358
FXUS63 KGRR 111129
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
729 AM EDT Fri Jul 11 2025

.KEY MESSAGES...

- Periods of showers/storms into Saturday evening

- Dry and Warm later Sunday through Tuesday

- Active period of convection possible Wednesday and Thursday

&&

.DISCUSSION...
Issued at 345 AM EDT Fri Jul 11 2025

- Periods of showers/storms into Saturday evening

Lots of complicated activity expected in the first 36-48 hours of
the forecast for the area, with multiple features interacting over
the region.

The first feature is a MCV/upper wave that is approaching the area
from the WSW this morning. This feature was responsible for
convection earlier toward the Quad Cities and east to near Chicago,
before diminishing after daytime heating was lost. The MCV is still
going strong, is is now starting to ramp up shower and storm
activity with it as the low level jet is ramping up ahead of it and
starting to nose into the area.

We expect a fairly solid area of showers and an embedded line of
storms to take shape, and move across at least the southern two
thirds of the forecast area through early this afternoon. The
strength of this convection this morning will be limited by a lack
of good instability ahead of it, as the wave is on instability
gradient. In addition, this feature coming through in the morning
will not allow much diurnal instability to develop as it moves
through. Would not be surprised for it to strengthen a bit as it
exits the area early this afternoon.

Initially the area will have been worked over, and limited heating
for the first few hours of the day will suppress temperatures a bit.
However, we will see the instability start to spread up over the
area with limited heating this afternoon, and for the warmer and
more humid air to make a run this way. This will be the result of
the system coming in Saturday starting to increase the upper flow
from the SW. Sitting on the instability gradient and it making a
move north will allow for a few showers and storms to pop this
afternoon. Any storm that forms has the potential to become strong
with CAPEs up over 2000 J/kg and deep layer shear values as high as
around 30 knots. Microbursts will be possible, and some hail with
mid level lapse rates a little high in the mid 6C/km range.

We will be waiting for more organized convection, likely later this
evening and overnight. The short wave/MCV out in Iowa this morning
will be steered a bit to the NE with the more organized synoptic
short wave coming in from the Upper Midwest. The low level jet ahead
of it is expected to fuel an additional wave of convection. There
seems to be some general agreement with the bulk of the convection
expected to affect the NW section of the forecast area. The
uncertainty with this is how the line may propagate ahead of it.
Some of it could run along the instability gradient over the
forecast area.

The uncertainty tonight then continues into Saturday, and will
affect how additional convection fires. The early morning convection
is likely to work the atmosphere over, and set up a boundary
over/near the area. This will be the focus of development Saturday
afternoon as synoptic scale forcing arrives ahead of the upper wave.
Where the atmosphere can remain unaffected my convection/outflow
boundaries, instability could build to over 3000 J/kg Saturday
afternoon. The further east, the better the instability with temps
approaching 90 and dew points in the 70s. The mid level winds
increasing with the wave will also support better shear of around 25
to 30 knots, and an increased severe potential. The SPC Day 2
outlook has this captured well.

The other threat with storms through the period will be the
potential of locally heavy rainfall. The atmosphere will have PWATS
up around 2 inches. Any training of cells, or prolonged storms over
any area could cause some local flooding.

- Dry and Warm later Sunday through Tuesday

The sfc front should be through the forecast area by 12z Sunday
morning, ending all of the storm threat and widespread rainfall. The
upper trough will be still poised to move through, so clouds and a
small chance of a residual shower may linger. We should see
conditions improve through the day on Sunday with the passage of the
trough.

We then see upper ridging build over the area late Sunday and hold
through Tuesday. This will bring the area dry conditions.
Temperatures will initially drop just a little bit initially, but it
will only be a few degrees. The humid air mass on Saturday will be
replaced by dew points about 10 degrees lower to make it less humid
by Monday. The heat will build though with the ridge in place and
then warmer 850 mb temps moving in.

- Active period of convection possible Wednesday and Thursday

The upper ridge will then build east by Wednesday, allowing lower
heights to settle in with an upper trough that will approach the
area. This trough will be rather broad in nature, and a front will
end up dropping over the area and stalling out as it becomes
parallel to the upper flow. Short waves are forecast to ride along
this front over the area, and could bring multiple rounds of
showers/storms to the area by later Wednesday and continue through
Thursday.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 729 AM EDT Fri Jul 11 2025

This batch of showers and embedded thunder is arriving about on
time, with just a little less thunder than earlier expected. We
will run with MVFR conditions in the showers, and a VCTS for the
northern terminals, and tempo thunder for the southern sites based
on latest radar trends.

Once this batch moves out of all of the terminals by early
afternoon, we will be left with mostly dry conditions for a few
hours. There may be a few pop up showers/storms, but the coverage
will be small enough to leave out of the forecast for now.

The next more organized chance of showers/storms will come very
late this evening and overnight for KMKG especially, and maybe
KGRR. The batch of storms is expected to move to the NNE over the
lake and clip KMKG and KGRR. The other sites are likely going to
stay mostly dry overnight and through 12z Sat.

&&

.MARINE...
Issued at 345 AM EDT Fri Jul 11 2025

We are cautiously optimistic in saying that we may not need any
marine headlines for a few days. We do see some increased wind aloft
move in starting this morning, and lingering through Saturday.
However with the temperature above the sfc of the lake being quite a
bit warmer, the stable conditions over the lake should prevent this
from coming down (outside of storms). There is not really much
colder air behind the system on Sunday to kick up winds/waves either.

&&

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

&&

$$

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