FXUS63 KARX 141732 AAA
AFDARX
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LA CROSSE WI
1132 AM CST MON DEC 14 2009
.UPDATE...
WILL BE ALLOWING WINTER WEATHER ADVISORY TO EXPIRE AT NOON. RADAR
SHOWING BACK EDGE OF ACCUMULATING SNOW MOVING RAPIDLY EASTWARD...
NOW EXTENDING FROM BLACK RIVER FALLS WISCONSIN...TO WEST UNION IOWA
LINE. DID PUSH BACK THE HIGHER PROBABILITIES INTO THE EARLY
AFTERNOON HOURS ACROSS THE FAR EAST...BUT OTHERWISE REMAINDER OF
FORECAST ON TRACK. UPDATED SET OF PRODUCTS WILL BE OUT AROUND NOON
OR SHORTLY THERE AFTER.
&&
JLR
.PREV DISCUSSION...
.SHORT TERM...THROUGH WEDNESDAY NIGHT
PRIMARY FORECAST CONCERNS WERE ASSOCIATED WITH PRECIPITATION
AND ITS IMPACT ON TRAVEL THROUGH THIS MORNING...THEN WIND CHILLS
LATE TONIGHT INTO TUESDAY MORNING.
SURFACE ANALYSIS EARLY THIS MORNING HAD ARCTIC FRONT EXTENDING FROM
SOUTHERN WI ACROSS CENTRAL IOWA TO LOW PRESSURE NEAR KMCI.
PRECIPITATION WAS INCREASING IN COVERAGE ACROSS THE UPPER MISSISSIPPI
VALLEY PER REGIONAL RADAR COMPOSITE. PRECIPITATION TYPE WAS FREEZING
DRIZZLE OVER CENTRAL AND SOUTH PARTS OF THE FORECAST AREA INTO
EASTERN AND CENTRAL IOWA. CALLS TO SHERIFF DEPARTMENTS INDICATED
THERE WAS SOME GLAZING OCCURRING...ESPECIALLY ON SECONDARY ROADS.
MEANWHILE...SNOW WAS EXPANDING FROM NORTHWEST WI INTO CENTRAL AND
SOUTHERN MN.
PRECIPITATION EXPANSION WAS DUE TO MANIFESTATION OF UPPER LEVEL
DIVERGENCE ASSOCIATED WITH COUPLED 300MB JET STRUCTURE...ALONG WITH
ONSET OF DEEP Q-G CONVERGENCE WITH EJECTING MID LEVEL AND SURFACE
WAVE FROM THE CENTRAL PLAINS. IN FACT...WATER VAPOR IMAGERY SHOWED
A VIGOROUS SHORT WAVE TROUGH ADVANCING ACROSS KANSAS...WHICH WAS
ENHANCING LARGE SCALE ASCENT OVER THE FORECAST AREA.
14.00Z MODELS IN BOTH THE NCEP AND EUROPEAN DATA SUITE HAD EXCELLENT
CLUSTERING OF UPPER LEVEL AND SURFACE FEATURES THROUGH THE MIDDLE
PART OF THE WEEK. THIS WAS CORROBORATED BY 14.00Z ARXWRF HIGH
RESOLUTION MODEL AND 14.03Z SREF DATA. THUS...FORECAST CONFIDENCE
WAS HIGH IN OVERALL DETAILS. AS A RESULT...CONFIDENCE WAS HIGH THAT
LARGEST SNOWFALL AMOUNTS OF 2 TO 4 INCHES EXPECTED ACROSS NORTHERN
PARTS OF THE FORECAST AREA THIS MORNING...WITH LESSER AMOUNTS
ELSEWHERE.
TODAY...UPWARD VERTICAL MOTION SIGNAL ASSOCIATED WITH PASSAGE
OF SURFACE AND UPPER LEVEL WAVE EXPECTED TO BE TRANSITORY...AS
INDICATED IN TIME/HEIGHT CROSS SECTIONS. THIS WAS REFLECTED WELL BY
Q-G CONVERGENCE EXITING QUICKLY THIS MORNING...FOLLOWED BY SUBSIDENCE
THIS AFTERNOON. ISENTROPIC UPGLIDE IN THE 280-290K LAYER ALSO
INDICATED TO EXIT BY ABOUT 14.15Z. THEREFORE...MOST PRECIPITATION
SHOULD OCCUR THROUGH THE MORNING AND HAVE MAINTAINED PROBABILITIES
AREA-WIDE IN THE 80-100 PERCENT RANGE.
WITH REGARD TO THE CURRENT WINTER WEATHER ADVISORY FOR NORTHERN
PARTS OF THE FORECAST AREA...WE HAVE OPTED TO MAINTAIN AREAL EXTENT
AS IS. THIS WAS BASED ON ONGOING SNOW FROM CENTRAL MN INTO NORTHERN
WI...AFFECTING OUR NORTH CENTRAL WI ZONES...PLUS SNOW ASSOCIATED
WITH KANSAS SHORT WAVE...WHICH SHOULD SPREAD ACROSS THE REMAINDER OF
THE FORECAST AREA. AS MENTIONED EARLIER...CONFIDENCE WAS HIGH IN
SNOW ACCUMULATIONS OF 2 TO 4 INCHES OVER NORTH CENTRAL WI ZONES...
