425
FXUS63 KGRR 022318
AFDGRR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
718 PM EDT WED SEP 2 2026
KEY MESSAGES
- HEAT THROUGH THIS EVENING, SOME STORMS OVERNIGHT
- PERIODIC STORMS THURSDAY AFTERNOON INTO LATE NIGHT
- SHORT BREAK MIDDAY FRIDAY, THEN MORE RAIN CHANCES
DISCUSSION
ISSUED AT 254 PM EDT WED SEP 2 2026
- HEAT THROUGH THIS EVENING, SOME STORMS OVERNIGHT
SURFACE DEW POINTS HAVE OVERACHIEVED THIS AFTERNOON COMPARED TO MOST
MODEL GUIDANCE, AND AS A RESULT HEAT INDICES HAVE NUDGED HIGHER.
WIDESPREAD INDICES OF 95 TO 100 ARE OCCURRING AND WILL CONTINUE TO
OCCUR ACROSS SOUTHERN LOWER MICHIGAN THROUGH THIS AFTERNOON AND
EVENING GIVEN AIR TEMPS IN THE UPPER 80S TO LOW 90S AND DEW POINTS
OF 70 TO 75 MAKING IT FEEL PRETTY MISERABLE IN THE SUN. REDUCE
STRENUOUS OUTDOOR ACTIVITIES THROUGH THE EVENING.
IN SPITE OF FAVORABLE MLCAPE VALUES OF 1500-2500 J/KG THIS AFTERNOON
AS WELL AS 0-6 KM SHEAR VALUES OF 40-50 KTS, SOME MLCIN OF 100-200
J/KG IS STILL PRESENT NORTH OF I-96 WHERE SOME CONVECTION HAS
OCCURRED BUT HAS LARGELY BEEN HELD IN CHECK. A LLJ OF 30-35 KTS IS
PRESENT OVERHEAD OUR REGION, THOUGH IT IS MORE CONVERGENT OVER SE
LOWER MICHIGAN TOWARD THE THUMB REGION. THAT IS WHERE SCATTERED
STRONG TO SEVERE STORMS ARE MOST LIKELY TO DEVELOP INTO THIS
EVENING. MUCH OF OUR REGION WILL BE STORM FREE THROUGH EARLY TONIGHT
WITH THE EXCEPTION OF NORTH OF M-20 AND NEAR/EAST OF US 127.
BEYOND MIDNIGHT TONIGHT, THE LLJ OVER WISCONSIN INCREASES BACK TO 30-
35 KTS AND ANGLES OVER LAKE MICHIGAN. SCATTERED SHOWERS/STORMS MAY
DEVELOP ESPECIALLY FROM MUSKEGON TO LANSING DURING THE 05Z-12Z TIME
FRAME. MOST ARE NOT LIKELY TO REACH SEVERE LEVELS. MODEL SOUNDINGS
DO SUPPORT GOOD CAPE THROUGH THE HAIL GROWTH REGION BUT AN INVERSION
AROUND 850 MB WILL LIKELY (THOUGH NOT WITH 100% CERTAINTY) LIMIT
DAMAGING WIND POTENTIAL. LOCALIZED DOWNPOURS MAY LEAD TO A FEW MINOR
LOWLAND AND/OR STREET FLOODING OCCURRENCES OVERNIGHT.
