871  
FXUS63 KGRR 210731  
AFDGRR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE GRAND RAPIDS MI  
331 AM EDT FRI JUL 21 2017  
 
LATEST UPDATE...  
SYNOPSIS/SHORT TERM/LONG TERM/MARINE  
   
SYNOPSIS
 
 
ISSUED AT 330 AM EDT FRI JUL 21 2017  
 
LOW PRESSURE AND A WARM FRONT WILL BRING SHOWERS AND THUNDERSTORMS  
TONIGHT INTO SATURDAY WITH A RISK OF LOCALLY HEAVY RAINFALL AND  
PERHAPS EVEN A DAMAGING WIND THREAT. RAIN CHANCES WILL DECREASE  
SUNDAY AND SUNDAY NIGHT, WITH A STRETCH OF QUIET AND LESS HUMID  
WEATHER EXPECTED FOR THE FIRST HALF OF NEXT WEEK.  
 

 
   
SHORT TERM
 
(TODAY THROUGH SUNDAY)  
ISSUED AT 330 AM EDT FRI JUL 21 2017  
 
MAIN FOCUS CONTINUES TO BE POTENTIAL FOR HEAVY RAIN/FLOODING AND  
SVR WX LATE TONIGHT INTO SATURDAY MORNING. CURRENTLY THE MAIN  
RISK LOOKS TO BE NEAR AND SOUTH OF I-96.  
 
ACTIVE WARM FRONT AND MAIN CORRIDOR OF INCREASED MCS POTENTIAL IS  
EXPECTED TO SHIFT INTO THE AREA LATER TONIGHT INTO SATURDAY  
MORNING AS UPR LOW/SHORTWAVE MOVES EAST ACROSS SRN SASKATCHEWAN  
AND MANITOBA.  
 
POOL OF VERY HIGH PRECIPITABLE WATER AIR OF 2.00 TO 2.25  
WILL PUSH INTO SW LWR MI WITH THE HELP OF A 30-50 KT SWLY LOW  
LEVEL JET. THIS SUPPORTS HIGH POPS FOR SHOWERS AND TSTMS LATE  
TONIGHT/SAT MORNING, HOWEVER THE SVR WX/HVY RAIN POTENTIAL IS  
SOMEWHAT LOW CONFIDENCE DUE TO UNCERTAINTIES ON BEHAVIOR OF THE  
LOW LEVEL JET AND MCS TRACK/PROPAGATION.  
 
LATE NIGHT MCS PLACEMENT/PROPAGATION SHOULD FAVOR THE INSTABILITY  
GRADIENT, WHICH SEVERAL GUIDANCE SOURCES SHOW TO BE JUST  
WEST/SOUTHWEST OF OUR AREA AT 06Z TONIGHT. THIS GRADIENT IS  
PROGGED TO NUDGE FARTHER NORTHEAST IN SW LWR MI AFTER 06Z, HOWEVER  
IF CONSIDERABLE CONVECTION IS DEVELOPING/TRACKING THROUGH WI THIS  
AFTERNOON/EVENING, THE ADVANCEMENT OF THE HIGHER INSTABILITY  
TOWARD SW LWR MI MAY BE DELAYED/SUPPRESSED. MAIN TIMEFRAME OF  
CONCERN FOR A DAMAGING WIND AND/OR FLOODING RISK SEEMS TO BE 08Z  
TO 14Z... NEAR AND SOUTH OF I-96.  
 
CONSIDERABLE CLOUDS AND LINGERING SHOWERS SATURDAY AS THE SFC LOW  
TRACKS THROUGH WILL PROBABLY HOLD DOWN HIGH TEMPS. HOWEVER  
REMAINING WARM AND VERY HUMID, FOLLOWED BY A FOG RISK SATURDAY  
NIGHT. THUNDER RISK ON SUNDAY LOOKS LIMITED DUE TO WESTERLY LOW  
LEVEL FLOW.  
 
IN THE NEAR TERM, PATCHY DENSE FOG WILL IMPACT THE MORNING COMMUTE  
TODAY, FOLLOWED BY HOT/HUMID CONDITIONS WITH HIGHS AROUND 90.  
SOUNDING ARE TOO DRY TO SUPPORT CONVECTION TODAY, ALTHOUGH WE MAY  
HAVE SOME THICKER DEBRIS CLOUDS AND LIGHT SHOWERS APPROACHING FROM  
THE WEST TOWARD 00Z.  
 
 
   
LONG TERM
 
(SUNDAY NIGHT THROUGH THURSDAY)  
ISSUED AT 330 AM EDT FRI JUL 21 2017  
 
WE CONTINUE TO EXPECT THAT RAIN CHANCES WILL GRADUALLY END SUNDAY  
NIGHT INTO MONDAY MORNING. UPPER TROUGH AND SECONDARY SFC COLD FRONT  
WILL PASS THROUGH THE AREA SUN EVENING. SOME INSTABILITY WILL REMAIN  
THROUGH EARLY EVENING FROM DIURNAL HEATING AND THE THERMAL TROUGH  
AXIS.  
 
WE THEN EXPECT A QUIET AND COMFORTABLE PERIOD FROM MONDAY THROUGH  
WEDNESDAY. UPPER RIDGE SITUATED WEST OF THE AREA OVER THE PLAINS  
WILL SUPPORT A CANADIAN SFC RIDGE OVERHEAD. A DRY AND COOL LOW LEVEL  
NE FLOW WILL BRING THE COMFORTABLE CONDITIONS TO THE AREA. THE SFC  
RIDGE WILL GRADUALLY SHIFT EAST AND ALLOW A SRLY FLOW TO START  
BRINGING IN SOME SLIGHTLY WARMER AIR.  
 
