740
FXUS63 KGRR 031832
AFDGRR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
232 PM EDT THU SEP 3 2026
KEY MESSAGES
- SHOWERS/STORMS THROUGH LATE AFTERNOON, THEN OVERNIGHT/FRIDAY AM
- LIKELY DRY FRIDAY AFTERNOON; SCATTERED STORMS POSSIBLE LATE
DISCUSSION
ISSUED AT 231 PM EDT THU SEP 3 2026
- SHOWERS/STORMS THROUGH LATE AFTERNOON, THEN OVERNIGHT/FRIDAY AM
THROUGH THE END OF THE AFTERNOON, THE MOST CONCENTRATED AREAS FOR
STRONGER CONVECTION WILL BE NEAR/NORTH OF I-96 AND EAST OF US 131
WHERE SCATTERED DAMAGING WIND GUSTS AND LARGE HAIL WILL BE POSSIBLE.
IN THIS REGION, EFFECTIVE SHEAR VALUES OF 40-45 KTS ARE PRESENT
ALONG WITH SBCAPE OF 2000-3000 J/KG. SURFACE VORTICITY ENHANCEMENTS
ALONG ANY OUTFLOW BOUNDARIES COULD ALSO AID IN A ROTATIONAL
COMPONENT TO STRONGER THUNDERSTORMS WITH A TORNADO NOT OUT OF THE
QUESTION ESPECIALLY ACROSS CENTRAL AND SOUTH CENTRAL LOWER MI. THERE
IS A CAM SIGNAL FOR SOME SWATHS OF CONVECTIVE WIND GUSTS EXCEEDING
60 MPH EAST OF US 131 INTO LATE AFTERNOON OR EARLY EVENING BEFORE
ENDING.
REDEVELOPMENT OF OVERNIGHT STORMS IS QUITE LIKELY. THIS ROUND OF
STORMS WILL BE FUELED BY AN INCREASING LLJ (30-40 KTS) OVER
WISCONSIN THAT WILL MOVE OUR DIRECTION, INTERSECTING A LARGELY
STATIONARY BOUNDARY OVER THE REGION. MODEL SIM REFS ARE INDICATING A
FEW POSSIBLE OUTCOMES INCLUDING A QLCS OR ELONGATED/TRAINING
CONVECTIVE EVENT, THE LATTER BEING MORE CONCERNING IN TERMS OF AN
URBAN AND POOR DRAINAGE FLOOD RISK. AREAS THAT WOULD BE MOST
SUSCEPTIBLE TO LOCALIZED FLOODING OVERNIGHT AND INTO FRIDAY MORNING
WOULD BE ALONG US 10 (WHERE 1"-2" OCCURRED TODAY) AS WELL AS THE
GREATER LANSING AREA WHERE SEVERAL ROUNDS OF THUNDERSTORMS HAVE
OCCURRED. HREF 6 HR PMM RAINFALL INDICATES 1"-2" ESPECIALLY NEAR AND
WEST OF US 131, THOUGH MAX 6 HR QPF FROM THE MEMBERSHIP INDICATES
AREAS OF 2"-3" OR SLIGHTLY MORE. PLACEMENT IS NOT CERTAIN AT THIS
TIME, AND COULD SHIFT EAST. IN TERMS OF A QLCS RISK, THERE IS
SOME SIGNAL FOR THIS TO OCCUR IN THE 09Z-13Z FRIDAY TIME FRAME.
NAM3KM AND HRRR SOUNDINGS AHEAD OF THIS POTENTIAL FEATURE INDICATE
400-500 J/KG OF CIN, LIKELY INDICATING STRONG WINDS WILL NOT MIX
DOWN TO THE SURFACE. HAIL AND HEAVY DOWNPOURS/LOCALIZED FLOODING
WOULD BE MORE LIKELY THREATS.
- LIKELY DRY FRIDAY AFTERNOON; SCATTERED STORMS POSSIBLE LATE
THE SURFACE BOUNDARY IN PLACE TODAY LOOKS TO SLIDE SOUTH TOWARD I-94
AND THE MI/IN BORDER BY LATE FRIDAY. SUBSIDENCE IN THE WAKE OF
MORNING RAIN ON FRIDAY WILL LIKELY INHIBIT CONVECTIVE DEVELOPMENT
FRIDAY AFTERNOON OUTSIDE OF AN ISOLATED POP UP SHOWER OR STORM THAT
COULD OCCUR ON ANY RESIDUAL OUTFLOW BOUNDARIES THAT REMAIN IN PLACE
DURING DAYTIME HEATING. PREDICTABILITY OF THESE IS LOW. BEST
INDICATIONS ARE SCATTERED SHOWERS AND THUNDERSTORMS COULD DEVELOP
LATE AFTERNOON/EVENING AND INTO FRIDAY NIGHT NEAR AND SOUTH OF A
GRAND RAPIDS TO LANSING LINE AFTER SUFFICIENT DAYTIME HEATING. A
HRRR SOUNDING NEAR LANSING AROUND 21Z INDICATES QUITE A BIT OF DCAPE
AT 1200 J/KG WITH FAIRLY SUBSTANTIAL 0-3 KM SHEAR OF 35 KTS. AN
ISOLATED SEVERE THUNDERSTORM CANNOT BE RULED OUT NEAR AND SOUTH OF
GRR TO LAN WITH DAMAGING WIND GUSTS THE PRIMARY HAZARD.
AVIATION /18Z TAFS THROUGH 18Z FRIDAY/
ISSUED AT 153 PM EDT THU SEP 3 2026
RADAR SHOWS ABUNDANT CONVECTION OVERSPREADING LOWER MI AND IT
MIGHT NOT BE THE ONLY ROUND WE SEE DURING THE PERIOD. MOST OF THE
TAF PERIODS HAVE A PROB30 FOR STORMS SINCE WE NEED TO ACCOUNT FOR
THE CURRENT CONVECTION AND ANOTHER ROUND LATER TONIGHT THAT
SEVERAL MODELS CONTINUE TO SHOW. A FEW OF THE STORMS COULD BE
STRONG TO SEVERE THIS AFTERNOON AND OVERNIGHT. THERE MAY BE A LULL
IN THE CONVECTION DURING THIS EVENING.
OUTSIDE OF THE STORMS, EXPECT MVFR FOG OVERNIGHT DUE TO HIGH
DEWPOINTS AND ABUNDANT MOISTURE FROM RECENT RAIN.
MARINE
ISSUED AT 231 PM EDT THU SEP 3 2026
WINDS AND WAVES SHOULD STAY BELOW HAZARDOUS LEVELS FOR MARINERS
THROUGH THE WEEKEND. MARINERS SHOULD STAY ABREAST OF THUNDERSTORM
POTENTIAL FROM THIS AFTERNOON INTO FRIDAY NIGHT AND HAVE PLANS TO
SEEK SAFE HARBOR WHEN NECESSARY.
GRR WATCHES/WARNINGS/ADVISORIES
MI...NONE.
MARINE...NONE.
DISCUSSION...HOVING
AVIATION...04
MARINE...HOVING
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