686
FXUS63 KGRR 111913
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
313 PM EDT Fri Jul 11 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 /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 150 PM EDT Fri Jul 11 2025

Going more pessimistic on ceilings this afternoon as there is a
lot of moisture in the lower atmosphere and many areas of
scattered to broken MVFR cloud bases that may not fully mix away.
Scattered shower development also can`t be ruled out through this
evening. The chance of an isolated thunderstorm prior to 02 Z is
below 30 percent but not zero. Thunderstorms crossing Lake
Michigan may head toward MKG and possibly GRR between 03 and 07 Z
per the 12z HREF.

&&

.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...CAS
MARINE...CAS