509  
FXUS63 KGRR 201139  
AFDGRR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE GRAND RAPIDS MI  
728 AM EDT SUN AUG 20 2017  
 
LATEST UPDATE...  
AVIATION  
   
SYNOPSIS
 
 
ISSUED AT 258 AM EDT SUN AUG 20 2017  
 
HIGH PRESSURE WILL BRING ONE MORE DAY OF FAIR WEATHER THEN CHANCES  
FOR SHOWERS AND THUNDERSTORMS INCREASE EARLY IN THE WEEK. A COLD  
FRONT MOVING IN ON TUESDAY COULD BRING A ROUND OF STRONG  
THUNDERSTORMS. FAIR AND COOLER WEATHER WILL FOLLOW FOR THE REST OF  
THE WEEK.  
 

 
   
SHORT TERM
 
(TODAY THROUGH TUESDAY)  
ISSUED AT 258 AM EDT SUN AUG 20 2017  
 
TWO ISSUES TO FOCUS ON IN THE NEAR TERM, FIRST IS THE CLOUD  
FORECAST FOR THE PARTIAL SOLAR ECLIPSE THAT WILL BE VISIBLE ACROSS  
WEST MICHIGAN MONDAY AFTERNOON AND THEN THE POTENTIAL FOR SEVERE  
THUNDERSTORMS ON TUESDAY.  
 
FOR MONDAY, THE SFC HIGH IS RETREATING AND A BROAD AREA OF WARM  
ADVECTION IS SETTING UP WITH SHOWERS AND THUNDERSTORMS FORECAST TO  
FORM SUNDAY NIGHT ACROSS WISCONSIN IN AREA OF MID LEVEL ISENTROPIC  
ASCENT NORTH OF A WARM FRONT EXTENDING FROM NEBRASKA TO SOUTHERN  
WISCONSIN. THESE STORMS SHOULD DISSIPATE AS THEY MOVE EAST MONDAY  
MORNING WITH THE WEAKENING OF THE LOW LEVEL JET, BUT ADDITIONAL  
STORMS ARE EXPECTED TO FIRE UP BY THE LATE MORNING ACROSS WISCONSIN  
AND MOVE EAST, REACHING WESTERN ZONES DURING THE AFTERNOON.  
 
THE AMOUNT OF DEBRIS CLOUD FROM THE OVERNIGHT CONVECTION COMBINED  
WITH HIGH AND MID CLOUDS FROM THE SECOND ROUND OF STORMS COULD  
OBSCURE SKIES ACROSS MUCH OF LOWER MICHIGAN BY THE AFTERNOON.  
IT IS WORTH MENTIONING HERE IS THAT EVEN THICK CIRROSTRATUS  
CLOUDS WOULDN'T NECESSARILY RUIN THE SHOW AS THE PARTIALLY  
ECLIPSED SUN MAY STILL BE VISIBLE, BUT IF COMBINED WITH EVEN A  
THIN DECK OF MID LEVEL CLOUDS SUCH AS A LAYER OF ALTOCUMULUS COULD  
OBSCURE IT.  
 
FOR TUESDAY, FLAT UPPER RIDGING BEGINS TO GET REPLACED BY AN  
AMPLIFYING TROUGH AND IMPRESSIVE HEIGHT FALLS FOR THIS TIME OF YEAR  
OCCUR LATE TUESDAY INTO WEDNESDAY, ALONG WITH FAVORABLE  
POSITIONING OF THE UPPER JET WHICH WILL BRING UPPER DIVERGENCE  
ACROSS LOWER MICHIGAN. THIS IS ATTENDED BY STRONG SPEED SHEAR AND  
IF THERE IS ENOUGH INSTABILITY TUESDAY AFTERNOON, A LINE OF  
STRONG TO SEVERE STORMS COULD FORM ALONG OR OUT AHEAD OF THE COLD  
FRONT. THE MAIN THREAT WILL BE STRONG DAMAGING WINDS AND FLOODING  
RAINS AS P-WATS ARE CLOSE TO 2 INCHES.  
   
LONG TERM
 
(TUESDAY NIGHT THROUGH SATURDAY)  
ISSUED AT 258 AM EDT SUN AUG 20 2017  
 
THE LONG TERM PORTION OF THE FORECAST WILL SEE RELATIVELY QUIETER  
CONDITIONS COMPARED TO THE POTENTIALLY ACTIVE WEATHER IN THE SHORT  
TERM ON MON NIGHT AND TUE MORNING.  
 
