899
FXUS63 KGRR 132358
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
758 PM EDT Mon Apr 13 2026
.KEY MESSAGES...
- Severe thunderstorm and flood potential tonight and late Tuesday
&&
.DISCUSSION...
Issued at 302 PM EDT Mon Apr 13 2026
- Severe thunderstorm and flood potential tonight and late Tuesday
This evening, multiple clusters of storms are favored to develop
across Wisconsin, northern Illinois, and Lower Michigan, with low-
mid level wind fields and low-level warm/moist advection
increasing ahead of a shortwave trough. HREF mean MUCAPE will be
around 1000-1500 J/kg with effective bulk shear around 40 knots.
Model sounding thermodynamic and kinematic profiles are favorable
for some hail- producing cells and scattered swaths of damaging
winds especially in the first half of the night, including in
southwest Michigan after 6 PM if the storms currently in Illinois
maintain strength and organization. Tornado potential is
relatively low but not zero, as there could be some curvature in
the low-level hodographs and 0-3 km shear near 30 knots, but
surface stabilization by later in the evening may tend to be the
limiting factor.
Tuesday evening, another wave of convection is favored with
supercells, clusters, and/or lines in a more potent thermodynamic
and kinematic environment. Mean HREF MUCAPE is between 1500-2500
J/kg and curved low-level hodographs and longer upper-level tails
(respectable low-level and deep-layer shear) are expected. HREF
Significant Tornado Parameter mean is about 2-3 during south of
M-46 during the evening before the surface begins stabilizing a
couple hours after sunset. Diffuse mesoscale warm-frontogenesis at
the surface may also be occurring over the area during the
evening. Hail and wind are both expected to be primary hazards.
For information on the increasing flood threat, see the Hydrology
section below.
&&
.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 757 PM EDT Mon Apr 13 2026
We are looking at a high impact forecast period for the terminals,
especially for the first 12 hours or so with lots of convection
and low level wind shear, then evolving to mainly lower clouds on
Tuesday for a bit.
There are isolated cells out there to start the period. The main
one looks to impact the KLAN terminal between 0030 and 02z, so
have accounted for that. Otherwise, we will see a few showers over
Lake Michigan as of 00z move ENE. These are not expected to do
much as they move through over the first 3-4 hours. Our main
concern is the convection over Wisconsin that will make a run at
the area toward/after midnight. Low level wind shear of 45-55
knots will develop ahead of the line.
The convection will affect KMKG the longest with training of cells
expected. They will eventually push SE and affect the other
terminals over time, but not as long in duration. IFR is expected.
The storms should move out of all areas by 12z or so. Then some
MVFR clouds will gradually lift to VFR into the afternoon.
&&
.MARINE...
Issued at 302 PM EDT Mon Apr 13 2026
Areas of dense fog in the nearshore will likely persist into
tonight and may or may not dissipate after thunderstorms pass
through tonight. After a slight lull this evening, hazardous winds
and waves for small craft are expected through Tuesday.
Thunderstorms, possibly severe thunderstorms with hail and wind,
are possible over Lake Michigan this evening/tonight and Tuesday
evening/night.
&&
.HYDROLOGY...
Issued at 302 PM EDT Mon Apr 13 2026
The further north you go in the Lower Peninsula, the more
significant the flooding situation becomes. Around an inch of rain
fell across much of the area along and north of the Muskegon
River watershed over the weekend, with a much heavier swath of 2-4
inches across the Manistee River watershed and the headwaters
portions of the Muskegon basin. Coming on the heels of one of the
wettest springs on record so far, and in the midst of a
significantly active (and wet) ongoing weather pattern, the water
that`s already on the ground is going to produce renewed flooding
on the Muskegon River (and many others rivers further north as
well). With another 2-3" of rain over the next few days in the
Muskegon, we`re now expecting the most significant flood on the
Muskegon River since the flood of 2018. If the next few days of
rains overperform (which is entirely possible), the forecasted
crests will go even higher. If you live near the Muskegon river,
now is the time to prepare for significant flooding.
Meanwhile, further south, conditions in the Grand River basin are
in slightly better shape, although water is still high from all
of the recent rains. We`ve finally dropped below stage at all
locations in the basin, but that trend will soon reverse as water
levels start to climb again throughout the week. With an
additional 2-3" of rain expected in this area as well, it`s
beginning to look like the region will generally repeat the flood
levels experienced last week.
In addition to the river flooding, some of the thunderstorms
we`re expecting over the next few days could be very slow moving
over the same areas repeatedly. If this happens, some localized
rain totals of 4+ inches are possible in the next few days. If
this happens over one of the urban areas, significant fast-
developing urban flooding is possible.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Wednesday morning for MIZ037>040-043>046-
050>052.
MARINE...Dense Fog Advisory until 8 AM EDT Tuesday for LMZ844>849.
Small Craft Advisory until 4 AM EDT Wednesday for LMZ844>849.
&&
$$
DISCUSSION...CAS
AVIATION...NJJ
MARINE...CAS
HYDROLOGY...AMD
NWS GRR Office Area Forecast Discussion
