FXUS63 KARX 110306 AAA
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
906 PM CST THU DEC 10 2009
.UPDATE...
STRATUS CLOUDS THAT DEVELOPED DURING THE AFTERNOON HOURS OVER THE
FORECAST AREA...ALSO PRODUCING A FEW FLURRIES...HAVE BASICALLY
DISSIPATED. THESE CLOUDS WERE MOSTLY A RESULT OF ISENTROPIC LIFTING
ON THE 265K SURFACE...JUST SOUTH OF A FAST MOVING SHORTWAVE CROSSING
CENTRAL WISCONSIN. THAT SURFACE NOW SHOWS ISENTROPIC DESCENT PER
10.18Z/11.00Z NAM...WHICH EXPLAINS THE DECREASE IN CLOUD COVER.
OTHER SCATTERED STRATUS CLOUDS EXTEND FROM WESTERN NORTH DAKOTA INTO
NORTH CENTRAL IOWA PER 11-3.9UM SATELLITE IMAGERY AND SURFACE OBS.
THESE ALSO ARE IN A VERY WEAK ISENTROPIC LIFTING REGIME ON THE 265K
SURFACE...WHICH LOOKS TO REMAIN BASICALLY IN PLACE THROUGH THE
NIGHT...OR PERHAPS A SLIGHT SHIFT TO THE EAST. EITHER WAY...IT
APPEARS MOST OF THE FORECAST AREA SHOULD END UP MOSTLY CLEAR THE
REST OF THE NIGHT. THEREFORE...HAVE ADJUSTED CLOUD COVER
DOWNWARD AND REMOVED THE FLURRY MENTIONS IN THE FORECAST. WINDS ARE
A LITTLE ISSUE AS WELL. MODELS AND EVEN MOS GUIDANCE ARE WAY
OVERDOING THE WINDS AT ALL OBSERVATION SITES...SUGGESTIVE A HIGHER
DENSITY AIRMASS THAN PROGGED. THIS IS REFLECTED IN SURFACE
TEMPERATURE PROGS FROM MODELS AND MOS GUIDANCE WHERE READINGS ARE
TOO HIGH VERSUS OBSERVATIONS. THE HIGHER DENSITY AIRMASS APPEARS TO
BE AFFECTING THE PRESSURE GRADIENT NOW PER LATEST SURFACE
ANALYSIS...AS IT DOES NOT LOOK AS TIGHT ACROSS THE FORECAST AREA AS
PROGGED. THEREFORE...DECREASED WIND SPEEDS AND LOWERED TEMPERATURES.
WIND CHILL ADVISORY FOR TAYLOR AND CLARK COUNTIES STILL LOOKS
GOOD...EVEN WITH THE DECREASE IN WIND SPEEDS AND TEMPERATURES. WOULD
NOT BE SURPRISED TO SEE A FEW OTHER SITES OUTSIDE OF THE ADVISORY
AREA TOUCH ADVISORY CRITERIA...SUCH AS ROCHESTER...BUT BOTH THE
AREAL AND TEMPORAL COVERAGE DOES NOT WARRANT EXPANSION AT THIS TIME.
&&
.SHORT TERM...TONIGHT THROUGH SUNDAY NIGHT
MAIN FOCUS OF THIS FORECAST PERIOD IS ON THE WIND CHILLS OVERNIGHT
INTO FRIDAY MORNING ALONG WITH THE CHANCES FOR AN ACCUMULATING
SNOW SATURDAY NIGHT INTO SUNDAY.
ALTO CU HAS BEEN ROLLING INTO THE REGION AHEAD OF A SHORT WAVE
TROUGH SLOWLY PROPAGATING THROUGH CENTRAL MINNESOTA. A FEW
AUTOMATED SITES ACROSS SOUTHERN MN HAVE BRIEFLY REPORTED SOME -SN
FROM TIME TO TIME...SO EXPECT THIS TREND TO CONTINUE EASTWARD INTO
OUR REGION TONIGHT. NAM AND GFS BUFKIT SOUNDINGS ARE NOT SHOWING
MUCH IN THE WAY OF OMEGA WITH THIS TROUGH PASSAGE...SO AM NOT
EXPECTING ANY SNOW ACCUMULATION OVERNIGHT.
WITH TEMPERATURES EXPECTED TO DROP BACK BELOW ZERO TONIGHT AND
WINDS REMAINING AROUND 10 MPH OR SO...WIND CHILLS ARE EXPECTED TO
MAINLY DROP INTO THE -10 TO -20 RANGE ACROSS MOST OF THE FORECAST
AREA. THE COLDEST AREA APPEARS TO BE TAYLOR AND CLARK COUNTIES
WHERE WIND CHILLS OF -20 TO -25 ARE EXPECTED. THEREFORE...WILL BE
ISSUING A WIND CHILL ADVISORY STARTING AT 9PM TONIGHT RUNNING TO
9AM FRIDAY MORNING. THE MAIN LIMITING FACTOR IS CLOUD COVER WHICH
AT THIS POINT SEEMS TO BE AT LEAST SCATTERED TO BROKEN...BUT IF
THIS DECK FILLS IN OR DOESN'T CLEAR OUT OVERNIGHT...TEMPS MAY NOT
DROP AS LOW AS ADVERTISED. CONDENSATION PRESSURE DEFICITS INCREASE
OUT AHEAD OF THE TROUGH...BUT SHIFT OFF TO THE EAST BY DAYBREAK
FRIDAY.
