630
FXUS63 KGRR 161144
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
744 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 /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 744 AM EDT Wed Jul 16 2025

The scattered showers and storms that were over portions of the
area earlier have moved away from the terminals, leaving a dry
period as of 1130z. We will see VFR conditions with the dry
weather until about 16-17z when we will see scattered showers and
storms form along a lake breeze from near KGRR and KAZO and for
areas east. These will remain scattered through about 21-23z
before they dissipate/move out.

Confidence is growing that all of the sites will see the line of
showers and storms move through late this evening and into the
early overnight hours. KMKG will see it first beginning around
00z, and KJXN will see it last moving out toward 06z. IFR and LIFR
conditions are likely in this line with heavy rainfall. Gusty
winds are expected also. Have not put those in yet, as we will
wait until we get closer for a tempo group.

In the wake of the storms, gusty winds from the sw will become
from the NW. Conditions ill improve right after the storms, to
only drop into the IFR category under low clouds on the backside
of the low. Those conditions will persist until the end of the
forecast period at 12z Thursday.

&&

.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