253  
FXUS63 KARX 212338  
AFDARX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE LA CROSSE WI  
638 PM CDT SAT OCT 21 2017  
   
SHORT TERM
 
(THIS EVENING THROUGH SUNDAY)  
ISSUED AT 230 PM CDT SAT OCT 21 2017  
 
WATER VAPOR SATELLITE THIS AFTERNOON SHOWS THE MEAN LONG WAVE TROUGH  
HAS PROGRESSED THROUGH THE ROCKIES. THE TROUGH AND EMBEDDED SHORT  
WAVE TROUGHS WILL CONTINUE TO WORK EAST THROUGH THE NIGHT AND THEN  
START TO SHEAR OUT SUNDAY AS A CLOSED UPPER LEVEL LOW LOOKS TO TAKE  
SHAPE ACROSS THE LOWER MISSISSIPPI RIVER VALLEY. THIS TROUGH AND  
EMBEDDED SHORT WAVE TROUGHS WILL PROVIDE THE NECESSARY LIFT FOR  
SHOWERS AND STORMS ALONG THE COLD FRONT. OUT AHEAD OF THE FRONT,  
THERE IS A STRONG CAP IN PLACE ABOVE 800 MB AND THE 21.17Z RAP AND  
21.12Z NAM FORECAST SOUNDINGS CONTINUE TO SHOW THIS CAP WILL REMAIN  
IN PLACE UNTIL THE FRONT ARRIVES THIS EVENING. THIS WILL PROVIDE A  
VERY NARROW WINDOW WHERE THERE WILL BE ENOUGH FORCING AND CAPE TO  
SUPPORT THUNDERSTORMS RIGHT ALONG THE FRONT. FORECAST SOUNDINGS FROM  
THE RAP SHOW BETWEEN 500 AND 750 J/KG OF CAPE ALONG THE FRONT IN A  
TALL, SKINNY PROFILE WITH DEEP LAYER SHEAR OF 30 TO 40 KNOTS, MOST  
OF WHICH RESIDES IN THE LOWEST 3 KM. THE MAIN CONCERN CONTINUES TO  
BE IF THIS CAPE PROFILE WILL BE ENOUGH TO BALANCE OUT THE SHEAR AND  
ALLOW SOME STRONGER TO POTENTIALLY SEVERE STORMS TO FORM. IF IT  
DOES, STRONG TO DAMAGING WINDS CONTINUE TO LOOK TO BE THE MAIN  
THREAT AND IN THAT REGARDS, NO REAL CHANGE TO CURRENT THINKING OR  
MESSAGING. THE STORMS SHOULD ALSO BE EFFICIENT RAIN PRODUCERS WITH  
PRECIPITABLE WATERS EXPECTED TO BE IN THE 1.5 TO 1.75 INCH RANGE AND  
WARM CLOUD DEPTHS AROUND 3.5 KM. EXPECTING THE FRONT WITH THE  
SHOWERS AND STORMS TO START MOVING INTO THE WESTERN SECTIONS OF THE  
AREA RIGHT AROUND 22.00Z AND THEN MOVE STEADILY EAST ACROSS THE AREA  
THROUGH THE NIGHT. THE THREAT FOR STRONG TO SEVERE STORMS SHOULD  
DIMINISH BY LATE EVENING AS THE CAPE BECOMES MINIMAL. LINGERING  
SHOWERS BEHIND THE FRONT SUNDAY MORNING, MAINLY OVER WISCONSIN, WILL  
COME TO AN END BY EARLY AFTERNOON. THE CLOUDS WILL TAKE A BIT LONGER  
TO CLEAR BUT LOOK FOR SKIES TO BECOME MOSTLY SUNNY STARTING IN THE  
MORNING ACROSS THE WEST AND DURING THE AFTERNOON FOR THE EASTERN  
SECTIONS.  
   
LONG TERM
 
(SUNDAY NIGHT THROUGH SATURDAY)  
ISSUED AT 230 PM CDT SAT OCT 21 2017  
 
MUCH COOLER WEATHER STILL EXPECTED FOR THIS COMING WEEK. OVERALL,  
LITTLE CHANGE MADE TO THE PREVIOUS FORECAST, ALTHOUGH FORECAST  
CONFIDENCE ERODES HEADING THROUGH THE MIDDLE AND LATTER STAGES OF  
THE WEEK.  
 
