556  
FXUS63 KMKX 171528 AAA  
AFDMKX  
 
AREA FORECAST DISCUSSION...UPDATED  
NATIONAL WEATHER SERVICE MILWAUKEE/SULLIVAN WI  
1028 AM CDT FRI AUG 17 2018  
   
UPDATE
 
 
A SUBTLE HEAVY RAIN EVENT LOOKS TO BE OCCURRING ACROSS PARTS OF  
THE AREA, WITH A SLOWLY SOUTHEASTWARD-MOVING 500 MB SHORTWAVE  
TROUGH OVER THE AREA. IN ADDITION, THERE IS UPPER DIVERGENCE FROM  
THE LEFT FRONT QUADRANT OF A JET STREAK OVER THE AREA, ALONG WITH  
LOW LEVEL CONVERGENCE/FRONTOGENESIS RESPONSE EXTENDING WEST TO  
EAST ACROSS THE CENTRAL PARTS OF THE FORECAST AREA. AREA FORECAST  
SOUNDINGS ARE SHOWING HIGH PRECIPITABLE WATER VALUES WITH TALL  
SKINNY MEAN LAYER CAPE.  
 
THIS HAS RESULTED IN FLASH FLOODING OCCURRING IN THE WATERTOWN  
AND ADJACENT AREAS FAR NORTHERN JEFFERSON COUNTY, MAINLY IN A WEST  
TO EAST BAND. THE RADAR TRENDS ARE INDICATING THAT THE BANDED  
HEAVY RAINFALL IS FINALLY STARTING TO WEAKEN AND MOVE TO THE NORTH  
AND WEST OF THE FLASH FLOOD WARNING AREA. THERE IS MORE ACTIVITY  
SHIFTING INTO THE LAKESHORE COUNTIES AROUND AND NORTH OF  
MILWAUKEE. ALL OF THIS ACTIVITY IS EXPECTED TO ROTATE SOUTHWEST TO  
SOUTH INTO THIS AFTERNOON, BECOMING MORE SCATTERED AS THE 500 MB  
SHORTWAVE TROUGH SLIDES TO THE SOUTHEAST OF THE AREA.  
 
COULD SEE ADDITIONAL SHOWERS AND STORMS DEVELOP WITH PEAK HEATING  
LATER THIS AFTERNOON, THOUGH NEED THE CLOUD COVER TO SCATTER OUT  
FOR THIS TO OCCUR. ANY SHOWER/STORM ACTIVITY SHOULD WIND DOWN  
EARLY THIS EVENING, WITH SKIES CLEARING OVERNIGHT. ADJUSTED HIGHS  
TODAY DOWNWARD SOMEWHAT TO ACCOUNT FOR CLOUDS HANGING AROUND  
LONGER INTO THE AFTERNOON.  
 
SOME FOG IS EXPECTED TO DEVELOP OVERNIGHT INTO EARLY SATURDAY  
MORNING, ESPECIALLY IN THE RIVER VALLEYS AND OTHER LOW LYING  
AREAS. HIGH PRESSURE SHOULD BRING DRY CONDITIONS FOR SATURDAY,  
WITH WARM AND HUMID CONDITIONS. ONSHORE WINDS SHOULD KEEP  
LAKESHORE AREAS COOLER.  
 

 
   
MARINE
 
 
THUNDERSTORMS WILL AFFECT THE NEARSHORE WATERS OF LAKE MICHIGAN  
AT TIMES THIS MORNING, BEFORE BECOMING MORE SCATTERED IN NATURE  
THIS AFTERNOON. CANNOT RULE OUT A WATERSPOUT OCCURRING LATE THIS  
MORNING INTO THE AFTERNOON, THOUGH THIS HAZARD WOULD BE ISOLATED.  
 
NORTH WINDS OF 10 TO 15 KNOTS ARE EXPECTED THIS AFTERNOON INTO  
SATURDAY ACROSS THE NEARSHORE WATERS OF LAKE MICHIGAN. WAVES OF 2  
TO 3 FEET ARE EXPECTED DURING THIS PERIOD, PERHAPS LINGERING INTO  
SATURDAY NIGHT. WINDS AND WAVES WILL THEN SUBSIDE ON SUNDAY.  
 