- PERIODIC STORMS THURSDAY AFTERNOON INTO LATE NIGHT
THERE ARE SOME MIXED SIGNALS REGARDING THURSDAY'S DAYTIME STORM
CHANCES, LEADING TO LESS CONFIDENCE THAN DESIRED (EG. NBM POPS ARE
AROUND 30%). DEEP LAYER SHEAR OF AROUND 40 KTS WILL BE PRESENT NORTH
OF I-96. HREF MEAN SBCAPE VALUES ARE LESS SUBSTANTIAL THAN TODAY BUT
STILL 1000-2000 J/KG. WE DO HAVE UPPER JET DIVERGENCE MOVING THROUGH
WHICH COULD AID IN THE DEVELOPMENT OF SOME ORGANIZED CONVECTION
DURING THE MIDDAY TO AFTERNOON PERIOD. THE LOW LEVEL JET WEAKENS TO
20-25 KTS. THERE IS AN INCOHERENT SIGNAL AMONGST HREF MEMBERS AS TO
THE PLACEMENT AND INTENSITY OF CONVECTION TOMORROW. THE HRRR/RRFS DO
BRING IN SOME ORGANIZED SWATHS OF STORMS WITH 60+ MPH GUSTS DURING
THE AFTERNOON, WHILE THE OTHER CAMS ARE NOT SUPPORTIVE. AT THIS
POINT, THE SPC MARGINAL RISK SEEMS TO COVER THE SPATIAL THREAT WELL,
WITH SOME ISOLATED/SCATTERED INSTANCES OF STRONG WIND GUSTS AND HAIL
POSSIBLE.
AFTER 00Z, A STRONGER NOCTURNAL LLJ DEVELOPS TO OUR WEST AND ANGLES
IN DURING THE LATE NIGHT HOURS. VERY HIGH PWAT VALUES ARE SHOWN BY
THE 12Z ECMWF DURING THIS TIME, WITH 2.00-2.50" MOVING IN. VERY
HEAVY DOWNPOURS AND SOME STREET FLOODING MAY DEVELOP IF ORGANIZED
CONVECTION GETS GOING ON THE NOSE OF THE LLJ THURSDAY NIGHT INTO
FRIDAY MORNING. LARGE HAIL ALSO LOOKS TO BE A THREAT WITH THESE
POTENTIAL STORMS.
- SHORT BREAK MIDDAY FRIDAY, THEN MORE RAIN CHANCES
A DECENT MODEL SIGNAL EXISTS FOR A BREAK IN THE RAIN CHANCES LATE
FRIDAY MORNING INTO THE EARLY AFTERNOON PERIOD. DAYTIME HEATING ON
FRIDAY COULD BRING SOME POP-UP SHOWERS AND THUNDERSTORMS BY
EVENING WITH PERHAPS ADDITIONAL RAIN CHANCES OVERNIGHT ALONG A
POTENTIAL SURFACE BOUNDARY IN THE AREA.
AVIATION /00Z TAFS THROUGH 00Z FRIDAY/
ISSUED AT 717 PM EDT WED SEP 2 2026
TWO MAIN CONCERNS WITH THE 00Z TAF PACKAGE. FIRST IS CONVECTION.
WILL MAINTAIN THE VCTS FOR A ROUND OF THUNDERSTORMS EXPECTED TO
DEVELOP LATE THIS EVENING INTO TONIGHT, FOLLOWED BY PROB30 FOR A
LESS CERTAIN SECOND ROUND SOME OF THE SHORT RANGE GUIDANCE NEAR
DAYBREAK THURSDAY. ANY THUNDERSTORMS MAY CONTAIN INCREASED WINDS
AND MVFR TO IFR CONDITIONS. IN ADDITION, RAIN TODAY MAY LEAD TO
PATCHY MVFR FOG. CONFIDENCE IS TO LOW (20-30%) FOR TAF INCLUSION
WITH THE BEST CHANCE AT GRR AND LAN. LIGHT AND VARIABLE WINDS
TONIGHT INCREASE FROM THE WEST TO AROUND 10 KNOTS THURSDAY.
MARINE
ISSUED AT 245 PM EDT WED SEP 2 2026
NO HAZARDOUS WINDS OR WAVES ARE EXPECTED FOR THE NEXT SEVERAL
DAYS WITH THE HIGHEST WAVES OF UP TO 2 FEET OCCURRING THIS
EVENING. HOWEVER, MARINERS WILL NEED TO BE AWARE OF THUNDERSTORM
CHANCES THROUGH SATURDAY.
GRR WATCHES/WARNINGS/ADVISORIES
MI...HEAT ADVISORY UNTIL 8 PM EDT THIS EVENING FOR MIZ056>059-064>067-
071>074.
MARINE...NONE.
DISCUSSION...HOVING
AVIATION...THOMAS
MARINE...HOVING
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