THE CHANCE FOR SHOWERS/STORMS WILL BEGIN TO INCREASE WED NIGHT AND  
CONTINUE THROUGH THURSDAY. AN UPPER LOW WILL ROTATE ALONG SOUTHERN  
CANADA, AND EVENTUALLY BRING A COLD FRONT DOWN INTO THE AREA. RIGHT  
NOW, IT APPEARS THAT THERE WILL NOT BE A GREAT INFLUX OF MOISTURE  
FROM THE GULF. THIS WILL KEEP INSTABILITY AND MOISTURE A BIT  
LIMITED, BUT STILL ENOUGH FOR A FEW STORMS.  
 

 
   
AVIATION
 
(FOR THE 06Z TAFS THROUGH 06Z FRIDAY NIGHT)  
ISSUED AT 137 AM EDT FRI JUL 21 2017  
 
OUR MAIN CONCERNS OVER THE NEXT 24 HOURS IS FOG POTENTIAL THROUGH  
DAYBREAK THIS MORNING, THEN CONVECTIVE POTENTIAL TOWARD THE END OF  
THE PERIOD.  
 
HIGH PRESSURE SETTLING IN IS BRINGING LIGHT WINDS TO THE AREA.  
THESE LIGHT WINDS COMBINED WITH THE MOIST LOW LEVELS WILL BRING A  
GOOD CHANCE OF FOG. SOME SITES HAVE BEEN SEEING SOME BRIEF BOUTS  
OF FOG ALREADY THIS MORNING. KMKG SEEMS TO BE MORE PRIMED TO SEE  
LOWER CONDITIONS, WHILE INLAND AREAS MAY NOT GO QUITE AS LOW. IFR  
AND LOWER DOES LOOK POSSIBLE AT ALL SITES THROUGH ABOUT 12-13Z.  
 
CONDITIONS WILL BE QUIET THEN FOR MOST OF THE REST OF THE PERIOD.  
MAINLY CLEAR SKIES WITH A LIGHT, AND SOMETIMES VARIABLE WIND WITH  
THE HIGH NEARBY. CONVECTION WILL DEVELOP TONIGHT ACROSS WI AND  
WILL TRY TO PUSH IN LATE IN THE PERIOD. IT LOOKS POSSIBLE FOR KMKG  
TO SEE SOME SHOWERS/STORMS TO MOVE IN TOWARD 06Z SAT. THE REST OF  
THE SITES SHOULD SEE THIS AFTER 06Z SAT.  
 

 
   
MARINE
 
 
ISSUED AT 330 AM EDT FRI JUL 21 2017  
 
CONVECTION LATE TONIGHT INTO SATURDAY MORNING CREATE PERIODS OF  
ROUGH CONDITIONS LAKE MICHIGAN. WILL NEED TO MONITOR FOR THE  
POSSIBILITY OF SOME HIGHER WINDS AND WAVES ON SUNDAY WITH COLD  
ADVECTION/NW FLOW BEHIND THE SFC LOW.  
 

 
   
HYDROLOGY
 
 
ISSUED AT 315 PM EDT THU JUL 20 2017  
 
THE BEST OPPORTUNITY FOR HEAVY RAINFALL COMES FRIDAY NIGHT INTO  
SATURDAY. A VERY MOIST AIRMASS MOVES IN WHICH, COMBINED WITH  
FAVORABLE JET STRUCTURES AT 300 MB AND 850 MB, WILL LEAD TO THE RISK  
FOR EFFICIENT RAIN PRODUCING THUNDERSTORMS. SINCE THESE STORMS DO  
NOT CURRENTLY EXIST, IT IS DIFFICULT TO SAY EXACTLY WHICH AREAS WILL  
GET THE HEAVIEST RAIN. MUCH MAY HINGE ON WHETHER ANY STORMS THAT  
DEVELOP IN WISCONSIN FRIDAY NIGHT TEND TO MIGRATE SOUTHWARD TOWARD  
CHICAGO AND SOUTHERN LAKE MICHIGAN (LIKE EARLY THURSDAY MORNING) OR  
EXPAND EASTWARD INTO WESTERN LOWER MICHIGAN. REPEATED THUNDERSTORM  
MOVEMENT OVER THE SAME AREAS IS POSSIBLE FOR SEVERAL HOURS, WHICH  
COULD LEAD TO SOME FLOODING OF ROADS AND RAPID RIVER RISES  
ESPECIALLY ON SMALL RIVERS AND STREAMS. RIVER RESPONSES TO THE 72  
HOUR ENSEMBLE QPF VALUES OVER OUR RIVER BASINS ARE NOT RAISING ANY  
BIG RED FLAGS AT THIS POINT, AS IT WOULD TAKE SEVERAL INCHES OF  
RAINFALL TO START FLOODING MAIN STEM RIVERS. THAT IS UNCERTAIN AT  
THIS TIME.  
 
PWAT VALUES APPROACHING OR EXCEEDING 2.00" IS EXPECTED FOR MOST, IF  
NOT ALL, OF THE REGION FRIDAY NIGHT AND SATURDAY. WITH THE SFC LOW  
REMAINING WEST OF THE REGION THROUGH SATURDAY AFTERNOON, THE THREAT  
FOR HEAVY RAINFALL CONTINUES WELL INTO THE DAY SHOULD ANY CONVECTION  
PERSIST.  
 

 
   
GRR WATCHES/WARNINGS/ADVISORIES
 
 
MI...NONE.  
LM...NONE.  

 
 

 
 
SYNOPSIS...MEADE  
SHORT TERM...MEADE  
LONG TERM...NJJ  
AVIATION...NJJ  
HYDROLOGY...HOVING  
MARINE...MEADE  
 
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