Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 160431
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1130 PM CDT MON SEP 15 2014

.SHORT TERM...(THIS EVENING THROUGH TUESDAY)
ISSUED AT 244 PM CDT MON SEP 15 2014

CURRENTLY AS OF 19Z...WATER VAPOR LOOP AND RAP 500MB HEIGHT ANALYSIS
SHOWED RIDGING OVER WESTERN NORTH AMERICA WHILE TROUGHING DOMINATED
EASTERN CANADA INTO THE GREAT LAKES REGION. THE MAIN SHORTWAVE OF
NOTE WITHIN THIS TROUGH WAS OVER SOUTHERN WI...WHICH WAS RESPONSIBLE
FOR BRINGING THE RAIN TO THE FORECAST AREA THIS MORNING AND EARLY
THIS AFTERNOON. THAT RAIN HAS NOW PUSHED OFF TO THE EAST...ALLOWING
SUBSIDENCE AND DRYING TO SPREAD INTO THE FORECAST AREA. GOES DERIVED
PRECIPITABLE WATER VALUES ARE 0.3-0.5 INCHES ACROSS THE DAKOTAS AND
MN. CLEARING SKIES ACROSS THE DAKOTAS ALONG WITH PEAKS OF SUN INTO
THE FORECAST AREA AS SEEN ON VISIBLE SATELLITE IMAGERY REFLECTS THIS
SUBSIDENCE AND CLEARING. HIGH PRESSURE AT THE SURFACE OVER EASTERN
SOUTH DAKOTA ALSO REFLECTS THE SUBSIDENCE. COOLER AIR HAS ALSO
SPREAD INTO THE REGION...WITH 925MB TEMPS ABOUT 4C COOLER OVER THE
PAST 24 HOURS TO AROUND 8C PER LATEST RAP ANALYSIS. AS SUCH...
TEMPERATURES ARE OFF 10 TO 15 DEGREES COMPARED TO THIS TIME
YESTERDAY.

NO PRECIPITATION IS EXPECTED IN THE SHORT TERM PERIOD...RESULTING
FROM PLENTY OF DRY ADVECTION AND SUBSIDENCE IN THE WAKE OF TODAYS
SHORTWAVE TROUGH. PRECIPITABLE WATER VALUES FALL INTO THE 0.3-0.5
INCH RANGE AS SEEN UPSTREAM THIS EVENING...THEN STAY THERE THROUGH
TUESDAY. THIS DRY AIR AND ASSOCIATED CLEARING WITH LOSS OF DAYTIME
HEATING MEANS POTENTIAL FOR DECENT RADIATIONAL COOLING TONIGHT.
ADDITIONALLY...IT APPEARS WE SHOULD HAVE A SURFACE RIDGE EXTENDING
FROM THE CENTER OF THE HIGH OVER SOUTH DAKOTA CROSSING THE FORECAST
AREA TONIGHT. ITS NOT NEARLY AS IDEAL AS HAVING THE HIGH COMING
OVERHEAD...WHICH INSTEAD LOOKS TO TRACK THROUGH WESTERN IA...BUT
CONDITIONS ARE STILL FAVORABLE. THIS SITUATION SUGGESTS FOLLOWING A
BLEND OF MOS GUIDANCE AND 30 DAY BIAS CORRECTED 2 METER OUTPUT FROM
THE RAW MODELS FOR LOWS. THE RESULT ENDS UP WITH MUCH OF THE
FORECAST AREA BETWEEN THE MID 30S AND LOWER 40S...THOUGH SOME OF THE
TYPICAL COLD SPOTS SUCH AS WESTERN TAYLOR COUNTY TO SPARTA...BLACK
RIVER FALLS AND VOLK FIELD WILL FALL INTO THE LOWER 30S. THOSE
TYPICAL COLD SPOTS WOULD BE UNDER THE MOST CONCERN FOR FROST...AND
HAVE COVERED THEM WITH A FROST ADVISORY. MEANWHILE...VALLEYS COULD
SEE SOME FOG AND STRATUS...ESPECIALLY THE SMALLER TRIBUTARIES WHICH
CAN DECOUPLE MORE THAN THE MAIN MISSISSIPPI CHANNEL.

