023  
FXUS63 KARX 181727  
AFDARX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE LA CROSSE WI  
1227 PM CDT TUE SEP 18 2018  
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 251 AM CDT TUE SEP 18 2018  
 
EARLY MORNING ANALYSIS SHOWED A SURFACE FRONT EXTENDING FROM EASTERN  
WISCONSIN SOUTHWESTWARD INTO CENTRAL IOWA. THIS FRONT CONTINUED TO  
MAKE PROGRESS SOUTHWARD AS A COLD FRONT, BUT WILL EVENTUALLY BECOME  
MORE OF A QUASI-STATIONARY FRONT AND SERVE AS THE FOCUS FOR REPEATED  
ROUNDS OF SHOWERS AND STORMS THROUGH THURSDAY. NORTH OF THE FRONT,  
SOME SHOWERS AND STORMS TIED TO INCREASING MOISTURE TRANSPORT WERE  
MOVING ACROSS SOUTHERN MINNESOTA AND INTO WESTERN WISCONSIN. PER  
LATEST SPC MESOANALYSIS, THESE STORMS SHOULD HAVE SOMEWHERE BETWEEN  
1000 AND 1500 J/KG OF MUCAPE TO WORK WITH, SO COULD BE A HAIL THREAT  
WITH THEM AS WE GO THROUGH THE MORNING.  
 
IN ADDITION TO THE ONGOING STORMS, MORE EXPECTED TO MOVE IN AFTER  
SUNRISE AS SHORTWAVE ENERGY MOVES IN FROM THE WEST. THINKING THE  
MAIN THREAT WILL CONTINUE TO BE HAIL GIVEN CAPPING IN PLACE THROUGH  
THE MORNING. EXPECTING TO SEE A BIT OF AN INCREASE IN DEEP LAYER  
SHEAR THROUGH THE MORNING WHICH COULD LEAD TO SOME BETTER STORM  
ORGANIZATION, AND COMBINED WITH INCREASING INSTABILITY AND 7-8 C/KM  
MID-LEVEL LAPSE RATES, COULD LEAD TO SOME SEVERE HAIL. COULD ALSO  
SEE SOME BRIEF HEAVY RAIN, BUT STORMS LOOK TO BE FAIRLY PROGRESSIVE  
IN MOVING THROUGH, EXITING THE AREA BY EARLY TO MID AFTERNOON.  
LOOKING LIKE ONLY LINGERING LIGHT SHOWERS AND PERHAPS SOME ISOLATED  
STORMS WILL REMAIN FOR THE REST OF THE AFTERNOON AND MUCH OF THE  
EVENING.  
 
LOW LEVEL JET AND MOISTURE TRANSPORT WILL RAMP UP AGAIN TONIGHT,  
LEADING TO MORE SHOWER AND STORM DEVELOPMENT. HEAVY RAIN ENVIRONMENT  
REMAINS IN PLACE (SEE HYDROLOGY DISCUSSION BELOW) AND ANY TIME  
MOISTURE TRANSPORT POINTS INTO A FAIRLY STATIONARY FRONT, FLOODING  
BECOMES A CONCERN. THE KEY WILL BE TO DETERMINE EXACTLY WHERE THE  
FRONT SETS UP TOMORROW NIGHT. CURRENTLY LOOKING LIKE IT WILL REMAIN  
OVER FAR SOUTHERN PORTIONS OF THE FORECAST AREA, WHERE RAINFALL  
AMOUNTS WOULD BE IN THE 1-3 INCH RANGE, WITH LOCALLY HIGHER AMOUNTS  
POSSIBLE BY WEDNESDAY MORNING.  
   
LONG TERM
 
(WEDNESDAY THROUGH MONDAY)  
ISSUED AT 251 AM CDT TUE SEP 18 2018  
 
SHOWERS AND STORMS WILL CONTINUE THROUGH WEDNESDAY AND THURSDAY AS A  
LOW PRESSURE SYSTEM FORMS OVER THE PLAINS AND MOVES NORTHEAST INTO  
THE AREA. 18.00Z RUNS OF THE GFS AND NAM MOVE THE SURFACE BOUNDARY  
NORTH AS A WARM FRONT FAIRLY QUICKLY WEDNESDAY INTO WEDNESDAY NIGHT,  
PLACING NEARLY ENTIRELY NORTH OF THE FORECAST AREA BY THURSDAY  
MORNING. THINKING THEY MIGHT BE TOO FAST IN DOING THIS GIVEN ONGOING  
CONVECTION ALONG AND TO THE NORTH OF IT, SO AXIS OF HEAVIEST  
RAINFALL MAY BE A BIT FURTHER SOUTH (MORE IN LINE WITH THE 18.00Z  
ECMWF AND CANADIAN).  
 
