452
FXUS63 KGRR 161946
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
346 PM EDT Wed Jul 16 2025

.KEY MESSAGES...

- A round of showers/storms move through later this evening

- Chances for showers/storms Saturday and again early next week

- Heat builds mid next week

&&

.DISCUSSION...
Issued at 345 PM EDT Wed Jul 16 2025

- A round of showers/storms move through later this evening

A well defined mesoscale convective vort (MCV) is seen spinning
east across Southern Wisconsin at 330p. The convectively produced
vort max has induced surface low pressure with a 1007mb low over
Southwest Wisconsin. An arc of showers and storms have developed
to the southeast and south of the low with multiple Severe
Thunderstorm and Tornado Warnings being issued this afternoon.

Here in Lower Michigan we have seen scattered to numerous diurnal
thunderstorms which have been non severe to this point. MUCAPE
values over Lower Michigan and points westward into Wisconsin have
reached 3,000 j/kg. Deep layer shear has reached 30-40 knots over
far Northern Illinois and Wisconsin. We will continue to see the
strong instability remain in place/advect in from the southwest.
In addition, at the MCV rotates east into Michigan we will see our
deep layer shear increase. Moisture (surface dew points in the
70s), instability (MUCAPE of 2,000-3,000 j/kg) and lift via the
MCV along with shear will all be in place this evening. We agree
with the slight risk area via SPC given the parameter space this
evening. The HREF 4 hour reflectivity max tells the story quite
well, in that our diurnal convection will fade into the evening
and the focus will shift to the incoming line. The lake is quite
warm and we do not expect a decrease in intensity as the lake may
actually give the convection as boost. The lift via the MCV will
help storms as well. So, bottom line...expecting a line of storms
this evening with wind being the main threat. An isolated tornado
like what has happened already this afternoon in Wisconsin is
certainly possible later this evening in a QLCS mode in areas
where flow backs to the southeast or east ahead of the line.
Showers and storms sweep out of the CWA by 2am-3am.

Locally heavy rain is certainly possible tonight with PWAT values
as high as 2.15 inches.

- Chances for showers/storms Saturday and again early next week

Two additional chances for showers and storms exist in the 7 day
forecast namely...Saturday and Saturday night with a weak cold
front and Monday night into Tuesday as a warm front lifts back in
from the south. At least scattered activity is expected in both
frontal passages.

- Heat builds mid next week

Something we are keeping an eye on is building heat as we move
from Tuesday to Thursday of next week. The ECMWF has +24C 850mb
temperatures moving in which would push highs well into the 90s F
if that verifies. Our current forecast is near 90 on Tuesday and
lower 90s on Wednesday. Thursday could go warmer than that if the
+24C verifies and precip holds off (the ECWMF has a front closing
in).




&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 120 PM EDT Wed Jul 16 2025

Scattered showers and thunderstorms have developed near and east
of AZO to GRR this afternoon. Brief periods of IFR conditions are
possible in any storms, though confidence in direct terminal
impacts is low. Will maintain the PROB30 and amend as needed.
Storms will diminish/exit the area after 21-23z from west to east.

A line of thunderstorms remains likely to reach the lakeshore
after 00z after arriving from Wisconsin. MKG will be the first
terminal to see storms around 00z, with storms moving east through
the late evening and exiting the JXN area towards 05z. IFR and
lower conditions and lightning are expected in any thunderstorms.
Gusty winds, with peak gusts possibly above 45 knots, may also
occur. Given storms over Wisconsin are just now developing, will
leave higher winds out of the TAFs at present but we will continue
to monitor upstream trends.

A brief period of VFR conditions are likely after the storms
before cigs fall to IFR overnight. Cigs remain IFR through mid
morning Thursday before lifting to MVFR. Gusty west to northwest
winds to 25 knots are also likely.

&&

.MARINE...
Issued at 345 PM EDT Wed Jul 16 2025

There are several concerns for marine interests and beachgoers over
the next 48 hours. First is a line of thunderstorms currently
developing across Wisconsin and Illinois. These storms will move
east reaching mid-lake after 00z (8PM EDT). If these storms can
maintain their strength, wind gusts in excess of 40 knots and
locally higher waves are possible.

Behind the front driving these storms, unstable over lake conditions
and increased northerly winds will bring solid Small Craft
Advisory/Beach Hazards Statement level conditions across the eastern
portions of Lake Michigan. Highest winds will be from late this
evening through mid Thursday afternoon up to 25 knots with waves of
4-7 ft. The current headlines cover the threat well. These northerly
winds may also lead to a period of upwelling on the lake, cooling
water temps for the later portion of the week.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from 2 AM EDT Thursday through late
     Thursday night for MIZ037-043-050-056-064-071.
     Air Quality Alert until midnight EDT tonight for MIZ050-056-057-
     064-071.
MARINE...Small Craft Advisory from 2 AM Thursday to 2 AM EDT Friday for
     LMZ844>849.

&&

$$

DISCUSSION...Duke
AVIATION...Thomas
MARINE...Thomas