PLENTY OF SUN TUESDAY MORNING SHOULD ALLOW ANY VALLEY FOG/STRATUS TO
MIX OUT QUICKLY. SOME DIURNAL CUMULUS MAY POP UP...BUT NOT ENOUGH TO
RESTRICT HEATING. WITH 925MB TEMPS PROGGED AROUND 10C ON TUESDAY AND
PLENTY OF DRY AIR...HIGHS SHOULD EASILY TOP OUT IN THE 60S WHILE
DEWPOINTS MIX DOWN INTO THE UPPER 30S/LOW 40S.

.LONG TERM...(TUESDAY NIGHT THROUGH MONDAY)
ISSUED AT 244 PM CDT MON SEP 15 2014

MODELS REMAIN IN GOOD AGREEMENT THAT MUCH OF THE LONG TERM PERIOD
THROUGH THURSDAY WILL BE DOMINATED BY HIGH PRESSURE. THERE IS A
BACKDOOR COLD FRONT THAT IS ON TAP TO DROP SOUTH THROUGH THE
FORECAST AREA WEDNESDAY AFTERNOON AND EVENING...BUT AT THIS TIME
THERE DOES NOT APPEAR TO BE ENOUGH MOISTURE FOR THE FRONT TO PRODUCE
PRECIPITATION. PRECIPITABLE WATER VALUES DO CLIMB SLIGHTLY TUESDAY
NIGHT SO IT SHOULD NOT BE AS COOL AS TONIGHT. HOWEVER...NAM/GFS
SOUNDINGS SUGGEST A LIGHTER WIND FIELD THROUGH A DEEPER DEPTH IN THE
MISSISSIPPI RIVER VALLEY...WHICH SHOULD TRANSLATE INTO A GREATER
POTENTIAL FOR VALLEY FOG/STRATUS DESPITE THE DRIER CONDITIONS FROM
TUESDAY. MORE VALLEY FOG COULD OCCUR WEDNESDAY NIGHT...BUT MAY HAVE
TO WATCH FOR POTENTIAL LINGERING CLOUDS ON AND BEHIND THE
AFOREMENTIONED BACKDOOR COLD FRONT. HIGH TEMPERATURES SHOULD GET
BACK UP CLOSE TO SEASONAL NORMS ON WEDNESDAY AS WARMER AIR COMES IN
AHEAD OF THE FRONT. MUCH OF THE COOL AIR BEHIND THE FRONT LOOKS TO
DEFLECT OFF TO THE EAST FOR THURSDAY...ALLOWING HIGH TEMPERATURES TO
END UP SIMILAR TO WEDNESDAY.

