482  
FXUS63 KGRR 242346  
AFDGRR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE GRAND RAPIDS MI  
746 PM EDT THU SEP 24 2020  
 
LATEST UPDATE...  
AVIATION  
   
SYNOPSIS
 
 
ISSUED AT 317 PM EDT THU SEP 24 2020  
 
- WARM WITH LITTLE PRECIPITATION THROUGH SATURDAY  
 
- COOLING SOME SUNDAY BUT STILL LITTLE RAIN  
 
- BREEZY AND MUCH COOLER WITH SHOWERS MUCH OF NEXT WEEK  
 

 
   
DISCUSSION
 
(THIS EVENING THROUGH NEXT THURSDAY)  
ISSUED AT 317 PM EDT THU SEP 24 2020  
 
-- WARM WITH LITTLE PRECIPITATION THROUGH SATURDAY--  
 
THROUGH SATURDAY IT FOR THE MOST PART WILL BE WARM AND DRY (LITTLE  
IF ANY RAIN). THE FRONTAL SYSTEM TIED TO THE NORTHERN STREAM JET  
WILL LIFT NORTH INTO SATURDAY. MID-LEVEL RIDGING 850 TO 300 MB  
WILL CAUSE A DEEP LAYER OF DRY AIR DURING MOST OF THIS TIME FRAME  
AND PREVENT AND UPSTREAM CONVECTION FROM GETTING INTO MOST OF OUR  
CWA.  
 
OUR NORTHWEST AND EXTREME NORTHERN CWA COULD SEE A FEW SHOWERS  
TONIGHT BUT ANY PRECIPITATION WOULD BE LIMITED IN AMOUNT AND BE  
HIT AND MISS.  
 
FRIDAY THE LEAD UPPER SHORTWAVE THAT IS ASSOCIATED WITH THE UPPER  
LEVEL SYSTEM THAT WILL FINALLY BRING IN COOLER AIR TO THIS REGION  
IS HELP TO BUILD MID TO UPPER LEVEL HEIGHTS AND STRENGTHEN THE  
RIDGING. THIS WILL LEAD TO A WARM DAY FRIDAY WITH ONLY A FEW  
CLOUDS.  
 
FRIDAY NIGHT THE LEAD SHORTWAVE TRACKS WELL NORTH OF THIS AREA BUT  
ENOUGH TRAILING ENERGY REMAINS THAT OUR NORTHWEST CWA MAY GET SOME  
SHOWERS AND MAYBE A THUNDERSTORM. ELSEWHERE IT WILL JUST STAY  
MILD AND DRY.  
 
SATURDAY A 140 KNOT SPEED MAX DIGS INTO THE WESTERN SIDE OF THE  
UPPER THROUGH OVER SOUTHWEST BRITISH COLUMBIA, WHICH STARTS TO  
DEEPEN THE TROUGH. THAT LEADS TO INCREASING WINDS HERE AS SURFACE  
LOW DEVELOPS ON THE FRONT OVER THE CENTRAL PLAINS AND TRACKS  
NORTHEAST. THIS WILL BE OUR WARMEST DAY FOR THE REST OF THE YEAR  
AND LIKELY THE LAST DAY WE WILL SEE HIGHS IN THE 80S. MY 1000/925  
THICKNESS TOOL GIVES GRR A HIGH OF 86 SATURDAY.  
 
-- COOLING SOME SUNDAY BUT STILL LITTLE RAIN--  
 
AS THAT FIRST SPEED MAX RACES THROUGH THE BOTTOM OF THE TROUGH AND  
STARTS CLIMBING OUT OF THE TROUGH OVER MN/WI SATURDAY NIGHT,  
SIGNIFICANT MID LEVEL INSTABILITY DEVELOPS OVER WESTERN MI WITH  
STEEP LAPSE RATES. HOWEVER THERE IS WAY TOO MUCH DRY AIR AT MID  
LEVELS TO GET MUCH CONVECTION HERE. OUR NORTHWEST CWA MAY SEE  
THUNDERSTORMS IN THE LATE EVENING BUT THE STORMS WILL STRUGGLE TO  
FIGHT THE DRY AIR SO THEY SHOULD NOT MAKE IT PAST GRR.  
 
