757
FXUS63 KGRR 090806
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
406 AM EDT Wed Jul 9 2025

.KEY MESSAGES...

- Storms possibly strong this afternoon

- Rain chances increase beginning Friday and peaking Saturday

&&

.DISCUSSION...
Issued at 406 AM EDT Wed Jul 9 2025

- Storms possibly strong this afternoon

So far tonight, storms have been well behaved, and no hydrology
concerns have cropped up yet. We are seeing a very marginal low
level jet of 15+ knots acting on over 1000 J/kg of MU CAPE in place
over the area. Deep layer shear is only around 20 knots, thus the
limited organization to them.

We will likely see a brief lull in the shower/storm activity, before
more pop up as early as this morning, and more so toward noon. These
showers and storms will be forming out ahead of the cold front that
will be moving through the forecast area. Places across the NW
section of the forecast area will see the front come through with no
diurnal instability, but as mentioned above, we do have some in
place.

The SE portion of the area will see the best coverage of
showers/storms this afternoon, and have the best potential for some
of them to become locally strong. Ahead of the cold front, CAPEs are
forecast to increase to over 2000 J/kg with good diurnal heating
taking place, and the front arriving later. Deep layer shear values
are forecast to stay below 25 knots, continuing to limit the
organization of the storms. Forecast soundings show mid level lapse
rates not impressive around 6 C/km. The better threat would be some
microbursts with DCAPE values up around 1000 J/kg. Again, not
expecting these to be widespread, but does justify the Marginal Risk
issued by the SPC.

We will see everything clear out of the area by 22-00z tonight as
the front will exit by then.

- Rain chances increase beginning Friday and peaking Saturday

We will see a break in the bonafide rain chances on Thursday and
possibly into Friday as upper ridging and a weak sfc ridge move
across the area. There is a non-zero chance of a shower showing up
across the NE section of the area, likely the result of convergence
of mesoscale/lake breeze interaction. Most areas will be dry, and it
is not high enough to include at this time.

There is some concern that the models may not have a great handle on
rain chances for Friday. More ensemble members had rainfall coming
in on Friday, where some of the latest members have lost this QPF.
Some are still holding on to it, which looks reasonable given the
expected strength of this wave. We will keep rain chances low for
the time being and watch the latest trends.

The main rain event for the area still looks to come in centered on
Saturday, but could start as early as late Friday night, and could
linger into Saturday evening. This system will have good moisture
transport ahead of the front. This will bring some pretty good
instability over the area ahead of it with 2000+ J/kg. Deep layer
shear is also sufficient to bring some potential organization to the
storms. The exact timing and amount of instability is in question,
but definitely something to watch.

Once the front moves out by Sunday, we will see drier conditions
ensue, with summerlike temperatures holding on with no significant
cooling taking place.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 154 AM EDT Wed Jul 9 2025

We are looking at a complicated forecast period through the first
18 hours or so before conditions improve for all of the terminals.

We have scattered showers and storms over most of the southern
third of the forecast area, or all of the terminals except KMKG,
and near KGRR. These scattered showers and storms are forecast to
persist through the first 4-6 hours of the forecast, and could
drop conditions down to MVFR or even IFR under the heaviest cells.
They should tend to diminish temporarily after 10-12z.

We will see additional showers and storms develop then this
afternoon as the front passes through. The front will first pass
through KMKG early, like 09-12z and have some showers and lower
clouds possibly with it, with light winds and the abundant
moisture in place.

We will see additional showers form and some storms as we see
heating of the day boost this activity. Coverage should be enough
for all of the sites except KMKG, to go with a tempo TSRA group
for a few hours this afternoon. It appears most of this activity
should move out of all of the sites by 21-22z this afternoon. Some
bkn VFR clouds may linger into the evening hours, but will clear
out before the end of this forecast period.

&&

.MARINE...
Issued at 406 AM EDT Wed Jul 9 2025

We will be issuing a Beach Hazards Statement and Small Craft
Advisory for the nearshore waters south of Whitehall for today. The
winds coming in behind the cold front, combined with the mesoscale
effects of the ridge building in will bring winds and waves close
to, and maybe just exceeding criteria. Given we are in the heart of
boating and swim season, we feel it is best to err on the side of
caution and issue the headlines. The worst of the conditions will be
mostly in the evening, but they will come up quick.

&&

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

&&

$$

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