FXUS63 KARX 112103
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
302 PM CST FRI DEC 11 2009
.SHORT TERM...TONIGHT THROUGH MONDAY
MAIN FORECAST CONCERNS ARE PRECIPITATION TYPE SATURDAY NIGHT...THEN
AGAIN SUNDAY NIGHT INTO MONDAY.
LATEST 19Z SURFACE ANALYSIS INDICATING ELONGATED RIDGE EXTENDING
FROM OVER THE OHIO RIVER VALLEY INTO SOUTHERN ONTARIO CANADA. DECENT
SUBSIDENCE UNDERNEATH SURFACE RIDGE IS ALLOWING FOR SUNNY SKIES
ACROSS THE UPPER MIDWEST AND GREAT LAKES REGION PER LATEST RUC TIME
HEIGHT CROSS SECTIONS AND VISIBLE SATELLITE IMAGERY.
THE 11.12Z GFS AND NAM ARE IN DECENT AGREEMENT WITH OVERALL UPPER
LEVEL PATTERN THROUGH THE PERIOD. THOUGH...BOTH THE 11.12Z GFS AND
NAM BEGIN TO DIVERGE ON PLACEMENT AND STRENGTH OF SHORTWAVE
TROUGH/SURFACE FEATURES AFTER 00Z SUNDAY. THIS WILL HAVE IMPACTS ON
PRECIPITATION TYPE FOR SUNDAY NIGHT INTO MONDAY...AS THE 11.12Z GFS
FARTHER NORTH WITH SURFACE/850MB LOW THAN NAM. BOTH MODELS ARE IN
GOOD AGREEMENT OF SURGING WARMER AIR ALOFT AND MOISTENING BOUNDARY
LAYER SATURDAY NIGHT INTO EARLY SUNDAY MORNING. CONFIDENCE IN
FORECAST THROUGH 00Z SUNDAY IS HIGH...THEN CONFIDENCE BEGINS
TO DECREASE CONSIDERABLY AFTER 00Z MONDAY.
TONIGHT AND SATURDAY...SURFACE RIDGE BUILDS EAST OF THE AREA...THIS
WILL ALLOW FOR SOUTHERLY WIND FLOW ALOFT AND AT SURFACE ACROSS
FORECAST AND WARMER AIR ALOFT TO ADVECT INTO THE REGION. MINIMUM
TEMPERATURES ARE EXPECTED TO BOTTOM OUT THIS EVENING AND REMAIN
NEARLY STEADY AFTER 06Z SATURDAY.
FOCUS THEN TURNS TO THE STRONG WARM AIR ADVECTION AND MOISTENING OF
BOUNDARY LAYER UNDER STRONG INVERSION AFTER 00Z SUNDAY. BOTH THE
11.12Z GFS AND NAM INDICATING DECENT ISENTROPIC LIFT AT THE 295K
SURFACE IN ASSOCIATION WITH THE MOISTENING OF BOUNDARY LAYER AFTER
00Z SUNDAY. THIS WILL PROVIDE MIXING AND WEAK VERTICAL MOTION TO
PRODUCE PRECIPITATION ACROSS FORECAST AREA...ESPECIALLY OVER
THE SOUTHERN FORECAST AREA. THE BIG QUESTION IS THE INTRODUCTION OF
ICE INTO THE SUB CLOUD LAYER...THOUGH BUFKIT FORECAST SOUNDINGS
INDICATE NO ICE INTRODUCTION FROM ALOFT INTO THE SUB CLOUD LAYER.
BOTH THE 11.12Z GFS AND NAM BUFKIT FORECAST SOUNDINGS DO INDICATE
TEMPERATURES ARE BELOW FREEZING IN THE SATURATED LAYER UP TO THE
INVERSION. THIS WILL HAVE THE POTENTIAL OF FREEZING DRIZZLE AND LIGHT
SNOW MIXTURE ACROSS FORECAST AREA...SINCE THE CLOUD DEPTH LAYER
UNDERNEATH INVERSION LAYER WILL BE MORE THAN 2000 FEET ACROSS MUCH OF
THE FORECAST AREA...PER 11.12Z GFS AND NAM BUFKIT FORECAST SOUNDINGS.
