615
FXUS63 KARX 210942
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
342 AM CST SAT NOV 21 2009
SHORT TERM
TODAY THROUGH MONDAY
AT 3 AM...HIGH PRESSURE...CENTERED OVER THE OHIO VALLEY...WAS IN
CONTROL OF THE WEATHER IN THE UPPER MISSISSIPPI RIVER VALLEY. THE
COMBINATION OF LIGHT WINDS AND A MOIST BOUNDARY LAYER HAS RESULTED
IN THE DEVELOPMENT OF DENSE FOG EARLY THIS MORNING. THIS FOG
SHOULD PERSIST ACROSS THE FORECAST AREA THROUGH 10 AM THIS
MORNING. AS A RESULT...NO CHANGES WILL BE NEEDED TO DENSE FOG
ADVISORY THAT WAS ISSUED ON FRIDAY EVENING.
THE BUFKIT SOUNDINGS CONTINUE TO SHOW A VERY DRY AIR MASS LOCATED
ABOVE THE INITIALLY MOIST BOUNDARY LAYER TODAY. AS DIURNAL HEATING
OCCURS...WE SHOULD EASILY MIX UP TO 900 MB. THIS IN TURN WILL
BRING THIS DRY AIR TO THE SURFACE. AS TYPICAL IN THESE
SITUATIONS...THE MOS DEW POINT GUIDANCE ARE TOO HIGH AND THE GFE
DEW POINT TOO WAS USED TO LOWER THESE DEW POINTS TO AROUND
30 DEGREES THIS AFTERNOON.
FROM TONIGHT INTO SUNDAY MORNING...THE MODELS CONTINUE TO SHOW THAT
THE MOISTURE BELOW 800 MB DRAMATICALLY INCREASES AHEAD OF AN
APPROACHING LONG WAVE TROUGH IN THE NORTHERN AND CENTRAL PLAINS.
THIS MOISTURE IS CURRENTLY PRODUCING LIFR/IFR CEILINGS AND
VISIBILITIES ACROSS OKLAHOMA THIS MORNING. THERE WAS ENOUGH
CONFIDENCE TO INCLUDE AREAS OF DENSE FOG IN THE GRIDS INTO THE
HIGHER ELEVATIONS OF NORTHEAST IOWA AND SOUTHEAST MINNESOTA AFTER
MIDNIGHT TONIGHT. THIS FOG WILL LIKELY PERSIST THROUGH MID TO LATE
MORNING. THE DRIZZLE IS STILL IN QUESTION FOR LATE TONIGHT...
BECAUSE THERE IS VERY LITTLE LIFT /IF ANY/ IN THE DEEP MOISTURE
LAYER.
IN ADDITION TO THE INCREASING MOISTURE...THE WARM AIR ADVECTION
SHOULD RESULT IN STEADY TO SLOWLY RISING TEMPERATURES ACROSS THE
FORECAST AREA TONIGHT. AS A RESULT...A NON DIURNAL TEMPERATURE
CURVE WAS USED ACCOUNT FOR THIS FROM LATE TONIGHT INTO SUNDAY
MORNING.
FROM SUNDAY NIGHT INTO MONDAY...IT WAS THE CONSENSUS THAT THE
SLOWER OPERATIONAL NAM/WRF...GEM...AND ECMWF SOLUTIONS WERE
PREFERRED OVER THE FASTER OPERATIONAL GFS. DUE TO THIS...THE
TEMPERATURES WERE RAISED SOME AND THE PRECIPITATION CHANCES WERE
LOWERED.
LONG TERM
MONDAY NIGHT THROUGH FRIDAY
MUCH OF THE TIME THIS MORNING WAS SPENT WORKING ON THE FORECAST
GRIDS FROM MONDAY NIGHT INTO WEDNESDAY NIGHT.
