510
FXUS63 KGRR 182330
AFDGRR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
730 PM EDT SAT OCT 18 2025
KEY MESSAGES
- MARGINAL SEVERE THREAT THIS EVENING
- HEAVIER RAIN OVERNIGHT, CONTINUED WET SUNDAY
- RAIN AND WIND MONDAY NIGHT THROUGH TUESDAY
- UNSETTLED WEATHER WEDNESDAY INTO THE WEEKEND
DISCUSSION
ISSUED AT 303 PM EDT SAT OCT 18 2025
- MARGINAL SEVERE THREAT THIS EVENING
LATEST MESOANALYSIS PROGS SHOW THAT 0-3KM MLCAPE IS EXPECTED TO
INCREASE, VIA INSOLATION AND MOIST ADVECTION, NEAR THE IN STATE LINE
AND SOMEWHAT JUXTAPOSED WITH A CONFLUENCE AXIS NOTED ON THE 18Z
SURFACE OBS PLOT. MORE FAVORABLE DEEP LAYER SHEAR APPEARS SHIFTED
POLEWARD OF WHERE THE LOW LEVEL BUOYANCY AND VERTICAL VORTICITY WILL
BE CENTERED. THIS REDUCES (BUT DOESN'T ELIMINATE) THE CHANCES FOR
TORNADO SPIN-UPS. ANYTHING THAT MANAGES TO OCCUR WOULD BE
ISOLATED AND SHORT-LIVED.
THE LIMITED SEVERE WEATHER THREAT SHIFTS EASTWARD AND SOUTHWARD
BY THIS EVENING. THESE EXPECTATIONS SEEM WELL CAPTURED BY THE SPC
MARGINAL RISK AREA THAT POKES INTO SOUTHEAST LOWER MI. DCAPE PROGS
ARE A LITTLE LOWER THAN BEFORE WHICH MAY LIMIT THE DAMAGING WIND
THREAT SOMEWHAT. HOWEVER, MARGINALLY SEVERE HAIL REMAINS POSSIBLE.
- HEAVIER RAIN OVERNIGHT, CONTINUED WET SUNDAY
LATEST ITERATIONS OF THE ECMWF ENSEMBLE ADVERTISE GREATER PRECIPITATION
AMOUNTS BETWEEN 00Z SUNDAY AND 00Z MONDAY. THIS IS NOT SURPRISING GIVEN
THAT DEEP FORCING FOR UPWARD VERTICAL MOTION (UVM) OVERNIGHT HAS COME
MORE INTO FOCUS AND IS IMPRESSIVE. THE MERIDIONALLY PHASED UPPER TROUGH
LOCATED EAST OF THE ROCKIES EARLIER TODAY WILL REACH THE WESTERN GREAT
LAKES AND BREAK CYCLONICALLY OVERNIGHT. STRONG QG FORCING ASSOCIATED
WITH TIGHTENING AND PIVOTING OF THE MID-LEVEL THERMAL GRADIENT WILL
ALLOW THE SURFACE LOW TO DEEPEN AS IT TRACKS NORTH ACROSS LOWER
MICHIGAN.
VICINITY OBSERVED 12Z SOUNDINGS THIS SATURDAY MORNING INDICATED
PRECIPITABLE WATER AROUND OR GREATER THAN 200 PERCENT OF NORMAL.
THIS AIRMASS WILL STILL BE IN PLACE AND WILL ALLOW FOR EFFICIENT
PRECIPITATION PROCESSES AMIDST THE STRONG UVM.