THE LOW PRESSURE SYSTEM WILL SLIDE THROUGH ON THURSDAY, BRINGING  
PLENTY OF DEEP LAYER SHEAR FOR A POTENTIAL SEVERE THREAT. THE KEY  
WILL BE TO DETERMINE HOW FAR THE WARM FRONT MAKES IT NORTH. SHOULD  
THE GFS/NAM'S FARTHER NORTH SOLUTIONS PLAY OUT, THE FORECAST AREA  
WILL BE SQUARELY IN THE WARM SECTOR, AND ASSUMING PRECIPITATION AND  
CLOUD COVER STAY CLOSE TO THE WARM FRONT, PLENTY INSTABILITY WILL BE  
ABLE TO BUILD IN, INCREASING THE SEVERE THREAT. SHOULD THE WARM  
FRONT STAY FARTHER SOUTH, THE PRECIPITATION AND CLOUD COVER WILL  
HINDER DESTABILIZATION AND LESSEN, BUT NOT COMPLETELY ELIMINATE, ANY  
SEVERE THREAT. FOR NOW, LEANING TOWARD THE WARM FRONT STAYING  
FARTHER SOUTH, BUT CONFIDENCE IS A BIT LOW.  
 
DRY AND COOL HIGH PRESSURE STILL LOOKS TO BUILD IN BEHIND THE LOW ON  
FRIDAY, LEADING TO HIGHS IN THE 60S TO LOWER 70S THROUGH THE WEEKEND  
AND LOWS GENERALLY IN THE 40S TO LOWER 50S, WITH A FEW 30S POSSIBLE  
IN THE FAVORED COLD SPOTS. SOME HINTS AT SMALL CHANCES FOR RAIN  
SUNDAY, WITH MORE SHORTWAVE ENERGY IN THE AREA, BUT THINKING BETTER  
CHANCES WILL BE TO THE NORTH. BETTER CHANCES FOR THE FORECAST AREA  
WILL MOVE IN MONDAY AFTERNOON AS ANOTHER COLD FRONT APPROACHES FROM  
THE WEST.  
 

 
   
AVIATION
 
(FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY AFTERNOON)  
ISSUED AT 1227 PM CDT TUE SEP 18 2018  
 
BULK OF MORNING SHRA/TS HAS SHIFTED SOUTH OF THE AREA LEAVING VFR  
CONDITIONS BEHIND THIS AFTERNOON. EXPECTING VFR CONDITIONS TO  
CONTINUE INTO TONIGHT BEFORE NEXT BATCH OF SHRA/TS PUSHES IN AFTER  
08Z. MOIST EASTERLY LOW LEVEL FLOW IS EXPECTED TO DRAW IN  
IFR/LIFR CEILINGS AND MVFR VISIBILITY AFTER 09Z AS THESE SHRA/TS  
MOVE IN. EXPECT THESE CONDITIONS TO LAST THROUGH THE REMAINDER OF  
THE FORECAST PERIOD.  
 

 
   
HYDROLOGY
 
 
ISSUED AT 251 AM CDT TUE SEP 18 2018  
 
A HEAVY RAINFALL ENVIRONMENT WILL REMAIN IN PLACE ACROSS THE AREA  
THROUGH THURSDAY, WITH PRECIPITABLE WATER VALUES IN THE 1.5 TO 2  
INCH RANGE AND WARM CLOUD DEPTHS NEAR 4 KM. GENERALLY ANTICIPATING  
RAINFALL AMOUNTS OF 2 TO 5 INCHES THROUGH THURSDAY, WITH LOCALLY  
HIGHER AMOUNTS POSSIBLE. HOWEVER, LOCATION OF THE HEAVIEST  
RAINFALL REMAINS A BIT UNCERTAIN, AS EACH DAY'S PRECIPITATION WILL  
DEPEND ON WHERE A SURFACE BOUNDARY SETS UP. WITH CURRENT RIVER  
LEVELS ABOVE NORMAL, RIVER RISES CAN BE EXPECTED AND RIVER  
FLOODING IS POSSIBLE ALONG WITH PERHAPS SOME FLASH FLOODING.  
 

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

 
 

 
 
SHORT TERM...CA  
LONG TERM...CA  
AVIATION...DAS  
HYDROLOGY...CA  
 
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