986
FXUS63 KARX 210418 AAA
AFDARX
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LA CROSSE WI
1017 PM CST FRI NOV 20 2009
UPDATED AVIATION DISCUSSION AND TO INCLUDE UPDATE SECTION
UPDATE
AFTER MUCH COORDINATION WITH SURROUNDING OFFICES AND WATCHING TRENDS
WITH SURFACE OBSERVATIONS...HAVE ISSUED A DENSE FOG ADVISORY ACROSS
ALL OF THE FORECAST AREA.
SKIES HAVE MOSTLY CLEARED OUT ACROSS THE AREA AS DRIER AIR SEEN ON
THE 00Z MPX SOUNDING MOVED IN. MOST OF THIS DRIER AIR APPEARS TO BE
COMING IN ALOFT...AS DEWPOINTS HAVE MOSTLY BEEN FALLING AS A RESULT
OF FALLING TEMPERATURES. WINDS ALSO REMAIN LIGHT...WITH A SURFACE
RIDGE IN PLACE...AIDING IN THE FALLING TEMPERATURES. THEREFORE...THE
REGIME IS NOT FAVORABLE FOR MUCH IN THE WAY OF LOW LEVEL DRY AIR
ADVECTION. THIS ALLOWS THE USE OF THE CROSS-OVER TECHNIQUE. LOOKING
BACK AT LATE AFTERNOON DEWPOINTS...READINGS WERE IN THE UPPER 30S TO
LOW 40S F. WITH READINGS DROPPING TO AT OR BELOW THESE DEWPOINTS...
VISIBILITIES OF 2-4 MILES IN MIST HAVE BECOME WIDESPREAD. A FEW
LOCALIZED AREAS ARE SEEING VISIBILITIES APPROACH ONE HALF OF A MILE
SUCH AS OELWEIN AND BOSCOBEL. WITH CONTINUED COOLING OVERNIGHT...
DROPPING FURTHER PAST THE LATE AFTERNOON DEWPOINTS...BELIEVE
VISIBILITIES SHOULD DROP TO ONE QUARTER OF A MILE OR LESS AT MANY
LOCATIONS. BEST BET WOULD BE IN THE RIVER VALLEYS GIVEN WATER
TEMPERATURES IN THE LOW TO MID 40S. COMBINATION OF SOUTH WINDS
PICKING UP AND DAYTIME HEATING SHOULD ALLOW THE FOG TO DISSIPATE BY
16Z SATURDAY.
SHORT TERM
TONIGHT THROUGH MONDAY NIGHT
BIGGEST FORECAST CONCERN IS ON DEGREE OF SATURATION LATE SATURDAY
NIGHT AND CORRESPONDING RAIN/DRIZZLE CHANCES...WITH RAIN CHANCES
SUNDAY THROUGH MONDAY NIGHT.
DATA ANALYSIS AT 19Z SHOWS HIGH PRESSURE LOCATED OVER KANSAS...WITH
CLEAR SKIES EXTENDING FROM THE SOUTHERN PLAINS INTO SOUTHERN CANADA.
LINGERING MID LEVEL CLOUDS PERSIST OVER MUCH OF THE FORECAST AREA...
WITH THE SLOW MOVING BACK EDGE OVER SOUTHEAST MINNESOTA.
BIGGEST DIFFERENCE IN THE 20.12Z MODELS ARE SATURDAY NIGHT AS GFS
CONTINUES TO GENERATE QPF ACROSS IOWA INTO MINNESOTA...WHILE NAM
REMAINS DRY. OTHERWISE MODELS APPEAR IN REASONABLE AGREEMENT.
IN THE NEAR TERM...UPPER LEVEL RIDGE AND SURFACE HIGH PRESSURE WILL
DOMINATE THE WEATHER INTO SATURDAY. MODELS CONSISTENT IN CLEARING
CLOUDS OUT OF THE AREA THIS EVENING. THUS EXPECTING A MOSTLY CLEAR
NIGHT WITH LIGHT WINDS. SURFACE OBSERVATIONS AND MODEL SOUNDINGS
INDICATING RESIDUAL LOW LEVEL MOISTURE OVER THE AREA. WITH THE
LIGHT WINDS AND CLEAR SKIES WILL LIKELY SEE SOME FOG FORMATION LATE
TONIGHT AN THUS HAVE ADDED THIS TO THE GRIDDED DATABASE. QUIET/SUNNY
AND MILD WEATHER ON TAP FOR SATURDAY AS HIGH PUSHES EAST AND
SOUTHERLY WINDS RETURN TO THE AREA.
WITH THE SOUTHERLY FLOW INCREASING SATURDAY NIGHT...MODELS
INDICATING LOW LEVEL SATURATION BEGINNING TO OCCUR AFTER MIDNIGHT.
BOTH THE NAM AND GFS INDICATING SOME WEAK LIFT THROUGH THE SATURATED
LAYER AND THUS HAVE CONTINUED THE MENTION OF DRIZZLE AFTER 09Z.
OTHERWISE COLUMN CONTINUES TO SATURATE SUNDAY/SUNDAY NIGHT IN MOIST
SOUTHERLY FLOW WITH WEAK WARM ADVECTION/ISENTROPIC LIFT AHEAD OF AN
APPROACHING TROUGH. ENOUGH LIFT GENERATED FOR LOWER END RAIN
PROBABILITIES AREA-WIDE...WITH SLIGHTLY BETTER CHANCES ACROSS THE
WESTERN COUNTIES.
ON MONDAY/MONDAY NIGHT...TROUGH DIGS OVER THE ROCKIES WITH WEAK
SHORTWAVE BEING EJECTED NORTHWARD TOWARD THE AREA. WITH CONTINUED
MOIST SOUTHERLY FLOW AND WEAK LIFT AHEAD OF THE DEEPENING TROUGH
WILL CONTINUE MID-LOW CHANCE RAIN PROBABILITIES.