BEYOND THIS THE SHORT TERM GUIDANCE IS IN GOOD AGREEMENT FOR
GRADUALLY BRINGING WARMER AIR BACK INTO THE REGION AS A RIDGE AXIS
OF HIGH PRESSURE PROPAGATES THROUGH...WHICH WILL BRING SFC WINDS
BACK AROUND TO THE SOUTHWEST. 850MB TEMPS WILL GRADUALLY IMPROVE
FROM AROUND -22C TONIGHT TO -15C BY FRIDAY AFTERNOON...AND TO -5C
BY SATURDAY. HIGHS WILL CLIMB BACK INTO THE MID TO UPPER TEENS ON
FRIDAY AND MID TO UPPER 20S ON SATURDAY AS THE GRADUAL WARMING
CONTINUES. DID CONTINUE THE TREND OF KNOCKING A FEW DEGREES OFF OF
THE TEMPS WITH THE FRESH SNOW PACK IN TACT.
NEXT BEST CHANCE FOR AN ACCUMULATING SNOWFALL APPEARS TO BE ON
SATURDAY NIGHT INTO SUNDAY AS A SHORTWAVE TROUGH CURRENTLY OFF THE
COAST OF CALIFORNIA PROPAGATES INTO THE UPPER MISSISSIPPI RIVER
VALLEY. SHOULD SEE AN AREA OF LIGHT SNOW DEVELOP ALONG THE LEADING
EDGE OF WARM AIR ADVECTION AT 850MB AND AHEAD OF THIS MID LEVEL
TROUGH. THIS TROUGH SEEMS TO DEEPEN AS IT GETS EAST OF US...WHICH
WOULD MATCH UP WITH THE BEST ISENTROPIC FORCING...SO THE BETTER
CHANCES FOR AN ACCUMULATING SNOW WILL LIKELY BE OFF TO THE EAST IN
SOUTHERN WISCONSIN. FOR NOW...WILL BE GOING FOR ABOUT AN INCH OF
SNOW IN SOUTHWEST WISCONSIN.
.LONG TERM...MONDAY THROUGH THURSDAY
MAIN FORECAST CHALLENGE WITH THE EXTENDED PERIOD IS WITH TIMING
ANY SHORTWAVE TROUGHS THAT DIG DOWN FROM CANADA FOR NEXT WEEK. HAVE
MAINLY LEFT THE FORECAST OVER THIS PERIOD DRY...BUT ADDED A SLIGHT
CHANCE FOR SOME LIGHT SNOW ON THURSDAY FOR WHAT APPEARS TO BE THE
STRONGEST TROUGH COMING ACROSS. TEMPERATURES WILL BE BELOW NORMAL
THROUGH MOST OF THE WEEK WITH NORTHWESTERLY UPPER LEVEL FLOW IN
PLACE. THE COLDEST AIR WILL LIKELY TUESDAY MORNING INTO WEDNESDAY
AS THE CORE OF A HIGH PRESSURE SYSTEM MOVES ACROSS THE PLAINS.
&&
.AVIATION...TONIGHT AND FRIDAY
LATEST WATER VAPOR SATELLITE IMAGERY INDICATING A COUPLE OF WEAK
SHORTWAVE TROUGHS EMBEDDED IN NORTHWEST FLOW ALOFT OVER THE NORTHERN
PLAINS AND UPPER MIDWEST. FIRST IMPULSE OVER CENTRAL MINNESOTA
PRODUCING SCATTERED STRATOCUMULUS DECK CLOUDS AND A FEW FLURRIES
OVER THE MINNESOTA RIVER VALLEY AND SOUTH CENTRAL MINNESOTA EARLY
THIS AFTERNOON. CEILINGS RANGE FROM 2500 TO 5000 FEET WITHIN
STRATOCUMULUS DECK. THIS DECK CLOUDS/WEAK WAVE WILL MOVE EAST OVER
THE AREA THROUGH THE AFTERNOON AND EARLY EVENING HOURS. SKY
CONDITIONS ARE EXPECTED TO REMAIN SCATTERED AND CEILINGS REMAIN VFR
AT BOTH KRST AND KLSE TAF SITES THIS AFTERNOON AND EVENING. SKY MVFR
CONDITIONS MAY BE POSSIBLE THOUGH FOR A SHORT DURATION TIME. HAVE
KEPT VFR CONDITIONS THROUGH THIS EVENING. NEXT SHORTWAVE OVER THE
WESTERN DAKOTAS PUSHES EAST INTO THE CENTRAL MINNESOTA AND NORTHERN
WISCONSIN. WEAK FORCING IN ASSOCIATION WITH SHORTWAVE TROUGH SHOULD
ALLOW FOR THE POSSIBILITY OF FLURRIES ACROSS THE REGION. DUE TO
TIMING ISSUES AND SHORT DURATION OF MVFR CONDITIONS...HAVE KEPT
SCATTERED 2000-2500 FEET AT BOTH KRST AND KLSE TAF SITES AFTER 07Z
FRIDAY.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...WIND CHILL ADVISORY UNTIL 9 AM CST FRIDAY FOR WIZ017-WIZ029.
MN...NONE.
IA...NONE.
&&
$$
UPDATE...AJ
SHORT/LONG TERM...HALBACH
AVIATION...DTJ
