926
FXUS63 KGRR 202208
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
608 PM EDT Fri Sep 20 2024

.KEY MESSAGES...

- Chance of showers and storms this evening

- Another chance for showers and storms Saturday night

- Likely showers and possible thunderstorms Sunday and Sun Night

- Unsettled next week, more seasonal temperatures, some rain
  possible

&&

.DISCUSSION...
Issued at 337 PM EDT Fri Sep 20 2024

- Chance of showers and storms this evening

Certainly not widespread rain this afternoon, but a few areas
picked up some measurable amounts over Southern and Western Lower
Michigan. Additional rainfall is expected this evening as a cold
front moves in off of Lake Michigan. Dew points have come up into
the middle to upper 60s across Western Lower Michigan with MUCAPE
values now pushing towards 2000 j/kg. Around 1100 j/kg of DCAPE
is in place currently, so previous thinking that the possibility
of strong to locally severe storms exists remains valid. Deep
layer shear per the SPC meso page shows 40 knots nosing into
areas near and southwest of Grand Rapids. We continue to think
that showers and storms will blossom potentially near MKG/BIV and
GRR and then move to the south and east as we head through the
evening. Wind remains the biggest threat with hail being a notch
below that threat. The most widespread precipitation is expected
this evening, becoming scattered overnight before moving east by
09Z or so.

- Another chance for showers and storms Saturday night

An upper shortwave trough moving from the Northern Plains into
Canada on Saturday will push a cold front towards Lower Michigan
from the west Saturday night. The front will be moving well away
from its source of synoptic scale lift associated with the wave
aloft. The fact that the front will be coming through in the
overnight period, that its running away from the upper wave and
that it will be moving into dry air does not bode well for
widespread precipitation. We may see some scattered showers late
moving towards the Western Lower Michigan shoreline, but not
expecting much more than that.

- Likely showers and possible thunderstorms Sunday and Sun Night

As noted before, ensembles have been very honed in on a solution of
shower and thunderstorm chances increasing during the day Sunday and
remaining high through Sunday night. A sharp upper trough will break
cyclonically over the extreme northwestern Great Lakes region. So,
although the strongest upper forcing will be displaced significantly
to our north, there will be an expansive surface front extending
from the Western Great Lakes into the Central Plains (as noted
previously). Per latest SPC guidance, severe weather is not
anticipated given the unfavorable placement of upper forcing, but
anomalously high PWATs certainly support localized heavier rainfall.

As also noted previously, there is relative uncertainty in maximum
temperatures Sunday due to questions about thunderstorm evolution
and associated diabatic cooling effects. To illustrate, the ECE
ensemble features an interquartile (25-75 percentile) range of
temperatures between 75 and 84 degrees F, with highs in the lower
70s also quite plausible.

- Unsettled next week, more seasonal temperatures, some rain possible

No changes to previous thinking. Rain chances are pretty well
assured Monday into early Tuesday. After this, we are looking at
increasingly unphased flow aloft. By mid-week there is evidence of a
southern stream cutoff low originating in the southwestern CONUS
region amidst very light flow and whose evolution is highly
uncertain. Predictability with this type of setup is extremely low
in general and the offered forecast is a bit of a hedge with low
precipitation chances each day. What is more certain is a reduction
in temperatures closer to seasonal normals during the course of the
week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 608 PM EDT Fri Sep 20 2024

Primarily VFR conditions this evening will gradually deteriorate
to IFR/LIFR overnight due to development of fog and low clouds.
These conditions will linger through mid morning Saturday before
gradually improving to VFR. Winds overnight and Saturday will be
rather light aob 10 kts.

&&

.MARINE...
Issued at 337 PM EDT Fri Sep 20 2024

Waves are near 2 feet at most of the nearshore buoys associated
with some stronger winds last night. Also, winds this morning with
a line of showers and storms helped build a wave field. The winds
are expected to be fairly light, 15 knots or less, tonight into
Saturday, so waves should settle below 2 feet. Saturday night the
southerly winds increase a bit ahead of our next front, with
conditions that could approach Small Craft Advisory criteria.
Sunday is our next good chance for winds and waves as north flow
takes hold behind the cold front. 2-4 footers look plausible on
Sunday with the chance for high than that.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

DISCUSSION...Duke/TJT
AVIATION...Laurens
MARINE...Duke