IN TERMS OF PRECIPITATION AMOUNTS, THERE IS AN UNUSUALLY GOOD CHANCE
FOR LOCALIZED OVER-PERFORMANCE. THE 12Z SPC HREF PMM/LPMM GUIDANCE
SHOWS AN AXIS OF HEAVIEST PRECIPITATION FROM ROUGHLY A SOUTH BEND IN
THROUGH ALPENA MI LINE WITH AMOUNTS REACHING 4 INCHES, PERHAPS
MORE, SOUTH AND EAST OF KALAMAZOO. EVEN MORE IMPRESSIVE, THE
18/00Z ECMWF ENSEMBLE EFI AND SHIFT OF TAILS (VALID 00Z SUN-00Z
MON) ARE >0.9 AND >=2, RESPECTIVELY, FOR THIS SAME REGION. THESE
ARE BOTH LARGE VALUES, AND THAT TYPICALLY MEANS UNUSUALLY HIGH
CONFIDENCE FOR AN EXTREME EVENT. HOWEVER, IT'S IMPORTANT TO NOTE
THAT REALIZED IMPACTS COULD BE QUITE LIMITED IN THIS CASE, BECAUSE
AS NOTED IN THE HYDROLOGY SECTION OF THIS DISCUSSION, WE ARE IN A
DROUGHT AND HAVE A LARGE CAPACITY FOR RAINFALL AT THIS TIME.
PLEASE ALSO NOTE THAT THE HYDROLOGY SECTION OF THIS DISCUSSION IS
NOW ABOUT 24 HOURS OLD, WHICH IS WHY IT MENTIONS LESSER
PRECIPITATION AMOUNTS COMPARED TO WHAT I AM DESCRIBING HERE.
PRECIPITATION IS EXPECTED TO CONTINUE WELL INTO SUNDAY THANKS
LARGELY TO A PERSISTENT AND SLOW-MOVING TROWAL. THIS FEATURE
INDICATES A LACK OF LOW LEVEL COLD AIR ADVECTION THAT IS
TYPICALLY REQUIRED FOR STRONG SURFACE WINDS. THUS, WE ARE NOT
OVERLY CONCERNED ABOUT WINDS OVER LAND AT THIS TIME.
- RAIN AND WIND MONDAY NIGHT THROUGH TUESDAY
WIND ON TUESDAY WITH THIS CLOSED LOW SYSTEM LOOKS MORE IMPRESSIVE
THANKS TO A MORE SIGNIFICANT COLD ADVECTION COMPONENT. GUSTS OVER 40
MPH LOOK LIKE A DECENT BET. LAKE EFFECT RAIN LOOKS LIKELY WITH LAKE
SBCAPE INCREASING TO A FEW HUNDRED J/KG.
- UNSETTLED WEATHER WEDNESDAY INTO THE WEEKEND
WEDNESDAY WILL SEE CONTINUED LAKE EFFECT RAIN SHOWERS LASTING FOR
MOST IF NOT ALL OF THE DAY.
OTHERWISE, THE UPPER LOW POSITION SLOWLY MOVES EASTWARD ACROSS
THE GREAT LAKES INTO THE WEEKEND, BUT THERE COULD BE A SUCCESSION
OF REINFORCING UPPER SHORTWAVE TROUGHS ACROSS THE WESTERN GREAT
LAKES, WHICH WOULD KEEP CONDITIONS UNSETTLED AND FORECAST
SPECIFICS DIFFICULT TO NAIL DOWN.
AVIATION /00Z TAFS THROUGH 00Z MONDAY/
ISSUED AT 728 PM EDT SAT OCT 18 2025
AREAS OF IFR IN HEAVY RAIN FROM AZO TO JXN WILL EXPAND ACROSS MUCH
OF LOWER MICHIGAN OVERNIGHT AS STEADY MOVES IN FROM THE SOUTH
WITH LIFR CONDITIONS EXPECTED BY 12Z. WINDS WILL BE LIGHT THIS
EVENING THEN GO NORTHWEST ON SUNDAY AND GUST OVER 20 KNOTS AFTER
12Z WITH SOME GUSTS OVER 30 KNOTS IN THE AFTERNOON. CONDITIONS
WILL SLOWLY IMPROVE FROM WEST TO EAST ON SUNDAY AFTERNOON.
MARINE
ISSUED AT 303 PM EDT SAT OCT 18 2025
NO CHANGES TO THE GALE WARNING AS WE EXPECT NORTHWEST GALES ON
SUNDAY MORNING AND AFTERNOON, THEN DECREASING BY LATE EVENING.
