969  
FXUS63 KGRR 200720  
AFDGRR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE GRAND RAPIDS MI  
320 AM EDT THU SEP 20 2018  
 
LATEST UPDATE...  
SYNOPSIS/DISCUSSION/MARINE  
   
SYNOPSIS
 
 
ISSUED AT 320 AM EDT THU SEP 20 2018  
 
SHOWERS AND THUNDERSTORMS WILL OCCUR TODAY AS A WARM FRONT LIFTS  
NORTH INTO LOWER MICHIGAN. COVERAGE OF RAIN WILL DECREASE TONIGHT  
AS THE WARM FRONT LIFTS WELL NORTH OF THE AREA, BUT A COLD FRONT  
APPROACHING FROM THE WEST WILL BRING ANOTHER CHANCE OF SHOWERS  
TOWARD DAYBREAK FRIDAY. BREEZY CONDITIONS WILL FOLLOW ON FRIDAY  
BEHIND THE COLD FRONT, THEN DRY BUT CHILLY WEATHER IS EXPECTED  
OVER THE WEEKEND WITH CANADIAN HIGH PRESSURE IN CONTROL.  
 

 
   
DISCUSSION
 
(TODAY THROUGH NEXT WEDNESDAY)  
ISSUED AT 320 AM EDT THU SEP 20 2018  
 
SHOWERS AND THUNDERSTORMS ASSOCIATED WITH THE ELEVATED WARM FRONT  
WILL BECOME MORE NUMEROUS OVER THE NEXT FEW HOURS AS LOW LEVEL  
JET THROUGH THE CENTRAL PLAINS BENDS/LEANS EAST IN OUR DIRECTION.  
THESE STORMS WILL BE ROOTED AROUND 850 MB, WITH THE MAIN THREAT  
BEING HEAVY RAINFALL THROUGH MID MORNING AS PWATS OVER 1.75 INCHES  
ARRIVE.  
 
PROBABILITY-MATCHED MEAN QPF GUIDANCE FROM THE CAMS INDICATE  
HEAVIEST RAIN FALLING GENERALLY NORTH OF A HOLLAND TO LANSING  
LINE THIS MORNING, WHERE WIDESPREAD 1 TO 2 INCH AMOUNTS APPEAR  
LIKELY. ISOLATED HIGHER AMOUNTS ARE POSSIBLE, WHICH MAY PRODUCE  
LOCALIZED NUISANCE FLOODING.  
 
AS MORNING CONVECTION SLOWLY FADES, SURFACE BASED INSTABILITY  
BEGINS TO BUILD FROM SOUTHWEST TO NORTHEAST THIS AFTERNOON WITH  
THE ARRIVAL OF THE SFC WARM FRONT. SB CAPES ON THE ORDER OF  
1500-2500 J/KG ARE PROGGED NEAR AND SOUTH OF THE FRONT, WHICH  
SHOULD BE LOCATED ROUGHLY ALONG THE I-96 CORRIDOR BY EARLY  
EVENING.  
 
PROGGED DEEP LAYER SHEAR AROUND 35 KTS WOULD BE SUFFICIENT FOR  
ORGANIZED STORMS LATER THIS AFTERNOON, AND THE PRESENCE OF THE  
SFC WARM FRONT IN THE AREA CREATES CONCERN FOR SPIN UPS. THE  
LATEST IDEA PRESENTED BY THE CAMS IN TERMS OF LATE DAY DIURNAL SFC  
BASED CONVECTION IS FOR SCATTERED STORMS TO DEVELOP SOUTH AND  
EAST OF GRR AROUND 4 PM ALONG AN OUTFLOW/MCV LEFTOVER BY MORNING  
CONVECTION.  
 
HREF GUIDANCE DOES SHOW ELEVATED VALUES OF 2-5 KM UPDRAFT  
HELICITY SOUTHEAST OF GRR AROUND 00Z, AND THE JXN HRRR FCST  
SOUNDING HAS A SHARPLY CURVED HODOGRAPH BETWEEN 0-1 KM. THUS ANY  
LATE DAY SFC BASED CONVECTION WILL NEED TO BE WATCHED VERY  
CLOSELY FOR ROTATION.  
 
MUCH OF TONIGHT SHOULD BE DRY AND QUITE WARM/MUGGY ONCE THE  
DIURNAL EVENING CONVECTION DIMINISHES AND THE WARM FRONT LIFTS  
OFF TO THE NORTH. THE BAND OF SHOWERS WITH THE COLD FRONT AFTER  
09Z DOES NOT LOOK VERY IMPRESSIVE IN THE CAMS BUT COULD CONTAIN  
SOME GUSTY WINDS AS IT COMES THROUGH. TURNING MAINLY DRY BUT  
BREEZY ON FRIDAY AS THE MID LEVEL DRY SLOT ARRIVES. WIND GUSTS TO  
35 MPH POSSIBLE ONCE DEEPER MIXING DEVELOPS.  
 
