865  
FXUS63 KGRR 041454  
AFDGRR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE GRAND RAPIDS MI  
1054 AM EDT FRI SEP 4 2015  
 
LATEST UPDATE...  
HYDRO  
   
SYNOPSIS
 
 
ISSUED AT 307 AM EDT FRI SEP 4 2015  
 
A WEAK AREA OF LOW PRESSURE WILL REMAIN OVER LOWER MICHIGAN  
THROUGH TODAY. THIS SYSTEM WILL GENERATE A FEW SHOWERS AND PERHAPS  
A THUNDERSTORM. THE VERY WARM AIRMASS RETURNS FOR SATURDAY.  
ACCOMPANYING THE ARRIVAL OF THIS WARM AIR COULD BE SOME  
THUNDERSTORMS. HIGH PRESSURE CONTROLS THE WEATHER FOR SUNDAY AND  
THAT SHOULD BE ONE OF THE WARMER DAYS OF SEASON. A COLD FRONT  
NEARS FOR LABOR DAY WHICH COULD LEAD TO SOME AFTERNOON  
THUNDERSTORMS WITH THE VERY WARM AIRMASS CONTINUING.  
 

 
   
UPDATE
 
 
ISSUED AT 1007 AM EDT FRI SEP 4 2015  
 
CANCELLED DENSE FOG ADVISORY BASED ON OBSERVATIONS IN THAT AREA  
SHOWING VISIBILITIES CLIMBING TO ABOVE A MILE. AS MENTIONED  
EARLIER...WIDESPREAD CEILINGS OF LOW CLOUDS AND THEIR ASSOCIATED  
ABSORPTION AND DOWN-WELLING OF LONGWAVE RADIATION HELPED TO  
MITIGATE THE FOG.  
 
ALSO REDUCED RAIN CHANCES BASED ON CURRENT RADAR AND SATELLITE  
TRENDS. CONVECTION ALLOWING GUIDANCE SUGGESTS SCATTERED DIURNALLY  
FORCED SHOWERS WILL DEVELOP THIS AFTERNOON MAINLY NORTHEAST OF  
THE AREA IN A WARM FRONTAL ZONE ASSOCIATED WITH A WEAK SURFACE  
LOW OVER CENTRAL MICHIGAN DRIFTING NORTH TODAY.  
 

 
   
SHORT TERM
 
(TODAY THROUGH SUNDAY)  
ISSUED AT 307 AM EDT FRI SEP 4 2015  
 
FOG HAS BEEN A CHALLENGE THIS MORNING. WE DEVELOPED SOME THICK FOG  
DURING THE EVENING...BUT THE THICKER MID TO UPPER LEVEL CLOUDS  
HAVE CAUSED THE FOG TO THIN OUT SOMEWHAT. I CHECKED WITH MANY OF  
OUR CENTRAL COUNTIES...ALONG WITH LOOKING AT WEBCAMS...AND IT  
APPEARS THAT GRATIOT AND MONTCALM ARE SEEING SOME DENSE FOG. I  
WILL FEATURE A DENSE FOG ADVISORY FOR THESE COUNTIES. IF CLEARING  
DOES DEVELOP ELSEWHERE WE MAY NEED TO EXPAND THE HEADLINE...BUT  
FOR NOW IT LOOKS LIKE STRATUS IT TAKING OVER.  
 
AS FOR THE RAIN POTENTIAL...WITH THE MID TO UPPER FLOW BEING  
CYCLONIC AND THE AIRMASS STILL VERY MOIST...WE SHOULD SEE SHOWERS  
AROUND TODAY AND INTO TONIGHT. INSTABILITY DEVELOPS THIS AFTERNOON  
SO I FEATURED A FEW THUNDERSTORMS AS WELL. AS FOR  
TONIGHT...INITIALLY WAS THINKING ONLY SHOWERS BUT MODELS ARE  
SHOWING ELEVATED INSTABILITY ARRIVING FROM THE SOUTH. THUS I WILL  
MENTION THUNDER INTO SATURDAY.  
 
