500
FXUS63 KGRR 091542
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1142 AM EDT Wed Jul 9 2025

.KEY MESSAGES...

- Storms possibly strong this afternoon

- Rain chances increase beginning Friday and peaking Saturday

&&

.UPDATE...
Issued at 1140 AM EDT Wed Jul 9 2025

Southern lower Michigan is in a lull as far as convection is
concerned. Daytime heating and an approaching front should allow
for showers and storms to form over the next few hours. Storms
should continue through the afternoon into the evening.
  A few cells have already started to spark just east of US 131.
Latest mesoanalysis shows the best DCCAPE valves to be north of I
96 through central Michigan with values of 900 J/kg. That region
should be the best region for damaging winds.
 Periods of heavy rain remain possible with any showers and
storms. The front does have enough westerly flow that should move
it through the region fairly quickly this evening. There was some
heavy rainfall overnight west of Jackson, and Lansing so those
regions will need to be watched. Minor flooding thus remains
possible but not expected.

&&

.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 /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 748 AM EDT Wed Jul 9 2025

The showers and storms that have been affecting the southern
terminals overnight are in the process of tapering off and moving
to the east. It does look like it may be a couple of more hours
before everything from this moves out. These areas have been
mainly VFR out of the rain, and then MVFR to IFR under heavier
downpours.

We will then see the front drop south through the day. It is
bringing some MVFR to IFR conditions with it in the form of clouds
and fog to KMKG. It will eventually fire additional showers and
storms this afternoon. The more coverage and better chance will
be further SE where we have gone with tempo. KGRR we have gone
with Prob30, and kMKG nothing. Again MVFR to LIFR conditions can
be expected under any showers/storms.

These will move out gradually from NW to SE by mid evening. Skies
should then clear out overnight.

&&

.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...Beach Hazards Statement from 2 PM EDT this afternoon through
     late tonight for MIZ050-056-064-071.
MARINE...Small Craft Advisory from 2 PM this afternoon to 2 AM EDT
     Thursday for LMZ844>847.

&&

$$

UPDATE...Ceru
DISCUSSION...NJJ
AVIATION...NJJ
MARINE...NJJ