CONDITIONS WILL REMAIN HAZARDOUS TO SMALL CRAFT THROUGH MONDAY, THEN
ANOTHER GALE WARNING WILL PROBABLY BE NEEDED FOR TUESDAY. AT THIS
POINT IT APPEARS THE WINDS ON TUESDAY MAY ACTUALLY BE STRONGER THAN
THOSE ON SUNDAY, WITH WEST GALES OVER 40 KNOTS POSSIBLE.
HYDROLOGY
ISSUED AT 318 PM EDT FRI OCT 17 2025
SIGNIFICANT RAINFALL IS EXPECTED OVER THE NEXT SEVERAL DAYS ACROSS
MUCH OF LOWER MICHIGAN. THE TOTAL AMOUNT OF RAIN WE ARE EXPECTING
CONTINUES TO SLOWLY NUDGE UPWARDS AS FORECAST DETAILS COME INTO
FOCUS. THE RAIN WILL BE SPREAD OUT OVER SEVERAL ROUNDS, PRIMARILY
SPANNING SATURDAY AND SUNDAY, WITH THE HEAVIEST RAIN CURRENTLY
EXPECTED BEFORE/DURING SUNRISE ON SUNDAY. OVERALL AVERAGES ACROSS
MUCH OF THE AREA WILL PROBABLY END UP IN THE 1 TO 2 INCH RANGE, WITH
SOME MULTI-COUNTY STREAKS OF 2-3"+. THE GOOD NEWS IS THAT BECAUSE
WE'RE IN A DROUGHT, THE RIVERS HAVE PLENTY OF ROOM TO PUT THIS
WATER. SOME OF THE MIDSIZED RIVERS IN THE AREA LIKE SYCAMORE CREEK
(HOLT), BUCK CREEK (GRANDVILLE), AND THE LOOKING GLASS RIVER (EAGLE)
ARE THE MOST LIKELY TO SEE RISES CLOSE TO BANKFULL, BUT NO RIVER
FLOODING IS EXPECTED UNLESS THIS STORM SERIOUSLY OVERPERFORMS OUR
EXPECTATIONS. IT WOULD PROBABLY TAKE 4"+ OVER A FAIRLY LARGE AREA TO
BEGIN FLOODING THE MIDSIZED TRIBUTARIES MENTIONED ABOVE.
NEVERTHELESS, WITH WATER LEVELS CURRENTLY RUNNING LOWER THAN NORMAL
DUE TO THE DROUGHT, NOTABLE WITHIN-BANK RISES ARE EXPECTED ACROSS
MINOR FLOODING DEVELOP IN THE CLASSIC LOW SPOTS AND OTHER POOR
DRAINAGE AREAS. IT'S HARD TO PIN DOWN WHICH URBAN AREAS WOULD BE
MOST AT RISK OF THIS HAPPENING, BUT RECENT MODEL TRENDS SUGGEST THE
EASTERN EDGES OF OUR AREA ARE MOST AT-RISK, WHICH WOULD INCLUDE
LANSING AND JACKSON AREAS. ANYONE WITH PERSONAL PROPERTY NEAR THE
WATERS EDGE LEFTOVER FROM THE SUMMER SHOULD PREPARE FOR RISING
WATER.
WHILE RIVER FLOODING ISN'T A BIG CONCERN, IF SOME SLOW-MOVING HEAVY
RAIN BANDS PARK OVER SOME OF OUR URBAN AREAS WE COULD SEE SOME
LOCALIZED MINOR FLOODING DEVELOP IN THE CLASSIC LOW SPOTS AND OTHER
POOR DRAINAGE AREAS. IT'S HARD TO PIN DOWN WHICH URBAN AREAS WOULD
BE MOST AT RISK OF THIS HAPPENING, BUT RECENT MODEL TRENDS SUGGEST
THE EASTERN EDGES OF OUR AREA ARE MOST AT-RISK, WHICH WOULD INCLUDE
LANSING AND JACKSON AREAS.
GRR WATCHES/WARNINGS/ADVISORIES
MI...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 8 PM EDT THIS EVENING FOR LMZ844>849.
GALE WARNING FROM 8 AM TO 11 PM EDT SUNDAY FOR LMZ844>849.
DISCUSSION...TJT
AVIATION...OSTUNO
MARINE...OSTUNO
HYDROLOGY...AMD
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