-- BREEZY AND MUCH COOLER WITH SHOWERS MUCH OF NEXT WEEK--  
 
THE SURFACE FRONT COMES THROUGH SOUTHWEST MICHIGAN DURING THE DAY  
ON SUNDAY. WE MAY SEE SOME JET ENTRANCE REGION (SOUTHWEST QUAD OF  
DEPARTING JET) LIFT HELP TO CAUSE POST FRONTAL RAIN DUE TO  
ISENTROPIC LIFT LATER SUNDAY INTO SUNDAY NIGHT.  
 
MEANWHILE A STRONG JET CORE DIGS INTO THE DEEPENING TROUGH AND  
CLOSING OFF AN UPPER LOW OVER THE CENTRAL PLAINS BY MONDAY  
MORNING. THEN AN EVEN STRONGER JET CORE, NEAR 180 KNOTS DIGS INTO  
THE NORTHERN STREAM TROUGH MONDAY AFTERNOON OVER WESTERN CANADA  
AND THAT CAUSES THE UPPER LOW AND ASSOCIATED SURFACE LOW TO TRACK  
NEARLY DUE NORTH TOWARD MICHIGAN. THE SYSTEMS MERGE ON TUESDAY  
NEAR MICHIGAN. THIS CAUSES AN NEGATIVE TILT UPPER TROUGH AND  
DEEPENING SURFACE LOW. I DO BELIEVE THIS WILL FINALLY BE ENOUGH  
TO BRING RAIN TO THE ENTIRE AREA. THERE IS ENOUGH MID LEVEL  
INSTABILITY FOR CONVECTION AND THUNDERSTORMS TUESDAY INTO  
WEDNESDAY. THERE ARE SOME SIGNIFICANT DIFFERENCES BETWEEN THE  
ECMWF AND GFS BUT BOTH GIVE US RAIN AND STRONG WINDS. IT MAY TAKE  
UNTIL FRIDAY OF NEXT WEEK FOR THIS SYSTEM TO CLEAR THE GREAT  
LAKES. EXPECT LAKE EFFECT RAIN SHOWERS, WATER SPOUTS, SOFT HAIL  
AND STRONG WINDS WEDNESDAY INTO THURSDAY. HIGHS MAY NOT GET OUT OF  
THE 40S IN SOME PARTS OF OUR CWA THURSDAY.  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z FRIDAY EVENING)  
ISSUED AT 746 PM EDT THU SEP 24 2020  
 
CONFIDENCE IS VERY HIGH THAT VFR WEATHER WILL CONTINUE TONIGHT  
AND FRIDAY. LIGHT SOUTH WINDS TONIGHT INCREASING TO AROUND 10 KTS  
BY NOON FRIDAY.  
 

 
   
MARINE
 
 
ISSUED AT 317 PM EDT THU SEP 24 2020  
 
WE HAVE TWO PERIODS OF STRONG WINDS IN THE SHORT TERM, THE FIRST  
IS FRIDAY NIGHT IN WARM ADVECTION PART OF THE SYSTEM. HOWEVER THE  
STRONGER EVENT IS EXPECTED SATURDAY NIGHT AS THE COLD FRONT HEADS  
INTO THE AREA WHILE A 992 MB LOW TRACKS NORTHEAST THROUGH  
WISCONSIN. THE GFS MODEL SHOWS THE MIXED LAYER OVER THE SOUTH BUOY  
IS MIXING DOWN 40 KNOTS. THE ECMWF SURFACE WIND SPEED IS IN THE 25  
TO 30 KNOT RANGE SATURDAY NIGHT WITH GUSTS OVER 45 KNOTS. I WOULD  
HAVE TO BELIEVE WE WILL HAVE AT LEAST A SMALL CRAFT ADVISORY FOR  
TOMORROW NIGHT INTO SUNDAY. WE MAY NEED A GALE WARNING NORTH OF  
WHITEHALL.  
 
OF COURSE WHEN THE SYSTEM MOVES THROUGH THE GREAT LAKES TUESDAY  
INTO WEDNESDAY I WOULD EXPECT GALES DURING THAT TIME FRAME TOO.  
 

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

 
 

 
 
SYNOPSIS...WDM  
DISCUSSION...WDM  
AVIATION...MEADE  
MARINE...WDM  
 
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