924
FXUS63 KGRR 211048
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
648 AM EDT Fri Aug 21 2026

.KEY MESSAGES...

- Scattered showers/thunderstorms today into Saturday

- Cooler and dry Sunday through Tuesday, midweek rain chances

&&

.DISCUSSION...
Issued at 317 AM EDT Fri Aug 21 2026

- Scattered showers/thunderstorms today into Saturday

The only change in thinking from 24 hours ago is that Convection
Allowing Models (CAMs) show widely dispersed convective cells
developing this afternoon over much of southwest Lower Michigan.
Please see the AVIATION section of this forecast discussion for
more specific details.

We expect precipitation coverage to be a bit more widespread
starting early tonight with additional convection approaching from
Lake MI. The latest SPC SWODY1 even has a marginal risk for wind
along our lakeshore. This would be predicated on already existing
organized multicell clusters making it into the area. Some Machine
Learning guidance, such as the NCAR suite, supports this idea.

Neither the thermodynamic environment nor the kinematics over
western Lower MI look particularly impressive for convective
maintenance, so would expect a fairly quick demise of any
organized convection that makes it this far east.

Models have been fairly consistent with a gradual west to east
shift of precipitation Saturday that mostly exits the area by
evening. It appears that the severe threat will remain east of our
area during this time.


- Cooler and dry Sunday through Tuesday, midweek rain chances

By late Sunday, the forecast area will be under deep, almost due
northerly flow with surface ridging. This is a dry pattern, but as
noted previously, can`t completely rule out isolated pop up
diurnal showers due to colder air aloft.

From Tuesday onward, flow aloft becomes more northwesterly and an
upper PV max in this flow will provide enough forcing for upward
vertical motion to reintroduce precipitation chances Tuesday night
into Wednesday. After this, mean troughing aloft continues with
unsettled conditions the rest of the work week. At this point,
there will be limited predictability regarding the timing of
subsequent precipitation chances.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 648 AM EDT Fri Aug 21 2026

Probability for VFR conditions at all terminals through 18Z is 90
percent. Main changes this update were to:

- delay onset of precipitation slightly
- reduce PROB30 group convective wind gusts
- add an extra PROB30 group for the overnight hours

These changes were based largely on most recent guidance, which
happens to show less overall coverage of showers and storms this
afternoon and tonight. Decided to trend this TAF package gently in
that direction with the understanding that future guidance and
radar/satellite trends could nudge things right back in the
opposite direction.


&&

.MARINE...
Issued at 317 AM EDT Fri Aug 21 2026

Although conditions will admittedly be marginally, we still opted
to issue a Small Craft Advisory and Beach Hazards Statement for
our northern two marine zones this afternoon into early evening.
There will be a risk for longshore currents and locally higher
waves, even though the overall wave field may be better described
by sub-advisory criteria. Nonetheless, this messaging lines up
well with APX`s immediately to the north of these zones.

There is another reason for erring on the side of caution here that
is not meteorologically based. We are looking at a Friday in the
summertime that may be the last Friday before many people return
to work or school. Consequently, there could be greater than
normal willingness to be exposed to a marginal risk; this gives a
net result of significant vulnerability.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from 2 PM EDT this afternoon through
     this evening for MIZ037-043.
MARINE...Small Craft Advisory from 2 PM this afternoon to 8 PM EDT this
     evening for LMZ848-849.

&&

$$

DISCUSSION...TJT
AVIATION...TJT
MARINE...TJT