FOR NOW HAVE KEPT MENTION OF FREEZING DRIZZLE AND LIGHT SNOW CHANCES
FOR 00-12Z SUNDAY AND INCREASED PRECIPITATION CHANCES. THE POTENTIAL
FOR ANY WINTER WEATHER ADVISORIES BEING ISSUED FOR SATURDAY NIGHT
WILL DEPEND ON IF MORE MOISTURE AND FORCING COMES INTO PLAY.
NEXT SHORTWAVE TROUGH/SURFACE LOW AFFECTS THE FORECAST AREA SUNDAY
NIGHT INTO MONDAY. BOTH THE 11.12Z GFS AND NAM ARE AT ODDS ON
PLACEMENT AND TIMING OF SURFACE LOW DEVELOPING ON LEE-SIDE OF THE
ROCKIES AND EJECTING OUT INTO THE UPPER MIDWEST. THE 11.12Z GFS IS
MUCH MORE PROGRESSIVE AND FARTHER NORTH THAN THE NAM. THE 11.15Z
SREF MEAN SURFACE LOW IS FASTER THAN BOTH THE GFS AND
NAM DETERMINISTIC RUNS...THOUGH PLACEMENT OF SURFACE LOW IS IN
AGREEMENT WITH THE 11.12Z GFS. DEPENDING ON THE TRACK WILL DETERMINE
ALL SNOW ACROSS FORECAST AREA TO A WINTRY MIX OF FREEZING
DRIZZLE/RAIN AND SNOW. AT THIS TIME...INCLUDED A MIX OF FREEZING
DRIZZLE AND LIGHT SNOW ACROSS THE SOUTHERN PORTIONS OF THE FORECAST
AREA AND ALL SNOW NORTH AND WEST OF LINE FROM NEAR NECEDAH...
VIROQUA...TO ALONG THE MINNESOTA AND IOWA BORDER. AS DETAILS BECOME
MORE CLEARER THE AMOUNT OF PRECIPITATION ASSOCIATED WITH SYSTEM
WILL DETERMINE ANY WINTER WEATHER HEADLINES ACROSS FORECAST AREA
SUNDAY NIGHT INTO MONDAY.
.LONG TERM...MONDAY NIGHT THROUGH FRIDAY
MAIN FORECAST CONCERNS ARE PRECIPITATION CHANCES THROUGH THE PERIOD.
THE 11.00Z GFS AND ECMWF ARE IN FAIRLY GOOD AGREEMENT WITH UPPER
LEVEL TROUGH OVER THE NORTHEAST U.S. AND RIDGING OVER THE WESTERN
U.S. THROUGH THE PERIOD. BIGGEST ISSUES ARE TIMING OF EMBEDDED
SHORTWAVE TROUGHS IN NORTHWEST FLOW ALOFT INTO THE UPPER MIDWEST AND
GREAT LAKES REGION THROUGH THE PERIOD. THE 11.00Z GEFS MEAN IS IN
GOOD AGREEMENT WITH THE 11.00Z DETERMINISTIC RUNS OVERALL UPPER
LEVEL FLOW PATTERN ACROSS THE CONTINENTAL U.S. THROUGH THE PERIOD.
