Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 170442
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1142 PM CDT SAT AUG 16 2014

.SHORT TERM...(THIS EVENING THROUGH SUNDAY)
ISSUED AT 341 PM CDT SAT AUG 16 2014

AT 3 PM...A LINE OF SHOWERS AND THUNDERSTORMS WERE LOCATED ALONG A
BACKDOOR CANADIAN COLD FRONT EXTENDED FROM LAKE ITASCA MINNESOTA
TO TWO RIVERS WISCONSIN. SURFACE BASED CAPES ALONG AND AHEAD OF
THIS FRONT ARE RUNNING IN THE 1000 TO 1500 J/KG RANGE. WHILE THERE
IS DECENT INSTABILITY...THE 0-6 KM EFFECTIVE SHEAR IS ONLY 25 TO
30 KNOTS AND THE DOWNDRAFT CAPE ARE LESS THAN 500 J/KG. THE MODELS
ARE IN GOOD AGREEMENT THAT THERE WILL BE LITTLE CHANGE IN THESE
PARAMETERS THIS EVENING...AS A RESULT NOT EXPECTING ANY SEVERE
WEATHER ALONG THIS FRONT THROUGH THIS EVENING. WITH PRECIPITABLE
WATER VALUES AROUND 1.8 INCHES AND WARM CLOUD DEPTHS RUNNING
BETWEEN 4 AND 4.5 KM...THESE SHOWERS AND THUNDERSTORMS WILL BE
VERY EFFICIENT RAIN PRODUCERS.

FOR LATE TONIGHT AND SUNDAY...THE 16.12Z AND 16.18Z MODELS ARE
SHOWING THAT THE 900 TO 800 MB FRONTOGENESIS WILL REMAIN MODERATE
TO STRONG THROUGH TONIGHT AND WEAK TO MODERATE ON SUNDAY. THESE
MODELS ARE IN VERY GOOD AGREEMENT THAT THIS FRONT WILL STRUGGLE
TO GET ANY FURTHER SOUTH THAN THE INTERSTATE 90 CORRIDOR. THIS
VERY MUCH DIFFERENT THAN YESTERDAY WHERE THE MODELS WERE WEAKENING
THE FRONT RAPIDLY DURING THE NIGHT AND IT WAS SOUTH OF THE AREA BY
SUNDAY. DUE TO THESE CHANGES...ADDED PRECIPITATION INTO THE
OVERNIGHT AND SUNDAY. LIKE THIS AFTERNOON...THE 0-3 AND 0-6 KM
SHEAR REMAINS LESS THAN 25 KNOTS...THUS NOT EXPECTING ANY SEVERE
WEATHER HOWEVER WITH PRECIPITABLE WATER VALUES AROUND 1.8 INCHES
AND WARM CLOUD DEPTHS RUNNING BETWEEN 4 AND 4.5 KM...THESE SHOWERS
AND THUNDERSTORMS WILL BE VERY EFFICIENT RAIN PRODUCERS.

.LONG TERM...(SUNDAY NIGHT THROUGH SATURDAY)
ISSUED AT 341 PM CDT SAT AUG 16 2014

ON SUNDAY NIGHT...A SHORT WAVE TROUGH...CURRENTLY OVER NORTH
DAKOTA...WILL MOVE EAST THROUGH THE FORECAST AREA. THIS FASTER
TREND SHOWED UP IN THE 16.00Z AND 16.06Z MODELS AND CONTINUED WITH
TODAY/S RUNS TOO. DUE TO THIS INCREASED AND SPEED UP THE RAIN
CHANCES ACROSS THE AREA. WHILE THE 0-3 KM AND 0-6 KM SHEAR ARE NOT
FAVORABLE FOR SEVERE WEATHER...THE PRECIPITABLE WATER /1.6 TO 1.8
INCHES/ AND WARM CLOUD DEPTHS /4 TO 4.5 KM/ WILL BE FAVORABLE FOR
HEAVY RAIN.

