859
FXUS63 KGRR 171105
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
705 AM EDT Thu Jul 17 2025

.KEY MESSAGES...

- Cooler, drier, and less humid through Friday

- Next threat of rain on Saturday

- Heat and humidity returns next week with storm chances

&&

.DISCUSSION...
Issued at 334 AM EDT Thu Jul 17 2025

- Cooler, drier, and less humid through Friday

Cold front is well evident on the radar and in the sfc observation
network this morning. As of 07z, the front stretched from near Alma
to Battle Creek with winds from the SW ahead of it and dew points in
the 70s, to gusty winds from the WNW behind it with dew points
falling quickly into the 60s.

A few remaining showers are in place over Central Lower Michigan on
the backside of the sfc low that is supported by the MCV that moved
through the region. There are a few sprinkles/very light showers
along the front. All of the showers will come to an end as one more
short wave moves through early this morning, and the sfc low and
cold front push east of the area.

We will see quite a bit of cloud cover stick around through most of
the day. Late this afternoon and evening is when we see clouds
scatter out, and eventually become mostly clear overnight and last
through Friday. Highs today with the clouds and cold air advection
will only reach the 70s, and again on Friday with much more
sunshine.

- Next threat of rain on Saturday

Our next chance for rain after the lingering showers this morning
will come centered around the Saturday time frame.

There is an upper trough over the desert SW that will eject some
short wave energy into the main jet and then approach the area. This
wave is expected to stay just south of the forecast area. At the
same time, the shear zone of the upper jet will be just to our
north. This wave will support a cold front that will be pushed
southeast through the area. Initially it looks like we will see the
two areas of rain split north and south of the area. As the front
drops down, that will help to bring rain to areas in between the two
waves.

As far as thunder potential is concerned, the models are showing
instability building to as low as -5C Lifted Indices with a sfc
based parcel, and similar values for 925 mb and 850 mb based
parcels. The best instability will be as you head further south,
especially south of I-96. CAPEs are forecast to be over 2000 J/kg
and effective shear values are forecast to increase to 35 to 40
knots. Those parameters are definitely something to monitor.

- Heat and humidity returns next week with storm chances

We will see any rain chances clear out by Sunday and likely stay
away through Monday. In the wake of the sfc low, upper waves, and
cold front, we will see a fairly expansive Canadian Sfc ridge build
over the area. Plenty of subsidence to squash anything that tries to
build.

Once the high moves out by late Monday, we will be looking at heat
trying to build back over the area from the SW. As the heat dome
builds this way, we will likely see the axis of more active short
waves and resultant MCS` pass through the area Tuesday into
Wednesday. This type of pattern is very difficult to try to nail
down any details, especially this far out. We will see the potential
for one or more rounds of storms potentially moving through as early
as Monday night and then being possible right through Wednesday. It
will be dry most of the time with building heat and humidity right
on the verge of moving over the area.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 705 AM EDT Thu Jul 17 2025

The cold front has made it through all of the terminals this
morning. Rain showers have pretty much come to an end, and much
cooler air is filtering in. Low clouds are in the IFR category
with ceilings of 500-800 ft agl. The ceilings will slowly lift up
through this afternoon/evening, and then scatter out slowly and
gradually. Gusty winds from the west as high as about 30 knots
this morning and afternoon will diminish this evening.

&&

.MARINE...
Issued at 334 AM EDT Thu Jul 17 2025

Our well advertised wind/wave event is well underway already this
morning. In fact, some of the initial gusts immediately behind the
frontal passage were touching Gales briefly. Those isolated Gale
gusts have disappeared, and we are seeing solid SCA and Beach
Hazards conditions at all of the coastal observations. The winds and
waves will continue through a good chunk of the day, before
diminishing this afternoon and evening.

Once this current event ends late today, we are looking at a fairly
quiet period for a few days with winds below 20 knots. We continue
to mention that the strong north flow going offshore will facilitate
a decent upwelling event where water temps could drop 20F fairly
quickly.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement through late tonight for MIZ037-043-050-
     056-064-071.
MARINE...Small Craft Advisory until 2 AM EDT Friday for LMZ844>849.

&&

$$

DISCUSSION...NJJ
AVIATION...NJJ
MARINE...NJJ/Thomas