293
FXUS63 KGRR 120530
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
130 AM EDT Sat Jul 12 2025

.KEY MESSAGES...

- Chance of strong/severe storms tonight/Saturday

- More storms possible middle of next week

&&

.DISCUSSION...
Issued at 312 PM EDT Fri Jul 11 2025

- Chance of strong/severe storms tonight/Saturday

Radar shows a few light rain showers moving across the cwa.
Mesoanalysis indicated a lot of juice remains in place. SBCAPE
around 2500 j/kg south of I-96, but only 20 kts of shear.
Additionally, a weak frontal boundary was draped over far
southern Lower, but creeping northward. In many respects, this is
the quintessential Michigan summer environment. We`re sitting on
abundant moisture and instability and just looking for a catalyst
for storm initiation. We may have to wait until later tonight for
that to occur.

While it`s possible that a storm or two could develop this
afternoon along the frontal boundary or even in the presence of
outflow boundaries from this morning`s convection...CAMs point
toward storms rolling eastward from Iowa/Illinois later this
evening 3z-6z ahead of an upper short wave that would interact
with the weak boundary drifting northward through the cwa. This
would mostly affect the northern/northwest half of the cwa.
Despite instability waning this evening, the low level jet will be
increasing which will increase shear to 30-35kts. Thus, it`s
possible that a few storms tonight could be strong, perhaps
severe. Heavy rainfall remains a threat too due to precipitable
water values aoa 2 inches.

A severe threat remains for Saturday too. SBCAPE is progd to be
aoa 2500 j/kg along with 30 kts of shear. Additionally, a cold
front over Wisconsin will be pushed east by a stronger upper
trough. However, if convection develops, it will likely be along a
prefrontal trough after 17z across the southeast cwa. If that
occurs, severe storms are quite possible with torrential rain and
strong winds; the eastern cwa remains in a Slight Risk for severe
storms.

Once the cold front pushes east Saturday night, we`ll see a few
days of dry weather.


- More storms possible middle of next week

Once the high that builds over the Great Lakes Sunday moves east
Tuesday, a weak trough/warm front will move north across the cwa
Wednesday, ahead of low pressure in the Plains. We see a 40-60
percent chance of showers/storms with that boundary
Wednesday/Wednesday night. It`s early yet to determine if a severe
threat will develop with that boundary, but it`s possible.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 130 AM EDT Sat Jul 12 2025

Scattered thunderstorms and MVFR conditions will be across Lake
Michigan and near MKG through about 08Z but chances of thunder in
other areas of western Lower Michigan are low overnight. Some
light fog could also bring patchy MVFR conditions through sunrise.

Showers and thunderstorms are expected to flare up Saturday
afternoon over roughly the eastern half of Lower Michigan with
some isolated IFR conditions in heavy rain along with winds
gusting over 30 knots and small hall in stronger storms.

&&

.MARINE...
Issued at 312 PM EDT Fri Jul 11 2025

Generally expecting 1 to 3 ft waves during the weekend, though
Saturday could sneak into 2 to 4 ft and moderate swim risk from
Grand Haven and north with southwest 10-20 knot winds. Thunderstorms
this afternoon in Wisconsin/Illinois will cross Lake Michigan late
this evening but will gradually weaken. However, they could bring
wind shifts and gusts over 30 knots.

&&

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

&&

$$

DISCUSSION...04
AVIATION...Ostuno
MARINE...CAS