THOUGH...THE 11.00Z GEFS MEAN DOES INDICATE STRENGTH AND TIMING
ISSUES WITH THE SERIES OF SHORTWAVE TROUGHS AFFECTING THE UPPER
MIDWEST AND GREAT LAKES REGION. CONFIDENCE THROUGH FORECAST ON
PRECIPITATION CHANCES THROUGH FORECAST PERIOD IS MODERATE...THOUGH
TIMING OF SHORTWAVE TROUGHS INTO FORECAST AREA IS LOW...BASED ON THE
TIMING DIFFERENCES BETWEEN THE 11.00Z DETERMINISTIC RUNS AND
ENSEMBLES. FIRST SHORTWAVE TROUGH AFFECTS THE FORECAST AREA LATE
MONDAY NIGHT AND TUESDAY. THE 11.00Z GFS IS MUCH MORE PROGRESSIVE
WITH SHORTWAVE THAN ECMWF. THE 11.00Z ECMWF IS SLOWER AND MORE
AMPLIFIED. WILL INTRODUCE SMALL CHANCES MONDAY NIGHT INTO TUESDAY
OVER PARTS OF THE FORECAST AREA. NEXT SHORTWAVE TROUGH EMBEDDED IN
NORTHWEST FLOW ALOFT TRACKS INTO THE UPPER MIDWEST AND GREAT LAKES
REGION THURSDAY. BOTH THE 11.00Z GFS AND ECMWF ARE INDICATING THE
FORCING/LIFT TO MOVE OVER NORTHERN WISCONSIN AND LAKE SUPERIOR...WILL
MAKE MINOR CHANGES TO PRECIPITATION CHANCES THURSDAY. TEMPERATURES
EARLY IN THE PERIOD WILL BE BELOW NORMAL...AS THE 11.00Z
DETERMINISTIC GFS AND ECMWF ARE INDICATING 850MB TEMPERATURES OF
MINUS 10 TO MINUS 20 DEGREES CELSIUS ACROSS FORECAST AREA THROUGH 12Z
THURSDAY AND THE 11.00Z GEFS IS INDICATING TWO METER SURFACE
TEMPERATURES OF MINUS ONE TO MINUS TWO STANDARD DEVIATIONS TUESDAY
AND INTO WEDNESDAY. TEMPERATURES ARE EXPECTED TO MODERATE AND BE
SLIGHTLY BELOW NORMAL OR NEAR NORMAL THURSDAY INTO FRIDAY.
&&
.AVIATION...TONIGHT AND SATURDAY
SURFACE HIGH PRESSURE RIDGE THAT GAVE THE AREA MOSTLY CLEAR/VFR
CONDITIONS TODAY WILL BE SLIDING OFF TO THE EAST TONIGHT. LIGHT
SOUTHERLY AIRFLOW EXPECTED OVERNIGHT ON THE BACKSIDE OF THE
DEPARTING RIDGE. IN THE MEANTIME...A WEAK MID-LEVEL TROUGH WILL BE
SCOOTING SOUTHEAST FROM THE DAKOTAS THROUGH NORTHERN MN INTO THE
LAKE SUPERIOR REGION TONIGHT THROUGH SATURDAY MORNING. ANY LIGHT
SNOW ASSOCIATED WITH THIS FEATURE WILL REMAIN WELL NORTH OF THE
KLSE/KRST TAF SITES...HOWEVER A FEW PATCHES OF MID-CLOUDS/
ALTOCUMULUS WITH BASES AOA 8KFT WILL BE SEEN FROM TIME TO TIME.
ONE AVIATION CONCERN DEALS WITH WARMING AND RESULTANT SNOW MELT
ACROSS THE CENTRAL PLAINS ON SATURDAY. SOUTHERLY AIRFLOW WILL CARRY
THIS BUILDING LOW-LEVEL MOISTURE IN THE FORM OF PROBABLE IFR STRATUS
INTO THE AREA BY SATURDAY EVENING. THICKENING STRATUS LAYER MAY ALSO
RESULT IN SOME FOG ALONG WITH PATCHY FREEZING DRIZZLE OR SNOW. THIS
IS BEYOND THIS AVIATION FORECAST PACKAGE...BUT WILL PASS CONCERN
ONTO THE EVENING SHIFT FORECASTER FOR FURTHER ASSESSMENT.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&
$$
SHORT/LONG TERM...DTJ
AVIATION..........DAS