FROM MONDAY INTO TUESDAY...ANOTHER SHORT WAVE TROUGH CURRENTLY
LOCATED OVER THE CANADIAN ROCKIES WILL MOVE SLOWLY EAST THROUGH
THE FORECAST AREA. THIS SYSTEM WILL PRODUCE VERY LITTLE 0-3 KM OR
0-6 KM SHEAR ACROSS THE AREA...SO NO SEVERE WEATHER IS
ANTICIPATED. THE HEAVY RAIN THREAT IS A BIT LOWER DUE TO LOWER
PRECIPITABLE WATER VALUES AROUND 1.5 INCHES AND WARM CLOUD LAYER
DEPTHS BETWEEN 3.5 AND 4 KM.

LOOKING TOWARD THE END OF THE WEEK...BOTH THE OPERATIONAL GFS AND
ECMWF ARE HINTING AT MUCH WARMER TEMPERATURES POTENTIALLY REACHING
90 DEGREES IN SOME LOCATIONS FROM FRIDAY INTO THE UPCOMING
WEEKEND. THE GFS IS A BIT FASTER WITH ITS COLD FRONT MOVING
THROUGH THE AREA THAN THE ECMWF. AS A RESULT...THE GFS ONLY HAS
ITS VERY WARM TEMPERATURES /FOR AT LEAST THIS SUMMER/ FOR ONE
DAY...AND THE ECMWF KEEPS THEM AROUND FOR AT 3 DAYS. HOWEVER AS
STATED YESTERDAY...ONE HAS TO BE CAREFUL WITH THIS BECAUSE
THE POSITIVE PACIFIC DECADAL OSCILLATION IS NOT THAT FAVORABLE FOR
A WESTERN LONG WAVE TROUGH LIKE THE DETERMINISTIC MODELS ARE
SHOWING. LOOKING AT THE 12 GFS ENSEMBLE MEMBERS...ONLY 2 ARE EVEN
CLOSE TO THE GFS DETERMINISTIC RUN.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SUNDAY NIGHT)
ISSUED AT 1142 PM CDT SAT AUG 16 2014

LOOKS LIKE ANOTHER GOOD NIGHT FOR THE FORMATION OF FOG AND LOW
STRATUS ACROSS THE AREA. THE WEAK COLD FRONT CONTINUES TO MAKE
SLOW SOUTHWARD PROGRESS AND SHOULD EVENTUALLY PASS THROUGH BOTH
SITES. HOWEVER...THE WIND FIELD AHEAD OF THE FRONT IS SO LIGHT
WITH ONLY A MINOR INCREASE IN SPEEDS BEHIND THE FRONT...THAT IT
WILL NOT MAKE MUCH OF AN IMPACT. SOME CLOUDS CURRENTLY WITH THE
FRONT...BUT GENERALLY MOSTLY CLEAR AHEAD OF IT. EXPECT TO SEE SOME
FOG AND STRATUS DEVELOP BEFORE THE PASSAGE OF THE FRONT AND LOCAL
WEB CAM ALREADY SHOWING SIGNS OF THIS OVER THE MISSISSIPPI RIVER
CHANNEL. ONCE THE FRONT GOES THROUGH...THE 17.00Z NAM AND 17.03Z
RAP BOTH SHOW SOME INCREASE IN THE LOW LEVEL WINDS OUT OF THE EAST
TO THE POINT THAT SURFACE DEW POINTS MAY START TO DRY OUT SOME AT
KLSE SO THAT THE VISIBILITY MAY NOT GET TOO LOW. A VISIBILITY
BELOW A MILE STILL POSSIBLE AT KRST THOUGH. CONDITIONS EXPECTED TO
SLOWLY IMPROVE SUNDAY MORNING WITH A RETURN TO VFR CONDITIONS BY
EARLY AFTERNOON. THE NAM SUGGESTS THE POSSIBILITY OF SOME SHOWERS
AND STORMS IN THE AFTERNOON AS A SHORT WAVE TROUGH COMES OUT OF
THE DAKOTAS AND ACROSS MINNESOTA. HOWEVER...ITS FORECAST SOUNDINGS
SHOW VERY LITTLE VERTICAL MOTION AND OPTED TO STAY WITH A DRY
FORECAST. BETTER CHANCES FOR RAIN MAY COME AFTER 06Z MONDAY AS A
STRONGER SHORT WAVE TROUGH APPROACHES.

&&

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

$$

SHORT TERM...BOYNE
LONG TERM....BOYNE
AVIATION...04



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