FOR THURSDAY NIGHT INTO EARLY NEXT WEEK...THERE HAS BEEN SOME
CHANGES TO THE EXTENDED FORECAST. FIRST...MODELS ARE SLOWER BRINGING
THE 850MB MOISTURE TRANSPORT AND ASSOCIATED INSTABILITY TOWARDS THE
REGION THURSDAY NIGHT...AS WELL AS SHIFTING ITS FOCUS FARTHER NORTH
INTO CENTRAL AND NORTHERN MN. AS A RESULT...ALL PRECIPITATION LOOKS
TO OCCUR WEST OF THE FORECAST AREA AND HAVE THEREFORE DRIED OUT THE
FORECAST. THAT 850MB MOISTURE TRANSPORT STREAM AND HIGHER
PRECIPITABLE WATER UPWARDS OF 1.5 INCHES STILL LOOKS TO MOVE INTO
THE FORECAST AREA FOR FRIDAY...HOWEVER...LIKE THURSDAY NIGHT THE
CONVECTIVE FOCUS APPEARS TO BE SHIFTING NORTH WITH MORE UPPER
RIDGING PROGGED OVER THE AREA. HAVE LOWERED CHANCES SOUTH OF I-90 AS
A RESULT. AN UPPER TROUGH STILL LOOKS ON TRACK TO DIG SOUTHEAST INTO
THE FORECAST AREA FRIDAY NIGHT AND SATURDAY. THIS TROUGH PICKS UP
THE REMNANTS OF CURRENT HURRICANE ODILE ON THE BAJA PENINSULA...
RESULTING IN PLENTIFUL MOISTURE FOR THE UPPER TROUGH TO PRODUCE
CONVECTION AS IT SLIDES THROUGH. FOLLOWED A CONSENSUS FORECAST WHICH
BRINGS CHANCES UP TOWARDS 60. IF CURRENT TRENDS CONTINUE THESE
CHANCES WILL NEED TO GO UP MORE. MAY STILL HAVE TO CONTEND WITH A
SEVERE THREAT...WITH THE TIMING NOW LOOKING MORE LIKE OVERNIGHT
FRIDAY NIGHT INTO SATURDAY MORNING. GENERALLY QUIET CONDITIONS WITH
TEMPERATURES A LITTLE BELOW NORMAL ARE EXPECTED FOR SUNDAY AND
MONDAY AS NORTHWEST FLOW SETS UP AGAIN OVER THE REGION.
HOWEVER...15.12Z ECMWF IS VERY AMPLIFIED IN BRINGING A POTENT UPPER
LOW THROUGH THE REGION ON MONDAY. SHOULD THIS OCCUR...RAIN CHANCES
WILL NEED TO BE INTRODUCED ALONG WITH AT LEAST A 10 DEGREE DROP IN
TEMPERATURES.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z TUESDAY NIGHT)
ISSUED AT 1130 PM CDT MON SEP 15 2014

THE MAIN AVIATION CONCERN IS THE VALLEY FOG POTENTIAL AT KLSE.
MORNING RAINS/CLOUDS GAVE WAY TO SCATTERED CLOUDS EARLY THIS
EVENING. T/TD SPREAD AT 03Z ONLY 3 F. WINDS WILL BE LIGHT AND
VARIABLE THROUGH THE OVERNIGHT. NEARLY A SLAM DUNK FOR VALLEY FOG.
THE DRAWBACK WILL BE THE DEPTH OF THE LIGHT WIND LAYER. LATEST RAP
SUGGESTS THE LAYER COULD REACH 1400 FT...BUT HAS SHOWN A LOT OF
VARIABILITY OVER ITS PAST FEW RUNS. 00Z NAM12 BUFKIT SOUNDING
STRETCHES TOWARD 2000 FT...AND HOLDS ALL NIGHT. HRRR NOT AS DEEP BUT
ALSO HOLDS ALL NIGHT. THIS NAM/HRRR SCENARIO WOULD FAVOR AT LEAST AN
HOUR OF 1/4SM. MEANWHILE...WINDS ARE BLOWING THE FLAG AROUND AT
GRANDAD/S BLUFF...AROUND 600 FT ABOVE THE VALLEY FLOOR...WHILE VAD
WIND PROFILERS HAVE SHOWN AN INCREASE TO 20-30 KTS AT 2 KFT OVER THE
LAST FEW HOURS. TOUGH CALL. CONFIDENCE LOWERED DUE TO THE DIFFERING
OPINIONS ON HOW THIS WIND LAYER WILL PLAY OUT. GOING TO LEAN TOWARD
FOG MOVING ACROSS KLSE FOR NOW...ALTHOUGH FOR A SHORT PERIOD. OBS
AND WIND FIELDS WILL BE MONITORED CLOSELY THROUGH THE OVERNIGHT.

CONCERNS FOR VALLEY FOG AT KLSE WILL CONTINUE TUE NIGHT...WITH A
SIMILAR SETUP...AND SIMILAR DETRACTOR. NAM12 BUFKIT SOUNDINGS PAINT
A SHALLOW LIGHT WIND LAYER...WHICH IS GENERALLY AN INHIBITOR TO
DENSE VALLEY FOG. BCFG PROBABLY A GOOD WAY TO COVER THE POTENTIAL.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...FROST ADVISORY FROM MIDNIGHT TONIGHT TO 8 AM CDT TUESDAY FOR
     WIZ017-029-034-042>044.

MN...NONE.
IA...NONE.
&&

$$

SHORT TERM...AJ
LONG TERM....AJ
AVIATION.....RIECK


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