Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 170444
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1144 PM CDT TUE SEP 16 2014

.SHORT TERM...(THIS EVENING THROUGH WEDNESDAY)
ISSUED AT 228 PM CDT TUE SEP 16 2014

CURRENTLY AS OF 19Z...WATER VAPOR LOOP AND RAP 500MB HEIGHT ANALYSIS
SHOWED RIDGING AGAIN OVER WESTERN NORTH AMERICA WHILE TROUGHING
DOMINATES EASTERN CANADA INTO THE GREAT LAKES AND OHIO VALLEY. THIS
PLACES THE FORECAST AREA IN A SUBSIDENT NORTHWEST FLOW REGIME...
FAVORING DRY CONDITIONS AND HIGH PRESSURE AT THE SURFACE. LATEST
SURFACE ANALYSIS SHOWED THE HIGH CENTERED OVER ILLINOIS WITH A RIDGE
AXIS EXTENDING INTO WESTERN WI. AIRMASS OVER THE AREA HAS WARMED
SLIGHTLY FROM YESTERDAY WITH 925MB TEMPS UP TO 10C. HOWEVER...MORE
SUNSHINE AND A DRIER AIRMASS REFLECTED BY PRECIPITABLE WATER VALUES
OF 0.3-0.5 INCHES PER PRECIPITABLE WATER IMAGERY HAS ALLOWED
TEMPERATURES TO CLIMB INTO THE 60S.

THE SHORT TERM PERIOD AGAIN LOOKS QUIET...RESULTING FROM PLENTIFUL
DRY AIR IN PLACE OVER THE NEXT 24 HOURS TO PREVENT PRECIPITATION.
HOWEVER...THERE ARE A FEW ISSUES TO CONTEND WITH...

1. POTENTIAL FOR VALLEY FOG TONIGHT. THE RIDGE AXIS EXTENDING FROM
THE SURFACE HIGH IN ILLINOIS INTO WESTERN WI IS PROGGED TO STAY IN
PLACE THROUGH WEDNESDAY MORNING. WITH DRY AIR IN PLACE...LIGHT WINDS
AND CURRENTLY CLEAR SKIES...TEMPERATURES SHOULD DROP QUICKLY.
HOWEVER...THERE ARE 3 PROBLEMS FOR VALLEY FOG. THE FIRST IS THAT THE
AIR IS MUCH DRIER TODAY COMPARED TO YESTERDAY AT THIS TIME...AND ALL
RAIN FROM YESTERDAY HAS BEEN ABSORBED. SECOND...THE GFS...NAM AND
RAP HAVE ALL TRENDED STRONGER WITH THE WIND FIELD AFTER MIDNIGHT...
WITH POTENTIAL OF UP TO 20 KT SUGGESTED RIGHT ABOVE THE SURFACE.
THESE WINDS COULD REALLY HAMPER FOG AND EVEN STRATUS TRYING TO STICK
AROUND IN THE MISSISSIPPI VALLEY. THIRD...A NEW DISCOVERY TODAY IS
THAT BOTH THE GFS AND NAM SHOW A BAND OF MID CLOUDS MOVING INTO THE
WESTERN HALF OF THE FORECAST AREA AFTER MIDNIGHT. THESE CLOUDS MATCH
UP VERY WELL WITH ISENTROPIC LIFT SEEN ON THE 300K SURFACE...WHICH
COULD ALSO HAMPER FOG AND STRATUS FROM STICKING AROUND...EVEN IN THE
TRIBS. THUS...HAVE BACKED DOWN THE COVERAGE OF VALLEY FOG AND
STRATUS OVERNIGHT INTO WEDNESDAY MORNING.

2. TEMPS AND FROST POTENTIAL TONIGHT. RELATED TO THE VALLEY FOG
DISCUSSION ABOVE... READINGS IN CENTRAL WISCONSIN LOOK TO PLUMMET
AGAIN. FOLLOWED PREVIOUS FORECAST IN DROPPING TEMPERATURES TOWARDS
THE MID 30S. THEY SHOULD STAY HIGH ENOUGH TO PRECLUDE FROST
FORMATION...THOUGH. ALONG/WEST OF THE RIVER...INCREASING WIND AND
INCOMING CLOUDS RESULT IN WARMER READINGS.

