750
FXUS63 KGRR 140001
AFDGRR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
801 PM EDT SUN SEP 13 2026
KEY MESSAGES
- GREATER CHANCES OF RAIN, MAINLY THIS AFTERNOON
- SHOWERS, THUNDERSTORMS, POSSIBLE HEAVY RAIN STARTS TUESDAY
- UNSETTLED INTO THIS WEEKEND
DISCUSSION
ISSUED AT 213 PM EDT SUN SEP 13 2026
- GREATER CHANCES OF RAIN, MAINLY THIS AFTERNOON
BASED ON RADAR AND SATELLITE TRENDS, COMBINED WITH MODEL
GUIDANCE, HAVE RAISED POPS TO NEAR CATEGORICAL THIS AFTERNOON.
THUNDERSTORMS ARE NOT EXPECTED BUT THERE SHOULD BE A FAIRLY
EXTENSIVE COVERAGE OF MEASURABLE PRECIPITATION. RAIN SHOULD BE
MOSTLY, IF NOT TOTALLY, OUT OF THE FORECAST AREA BY THIS EVENING.
AS NOTED BEFORE, WINDS SHOULD DECREASE RAPIDLY THIS EVENING AS THE
SURFACE HIGH MOVES INTO LOWER MI. WE ARE STILL LOOKING AT LOWS IN
THE 40S MONDAY MORNING WITH UPPER 30S NOT OUT OF THE QUESTION IN THE
FAVORABLY COOLER SANDY SOIL REGIONS TO THE NORTH.
- SHOWERS, THUNDERSTORMS, POSSIBLE HEAVY RAIN STARTS TUESDAY
A CYCLONICALLY BREAKING UPPER SHORTWAVE TROUGH ORIGINATING FROM THE
NORTHERN ROCKIES WITH A SURFACE LOW TRACKING INTO CANADA BY TUESDAY.
THIS WILL AMPLIFY THE DOWNSTREAM UPPER RIDGE AND PUSH IT EASTWARD
ACROSS THE GREAT LAKES. THE ASSOCIATED EASTWARD PROGRESSION OF THE
SURFACE HIGH WILL INTRODUCE INCREASING SOUTHERLY FLOW IN ITS WAKE.
THE COLD FRONT AN ASSOCIATED LOW LEVEL THETA-E RIDGING WILL ENTER
THE FORECAST AREA EARLY TUESDAY MORNING. WITH MUCAPE APPROACHING
1000 J/KG, THERE WILL BE A CHANCE FOR THUNDERSTORMS.
AS NOTED PREVIOUSLY, THE COLD FRONT IS EXPECTED TO STALL SOMEWHERE
IN THE VICINITY OF THE SOUTHERN CWA. THERE WILL BE A SHARP THETA-E
AND CONDITIONAL INSTABILITY GRADIENT WITH THIS FEATURE. ECMWF ENSEMBLE
(ECE) MEAN GUIDANCE INDICATES PRECIPITABLE WATER VALUES APPROACHING
2 INCHES, WHICH IS HIGHLY ANOMALOUS WITH RESPECT TO BOTH CLIMATOLOGY
FOR THIS TIME OF YEAR AND WITH RESPECT TO THE PROPENSITY OF THE ECE
TO FORECAST VALUES THIS HIGH FOR THIS TIME OF YEAR. WPC IS INDICATING
A MARGINAL RISK FOR EXCESSIVE RAINFALL, WHICH IS QUITE REASONABLE.
HOWEVER, ACCURATE PLACEMENT OF THE ACTUAL THREAT AREA WILL BE TRICKY
AND HIGHLY LOCATION-DEPENDENT GIVEN THE SHARPNESS OF THE BOUNDARY.
- UNSETTLED INTO THIS WEEKEND
ENSEMBLE GUIDANCE REALLY SPEAKS TO THE POOR PREDICTABILITY LEADING
INTO THIS WEEKEND. OFTEN IT'S POSSIBLE TO CARVE OUT A PARTICULAR DAY
THAT'S LIKELY TO BE DRY. NOT SO THIS TIME. WHILE THERE IS A SIGNAL
FOR A DRY DAY HAPPENING AT SOME POINT, WHICH DAY THAT IS HAS VARIED
FROM RUN TO RUN. HENCE, WE ARE FEATURING PERSISTENT 20-40 PERCENT
POPS THROUGH THE PERIOD.
AVIATION /00Z TAFS THROUGH 00Z TUESDAY/
ISSUED AT 801 PM EDT SUN SEP 13 2026
SURFACE HIGH PRESSURE IS LOCATED JUST UPSTREAM OF THE SOUTHWEST
LOWER MICHIGAN TAF SITES OVER MINNESOTA AND WISCONSIN AT 00Z. THE
HIGH WILL SHIFT EASTWARD TONIGHT AND BE LOCATED DIRECTLY OVER OUR
AREA ON MONDAY. NO SIGNIFICANT AVIATION CONCERNS ARE EXPECTED THE
NEXT 24 HOURS WITH HIGH PRESSURE PROVIDING VFR WEATHER.
AS FOR THE DETAILS...
WINDS WILL BECOME LIGHT AND VARIABLE THIS EVENING FAIRLY QUICK,
OR BY 03Z. LIGHT AND VARIABLE WINDS WILL CONTINUE OVERNIGHT AND
THROUGH THE MORNING HOURS OF MONDAY. A LIGHT SOUTHEAST WIND WILL
DEVELOP MONDAY AFTERNOON AT 4-8 KNOTS. SKIES WILL GENERALLY BE
CLEAR TONIGHT, BUT THERE WILL BE SOME SCATTERED CUMULUS TOWARDS
THE LAKESHORE AT AROUND 5,000 FEET. THESE CLOUDS ARE ASSOCIATED
WITH LAKE INSTABILITY (COLDER AIR OVERRIDING WARMER WATER). THE
CIRRUS CLOUDS TO THE SOUTH WILL MOVE AWAY THIS EVENING. CIRRUS
WILL SPREAD BACK IN FROM THE WEST AFTER 20Z ON MONDAY. CIRRUS
BASES SHOULD BE AT OR ABOVE 20,000 FEET.
MARINE
ISSUED AT 213 PM EDT SUN SEP 13 2026
NO CHANGES TO THE MARINE HEADLINES AT THIS TIME. AS NOTED
PREVIOUSLY, GALE CONDITIONS ARE EXPECTED TO COMMENCE MONDAY
NIGHT. HOWEVER, TO PREVENT CONFUSION, WE WILL WAIT FOR CURRENT
HEADLINES TO EXPIRE TONIGHT BEFORE ADDRESSING THIS NEXT THREAT
WINDOW. AFTER THE ONSET OF GALE CONDITIONS MONDAY NIGHT, HAZARDOUS
CONDITIONS SHOULD PERSIST THROUGH TUESDAY AND INTO TUESDAY NIGHT.
GRR WATCHES/WARNINGS/ADVISORIES
MI...BEACH HAZARDS STATEMENT UNTIL 11 PM EDT THIS EVENING FOR MIZ037-
043-050-056-064-071.
MARINE...SMALL CRAFT ADVISORY UNTIL 11 PM EDT THIS EVENING FOR LMZ844>849.
DISCUSSION...TJT
AVIATION...DUKE
MARINE...TJT
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