NOT SURE WE WILL STAY DRY ON SUNDAY. THE ATMOSPHERE DOES BECOME  
UNSTABLE IN THE AFTERNOON. NO SIGNIFICANT FORCING. WE MAY NEED TO  
MONITOR FOR POSSIBLE CONVECTION IF A LAKE BREEZE CAN DEVELOP. WILL  
KEEP IT DRY FOR NOW.  
 
VERY WARM TEMPERATURES ARE EXPECTED BOTH SATURDAY AND SUNDAY.  
HIGHS SHOULD TOP MID 80S AT MOST LOCATIONS AS 925 MB TEMPERATURES  
REACH INTO THE MID 20 DEG C LEVEL. SOME 90S ARE POSSIBLE.  
   
LONG TERM
 
(SUNDAY NIGHT THROUGH THURSDAY)  
ISSUED AT 307 AM EDT FRI SEP 4 2015  
 
PRIMARY CONCERN IS WITH THUNDERSTORM POTENTIAL MONDAY AFTERNOON INTO  
WEDNESDAY. THE BEST CHANCE FOR RAINFALL APPEARS TO BE TUESDAY AND  
TUESDAY NIGHT WITH SOME LOCATIONS LIKELY GETTING AN INCH OR TWO OF  
RAINFALL. I WOULD NOT BE SURPRISED TO SEE A FEW STRONGER STORMS ON  
TUESDAY.  
 
THERE ARE DIFFERENCES IN THE TIMING OF THIS SYSTEM WITH THE 00Z  
GUIDANCE...WITH THE ECMWF NOW THE QUICKEST TO EXIT THE BOUNDARY  
TUESDAY NIGHT. MODELS GENERALLY AGREE THAT A COLD FRONT WILL WASH  
OUT ACROSS THE REGION MONDAY. SOME SCATTERED SHOWERS AND STORMS WILL  
BE POSSIBLE MONDAY AFTERNOON AND EVENING AS THIS OCCURS. THE  
BOUNDARY LIFTS THROUGH AS A WARM FRONT TUESDAY AS THE RIGHT ENTRANCE  
REGION OF THE UPPER LEVEL JET CORE MOVES OVERHEAD. WITH UL DIV...A  
MID LEVEL VORT LOBE MOVING THROUGH BETWEEN 18Z-00Z...AND THE SFC  
BOUNDARY IN THE AREA THERE COULD BE SOME STRONGER STORMS. THE LLJ  
HOLDS UP AT 30-35 KTS SO THERE MAY BE AT LEAST MODEST SHEAR DOWN  
LOW.  
 
MODEL QPF INDICATES POCKETS OF HEAVIER RAIN ARE POSSIBLE TUESDAY  
INTO EARLY WEDNESDAY. THE ECMWF SWEEPS THE FRONT THROUGH QUICKER  
THAN THE GFS AND GEM...WHICH ARE ALMOST IN LOCK STEP AND LINGER SOME  
HEAVIER RAINS ACROSS THE SOUTHERN CWA WELL INTO WEDNESDAY AFTERNOON  
AND EVENING. GFS PW VALUES ARE QUITE HIGH TUESDAY NIGHT AND  
WEDNESDAY...APPROACHING 2.00" FROM I-96 TO THE SOUTH. ANY  
SHOWERS/STORMS IN THIS ENVIRONMENT WOULD BE HEAVY RAIN PRODUCERS  
WITH IMPRESSIVE HOURLY RAINFALL RATES.  
 
COOLER AND DRIER WEATHER SETTLES IN FOR THE SECOND HALF OF THE WEEK.  
THERE MAY BE SOME SYNOPTIC SCALE INFLUENCE FOR THE GREAT LAKES  
REGION LATE IN THE WEEK WITH TROPICAL STORM IGNACIO EXPECTED TO GET  
ABSORBED IN UPPER LEVEL ENERGY ACROSS ALASKA...WHICH MAY DEEPEN  
LONGWAVE TROUGHING OVER THE GREAT LAKES BY THE END OF THE WEEK. COOL  
AND PERHAPS SHOWERY CONDITIONS ARE POSSIBLE BY FRIDAY AND SATURDAY  
AS THIS OCCURS.  
 

