274
FXUS63 KGRR 311033
AFDGRR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
633 AM EDT MON AUG 31 2026
KEY MESSAGES
- MULTIPLE ROUNDS OF SHOWERS AND THUNDERSTORMS, WARM AND HUMID
DISCUSSION
ISSUED AT 320 AM EDT MON AUG 31 2026
OVERALL GUIDANCE IS STRUGGLING WITH THE CURRENT PATTERN OVER THE
NEXT FEW DAYS, THEREFORE CONFIDENCE IS NOT HIGH IN THE EXACT
TIMING AND PLACEMENT OF RAIN AND THUNDERSTORM CHANCES. HOWEVER
SOME SYNOPTIC FEATURES GIVE SLIGHTLY HIGHER CONFIDENCE AT TIMES.
TODAY:
WARM AIR ADVECTION WILL SHIFT INTO THE AREA THIS MORNING ALONG
WITH THE LOW LEVEL JET SPARKING OFF SHOWERS AND THUNDERSTORMS.
BEST CHANCES (GREATER THAN 40 PERCENT) WILL BE NORTH OF I-96 WITH
A 20 TO 40 PERCENT CHANCE ALONG AND SOUTH OF I-96. A BRIEF LULL IN
SHOWERS AND THUNDERSTORMS IS EXPECTED LATE THIS MORNING INTO THE
EARLY AFTERNOON. A PUSH OF 700MB WARM AIR ADVECTION AND A WEAK LOW
TO MID LEVEL FRONT MAY SPARK OFF ADDITIONAL SHOWERS AND
THUNDERSTORMS THIS AFTERNOON. BOTH THE HREF AND REFS HIGHLIGHT
THIS ALONG AND SOUTH OF I-96 THIS AFTERNOON. THUNDERSTORMS WILL BE
ELEVATED WITH PLENTY OF DRY AIR TOWARD THE SURFACE. THIS COULD
RESULT IN SOME GUSTIER WINDS WITH THUNDERSTORMS. SOME MACHINE
LEARNING GUIDANCE HIGHLIGHTS THIS POTENTIAL FOR STRONGER WINDS.
TUESDAY:
LATE TONIGHT INTO TUESDAY ADDITIONAL THUNDERSTORMS ARE POSSIBLE
WITH THE BEST CHANCES DURING THE DAY (30 PERCENT) WITH THE LOW
LEVEL JET MOVING OVER THE AREA AND POSITIVE VORTICITY ADVECTION.
MEAN MUCAPE VALUES ARE AROUND 2000 J/KG WITH MEAN SFC-500MB SHEAR
VALUES AROUND 30 TO 40 KNOTS. DIFFERENCES WITHIN THE VARIOUS
SOLUTIONS BRING LOWER CONFIDENCE TO TUESDAY'S FORECAST. IF STORMS
DEVELOP THERE IS A MARGINAL RISK FOR THEM TO BE SEVERE WITH WINDS
AS THE MAIN CONCERN. TUESDAY WILL ALSO BE ONE OF THE HOTTEST DAYS
THIS WEEK WITH HIGHS IN THE MID 80S TO LOW 90S AND HEAT INDEX
VALUES IN THE 90S WITH THE POTENTIAL FOR VALUES TO APPROACH 100 IN
SOUTHERN MICHIGAN.
TUESDAY NIGHT THE WARM FRONT LIFTS NORTH WITH AREAS ALONG AND
NORTH OF I-96 SEEING AN ADDITIONAL CHANCE FOR THUNDERSTORMS.
REST OF WEEK:
WEDNESDAY WILL BE ANOTHER HOT DAY WITH HEAT INDEX VALUES IN THE
90S MAINLY ALONG AND SOUTH OF A GRAND HAVEN TO MOUNT PLEASANT
LINE. AREAS ALONG AND SOUTH OF I-94 COULD SEE HEAT INDEX VALUES
AROUND 100. THIS WILL BE HIGHLY DEPENDENT ON THUNDERSTORM CHANCES.