ONSHORE WINDS MAY CAUSE WAVES TO REACH SMALL CRAFT ADVISORY  
LEVELS AT TIMES NEXT WEEK.  
 

 
   
PREV DISCUSSION
 
(ISSUED 703 AM CDT FRI AUG 17 2018)  
 
UPDATE...  
 
THERE IS A BAND OF MODERATE RAIN ACROSS SOUTHERN WI THIS MORNING.  
THIS IS ASSOCIATED WITH 250-850MB DIFFERENTIAL DIVERGENCE AS AN  
UPPER LOW TRACKS ALONG THE WI/IL BORDER. THE RAIN WILL MAKE THE  
MORNING COMMUTE SLOWER FOR THOSE IN THE MADISON TO JOHNSON CREEK  
AREA ALONG I-94. THERE IS ALSO SOME PATCHY DENSE FOG OUT THERE.  
NOT WIDESPREAD ENOUGH FOR A HEADLINE.  
 
THERE IS NO THUNDER WITH THIS PRECIP YET, BUT EXPECT BETTER  
INSTABILITY TO DEVELOP WITH DAYTIME HEATING BY LATE MORNING. THIS  
BAND WILL PERSIST THROUGH THE MORNING AND THEN EXPECT PRECIP TO  
BECOME MORE SCATTERED THIS AFTERNOON.  
 
ADDITIONALLY, WE HAD SEVERAL REPORTS OF WATERSPOUTS JUST NORTH OF  
MILWAUKEE THIS MORNING. THE STORMS ARE STILL SHOWING SIGNS OF WEAK  
ROTATION, SO WE ISSUED A SPECIAL MARINE WARNING FOR THE  
POSSIBILITY OF MORE WITH THESE PARTICULAR STORMS.  
 
AVIATION(12Z TAFS)...  
 
THERE IS A BAND OF MODERATE RAIN ACROSS SOUTHERN WI, MAINLY ALONG  
AND JUST NORTH OF I-94. THIS BAND WILL PERSIST IN THIS VICINITY  
THROUGH THE MORNING. LOOK FOR SHOWERS AND THUNDERSTORMS TO BECOME  
MORE SCATTERED THIS AFTERNOON BEFORE DISSIPATING BY EARLY EVENING.  
 
PREV DISCUSSION... (ISSUED 303 AM CDT FRI AUG 17 2018)  
 
DISCUSSION...  
 
TODAY AND TONIGHT...FORECAST CONFIDENCE IS MEDIUM.  
 
AN UPPER TROUGH SLIDING ACROSS IOWA AND NORTHERN ILLINOIS AND  
SOUTHERN WISCONSIN WILL CONTINUE TO PRODUCE SHOWERS ACROSS  
PORTIONS OF SOUTHERN WI TODAY. INSTABILITY WILL BE SUFFICIENT FOR  
SCATTERED THUNDERSTORMS ONCE AGAIN, BUT THE CAPE VALUES ARE LOWER  
THAN THEY WERE YESTERDAY. PRECIPITABLE WATER IS HIGH, SO EXPECT  
EFFICIENT RAINFALL. CLOUDS WILL KEEP HIGH TEMPERATURES IN CHECK  
ONCE AGAIN, SO EXPECT HIGHS IN THE LOWER TO MID 80S.  
 
THE TROUGH WILL FINALLY MOVE OUT OF THE AREA BY MID EVENING,  
ENDING THE PRECIP CHANCES FOR THE OVERNIGHT HOURS. WE CAN EXPECT  
AREAS OF FOG TO DEVELOP DUE TO CLEARING SKIES, LINGERING MOISTURE,  
AND LIGHT WINDS.  
 
SATURDAY AND SUNDAY - CONFIDENCE...MEDIUM TO HIGH  
LARGE HIGH PRESSURE AREA EXPECTED TO KEEP THINGS ON THE QUIET  
SIDE. A NORTHWEST MID LEVEL FLOW TRANSITIONS TO A MID LEVEL RIDGE  
AXIS BEING SITUATED ACROSS THE AREA ON SUNDAY, BUBBLING UP AHEAD  
OF THE APPROACHING TROUGH.  
 
