246  
FXUS63 KARX 250341  
AFDARX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE LA CROSSE WI  
1040 PM CDT MON SEP 24 2018  
   
SHORT TERM
 
(THIS EVENING THROUGH TUESDAY)  
ISSUED AT 244 PM CDT MON SEP 24 2018  
 
AFTERNOON ANALYSIS SHOWED A SURFACE COLD FRONT STRETCHED FROM  
NORTHERN MINNESOTA, SOUTHWESTWARD INTO FAR NORTHWEST IOWA AND INTO  
NEBRASKA. PLENTY OF CLOUD COVER ASSOCIATED WITH THE FRONT, ALONG  
WITH SOME SHOWERS AND AN ISOLATED STORM OR TWO. FARTHER WEST,  
SATELLITE WATER VAPOR IMAGERY SHOWED A SHORTWAVE SLIDING THROUGH THE  
IDAHO/MONTANA/WYOMING AREA. THIS WILL BECOME THE MAIN PLAYER IN  
SHOWER AND STORM CHANCES ON TUESDAY.  
 
FOR TONIGHT, THE COLD FRONT WILL MOVE INTO THE AREA AND PERHAPS  
PRODUCE A FEW SHOWERS, BUT INSTABILITY LOOKS PRETTY MARGINAL (500  
J/KG), SO NOT ANTICIPATING MUCH MORE THAN A COUPLE ISOLATED STORMS.  
 
BETTER RAIN CHANCES WILL MOVE IN AS THE AFOREMENTIONED SHORTWAVE  
WILL DIG ACROSS THE CENTRAL PLAINS AND APPROACH THE FORECAST AREA  
FROM THE SOUTHWEST TUESDAY MORNING. THE COLD FRONT WILL BE DRAPED  
NORTHEAST TO SOUTHWEST ACROSS WISCONSIN BY EARLY TUESDAY AFTERNOON.  
MODELS PROG SOME DECENT MOISTURE ADVECTION AHEAD OF THE FRONT AND  
BUILD IN AROUND 1500 J/KG OF MUCAPE, THOUGH CLOUD COVER COULD HINDER  
DESTABILIZATION A BIT. STRONGEST DEEP LAYER SHEAR LOOKS TO BE BEHIND  
THE COLD FRONT, BUT STILL LOOKING AT PERHAPS 40 KTS OF 0-6 KM SHEAR  
ALONG WITH 30 KTS IN THE 0-3 KM LAYER. NOT A LOT OF LOW LEVEL SHEAR  
UNTIL THE LOW LEVEL JET RAMPS UP LATER IN THE AFTERNOON OR TOWARD  
EARLY EVENING, BUT THE COLD FRONT SHOULD BE WELL EAST OF THE  
FORECAST AREA BY THEN. SO BOTTOM LINE, THINKING THERE COULD BE A  
STRONG TO SEVERE STORM OR TWO POSSIBLE IN THE FORECAST AREA AHEAD OF  
THE COLD FRONT WHERE THE BEST INSTABILITY CAN BUILD. AT THIS TIME,  
THIS LOOKS TO BE CONFINED TO FAR SOUTHERN/SOUTHEASTERN PORTIONS, BUT  
TRENDS HAVE BEEN PUSHING THE THREAT FARTHER NORTHWEST WITH TIME. AS  
FAR AS HAZARDS, FORECAST SOUNDINGS SHOW RATHER "SKINNY" CAPE AND  
GOOD MIXING TO 900 MB, SO THINKING THE MAIN SEVERE THREAT WOULD BE  
DAMAGING WINDS.  
 
OTHERWISE, TEMPERATURES BEHIND THE FRONT WILL TOP OUT IN THE UPPER  
50S TO LOWER 60S ON TUESDAY, WHILE THOSE AREAS THAT STAY AHEAD OF  
THE FRONT LONGER COULD GET INTO THE LOWER 70S.  
   