DRIER WEATHER IS LIKELY TO HAVE ARRIVED BEFORE THE FORECAST PERIOD  
BEGINS ON TUE NIGHT. THE COLD FRONT SHOULD HAVE MOVED THROUGH THE  
ENTIRE AREA BY THEN, SWEEPING ALL OF THE RICH MOISTURE OUT OF THE  
AREA. WE WILL BE LEFT WITH THE UPPER TROUGH LINGERING OVER THE AREA.  
THIS, ALONG WITH A NORTHERLY LOW LEVEL FLOW HOLDING ON, WILL KEEP  
COOLER AIR IN PLACE FOR MUCH OF THE WEEK AFTER TUE.  
 
WE HAVE ROLLED WITH A DRY FORECAST FOR NOW FOR WED AND BEYOND. THIS  
IS A CAUTIOUSLY OPTIMISTIC FORECAST FOR NOW. THE UPPER TROUGH WILL  
LINGER THROUGH NEXT FRI, WITH THE UPPER JET ALMOST OVERHEAD THROUGH  
THE PERIOD. MOISTURE IS FORECAST TO BE RATHER LIMITED. THIS WILL  
LIMIT RAIN POTENTIAL FOR SHORT WAVES THAT COULD APPROACH THE AREA  
FROM THE NW. TOWARD THE END OF THE WEEK, THE TROUGH IS EXPECTED TO  
GET PUSHED OUT. THIS WILL ALLOW FOR A MODERATING TREND.  
 

 
   
AVIATION
 
(FOR THE 12Z TAFS THROUGH 12Z MONDAY MORNING)  
ISSUED AT 726 AM EDT SUN AUG 20 2017  
 
VFR CONDITIONS WILL PREVAIL INTO TONIGHT. SOME SHOWERS AND  
THUNDERSTORMS COULD BE ARRIVE AFTER 06Z TONIGHT ACROSS WESTERN LOWER  
MICHIGAN. WINDS WILL BE SOUTHWEST AOB 10 KNOTS.  
 

 
   
MARINE
 
 
ISSUED AT 258 AM EDT SUN AUG 20 2017  
 
WINDS AND WAVES WILL BE RELATIVELY LIGHT THROUGH MONDAY THEN  
PICK UP TUESDAY AND TUESDAY NIGHT WITH STRONG THUNDERSTORMS  
POSSIBLE AT THAT TIME.  
 

 
   
HYDROLOGY
 
 
ISSUED AT 315 PM EDT SAT AUG 19 2017  
 
OUR HYDROLOGIC FOCUS REMAINS ON EARLY THIS COMING WEEK. CONFIDENCE  
IS INCREASING THAT MONDAY INTO TUESDAY COULD FEATURE LOCALLY HEAVY  
RAINS WITH A FAVORABLE SETUP FOR THUNDERSTORM ACTIVITY ACROSS OUR  
REGION, MORE SO THAN MANY OF OUR EVENTS THAT WE HAVE HAD THIS  
SUMMER. MEDIUM RANGE GUIDANCE IS GENERALLY INDICATING A POTENTIAL  
FOR 1" OR MORE OF RAIN WITH THIS SYSTEM, WITH POCKETS OF LESS THAN  
1" MAINLY EAST OF US 131. KEEP IN MIND THAT MEDIUM RANGE MODELS ARE  
NON-CONVECTION ALLOWING SO THESE AMOUNTS WILL LIKELY BE LOCALLY  
UNDERDONE. WITH PWAT VALUES ONCE AGAIN APPROACHING 2.00" AND  
FAVORABLE H300 JET DIVERGENCE / H850 LLJ OF 30-40 KTS AND A SFC  
FRONTAL BOUNDARY DRAPED ACROSS THE LOWER PENINSULA, IT IS NOT HARD  
TO ENVISION LOCALIZED FLOODING (IE. SOME WATER COVERED ROADS) WITH  
THIS SYSTEM. RIVER RESPONSES DEPEND ON PLACEMENT OF RAINFALL AND  
DURATION OF HEAVY RAINS. SMALL RIVERS AND STREAMS WILL BE MOST  
SUSCEPTIBLE TO QUICK RISES.  
 

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

 
 

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