MONDAY AND TUESDAY - CONFIDENCE...LOW TO MEDIUM  
PATTERN STILL LOOKING UNSETTLED. THE GFS IS STAYING THE COURSE IN  
KEEPING A WEAKER LOW WELL TO OUR SOUTH AND SOUTHEAST THIS PERIOD.  
MEANWHILE THE ECMWF AND GEM ARE STAYING REASONABLY CONSISTENT WITH  
A STRONGER AND MORE NORTHERN TRACK OF THE LOW. THE KEY UPPER  
FEATURES IN THE LATTER CASE ARE ALSO IN A MORE NORTHERN POSITION  
WITH MORE DYNAMICAL CONTRIBUTIONS THAN THE WEAKER AND MORE  
SOUTHERN WAVE OF THE GFS. EVEN IN THE GFS CASE IT WOULD STILL BE  
SHOWERY AND COOL FOR US WITH A NORTHEAST WIND REGIME. IN THE  
GEM/ECMWF WE'D BE LOOKING AT HEAVIER RAINS AND POTENTIAL FOR  
THUNDER AS WELL. THE PERIOD OF HEAVIEST RAINS WOULD LIKELY END UP  
BEING MONDAY AFTERNOON INTO MONDAY EVENING. SO STILL SOME  
UNCERTAINTIES ON HOW THIS WILL EVOLVE BUT IF ANYTHING THE ECMWF  
AND GEM ARE TRENDING JUST A LITTLE MORE SOUTHEAST, SO WILL BE  
INTERESTING TO SEE IF IN FUTURE RUNS THERE IS MORE CONVERGENCE  
TOWARDS THE WEAKER MORE SOUTHERN TRACK OF THE GFS. SECONDARY WAVE  
RIDES IN FROM THE NORTHWEST ON TUESDAY WITH COLD ADVECTION REGIME  
AND LOW LEVEL TROUGH IN PLACE COULD END UP BEING A PRETTY SHOWERY  
DAY WITH COOLER CONDITIONS NORTH OR NORTHWEST WINDS.  
 
WEDNESDAY AND THURSDAY - CONFIDENCE...MEDIUM  
SURFACE HIGH BUILDS ACROSS INTO WEDNESDAY NIGHT AND WE'LL LIKELY  
SEE IF OFF TO THE EAST ALREADY FOR THURSDAY WITH AN EVOLVING  
RETURN FLOW/WAA REGIME SETTING UP.  
 
AVIATION(09Z TAFS)...LIGHT RAIN SHOWERS AND SCATTERED THUNDERSTORMS  
WILL PERSIST ACROSS PORTIONS OF SOUTHERN WISCONSIN FOR MOST OF THE  
DAY. EXPECT PATCHY FOG WITH A FEW TRANSIENT POCKETS OF DENSE FOG  
EARLY THIS MORNING. CEILINGS ARE HIGHLY VARIABLE AS WELL. THERE  
WILL LIKELY BE MORE WIDESPREAD FOG TONIGHT IN SOUTH CENTRAL WI,  
WITH DENSE FOG POSSIBLE.  
 
MARINE...INCREASING NORTH-NORTHEAST WINDS WILL BUILD 2 TO 3 FOOT  
WAVES IN THE NEARSHORE AREAS BY THIS AFTERNOON AND THEY WILL PERSIST  
THROUGH SATURDAY BEFORE GRADUALLY DIMINISHING SUNDAY. NO HEADLINES  
EXPECTED.  
 
LOOK FOR INCREASING WINDS MONDAY AND TUESDAY IN ASSOCIATION WITH  
A SOMEWHAT STRONGER STORM SYSTEM THAT WILL AFFECT THE LOWER GREAT  
LAKES. THERE IS STILL UNCERTAINTY WITH THE STRENGTH AND TRACK OF  
THE SURFACE LOW, SO CHANGES ARE LIKELY TO THE FORECAST IN THE COMING  
DAYS. AT THIS POINT, IT LOOKS LIKE WINDS AND/OR WAVES MAY BE STRONG  
ENOUGH TO WARRANT A SMALL CRAFT ADVISORY SOMETIME BETWEEN MONDAY  
NIGHT AND TUESDAY NIGHT.  
 

 
   
MKX WATCHES/WARNINGS/ADVISORIES
 
 
WI...NONE.  
LM...NONE.  

 
 

 
 
UPDATE...WOOD  
TODAY/TONIGHT AND AVIATION/MARINE...CRONCE  
SATURDAY THROUGH THURSDAY...COLLAR  
 
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