3. BACKDOOR COLD FRONT WEDNESDAY AFTERNOON. THIS FRONT HAS BEEN
SHOWN FOR MANY DAYS TO COME INTO THE REGION FROM THE NORTH. DRY AIR
WILL PREVENT PRECIPITATION...HOWEVER...THE FRONT SHOULD PULL SOME
WARMER AIR IN AHEAD OF IT TO ALLOW FOR HIGHS TO BE ABOUT 5 DEGREES
WARM THAN TODAY.

.LONG TERM...(WEDNESDAY NIGHT THROUGH TUESDAY)
ISSUED AT 228 PM CDT TUE SEP 16 2014

QUIET WEATHER CONTINUES FOR WEDNESDAY NIGHT THROUGH THURSDAY EVENING
AS A CANADIAN HIGH DROPS DOWN INTO THE GREAT LAKES REGION BEHIND THE
BACKDOOR COLD FRONT. MAY HAVE TO CONTEND WITH SOME STRATUS IN THE
WAKE OF THE FRONT...AS WELL AS AN INCREASING EAST-SOUTHEAST
WIND...BOTH OF WHICH COULD IMPACT VALLEY FOG FORMATION/PERSISTENCE.
MUCH OF THE COOLING BEHIND THE FRONT STILL APPEARS TO GET DEFLECTED
TO THE EAST...THUS HIGHS ON THURSDAY ARE ONLY ANTICIPATED TO BE
SLIGHTLY COOLER THAN WEDNESDAY.

MUCH OF THE LONG TERM FOCUS IS AFTER MIDNIGHT THURSDAY NIGHT THROUGH
SATURDAY. MODELS REMAIN IN GOOD AGREEMENT ON BUILDING UPPER RIDGING
INTO THE REGION ON FRIDAY...AHEAD OF A SHORTWAVE TROUGH CROSSING THE
NORTHERN ROCKIES. THIS SETS UP A FAIRLY SIGNIFICANT WARM ADVECTION
REGIME OVER THE FORECAST AREA...WITH 850MB TEMPS CLIMBING 8-10C
BETWEEN 00Z FRI AND 00Z SAT. THIS REGIME ALSO COMES WITH MORE
MOISTURE...AS PRECIPITABLE WATER VALUES CLIMB FROM AROUND 0.7 INCHES
AT 00Z FRI TO AROUND 1.5 INCHES BY 18Z FRI. THESE TWO PARAMETERS
INCREASING SIGNIFICANTLY BRINGS UP A CONCERN FOR ELEVATED
CONVECTION. 850MB MOISTURE TRANSPORT PROGS FROM THE 16.12Z
NAM/GFS/ECMWF WOULD SUGGEST POSSIBLE CONVECTION FIRING WEST OF I-35
IN MN AND IA BETWEEN 06-12Z FRI...THEN LIFTING NORTHEAST INTO
NORTHERN WI BY 00Z SAT AS CAPPING BUILDS IN. THUS...SOME SHOWER AND
THUNDERSTORM CHANCES PERSIST FOR FRI...ESPECIALLY NORTH OF I-90.
CAPE AND SHEAR PROFILES WOULD SUGGEST NO SEVERE WEATHER WITH THIS
ACTIVITY. GREATER CHANCES FOR WIDESPREAD RAIN LOOK TO COME AFTER
MIDNIGHT FRIDAY NIGHT INTO SATURDAY. THIS IS WHEN THE TROUGH MAKES
ITS WAY EAST AND THROUGH THE REGION...FOLLOWED WITH AN ASSOCIATED
SURFACE COLD FRONT FOR CONVERGENCE. PLENTIFUL MOISTURE CHARACTERIZED
BY THE AFOREMENTIONED 1.5 INCH PRECIPITABLE WATER WILL ALSO HELP.
HARD TO SAY AT THIS TIME IF THE MID LEVEL CIRCULATION OF TROPICAL
STORM ODILE OVER BAJA CALIFORNIA WILL GET PULLED UP INTO THE REGION
OR NOT. IF IT DOES...THE CHANCES FOR PRECIPITATION...AND HEAVY
PRECIPITATION AT THAT WOULD INCREASE. TIMING OF THE FRONT WILL BE
CRITICAL TO WHEN AND WHERE POTENTIAL EXISTS FOR SEVERE WEATHER.
16.12Z ECMWF POINTS TO FRIDAY EVENING ACROSS THE NORTHWEST HALF OF
THE FORECAST AREA...WHILE THE 16.12Z GFS WOULD BE SATURDAY AFTERNOON
OVER THE SOUTHEAST HALF.

