598
FXUS63 KGRR 111159
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
759 AM EDT Mon Aug 11 2025

.KEY MESSAGES...

- Periodic chances for showers and storms through Tuesday night

- Cooler and less humid second half of the week

- Heat and humidity return for next weekend

&&

.DISCUSSION...
Issued at 343 AM EDT Mon Aug 11 2025

- Periodic chances for showers and storms through Tuesday night

Warm and uncomfortably humid air mass remains in place through Tue
night, with a series of weak disturbances rippling through to
touch off showers/storms until a cooler and drier air mass arrives
midweek.

Thunderstorms ongoing around Ludington and isolated showers north
of M-20 should trend less probable as the day goes on. Then the
chance of thunderstorms shifts east of a South Haven / Ionia /
Alma line this afternoon along a wind confluence zone. Most of the
CAMs develop some convection in that area despite neutral/unfavorable
upper-level dynamics. The HRRR is among the weaker solutions but
that is probably due to it drying out the boundary layer well
below the NBM mean. If storms develop, there will be 1000-1500
J/kg of CAPE but little deep-layer shear. The mode will continue
to be hit-or-miss downpours and isolated stronger wind gusts over
40 mph.

The next wave originating from Kansas thunderstorms along with a
weak LLJ will set up additional scattered storms to develop over a
decent portion of our area late Mon night to early Tue morning.
Additional storm redevelopment becomes most likely within the HREF
for Tue afternoon, again south of a South Haven - Alma line. Both
waves of thunderstorms may carry a threat of gusty winds, perhaps
isolated severe. Rainfall will also be efficient. HREF LPMM for
Tue has a potential for spotty rain totals greater than 3 inches
south of Holland-Lansing where storms might repeat.

- Cooler and less humid second half of the week

The cold front will be mostly through the forecast area by 12z
Wednesday, but there may be a few lingering showers early Wednesday
across the far southern portion of the area.

Otherwise a nice Canadian sfc ridge will follow in the wake of the
cold front. This feature, and a flow from the NE, along with lower
upper heights will all point toward a cool down compared to the
recent 90s much of the area has seen. Dew points will also drop to
more comfortable levels for most people as they drop down to the
upper 50s to around 60F. The drier air, and ridging at the sfc
should keep most areas dry through Friday.

- Heat and humidity return for next weekend

The brief respite in the heat and humidity will come to an end by
Saturday. The lower heights over the region late in the week will
give way to another building ridge across the Upper Midwest. This
scenario will help to push the stalled front that went through, back
north as a strengthening warm front with increasing southerly
flow.

There is a bit of uncertainty regarding precipitation chances for
next weekend. This is because with the heat dome building, plenty of
fuel will become available for storms to erupt along and just north
of the front with the wave of low pressure moving along it. How this
potential MCS propagates will depend on the position of it and the
instability gradient. The showers and storms could stay well north
of the area, but they could also propagate their way down into
the area.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 759 AM EDT Mon Aug 11 2025

This is not a particularly high-confidence forecast, as there is
potential for showers or scattered thunderstorms much of the time
but it will be difficult to pin down if and exactly when one will
affect a given aerodrome.

Starting with MKG this morning, ceilings below 2,000 feet (and
some IFR) over Wisconsin/Illinois have made inroads across part
of Lake Michigan. Models have been indicating Holland (BIV) to
Ludington (LDM) including MKG are most likely to be affected.
Farther east, daytime cumulus clouds above 3,000 feet are more
favored to develop later this morning.

A good number of short-term models pop up scattered showers or
thunderstorm cells this afternoon into evening around AZO, BTL,
LAN, and/or JXN. If a more robust convective cell were to develop,
it could produce temporary IFR under heavy rain, and also some
localized LLWS and surface gusts greater than 30 knots.

There is another chance of thunderstorms during the overnight
period into Tuesday morning, which would also have a threat of
LLWS and surface wind shifts/gusts. Models differ quite a lot on
the evolution of this threat for now.

&&

.MARINE...
Issued at 343 AM EDT Mon Aug 11 2025

We are looking at most of this upcoming week seeing lighter winds
and smaller waves for our nearshore waters, aside from the short-
term wind and wave impacts that thunderstorms may have through
Tuesday night.

The first half of the week will see a front laid across the area,
slowly sinking south through Tuesday night. There are no
significant waves riding along this front, so the pressure
gradient remains relatively light. Even behind the front on
Wednesday. What we will see behind the front is an offshore flow
set up. This will support another likely upwelling event.

The next system will approach the area late Friday into Saturday.
It is looking like the winds will pick up in this time frame as
the gradient increases with a developing warm front to our SW. It
looks like any headline potential will likely stay low until at
least Friday.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

DISCUSSION...CAS/NJJ
AVIATION...CAS
MARINE...NJJ