968
FXUS63 KGRR 182332
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
732 PM EDT Fri Jul 18 2025

.KEY MESSAGES...

- Thunderstorms expected on Saturday...possibly severe

- Additional chances for showers/storms Wednesday through Friday

- Heat builds through the week...Tuesday through Friday

&&

.DISCUSSION...
Issued at 341 PM EDT Fri Jul 18 2025

- Thunderstorms expected on Saturday...possibly severe

Thunderstorms that fire this evening in Minnesota will try to make
a run into West Central Lower Michigan up near Big and Little
Sable Points towards daybreak. The HREF has supported this idea
for a few runs now. That convection will likely work/build south
and east with time moving across Central Lower Michigan Saturday
morning, likely in a non severe mode as it will be decaying.

Instability will build through the morning to the south of the
ongoing convection with the HREF indicating 1,000 to 2,000 j/kg of
MUCAPE in the afternoon near and south of I-96. The mid level jet
up at 500mb is on the order of 40 knots with a rather meager 850mb
jet of 20-25 knots. These values are enough for storm organization
with deep layer shear values around 30 knots. Mid level lapse
rates are not steep, so hail is not a big threat on Saturday. Wind
is the top threat as storms may tend to translate into lines or
bows. The main time frame of concerns is roughly 1pm through 7pm.

- Additional chances for showers/storms Wednesday through Friday

A building ridge will be in place across much of the Ohio Valley
into the Southern Great Lakes region next week. A surface front
will be situated to the north of the upper ridge oscillating
through the Lower Peninsula over the course of those 3 days.
Shower and thunderstorms chances look to be in place all 3 days as
there will be more than enough instability and a trigger in the
form of a front. So at this point we are carrying a deep summer
type of forecast with scattered showers and storms each period.
Certainly not a wash out by any means but the chance of
precipitation is there (30-40 pct).

- Heat builds through the week...Tuesday through Friday

Heat builds in the Tuesday through Friday time frame as 500mb
heights will be in the 590s and 850mb temps reach +24C at times.
These values are very high for us and equate to highs in the lower
90s most likely. The temperatures will be moderated by clouds and
showers and storms however. The heat and humidity will be there
though outside of thunderstorm activity.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 732 PM EDT Fri Jul 18 2025

The first half of this forecast period will be quiet with little
to no impacts expected. Any diurnal cumulus will quickly dissipate
by 02z, leaving only some high clouds for the overnight hours. Fog
potential is fairly low with some wind aloft remaining in place,
and with crossover temps this afternoon 2-3 degrees lower than
forecast low temperatures. Can not rule it out KJXN, but will hold
off mentioning it for now.

We will see mid clouds move in Saturday morning in advance of
showers and a few storms trying to move in. The best chance for
showers and a few storms will be after 15-16z at KMKG, and then
progressively later as you head SE through the terminals. KMKG,
KGRR, and KLAN have a lower threat of thunder. The other sites it
is high enough to mention as a prob30 at this time. The threat
then looks to end from NW to SE from 18-00z.

&&

.MARINE...
Issued at 341 PM EDT Fri Jul 18 2025

Winds and waves will remain below hazardous levels as the overall
gradient flow remains weak. Southerly flow will increase tomorrow to
10-15 knots. It is likely thunderstorms will cross the lake in the
late morning into the early afternoon hours on Saturday. Locally
higher winds and waves are possible in any storms. The next chance
for non-thunderstorm driven hazardous winds and waves is Sunday. A
classic advancing high setup will lead to an increase of winds along
the shore Sunday afternoon. Available short range model guidance is
showing 15-20 knot winds along the lake in the afternoon, so this
period will need to be monitored.

&&

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

&&

$$

DISCUSSION...Duke
AVIATION...NJJ
MARINE...Duke