LONG TERM
 
(TUESDAY NIGHT THROUGH MONDAY)  
ISSUED AT 244 PM CDT MON SEP 24 2018  
 
FOR TUESDAY NIGHT THRU THURSDAY NIGHT: MAIN FCST CONCERNS THIS  
PERIOD ARE COOLER TEMPS THRU THE PERIOD AND SHRA CHANCES WITH A  
FRONT ON THU.  
 
MODEL RUNS OF 24.12Z IN GOOD AGREEMENT ON LONGWAVE TROUGHING OVER  
THE UPPER MIDWEST THRU THIS PERIOD. ONE SHORTWAVE IN THIS TROUGH TO  
BE PUSHING EAST OF THE AREA TUE NIGHT, WITH ANOTHER STRONGER ONE TO  
DROP INTO/ACROSS NORTHERN MN/NORTHERN GREAT LAKES THU/THU NIGHT.  
FCST CONFIDENCE THIS PERIOD IS AVERAGE TO GOOD THIS CYCLE.  
 
MID LEVEL TROUGH AXIS/SHORTWAVE TO PASS TUE NIGHT, BUT SFC-700MB  
FRONT/TROUGH PROGGED TO ALREADY BE ON THE SE EDGE OF THE FCST AREA  
BY 00Z WED. SOME LINGERING SHRA CHANCES ACROSS THE EAST SIDE OF THE  
FCST AREA REASONABLE TUE EVENING, OTHERWISE HIGH PRESSURE WITH A  
COOLER, DRIER AIRMASS TO BUILD IN FOR TUE NIGHT INTO WED NIGHT. NW  
GRADIENT WINDS TO KEEP BL STIRRED TUE NIGHT AND LOWS FROM BOTTOMING  
OUT TOO FAR. LOW LEVEL COLD ADVECTION SENDS 925MB TEMPS INTO THE 7-  
10C RANGE FOR WED AFTERNOON, FOR WHAT IS PROMISING TO BE A COOL DAY.  
THIS EVEN AS THE HIGH QUICKLY SLIPS OFF AND SW TO W WINDS/LOW LEVEL  
WARM ADVECTION RETURNS BY LATE IN THE DAY. BLEND OF GUIDANCE HIGHS  
MOSTLY IN THE UPPER 50S-LOW 60S REASONABLE. LOW LEVEL WARM ADVECTION  
AND SW WINDS FOR BL MIXING SHOULD KEEP WED NIGHT LOWS MOSTLY IN THE  
40S. NEXT SHORTWAVE INTO THE LONGWAVE TROUGH PUSHES ANOTHER SFC-  
700MB TROUGH/FRONT WITH ITS ROUND OF LOWER LEVEL THERMO-DYNAMIC  
FORCING INTO/ACROSS THE AREA LATER WED NIGHT/THU. MOISTURE RETURN  
AHEAD OF THIS ONE APPEARS A BIT LIMITED, WITH DEEPER/STRONGER  
FORCING MAINLY ACROSS THE NORTH 1/2 OF THE FCST AREA. CAPE APPEARS  
QUITE LIMITED AS WELL. 30-60% SHRA CHANCES MAINLY ACROSS THE NORTH  
HALF OF THE FCST AREA, CENTERED ON THU MORNING, OK FOR NOW.  
 
FOR FRIDAY THRU MONDAY (DAYS 4-7): MAIN FCST CONCERNS THIS PERIOD  
ARE BELOW NORMAL TEMPS AND PERIODIC RA/SHRA CHANCE.  
 
MEDIUM RANGE MODELS OF 24.00Z/24.12Z IN REASONABLE AGREEMENT FOR  
RIDGING/REX BLOCKING FLOW ALONG THE WEST COAST OF NOAM WITH  
DOWNSTREAM TROUGHING OVER THE CENTRAL/EAST PARTS OF THE CONTINENT  
THIS PERIOD. PLENTY OF DETAIL DIFFERENCES WITH SHORTWAVE THRU THE  
TOUGHING FLOW OVER THE NORTH-CENTRAL CONUS FLOW THOUGH. FCST  
CONFIDENCE THIS PERIOD IS GOOD ON BELOW NORMAL TEMPS AND AVERAGE FOR  
SHRA CHANCES.  
 