THE 21.00Z MODELS CONTINUE TO STRUGGLE WITH THE EVOLUTION AND
TIMING OF A LOW PRESSURE SYSTEM MOVING THROUGH THE UPPER
MISSISSIPPI RIVER VALLEY SOMEWHERE FROM MONDAY NIGHT THROUGH
THANKSGIVING. MUCH OF THIS HAS TO DEAL WITH HOW THIS SYSTEM
INTERACTS WITH ANOTHER SYSTEM DROPPING INTO THE MEAN TROUGH BEHIND
THE INITIAL STRONG WAVE. WHILE THE OPERATIONAL GFS HAS SLOWED ITS
MOVEMENT OF THIS SYSTEM FROM 24 HOURS AGO...IT CONTINUES TO BE
MUCH FASTER THAN THE ECMWF AND GEM. ABOUT TWO THIRDS /8 OUT OF 12/
OF ITS FAMILY MEMBERS SUPPORT THIS FASTER SOLUTION. MEANWHILE THE
REMAINING GFS MEMBERS ARE CLUSTERED WITH THE SLOWER ECMWF WHICH
MOVES THE SYSTEM THROUGH OUR AREA BETWEEN TUESDAY THROUGH
THANKSGIVING. THIS ADDS TO QUITE A BIT OF UNCERTAINTY TO THIS TIME
PERIOD...BUT THE OVERALL CONSENSUS OF OUR OFFICE AND THE
SURROUNDING WFOS WAS TO CONTINUE TO TREND OUR FORECAST GRIDS
TOWARD THE SLOWER ECMWF AND GEM SOLUTIONS. THIS ULTIMATELY
RESULTED IN SOME CHANGES TO TEMPERATURES /RAISED TEMPERATURES FROM
MONDAY NIGHT INTO TUESDAY NIGHT/...PRECIPITATION CHANCES /LOWERED
CHANCES ON MONDAY NIGHT AND THEN INCREASED THEM SOME FROM TUESDAY
INTO WEDNESDAY NIGHT/...AND WEATHER /DELAYED CHANGE OVER TO SNOW/
GRIDS FROM MONDAY NIGHT THROUGH WEDNESDAY NIGHT.
THE ECMWF ACTUALLY GENERATES QUITE A BIT OF SNOW /4 TO 8 INCHES/
ACROSS THE NORTHERN IOWA AND SOUTHERN MINNESOTA...AND THEN
NORTHEAST INTO UPPER MICHIGAN FROM LATE WEDNESDAY INTO
THANKSGIVING. THE OPERATIONAL GFS ACTUALLY SHOW A SIMILAR
TOTALS...BUT IT IS FURTHER NORTHWEST AND SLIGHTLY EARLIER. WHILE
THESE MODELS ARE SHOWING THIS...THE COBB DATA ONLY SHOWS MINOR
SNOW ACCUMULATIONS IN THE GFS. WITH SUCH DIFFERENCES...WE DECIDED
NOT TO HIGHLIGHT THIS SYSTEM IN OUR HAZARDOUS WEATHER OUTLOOK.
HOPEFULLY THIS WILL BECOME A BIT CLEARER IN THE UPCOMING DAYS.
AVIATION
TODAY AND TONIGHT
PRIMARY CONCERNS WERE FOCUSED ON IFR CONDITIONS IN LOW CLOUDS AND
FOG DURING THE VALID TAF PERIOD.
RADIATIONAL COOLING AND A MOIST BOUNDARY LAYER HAS ALLOWED FOR
WIDESPREAD IFR CONDITIONS IN FOG EARLY THIS MORNING. AS SOUTH WINDS
INCREASE TODAY AND MIXING COMMENCES...IMPROVEMENT IS EXPECTED.
HOWEVER...HAZE SHOULD LINGER THROUGH THE DAY WITH PERSISTENT
SOUTHERLY FLOW. THIS SAME SOUTHERLY FLOW CONTINUES DRAWING HIGHER
RELATIVE HUMIDITY NORTHWARD TONIGHT. VARIOUS DATA SETS WERE STRONGLY
SUGGESTIVE OF IFR AND/OR MVFR IN LOW CLOUDS/FOG AGAIN TONIGHT.
POOREST CONDITIONS ANTICIPATED ACROSS HIGHER ELEVATION AIRPORTS
MAINLY WEST OF THE MISSISSIPPI RIVER...SUCH AS KRST TAF SITE...WHERE
CONFIDENCE WAS HIGH ENOUGH TO FORECAST CONDITIONS AT INSTRUMENT
APPROACH MINIMUMS.
ARX WATCHES/WARNINGS/ADVISORIES
WI...DENSE FOG ADVISORY UNTIL 10 AM CST THIS MORNING FOR WIZ017-
WIZ029-WIZ032-WIZ033-WIZ034-WIZ041-WIZ042-WIZ043-WIZ044-
WIZ053-WIZ054-WIZ055-WIZ061.
MN...DENSE FOG ADVISORY UNTIL 10 AM CST THIS MORNING FOR MNZ079-
MNZ086-MNZ087-MNZ088-MNZ094-MNZ095-MNZ096.
IA...DENSE FOG ADVISORY UNTIL 10 AM CST THIS MORNING FOR IAZ008-
IAZ009-IAZ010-IAZ011-IAZ018-IAZ019-IAZ029-IAZ030.
SHORT/LONG TERM...BOYNE
AVIATION...THOMPSON/AJ
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