419
FXUS63 KGRR 181045
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
645 AM EDT Tue Aug 18 2026

.KEY MESSAGES...

- Showers and storms this afternoon and overnight

- Precipitation chances return Friday

&&

.DISCUSSION...
Issued at 324 AM EDT Tue Aug 18 2026

- Showers and storms this afternoon and overnight

Now that we are at a closer forecast projection time to better
leverage convection-allowing models (CAMs), a couple of differences
are observed compared to this time yesterday. First, we see a
greater amount of forecast conditional instability, which is now
sufficient for thunderstorms across most, if not all, of the
forecast area. The 18/00Z HRRR, for example, indicates a ribbon of
MUCAPE approaching 2000 J/kg immediately ahead of the cold front
that will traverse the forecast area tonight.

The second difference is that precipitation is now expected to exit
the forecast earlier than previously thought, thanks to a more
progressive cold front. Rain may even be totally out of the forecast
area by sunrise Wednesday.

Both model guidance and satellite suggest stronger forcing aloft to
hasten progress of the cold front. GOES full disk imagery shows a
seasonably strong upper low entering the northern Ontario/southern
Hudson Bay region from Manitoba. A strong upper jet is diving
southward over Saskatchewan on the back side of this low and this
likely will be the feature that that emerges as strong PV lobe
rotating around the southern periphery of the low and crossing Upper
MI and northern Lower MI early tonight.

Greater conditional instability naturally raises the question of
severe potential with these thunderstorms. Several CAMs, both within
the SPC HREF suite and also external members such as RRFS and MPAS-
NSSL, depict discrete cells with notable mid-level updraft helicity
moving across the northern forecast area early tonight; this speaks
to the expected presence of marginally favorable deep layer vertical
shear. Low level updraft helicity is much less pronounced, which is
not surprising given that the best low level shear should be
restricted to the poleward side of the front and away from the
convection.

Estimated DCAPE will be respectable at 800+ J/kg, thanks in part to
moderately steep mid-level lapse rates approaching 8C/km. Thus, it
is no surprise that wind will be the most plausible severe weather
type, as noted by SPC guidance across the west-northwest CWA. Still,
with the potential for discrete supercell structures depicted by the
CAMs, we can`t rule out some marginally severe hail given the
potential for mesocyclones in this steep mid-level lapse rate
environment. We are a bit less concerned about heavy rain given the
anticipated more progressive nature of the front, but do note that
WPC continues a marginal risk for excessive rainfall across far
southwest Lower MI.


- Precipitation chances return Friday

Northwest flow aloft will yield dry conditions Thursday. Broad upper
troughing across the northeastern U.S., including the Great Lakes,
will continue. Deterministic solutions show upper PV maxima working
their way through this pattern during this time. The first one
arrives Friday, bringing a chance for showers and thunderstorms by
Friday night. The weekend continues to look potentially unsettled
with a significant upper PV max diving southward and potentially
evolving into a closed low through a deep layer of the troposphere
somewhere over the western Great Lakes.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 645 AM EDT Tue Aug 18 2026

Through 18Z, VFR conditions are highly likely (95 percent chance).

Main change this update was to provide extra details regarding
expected evolution of showers and thunderstorms tonight,
specifically through and after 06Z. Terminals, especially MKG,
could see convective wind gusts exceeding 30 knots at some point
early tonight but it was too low probability to include even in a
PROB30 group. Once storms develop, it will be possible to add more
specific detail, but this likely won`t really be feasible until
well after 18Z.


&&

.MARINE...
Issued at 324 AM EDT Tue Aug 18 2026

Based on latest wind and wave guidance, decided to issue a Small
Craft Advisory for this afternoon into early tonight chiefly for
our northern marine zones. There will be a brief window of brisk
west-southwest winds ahead of the front which will grow waves into
the 3-5 foot range.

Given that the hazard will begin late in the day and likely will
be accompanied by precipitation, this is not a situation likely to
involve many beachgoers. So, will hold off on a beach hazards
statement for now, but the day shift may elect to issue this if
the hazard begins earlier or precipitation begins later.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...Small Craft Advisory from 5 PM this afternoon to 11 PM EDT this
     evening for LMZ846>849.

&&

$$

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