LOW LEVEL TROUGH/FRONT FROM THU USHERS ANOTHER SEASONABLY COOL  
AIRMASS INTO THE AREA FOR FRI. NEXT WAVE THROUGH THE TROUGH QUICKLY  
SPREAD SOME MOISTURE AND MID LEVEL ISENTROPIC UPGLIDE FLOW OVER THIS  
COOL DOME, WITH RA/SHRA CHANCES INCREASING AND SPREADING NE ACROSS  
THE AREA STARTING FRI AFTERNOON AND CONTINUING THRU SAT. PRESENT  
TIMING WOULD GIVE THE AREA A BETWEEN-SYSTEMS BREAK FROM THE RA SAT  
NIGHT, WITH A STRONGER WAVE LOOKING TO SPREAD INCREASING MOISTURE/  
LIFT ACROSS THE AREA SUN THRU MON. AGAIN, PLENTY OF DETAIL/TIMING  
DIFFERENCES WITH SHORTWAVES AND RESULTING LOWER LEVEL FEATURES/LIFT  
IN THE FRI-MON PERIOD, SO TIMING OF HIGHER RAIN CHANCES THESE DAYS  
IS SUBJECT TO CHANGE. EACH WAVE WILL BRING A SEASONABLY COOL AIRMASS  
INTO THE REGION BEHIND IT. WHAT MAY WELL BE PLENTIFUL CLOUD COVER  
AND ANY RA/SHRA WOULD ALSO HELP HOLD DAYTIME TEMPS DOWN AND  
NIGHTTIME LOW UP. FOR NOW, THE BLEND OF GUIDANCE HIGHS MOSTLY IN THE  
50S FRI/SAT, MODERATING TO THE 60S BY MON APPEARS REASONABLE.  
 

 
   
AVIATION
 
(FOR THE 06Z TAFS THROUGH 06Z TUESDAY NIGHT)  
ISSUED AT 1040 PM CDT MON SEP 24 2018  
 
COLD FRONT TRACKS ACROSS THE REGION OVERNIGHT, BRINGING A FEW  
SHOWERS. LOW CLOUDS WILL FOLLOW IN WAKE OF FRONT, WITH MORE  
WIDESPREAD SHOWERS AS AN UPPER LEVEL SHORTWAVE TROUGH SPINS ACROSS  
THE REGION TUE. ISOLD THUNDER THREAT, BUT NOT ENOUGH CONFIDENCE TO  
INCLUDE IN FORECAST.  
 
CIGS: EXPECT IFR/MVFR CIGS MOVING IN OVERNIGHT, IN THE WAKE OF A  
PASSING COLD FRONT. LOW CIGS WILL HOLD THROUGH THE AFTERNOON TUE,  
IMPROVING TUE NIGHT.  
 
WX/VSBY: CURRENT RADAR POINTING TO SHOWERS AND ISOLD STORMS OVER  
NORTHWEST WI - ALONG THE FRONT AND INCONCERT WITH THE LOW LEVEL JET.  
FARTHER SOUTH THE SHOWERS HAVE DIMINISHED AS THE FRONTOGENETIC  
FORCING WEAKENS AND THE PUSH OF THE LOW LEVEL JET STAYS NORTH. DON'T  
SEE MUCH OF IMPACT FOR KRST (OR KLSE FOR THAT MATTER). SHOWERS  
SHOULD BE MORE WIDESPREAD MID TO LATE TUE MORNING, EXITING EAST BY  
LATER IN THE AFTERNOON. SOME VSBY RESTRICTIONS POSSIBLE, BUT WILL  
HOLD VFR FOR NOW.  
 
WINDS: SOUTH SWINGS TO WEST/NORTHWEST LATER TONIGHT, HOLDING THERE  
INTO TUE NIGHT.  
 

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

 
 

 
 
SHORT TERM...CA  
LONG TERM...RRS  
AVIATION...RIECK  
 
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