SFC RIDGE AND DRY AIR OVER THE WEEKEND WILL PROMOTE PLEASANT  
WEATHER AND COLD NIGHTS WITH LOWS IN THE 40S. WE COULD EVEN SEE  
SOME PATCHY FROST NEAR/NORTH OF U.S 10. OUR NEXT CHANCE OF RAIN  
ARRIVES MONDAY NIGHT/TUESDAY AS MOISTURE STREAMING NORTH FROM THE  
GULF INTERACTS WITH A COLD FRONT APPROACHING FROM THE WEST. THIS  
HAS THE POTENTIAL TO PRODUCE SOAKING RAINS WHICH WOULD BE FOLLOWED  
BY A SIGNIFICANT COOL DOWN FOR THE MID TO LATE PART OF NEXT WEEK.  
 

 
   
AVIATION
 
(FOR THE 06Z TAFS THROUGH 06Z THURSDAY NIGHT)  
ISSUED AT 136 AM EDT THU SEP 20 2018  
 
STORM TRENDS THROUGH THE PERIOD ARE OF MOST CONCERN WITH THE 06Z  
FCSTS THIS MORNING.  
 
THE FIRST ROUND OF SHOWERS AND STORMS ARE NOW JUST STARTING TO POP  
JUST SW OF KGRR. WE EXPECT THESE TO FILL IN, AND AFFECT THE I-96  
TERMINALS THROUGH ABOUT 12Z OR SO. ANY STORMS WILL HAVE THE  
POTENTIAL TO BRING LOCALIZED IFR AND LOWER CONDITIONS. THE MORE  
WIDESPREAD IFR SHOULD DEVELOP NORTH OF THE TERMINALS THIS MORNING.  
 
THERE WILL LIKELY BE A SHORT BREAK AFTER THE FIRST ROUND OF STORMS  
THIS MORNING, AND THE NEXT ONE EXPECTED TO COME IN FROM WI AND  
LAKE MICHIGAN TOWARD LATE MORNING. THIS WILL PROGRESS THROUGH THE  
TERMINALS INTO MID AFTERNOON. ADDITIONAL STORMS ARE EXPECTED TO  
FIRE ALONG THE I-94 TERMINALS WHERE THE MOST INSTABILITY WILL BE  
FOUND AND OUTFLOW FROM THE MORNING STORMS COULD TRIGGER NEW  
DEVELOPMENT. THESE SHOULD LAST INTO THE EARLY EVENING HOURS BEFORE  
DISSIPATING WITH THE LOSS OF HEATING OF THE DAY. ANOTHER LINE OF  
DISSIPATING SHOWERS AND A STORM WILL POTENTIALLY COME IN AFTER THE  
END OF THIS FCST PERIOD.  
 
WINDS WILL BECOME GUSTY OVERNIGHT TONIGHT WITH THE APPROACH OF  
THE FRONT.  
 

 
   
MARINE
 
 
ISSUED AT 320 AM EDT THU SEP 20 2018  
 
WILL CONVERT THE GALE WATCH TO A GALE WARNING, BUT ONLY IN THE  
NORTHERNMOST ZONE NORTH OF PENTWATER. ELSEWHERE WILL HAVE SMALL  
CRAFT ADVISORIES. INITIALLY TONIGHT THE HAZARD IS THE STRONG  
SOUTHERLY FLOW AHEAD OF THE COLD FRONT, THEN ON FRIDAY IT WILL BE  
THE STRONG NORTH-NORTHWEST FLOW BEHIND THE FRONT. WAVES UP TO 10  
FEET ARE EXPECTED.  
 

 
   
HYDROLOGY
 
 
ISSUED AT 1056 AM EDT WED SEP 19 2018  
 
RIVER LEVELS ARE AROUND TO A LITTLE ABOVE NORMAL FOR THE TIME OF  
YEAR. LEVELS ARE ALSO STEADY OR SLOWLY FALLING. RAIN AND  
THUNDERSTORMS ARE EXPECTED LATE WEDNESDAY NIGHT INTO THURSDAY  
MORNING AND AGAIN THURSDAY NIGHT INTO FRIDAY MORNING. TOTAL RAINFALL  
WILL RANGE UP TO AROUND TWO INCHES THROUGH FRIDAY. LOCAL TOTALS  
COULD BE HIGHER, IF THUNDERSTORMS TRAIN OVER THE SAME AREA. RAINFALL  
RATES IN EXCESS OF ONE INCH PER HOUR ARE POSSIBLE. THE QUICK-HITTING  
NATURE OF THESE ROUNDS OF HEAVY RAIN WILL BE MORE LIKELY TO CAUSE  
LOCALIZED FLOODING ON ROADS, IN LOW-LYING AREAS, AND ALONG SMALL  
CREEKS, THAN FLOODING OF STREAMS AND RIVERS. DRIER WEATHER IS  
EXPECTED FRIDAY AFTERNOON THROUGH SUNDAY.  
 

 
   
GRR WATCHES/WARNINGS/ADVISORIES
 
 
LM...GALE WARNING FROM 11 PM THIS EVENING TO 8 PM EDT FRIDAY FOR  
LMZ849.  
 
SMALL CRAFT ADVISORY FROM 8 PM THIS EVENING TO 8 AM EDT SATURDAY  
FOR LMZ844>848.  
 

 
 

 
 
SYNOPSIS...MEADE  
DISCUSSION...MEADE  
AVIATION...NJJ  
HYDROLOGY...63  
MARINE...MEADE  
 
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