SOME SOLUTIONS HAVE A WEAK SHORTWAVE MOVING THROUGH DURING THE
AFTERNOON AND EVENING AND OTHERS HAVE A STRONGER TROUGH AND LOW
PRESSURE SYSTEM ALONG WITH A STRONGER JET. IF THIS LATTER SOLUTION
PANS OUT THERE WILL BE A HIGHER RISK FOR SEVERE THUNDERSTORMS.
STAY TUNED.
NORTHWEST FLOW RETURNS TO THE AREA THURSDAY INTO THE WEEKEND.
WE'LL BE ON THE NORTHEAST SIDE OF THE RIDGE PARKED OVER THE
CENTRAL CONUS. WITHIN THIS FLOW WE'LL NEED TO WATCH FOR RIDGE
RIDERS BRINGING ADDITIONAL RAIN AND THUNDERSTORM CHANCES. HIGH
TEMPERATURES WILL BE COOLER IN THIS FLOW (70S AND 80S), BUT DEW
POINTS WILL LARGELY BE IN THE 60S.
AVIATION /12Z TAFS THROUGH 12Z TUESDAY/
ISSUED AT 633 AM EDT MON AUG 31 2026
THUNDERSTORMS HAVE DEVELOPED IN EASTERN WISCONSIN AND ARE TRACKING
EAST. THESE STORMS ARE EXPECTED TO REACH MKG AROUND 11-12Z, GRR
AROUND 13Z AND AZO/BTL AROUND 14Z. GUIDANCE SUGGESTS THAT STORMS
SHOULD WEAKEN AS THEY APPROACH LAN AND JXN, BUT IT ISN'T HANDLING
THE CURRENT SITUATION WELL. FOR NOW HAVE PROB30 GROUPS FOR
THUNDERSTORMS, BUT AN AMENDMENT MAY BE NEEDED IF THIS AREA OF
STORMS HOLDS TOGETHER. LOWER VISIBILITIES WILL BE POSSIBLE WITH
ANY THUNDERSTORM ALONG WITH HEAVY RAINFALL.
DRY CONDITIONS RETURN BY THIS AFTERNOON, HOWEVER POP-UP
THUNDERSTORMS WILL BE POSSIBLE BETWEEN 19-00Z THIS AFTERNOON AND
EARLY EVENING. SOUTHWEST WINDS GUSTING TO 20 TO 25 KNOTS ARE
EXPECTED ACROSS MUCH OF THE AREA, WIND GUSTS MAY REMAIN BELOW 20
KNOTS AT JXN. AT THIS TIME HAVE A DRY FORECAST TONIGHT INTO EARLY
TUESDAY MORNING, BUT THUNDERSTORM CHANCES MAY NEED TO BE
INTRODUCED.
MARINE
ISSUED AT 320 AM EDT MON AUG 31 2026
ANOTHER ROUND OF BREEZY SOUTHERLY WINDS IS EXPECTED TODAY CAUSING
WAVES TO BUILD TO 2 TO 5 FEET. A SMALL CRAFT ADVISORY AND BEACH
HAZARD STATEMENT ARE IN EFFECT. THUNDERSTORMS WILL BE POSSIBLE
THIS MORNING AND DURING THE AFTERNOON. ADDITIONAL CHANCES FOR
THUNDERSTORMS EXIST LATE TONIGHT THROUGH TUESDAY WITH THE
POTENTIAL FOR GUSTS 34 KNOTS OR GREATER WITH THE STRONGEST STORMS.
FOR THE REST OF THE WEEK WINDS AND WAVES ARE EXPECTED TO REMAIN
BELOW ADVISORY OR STATEMENT LEVELS. THUNDERSTORMS WILL BE POSSIBLE
AGAIN ON WEDNESDAY AND THURSDAY WITH WINDS BECOMING MORE OFFSHORE
FRIDAY INTO THE WEEKEND.
GRR WATCHES/WARNINGS/ADVISORIES
MI...BEACH HAZARDS STATEMENT FROM 11 AM EDT THIS MORNING THROUGH THIS
EVENING FOR MIZ037-043-050-056.
MARINE...SMALL CRAFT ADVISORY FROM 11 AM THIS MORNING TO 11 PM EDT THIS
EVENING FOR LMZ846>849.
DISCUSSION...RAH
AVIATION...RAH
MARINE...RAH
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