331
FXUS63 KGRR 181739
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
139 PM EDT Tue Aug 18 2026
.KEY MESSAGES...
- Showers and Storms Likely Tonight With A Few Severe Storms Possible
- Rainfall Chances Return Friday
&&
.DISCUSSION...
Issued at 138 PM EDT Tue Aug 18 2026
- Showers and Storms Likely Tonight With A Few Severe Storms Possible
A cold front will traverse the region tonight, with instability
sufficient for the development of showers and thunderstorms after
approximately 7pm. Upstream VWP from KMPX shows a mid-level jet max
that will progress into the area this evening supporting enhanced
deep layer shear values. This will be in combination with moderate
bouyancy with CAPE values over 1000 J/kg, highest closer to the
lakeshore.
This poses the question of whether any storms will be severe.
DCAPE/low-level lapse rates will be on the marginal side but, with
strong shear/mid-level flow, sufficient for a cold-pool driven
damaging wind threat. In addition, the long hodograph shape favors
the potential for initially discrete storms to pose a hail risk.
REFS and other CAMs indicate some UH swaths favoring potential
supercell development matching this thinking. Hail risk will be
favored northwest of a Muskegon to Cadillac line. The thinking is
then storms will begin to cluster transitioning to mainly a wind
risk. Storms look to clear the area by 1-2am.
- Rainfall Chances Return Friday
Surface ridging keeps things dry much of Wednesday and Thursday
before unsettled conditions return Thursday night. Several
shortwaves are then forecast to progress across the forecast area
within a larger longwave troughing pattern. While as expected
variability in small-scale details exist, a solid signal exists
across ensemble and deterministic guidance for a closed low to
develop across the Central Great Lakes by this weekend.
&&
.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 128 PM EDT Tue Aug 18 2026
A cold front will move across the terminals tonight and there may
be come convective development along/ahead of it this evening.
We`ve included a prob group for this possibility. Otherwise, the
southern terminals may see some IFR fog/cigs late tonight before
improving conditions mid Wednesday morning.
&&
.MARINE...
Issued at 138 PM EDT Tue Aug 18 2026
Have elected to maintain the ongoing Small Craft Advisory, but
refrrain from adding a Beach Hazards Statement at this time.
Forecast soundings indicate southwest flow increasing late afternoon
into this evening ahead of a cold front. Scattered gusts to 25 knots
are possible. However, short range model guidance favors winds being
just low enough to keep waves generally in the 2-4 foot range. The
caveat is thunderstorms are likely 6pm-12am tonight across the
waters. Locally higher waves, large hail, and winds in excess of 34
knots are possible in any thunderstorms.
&&
.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...Thomas
AVIATION...04
MARINE...Thomas
NWS GRR Office Area Forecast Discussion