SOMETHING ALSO TO KEEP A CLOSE EYE ON IS WINDS ON FRIDAY ACROSS
SOUTHEAST MN. WITH 40 TO PERHAPS AS HIGH AS 60KT SOUTHWEST
WINDS...STRONGEST IF YOU BELIEVE THE NAM...PROGGED BETWEEN 3000-5000
FT ABOVE THE GROUND...MIXING SHOULD EASILY CREATE GUSTS NEAR 30 KT.
THESE IN LATER FORECASTS COULD GET CLOSE TO WIND ADVISORY CRITERIA
IF MORE SUN OCCURS.

FOR SUNDAY INTO TUESDAY...MODELS REAMPLIFY TROUGHING OVER EASTERN
CANADA INTO THE GREAT LAKES REGION...SIMILAR TO WHAT IS GOING ON
TODAY. THERE REMAINS UNCERTAINTY IF A MORE POTENT SHORTWAVE WILL
DIVE THROUGH THE TROUGH AND THROUGH THE FORECAST AREA...BUT TRENDS
SEEM TO SUGGEST THAT THE TRACK OF SUCH WAVE WILL END UP NORTHEAST OF
THE AREA. THEREFORE...THE FORECAST STAYS DRY WITH THE COOLER AIR
STAYING TO THE NORTH AND EAST. STILL...WE SHOULD SEE TEMPERATURES
FALL BACK TO JUST BELOW NORMAL.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z WEDNESDAY NIGHT)
ISSUED AT 1144 PM CDT TUE SEP 16 2014

STILL SOME UNCERTAINTIES AS TO WHETHER FOG WILL FORM IN THE
MISSISSIPPI RIVER VALLEY TONIGHT. THE 17.02Z RAP AND 17.00Z NAM
BOTH CONTINUE TO SHOW THE WINDS JUST ABOVE THE SURFACE INCREASING
OVERNIGHT AS THE SURFACE HIGH OVER ILLINOIS MOVES OFF INTO THE
OHIO RIVER VALLEY. HAVE NOTICED THROUGH THE EVENING THAT THE FLAG
HERE AT THE OFFICE HAS BEEN MOVING AROUND AT TIMES AND THE VWP
WINDS FOR THE KARX RADAR HAVE BEEN SHOWING 10 KNOTS AT 1000 FEET
ALL EVENING WITH INCREASING WINDS ABOVE THAT. BOTH MODELS ALSO
CONTINUE TO SHOW SATURATION OCCURRING RIGHT AT THE SURFACE AND
WITH A 4 DEGREE TEMP/DEW POINT SPREAD AT 17.04Z THIS COULD ALLOW
SOME FOG TO FORM NEAR THE AIRPORT AND WILL CONTINUE WITH BCFG BUT
NOT SHOW ANYTHING MORE THAN THAT GIVEN THE WINDS ALOFT.
OTHERWISE...VFR CONDITIONS EXPECTED THROUGH THE PERIOD AS A WEAK
COLD FRONT SLIDES THROUGH WEDNESDAY AFTERNOON FOLLOWED BY ANOTHER
AREA OF HIGH PRESSURE.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&

$$

SHORT TERM...AJ
LONG TERM...AJ
AVIATION...04



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