169
FXUS63 KGRR 160740
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
340 AM EDT Wed Jul 16 2025

.KEY MESSAGES...

- Strong to severe storms possible this evening

- Cooler and drier Thursday-Friday

- Rain chances Saturday

&&

.DISCUSSION...
Issued at 340 AM EDT Wed Jul 16 2025

- Strong to severe storms possible this evening

Going to be an unsettled day with three potential periods of
showers/storms for the area today through tonight.

The scattered showers and isolated storms ongoing first this morning
are a result of a weak short wave that is drifting NE just south of
the area, and being at the leading edge of better warm and unstable
air. These showers and storms will tend to dissipate toward daybreak
as the upper short wave will move east of the area.

The second batch of storms today will be air mass type of showers
and storms that will form starting toward noon, and last until just
before sunset. The aforementioned arrival of the warmer and more
unstable air mass will be available for diurnal heating to push
temperatures to fire showers and storms near any type of
convergence. The initial convergence may be right along the lake
breeze convergence. Weak deep layer shear of 20-25 knots this
afternoon will lead to mainly pulse type of storms this afternoon,
spreading outflow boundaries from the storms. Small hail, wind gusts
of 40 mph or so, and heavy downpours will be the main threats this
afternoon.

The main show looks to be a mid-late evening timing for the area. A
MCV that is currently associated with convection will translate ENE,
and fire additional convection over IA, WI, and IL this afternoon.
We will see the MCV is some way approach the area by mid evening.
There is a decent amount of uncertainty with regards to how much
convection makes it this far east.

What we do know is that the atmosphere should remain quite unstable
until the convection across the lake and MCV arrive here. Mean MU
CAPE values from the HREF are forecast to still be up around 1500+
J/kg for a lot of the area. We also know that deep layer shear
values increase with the arrival of the wave/MCV up to around 35-40
knots. This is more than enough for organized storms. We are in full
agreement with the upgrade of the western portion of the area to a
Slight Risk by the SPC, in a conditional sense of storms are present
or not.

The uncertainty comes with what shape is the MCV/upper wave in as it
arrives. The further north over our area, the better the forcing
from the wave is expected, but instability is less. As you head
south, instability is the highest, but forcing is the least. It
really ends up being how far south the sufficient forcing will be to
maintain storms over the area. That is the million dollar question,
and we will continue to monitor the latest trends.

- Cooler and drier Thursday-Friday

The MCV/wave moving through tonight will pass by to our east by
daybreak on Thursday. The result of this is that the sfc cold front
is pushed well through the area (about 2/3rds through) by 12z
Thursday. With brief ridging behind the wave and it being the
coolest portion of the day, not expecting much precipitation with
the front Thursday morning. The only thing to help it briefly is
another short wave moving across Northern Lower.

The stronger gradient behind the front will usher in much cooler
air, and cloud cover will keep temps from rising too much on
Thursday. Highs will likely stay in the 70s for both Thursday and
Friday with 850 mb temperatures dropping to around +9 to +11C. High
pressure moving directly overhead on Friday will clear the area out
with the front well south of the area.

- Rain chances Saturday

We continue to see a short wave that is expected to lift out of an
upper low over Baja California and California itself. This wave has
continue to be shown to bring the front to our south back up to near
the area with low pressure moving across the area. This will supply
the area with shower and storm chances centered on Saturday. We
should dry out for Sunday and Monday as the front drops south of the
area again.

Quite a bit uncertainty exists for the area regarding details of the
forecast starting Monday night and Tuesday. We start seeing heights
building toward the area again as the heat dome to our SW starts to
build/expand over the country. This set up is one that helps to
facilitate a ring of fire pattern. The axis of the ring of fire is
uncertain, but it will be close enough that we could see one or more
MCS` potentially affect the area. Mid-late July is after all about
the peak of MCS activity for our area under the ring of fire
pattern.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 150 AM EDT Wed Jul 16 2025

We are looking at a more active forecast period this morning as
compared to the last couple of days. We have a few showers over
the area and even a couple of isolated storms. KMKG and KJXN have
the best immediate threat of the showers and isolated storms
affecting them this morning. Can not rule it out elsewhere, but
will leave it out until they approach the other terminals, if they
do.

We will see these showers/storms dissipate around daybreak, and
have a brief break of no precipitation. Then toward or just after
noon, a few showers and storms will fire close to the lake breeze
boundary. We have a mention at all sites except KMKG who will
likely be west of the activity. These showers/storms will then
dissipate after 22-00z.

Our attention then turns to expected convection over WI and IL
this afternoon and evening. This convection is somewhat uncertain
on how long it will hold on before diminishing later this evening.
For now, the best chance will be at the northern terminals, with a
lower chance at the southern terminals. We have a 2-4 hour period
of this, longest at KMKG where it may persist. Then conditions
should improve just before the end of this forecast period.

&&

.MARINE...
Issued at 340 AM EDT Wed Jul 16 2025

We continue to focus on a potentially higher impact event for
mariners and swimmers for Thursday behind a frontal passage. There
is not significant wind ahead of the front as the pressure gradient
remains fairly loose. Behind the front is a different story however.
This looks like a solid headline event with higher wind and unstable
conditions over the warm lake allowing winds and waves to max out.
The worst of the wind looks to come in the morning through first
half of the afternoon before the gradient weakens a bit.

Another point to mention is a noticeable upwelling event is looking
quite possible. The strong winds from the north going offshore as
they weaken is a recipe for a good water temp drop Thu-Fri.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Air Quality Alert until midnight EDT tonight for MIZ050-056-057-
     064-071.
MARINE...None.

&&

$$

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