TEMPERATURES LOOK TO REMAIN MILD THROUGH THE PERIOD AVERAGING SOME
10 TO 15 DEGREES ABOVE NORMAL THROUGH MONDAY. GUIDANCE VALUES LOOK
REASONABLE AD NOT MUCH DEVIATION FROM THEM OR PREVIOUS FORECAST.
LONG TERM
TUESDAY THROUGH FRIDAY
MEDIUM RANGE MODELS IN BETTER AGREEMENT WITH THEIR LATEST
RUNS. 20.06Z GFS IS THE OUTLIER WITH THE WEAKER AND FASTER SOLUTION
IN HANDLING THE TROUGH/SURFACE AND MID LEVEL FEATURES...WHILE THE GEM
HAS COME AROUND TO THE ECMWF SOLUTION. HOWEVER...WITH THE
20.12Z GFS RUN IT IS COMING MORE IN-LINE WITH THE ECMWF/GEM
SOLUTIONS. PREFERENCE WAS THE BETTER CLUSTERING OF THE ECMWF/GEM.
PATTERN WILL BE DOMINATED BY LONGWAVE TROUGH ACROSS THE CENTRAL CONUS
AT THE BEGINNING OF THE PERIOD...WITH CLOSED MID LEVEL LOW OVER IOWA.
THIS FEATURE WILL THEN LIFT NORTHEAST THROUGH THE UPPER MISSISSIPPI
RIVER VALLEY REGION TUESDAY AND TUESDAY NIGHT...WITH A SECONDARY WAVE
DROPPING ACROSS THE AREA WEDNESDAY AND WEDNESDAY NIGHT...KEEPING
TROUGH FIRMLY ENTRENCHED ACROSS THE AREA. SOUNDINGS FROM THE ECMWF
SHOWING 5000 FOOT SURFACE WARM LAYER TUESDAY ACROSS MUCH OF THE AREA
AND THUS EXPECT MAINLY RAIN. AS COLUMN COOLS TUESDAY NIGHT AND
WEDNESDAY WILL CONTINUE TO MENTION RAIN/SNOW POSSIBILITIES. WRAP
AROUND MOISTURE WITH CYCLONIC FLOW WILL KEEP LOWER END PRECIPITATION
CHANCES WEDNESDAY NIGHT...WITH ENOUGH COLDER AIR TO RESULT IN ALL
SNOW.
TROUGH TRANSLATES INTO THE EASTERN CONUS THURSDAY AND FRIDAY AS
RIDGE BUILDS ACROSS THE CENTRAL CONUS. THUS EXPECT RISING HEIGHTS
AND QUIET WEATHER.
ABOVE NORMAL TEMPERATURES AT THE BEGINNING OF THE PERIOD WILL FALL
BACK TO NEAR OR SLIGHTLY BELOW FROM WEDNESDAY THROUGH FRIDAY.
ALL IN ALL THE SYSTEM TUESDAY/TUESDAY NIGHT BEARS WATCHING. IF
MORE COLDER AIR GETS WRAPPED INTO THE SYSTEM ACCUMULATING SNOW IS NOT
OUT OF THE QUESTION FOR PARTS OF THE AREA.
AVIATION
TONIGHT AND SATURDAY
SKIES HAVE CLEARED ACROSS BOTH TAF SITES...ALLOWING FOR MVFR
VISIBILITIES IN BR TO DEVELOP. THESE VISIBILITIES SHOULD CONTINUE TO
FALL THROUGH THE NIGHT AS TEMPERATURES COOL FURTHER...AIDED BY LIGHT
WINDS AND DRY AIR ALOFT. RIGHT NOW HAVE FORECAST VISIBILITIES
BOTTOMING OUT AT IFR AT KRST AND LIFR AT KLSE LATE TONIGHT...BUT
THESE WILL NEED MONITORING...ESPECIALLY AT KLSE WITH THE WARMER
MISSISSIPPI RIVER NEARBY ADDING EXTRA MOISTURE. IN ADDITION...WOULD
NOT BE SURPRISED IF A LIFR CEILING DEVELOPS AT KLSE LATE TONIGHT IF
INDEED A STRATUS DECK DEVELOPS WITHIN THE RIVER VALLEY. SOUTH WINDS
SHOULD PICK UP DURING THE DAY SATURDAY AS A SURFACE RIDGE OVER THE
TAF SITES MOVES OFF TO THE EAST. THE COMBINATION OF THESE WINDS AND
DAYTIME HEATING WILL ALLOW MOST FOG TO MIX OUT AROUND 15Z. VFR
CONDITIONS ARE EXPECTED TO PREVAIL FOR THE REST OF THE DAY WITH A
DRY AIRMASS IN PLACE.
ARX WATCHES/WARNINGS/ADVISORIES
WI...DENSE FOG ADVISORY UNTIL 10 AM CST SATURDAY FOR WIZ017-WIZ029-
WIZ032-WIZ033-WIZ034-WIZ041-WIZ042-WIZ043-WIZ044-WIZ053-
WIZ054-WIZ055-WIZ061.
MN...DENSE FOG ADVISORY UNTIL 10 AM CST SATURDAY FOR MNZ079-MNZ086-
MNZ087-MNZ088-MNZ094-MNZ095-MNZ096.
IA...DENSE FOG ADVISORY UNTIL 10 AM CST SATURDAY FOR IAZ008-IAZ009-
IAZ010-IAZ011-IAZ018-IAZ019-IAZ029-IAZ030.
UPDATE...AJ
SHORT/LONG TERM...RABERDING
AVIATION...AJ
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