.
&&
.PREV DISCUSSION...
INIT SHOT OF PCPN MOVG INTO SRN FCST AREA AND BACKING NWWD PRETTY
MUCH AS EXPECETD. PCPN BEGAN AS ZR/IP FROM SUX E AND QUICKLY WENT
TO SNOW AT SUX AS EXPECTED. SOME ZR/IP WILL LINGER LONGER E OF SUX
BUT XCP FOR SLB AREA...PCPN SHUD GO MOSTLY TO SNOW THIS EVE.
MEANWHILE...SNOW SHUD BACK UP INTO THE FAR WRN FCST AREA THO WITH
SOMEWHAT LESS INTENSITY TO THE W. SNOW ACCUM FOR TNGT SHUD BE MOSTLY
2-4 INCHES AS SUPPORT FOR ANY REALLY DECENT SNOWBURSTS WANES WITH
TIME. CONTINUED INDICATIONS OF SOME WARMING ALF SHUD BRNG A MINOR
RESURGENCE OF ZR/IP E OF SUX AND UP N TO ABOUT SPW LATE TNGT...
OUSIDE OF THIS AREA PCPN WILL BE SNOW. MEANWHILE...WITH MOISTURE BUT
ONLY WEAK SUPPORT BEHIND THIS BAND AS IT MOVES ON N/NW...PCPN
INTENSITY SHUD DROP OFF FROM THE S OVER ERN FCST AREA DURG THE
NIGHT...THO WILL KEEP HI 80 TYPE POPS FOR LATE TNGT AS LIGHT LOW
BASED PCPN LIKELY TO CONT.
COLDER AIR FLOWING INTO WRN FCST AREA AS SYS CONTS TO DVLP SHUD DROP
TEMPS INTO THE TEENS FAR W LATE TNGT...OR ABOUT 10 DEGS COLDER THAN
CURRENT VALUES. FRTHR E...LOWS SHUD TEND TO BE 5 DEGS OR LESS COLDER
THAN CURRENT READINGS. THE COLD AIR SURGE IN THE FAR W WILL BE AIDED
BY INCREASING NNWRLY WINDS. WINDS SHUD BE STG ENUF TO CAUSE AREAS OF
BLOWING SNOW FAR W LATE TNGT. THO THERE MAY NOT BE A LOT OF BLOWABLE
SNOW YET...FALLING SNOW WILL BE ONGOING OUT W SO THE BLOWING SNOW
SEEMS APPROPRIATE. /WILLIAMS
FIRST SHOT OF SNOW WL BE EXITING CWA IN THE EARLY MORN...AND MOST OF
DAY MAY ONLY SEE LIGHT SNOW AHEAD OF MAIN UPPER LOW THAT WL LIFT NWD
THU NITE INTO FRI. SO WL KEEP WDSPRD SNOW ON THU BUT GENLY ON THE
LIGHT SIDE DURING THE DAY WITH MAYBE AND INCH OR TWO AT BEST. WINDS
SHUD GRADUALLY PICK UP DURING THE DAY ESPEC WRN CWA AND EXPECT TO
SEE BLOWING SNOW DEVELOP ALONG AND W OF I29 BY AFTERNOON.
NRN STREAM UPPER LOW DIGGING SEWD ACRS ERN MT WL BEGIN TO PHASE IN
WITH SRN WAVE COMING OUT TX ON THU AND THIS SHUD REALLY HELP TO
INTENSIFY SFC LOW ACRS NRN ARK THU AFTN AND THEN MOVE NWD INTO IA BY
FRI MORN. VERY MOIST ELY FLOW ALOFT WILL CONT TO THEN ADVECT WWD
OVER COLDER AIR IN THE LOW LEVELS TO CREATE A WDSPRD OVER RUNNING
SNOW ACRS THE CWA THU NIGHT ALL THE WAY INTO SAT MORN. THIS SHUD
EASILY ALLOW SNOWFALL AMOUNTS TO BE IN THE 12 TO 20 INCH RANGE ACRS
THE CWA WITH THE LOWER AMOUNTS ACRS FAR WRN CWA. THE OTHER FACTOR WL
BE THE WINDS THAT WL INCREASE ACRS THE REGION THU NITE INTO SAT AS
NRN UPPER LOW PHASES WITH SRN STREAM. EVEN THO NOT EXPECTED TO SEE
STG CAA WITH THE PHASING...STILL ENOUGH TIGHTENING OF SFC GRAD TO
PRODUCE GUSTS TO 35 TO 40 MPH THU NITE INTO FRI NITE. AT LEAST TEMPS
WL NOT BE BITTERLY COLD WITH THIS SYS AS ISOTHERMAL SOUNDING UP TO
AROUND H7 SO NOT EXPECTING SUB ZERO TEMPS WITH THIS STORM. IN
FACT...TEMPS MAY HOLD STEADY THRU FRI NIGHT BEFORE FINALLY STARTING
TO COOL AS UPPER LOW BEGINS TO PULL AWAY. WL NEED TO KEEP CHC OF S-
AND BS ACRS CWA THRU SAT EVE...ALTHO BULK OF ACCUM SNOW SHUD BE OVER
BY FRI EVE.
SUN WL REMAIN CLOD AND WITH UPPER LOW OVR GREAT LAKES REGION...PBLY
WL NOT SEE MUCH SUN. WL KEEP TEMPS BLO NORM MON THRU WED. MAY SEE A
CHC OF S- ON WED WITH WEAK S/W. FOR NOW WL LEAVE SUN THRU WED DRY
WITH MAIN FOCUS OF THIS FCST ON THIS BIG STORM.
&&
.FSD WATCHES/WARNINGS/ADVISORIES...
IA...WINTER STORM WARNING UNTIL 6 AM CST SATURDAY FOR IAZ001>003-
012>014-020>022-031-032.
MN...WINTER STORM WARNING UNTIL 6 AM CST SATURDAY FOR MNZ089-090-098.
WINTER WEATHER ADVISORY UNTIL 6 PM CST THIS EVENING FOR MNZ071-
072-080-081-097.
WINTER STORM WARNING FROM 6 PM THIS EVENING TO 6 AM CST
SATURDAY FOR MNZ071-072-080-081-097.
NE...WINTER STORM WARNING UNTIL 6 AM CST SATURDAY FOR NEZ013-014.
SD...WINTER STORM WARNING UNTIL 6 AM CST SATURDAY FOR SDZ050-057>071.
WINTER WEATHER ADVISORY UNTIL 6 PM CST THIS EVENING FOR
SDZ038>040-052>056.
WINTER STORM WARNING FROM 6 PM THIS EVENING TO 6 AM CST
SATURDAY FOR SDZ038>040-052>056.
&&
$$
