358
FXUS63 KGRR 121925
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
325 PM EDT Sun Apr 12 2026
.KEY MESSAGES...
- Excessive rainfall threat this week
- Severe thunderstorms possible Monday night through Tuesday night
&&
.DISCUSSION...
Issued at 325 PM EDT Sun Apr 12 2026
- Excessive rainfall threat this week
Multiple chances for showers and thunderstorms tonight, Monday
night, Tuesday night, and beyond will lead to an increasing flood
threat after what has already been an abnormally wet spring since
the beginning of March. Rivers have come down from their crests
last week but will be prone to go back up again as thunderstorms
dump rain onto fairly moist soil. The usual caveats of convective
hit-or-miss rain swaths are in play, but any well developed
storms that repeat over the same areas will be in an environment
with PW values 1.25 to 1.5 inches, near the climatological maximum
this time of year, so rainfall production should be efficient.
The river forecasts issued Sunday morning contain forecast
rainfall through Monday morning. River forecasts are susceptible
to change tomorrow once tonight`s actual rainfall is counted and
Monday night`s thunderstorm rainfall enters the forecast. An areal
flood watch covers portions of central Michigan (and to the
north) through Wednesday morning, but could be extended south in
the next day or two as the thunderstorm threat eventually migrates
that direction.
- Severe thunderstorms possible Monday night through Tuesday night
This evening... a well mixed warm sector has moved into southern
Michigan up to Muskegon-Alma. A convectively enhanced shortwave
ejecting out of the south-central Plains today will bring another
potential for showers and isolated storms this evening as low-
level wind fields increase ahead of the wave. The relatively dry
air beneath the showers may facilitate mixing down gusts over 40
or 50 mph in some locations into tonight, as multiple runs of the
HRRR have been depicting.
Monday evening... Thunderstorms are likely to develop in the
vicinity of a frontal wave and surface low over Wisconsin and
Michigan during the evening and overnight. CAMs show a variety of
solutions regarding if and where storms develop over the area on
the warm advection side Monday evening, so confidence is on the
lower side for now. If storms develop, elevated supercells with
wind and hail hazards are possible. A squall line late in the
night may then arrive in west-central Michigan with a wind hazard.
Tuesday... a lingering tail of convection during part of the
morning can`t be ruled out, but much of the day is likely to be
rain-free before the next shortwave arrives in the evening or
night. A plume of 1000-2000 J/kg of CAPE will be stretched over
much of the Midwest including Michigan, with sfc-500 mb shear
around 40 knots, so severe weather hazards are again possible.
&&
.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 148 PM EDT Sun Apr 12 2026
Largely VFR conditions are expected until later tonight when rain
and thunderstorms expand over the entire area bringing a period of
MVFR ceilings with MKG having the best chance for IFR to LIFR
ceilings into Monday morning. Southwest winds will be breezy this
afternoon into tonight with the potential for some stronger gusts
toward 40 knots mixing down later this afternoon into the early
evening. Already observing these gusts off of the lake boundary at
BIV and MKG. Rain exits the area between 6-12Z Monday with dry
conditions through the remainder of the TAF forecast.
&&
.MARINE...
Issued at 325 PM EDT Sun Apr 12 2026
Hazardous winds and waves for small craft continue through much
of Monday, followed by a slight lull, then reinvigorating again
Monday night into Tuesday morning. So far today, the cool and
stable marine layer has prevented a lot of gale force winds aloft
from mixing down (there have been 40 mph wind gusts crashing to
the ground a couple miles inland). Some southwesterly gales are
possible late this evening, as have been depicted by the HRRR, as
showers and scattered thunderstorms move over the lake. Additional
thunderstorm hazards over the lake are possible between Monday
evening and Tuesday night.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Wednesday morning for MIZ037>040-043>045.
MARINE...Small Craft Advisory until 4 PM EDT Monday for LMZ844>849.
&&
$$
DISCUSSION...CAS
AVIATION...RAH
MARINE...CAS
NWS GRR Office Area Forecast Discussion