 
   
AVIATION
 
(FOR THE 12Z TAFS THROUGH 12Z SATURDAY MORNING)  
ISSUED AT 719 AM EDT FRI SEP 4 2015  
 
IFR AND LOWER CONDITIONS WILL PREVAIL FOR A FEW HOURS THIS  
MORNING. THE TREND SUPPORTS DIMINISHING THESE LOW CONDITIONS  
SOONER AS THE FOG/STATUS IS NOT QUITE AS LOW AS FORECASTED  
EARLIER. THE ATMOSPHERE BECOMES UNSTABLE THIS AFTERNOON...SO SOME  
THUNDERSTORMS COULD DEVELOP. FOG COULD REDEVELOP THIS EVENING.  
LATER AT NIGHT AROUND KAZO AND KBTL WE COULD SEE A FEW STORMS  
DEVELOP AS THE WARMER AIRMASS STARTS TO RETURN.  
 

 
   
MARINE
 
 
ISSUED AT 307 AM EDT FRI SEP 4 2015  
 
RELATIVELY WEAK PRESSURE GRADIENT INTO SATURDAY WILL ACT TO KEEP  
THE WINDS AND WAVES DOWN. SOME INCREASE IN THE WIND SHOULD OCCUR  
ON SUNDAY AND INTO MONDAY AS A COLD FRONT NEARS FROM THE WEST  
CAUSING THE PRESSURE GRADIENT TO WEAKEN. WAVE WILL BUILD  
SLIGHTLY BUT HAZARDOUS CONDITIONS SHOULD STILL BE LIMITED.  
 

 
   
HYDROLOGY
 
 
ISSUED AT 1054 AM EDT FRI SEP 4 2015  
 
NO CONCERNS EXIST IN THE NEAR TERM. QUESTIONS ARISE GOING INTO NEXT  
WEEK AS A BOUNDARY DRAPES ITSELF SOMEWHERE THROUGH OR NEAR THE GREAT  
LAKES REGION.  
 
AREA RIVERS HAVE RESPONDED PRETTY WELL TO YESTERDAYS RAINFALL.  
LOCALIZED TOTALS OF MORE THAN TWO INCHES RESULTED IN DECENT UPWARD  
TRENDS AT A FEW FORECAST POINTS. MOST NOTABLE IS SYCAMORE CREEK AT  
HOLT...WHICH HAS LEVELED OFF ABOUT A FOOT BELOW ADVISORY CRITERIA.  
PROVIDED THAT ADDITIONAL RAINFALL IS LIMITED...HOLT SHOULD CONTINUE  
ITS DOWNWARD TREND. OTHER SITES HAVE ALSO LEVELED OFF AND RESPONSES  
HAVE BEEN MINOR.  
 
RAINFALL THROUGH THE WEEKEND IS EXPECTED TO BE SCATTERED WITH  
LOCALIZED HEAVY RAIN POSSIBLE IN OR AROUND THUNDERSTORMS. CONDITIONS  
ARE NOT FAVORABLE FOR WIDESPREAD HEAVY RAIN. A PATTERN CHANGE WILL  
TAKE PLACE NEXT WEEK AND ITS ASSOCIATED BOUNDARY LOOKS TO BE MOVING  
SLOWLY. DIFFERENCES IN LONG RANGE MODELS PUT A CAP ON ANY HIGH  
CONFIDENCE. HOWEVER...PWATS ARE 1.5-2.0 INCHES AND THE SLOW  
PROGRESSION OF THE BOUNDARY COULD RESULT IN PRETTY DECENT RAINFALL  
TOTALS MONDAY THROUGH WEDNESDAY. THE SETUP MAY SUGGEST MORE THAN AN  
INCH...WITH AREAS EXCEEDING A COUPLE OF INCHES IF STORMS ARE SLOW  
MOVERS. THIS WILL LIKELY BE THE FOCUS HYRO-WISE OVER THE NEXT FEW  
DAYS.  
 

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

 
 

 
 
UPDATE...TJT  
SYNOPSIS...MJS  
SHORT TERM...MJS  
LONG TERM...HOVING  
AVIATION...MJS  
HYDROLOGY...JAM  
MARINE...MJS  
 
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