WHICH COBB OUTPUT CORROBORATED. LESSER AMOUNTS ANTICIPATED ELSEWHERE.
THERE WAS CONCERN THAT FREEZING PRECIPITATION COULD LINGER OVER
MAINLY SOUTH AND EAST SECTIONS OF THE FORECAST AREA...BETWEEN
14.12Z-14.15Z...WHICH WOULD IMPACT THE MORNING COMMUTE. BUFKIT
SOUNDINGS CERTAINLY SUGGEST THIS COULD BE THE CASE BEFORE DEEPER ICE
CLOUDS MOVE IN AND CHANGE PRECIPITATION TO SNOW. THEREFORE...WE
LINGERED FREEZING DRIZZLE ACROSS LOCATIONS MAINLY IN CENTRAL AND
SOUTHWEST WI EARLY THIS MORNING. BECAUSE OF GLAZING THAT OCCURRED
OVERNIGHT...WE DECIDED TO EXPAND WINTER WEATHER ADVISORY FOR REST OF
THE FORECAST AREA TOO...ESPECIALLY SINCE MORNING COMMUTE WILL BE
IMPACTED.
LATE TONIGHT INTO TUESDAY MORNING...ARCTIC AIR FILTERS IN DURING
THIS TIME FRAME AS SURFACE HIGH PRESSURE BUILDS IN. COMBINATION OF
EXPECTED TEMPERATURES AND WIND SPEEDS BEGIN TO PLACE WIND CHILL
VALUES INTO THE ADVISORY CRITERIA FOR PARTS OF SOUTHEAST MN AND
NORTHEAST IOWA. WITH WINTER WEATHER ADVISORIES IN EFFECT THIS
MORNING...OPTED TO LET THIS ISSUE BE ADDRESSED WITH SUBSEQUENT
FORECASTS.
.LONG TERM...THURSDAY THROUGH SUNDAY
PRIMARY FORECAST CONCERNS REMAIN FOCUSED ON PRECIPITATION CHANCES.
14.00Z DETERMINISTIC GFS AND ECMWF WERE IN AGREEMENT THAT 500MB
PATTERN AMPLIFIES CONSIDERABLY. THIS WAS BASED ON GROWING CONSENSUS
THAT THERE COULD BE RETROGRESSION OF LARGE MID LEVEL LOW PRESSURE
FROM SOUTHEAST CANADA WESTWARD INTO THE GREAT LAKES REGION. MODEL
DIFFERENCES NOTED WERE WITH HOW QUICKLY THIS RETROGRESSION OCCURS.
IN ADDITION...TIMING OF ANY SHORT WAVE TROUGHS CONTINUES TO BE
PROBLEMATIC WITH MODEL VARIABILITY. THEREFORE...CONFIDENCE REMAINS
LOW WITH REGARD TO DETAILS ASSOCIATED WITH PRECIPITATION CHANCES
AND THUS NO CHANGES WERE MADE TO THE EXTENDED OUTLOOK.
&&
.AVIATION...TODAY AND TONIGHT
PCPN WILL IMPACT AVIATION EARLY THIS MORNING...IN THE FORM OF -SN
AND/OR -FZDZ. MOSTLY -SN AT KRST NOW...WHILE -FZDZ CONTINUES AT
KLSE. BUFKIT SOUNDINGS SUGGEST THAT THIS SHOULD CHANGE OVER TO ALL
SNOW AT KLSE BETWEEN 12-14Z THIS MORNING...AS AN UPPER LEVEL
SHORTWAVE APPROACHES...SATURATING OUT A MID LEVEL DRY LAYER...AND
INTRODUCING ICE INTO THE COLUMN. HOWEVER...SOME LIGHT ICING WILL
HAVE TO BE ACCOUNTED FOR AT KLSE THROUGH THE EARLY MORNING HOURS.
THE BULK OF THE ISENTROPIC LIFT/QG CONVERGENCE WITH THE SHORTWAVE
EXIT AFTER 18Z...AND EXPECT THE BULK OF ANY ACCUMULATING PCPN TO
SHIFT EAST OF THE TAF SITES BY THIS TIME.
FOR CLOUDS...IFR TO MVFR CEILINGS SHOULD HOLD THOUGH AT LEAST
00Z...BUT A SFC HIGH WILL START TO INFLUENCE THE REGION FROM THE
WEST...WITH THIS HIGH MOVING JUST SOUTH OF THE LOCAL AREA BY 06Z
WED. RH FIELDS...TIME/HEIGHT X-SECTIONS...AND BUFKIT SOUNDINGS
SUGGEST DECREASING SATURATION TONIGHT...AND WILL LIKELY TREND TOWARD
MORE SCT SKIES OVERNIGHT AT THE TAF SITES.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&
$$
SHORT/LONG TERM...THOMPSON
AVIATION..........RIECK
