478
FXUS63 KGRR 170156
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
956 PM EDT Fri Aug 16 2024

.KEY MESSAGES...

- Potential for Thunderstorms into Saturday

- Mostly dry and cool next week

&&

.UPDATE...
Issued at 956 PM EDT Fri Aug 16 2024

Solid line of showers with embedded thunder, associated with the
next shortwave rotating around the upper low, continues to move
slowly east across Lk MI. Other scattered cells have been
developing ahead of the main line with MLCape around 1500 J/KG.
Deep layer shear remains limited at only about 20-25 kts so not
much organization to this line. Main impact should be locally
heavy downpours and perhaps a few cloud to ground lighting
strikes.

&&

.DISCUSSION...
Issued at 252 PM EDT Fri Aug 16 2024

- Potential for Thunderstorms into Saturday

The mid level low in northern WI will slowly drop down into lower MI
on Saturday. The deeper cyclonic flow associated with this
feature along with the cooling aloft will support a continued
environment favorable for shower formation. PWAT`s remain
elevated with values up around 1.5 inches based off of the
ensemble plots. Ensemble qpf plots suggest most locations will be
seeing some rain during this period as most members are showing
qpf. We also maintain some instability through the period so a
risk for thunderstorms exists. The convection that has persisted
this morning along the lakeshore was weakening. Overall the
forcing remains weak this afternoon but decent surface based CAPE
is expected with inland values up over 1000 J/kg. There will also
be weak low level convergence inland. Based on this, any renewed
convection this afternoon should be further inland with the
coverage forecast to be isolated/scattered. Early this evening a
stronger mid level vort max moves in and the lift associated with
this feature is expected to lead to a an area of storms tracking
eastward into the CWA. As for the severe thunderstorm risk, the
core of stronger mid levels winds stays south and west of the CWA
which will limit the deep layer shear and degree of storm
organization. As for Saturday, much less instability is forecast.
However with the cold temperatures aloft and deep cyclonic flow,
any heating will generate showers and a few storms. We will
maintain the risk for storms in the forecast into Saturday.

- Mostly dry and cool next week

A few lingering showers on Sunday will exit the area as the upper
low in our neck of the woods finally moves off to the east. In it`s
wake, Lower Michigan will spend the bulk of the upcoming week under
the influence of northwest flow, thanks to an upper level ridge that
reasserts itself over the plains. What this means for us is a cooler-
than-normal airmass (highs pleasantly in the 70s), low humidity,
plenty of sunshine, and relatively light winds. Overall, a very
tranquil week appears to be on track for us.

There are some indications that a shortwave comes over the ridge
sometime in the 2nd half of the week and could produce a few showers
for us, but this is far from certain, and regardless is not likely
to "ruin" any days. Of note, overnight temperatures especially early
in the week are likely to be fairly chilly and will be dropping down
into the 40s - especially in rural/northern areas.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 803 PM EDT Fri Aug 16 2024

Thunderstorms approaching from the west at 00Z will impact the
terminals between roughly 02Z and 06Z. Brief vsbys reductions to
IFR in heavy rain and a local wind gust up to 30 kts are possible
if any of the heavier cells pass over terminal.

Even after those storms pass through later tonight, some
lingering showers will persist overnight into Saturday. Cigs
trending into the MVFR or possibly IFR category for late
tonight/Saturday morning, then gradual improvement to cigs
expected after 18Z Saturday. Cigs likely recovering to mostly VFR
levels by 21Z, although scattered showers and a few tstms
remaining possible, mainly inland from the Lk MI shoreline.

&&

.MARINE...
Issued at 252 PM EDT Fri Aug 16 2024

The winds and waves have decreased this afternoon as expected but
they will increase again on Saturday, especially later in the day.
Based on recent wave model forecasts, I decided to issue a small
craft advisory and beach hazards statement for Saturday south of
Grand Haven. It`s looking like the waves will reach 4 feet there
tomorrow.


&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from 8 AM EDT Saturday through Saturday
     evening for MIZ056-064-071.
MARINE...Small Craft Advisory from 8 AM to 10 PM EDT Saturday for
     LMZ844>846.

&&

$$

UPDATE...Meade
DISCUSSION...MJS/AMD
AVIATION...Meade
MARINE...MJS