THERE WILL BE ONE MORE MILD/SEASONABLE DAY MONDAY AHEAD OF AN  
UPPER TROUGH THAT WILL DIG SOUTHEASTWARD TOWARDS THE GREAT LAKES  
AND DRIVE A STRONG COLD FRONT THROUGH THE AREA ON MONDAY.  
FOLLOWING THE FRONTAL PASSAGE, TEMPS WILL FALL BELOW AVERAGE ON  
TUESDAY WITH HIGHS IN THE 40S TO NEAR 50. AS THE COLD AIR ALOFT  
AND STEEP LOW-LEVEL LAPSE RATES INTERACT WITH WEAK RIPPLES WITHIN  
THE UPPER TROUGH, SOME SHOWERS ARE POSSIBLE ESPECIALLY MONDAY  
NIGHT INTO TUESDAY, WITH OTHERWISE MOSTLY CLOUDY, BREEZY AND  
CHILLY CONDITIONS. SOME DIFFERENCES EXIST TUESDAY NIGHT INTO  
WEDNESDAY WITH THE 21.12Z GFS A LITTLE FARTHER SOUTHWEST WITH A  
REINFORCING SHORTWAVE TROUGH IN NORTHWEST FLOW COMPARED WITH THE  
ECMWF, BRINGING LIGHT PRECIP AND INCREASED CLOUDS TO THE AREA.  
THIS COULD ULTIMATELY IMPACT LOW TEMPS AND POSSIBLE FROST CONCERNS  
WEDNESDAY MORNING.  
 
SHORT-LIVED SHORTWAVE RIDGING IS EXPECTED ON THURSDAY WITH RETURN  
SOUTHERLY FLOW HELPING TEMPS MODERATE TO NEAR OR SLIGHTLY ABOVE  
SEASONAL NORMS AHEAD OF LOW PRESSURE DEVELOPING TO THE WEST.  
 
BEYOND THURSDAY, CONFIDENCE IN FORECAST DETAILS REMAINS LOWER,  
ESPECIALLY REGARDING ANY PRECIP CHANCES. DETERMINISTIC AND  
ENSEMBLE GUIDANCE BOTH SUPPORT ANOTHER UPPER TROUGH MOVING INTO  
THE CENTRAL US LATE THIS WEEK, A LITTLE FARTHER WEST THAN THE  
EARLY WEEK TROUGH. A REINFORCING SURGE OF COOLER, BELOW AVERAGE  
TEMPS IS EXPECTED FOR FRIDAY/SATURDAY (850 MB TEMPS FALLING TO -6  
TO -9C BY SATURDAY), WITH ANOTHER CHANCE FOR A WIDESPREAD  
FROST/FREEZE BY LATE THIS WEEK. HOWEVER, RUN-TO-RUN CONSISTENCY  
AMONG MODELS HAS BEEN POOR DURING THIS TIME WITH THE EVOLUTION OF  
THE UPPER TROUGH, SO STUCK WITH MODEL CONSENSUS POPS LATE THIS  
WEEK FOR NOW. IT'S POSSIBLE IT COULD BE COLD ENOUGH FOR SOME  
SNOWFLAKES BY LATE THIS WEEK WITH ANY PRECIP.  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z SUNDAY EVENING)  
ISSUED AT 638 PM CDT SAT OCT 21 2017  
 
A COLD FRONT CONTINUES TO APPROACH THE AREA, AND WILL ARRIVE AT  
RST AROUND 02Z AND LSE BY 06Z. AHEAD OF THAT FRONT, A RELATIVELY  
BRIEF PERIOD OF SHOWERS AND THUNDERSTORMS IS EXPECTED, WITH  
TEMPORARY MVFR CONDITIONS POSSIBLE ALONG WITH POTENTIALLY SOME  
BRIEF GUSTY WINDS TO AROUND 25 KNOTS. BEHIND THE FRONT, RST  
SHOULD DRY OUT FAIRLY QUICKLY BY AROUND MIDNIGHT AT THE LATEST,  
BUT ADDITIONAL SHOWERS ARE EXPECTED TO DEVELOP NEAR AND EAST OF  
THE MISSISSIPPI RIVER, IMPACTING LSE THROUGH SUNRISE IF NOT A  
LITTLE LATER. ALSO BEHIND THE FRONT, CEILINGS WILL GRADUALLY RISE  
INTO SUNDAY, WITH SKIES EXPECTED TO CLEAR ENTIRELY FOR RST BY  
EARLY AFTERNOON WHILE THICKER CIRRUS HANGS ON AROUND LSE.  
 

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

 
 

 
 
SHORT TERM...04  
LONG TERM...JM  
AVIATION...LAWRENCE  
 
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