120
FXUS63 KGRR 190716
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
316 AM EDT Tue Aug 19 2025

.KEY MESSAGES...

- Showers and Thunderstorms Through This Afternoon

- Dry Wednesday through Friday

- Cooler with a Few Showers Possible This Weekend

&&

.DISCUSSION...
Issued at 316 AM EDT Tue Aug 19 2025

- Showers and Thunderstorms Through This Afternoon

Little change to previous thinking. The surface low that was
over southern WI early this morning will shift east across Lower
MI today ahead of an amplifying upper shortwave trough/PV maximum
that will be east of Lower MI by early tonight. We are starting to
see an increasing coverage of showers over Lake Michigan
associated with our next round of precipitation. This area of
shower development is likely well represented by the GRR VAD Wind
profile, which shows strongly veered low level flow -
specifically, brisk southeasterly near-surface winds that become
southwesterly at 25 knots at 1500m AGL. This belt of isentropic
ascent should be maintained overnight as we continue to experience
augmentation of the nocturnal low level jet via the approaching
upper PV max. This favored area will remain east of the slowly
approaching surface low, meaning that chance PoPs or greater will
be needed through at least noon for the US-127 corridor.

- Dry Wednesday through Friday

The previously mentioned shortwave trough will continue to traverse
the rest of the Great Lakes through Wednesday. Subsident northwest
flow in its wake will induce surface pressure ridging across the
region with substantially drier conditions and light winds through
Friday. Expect afternoon high temperatures to incrementally climb
each day from around 80 Wednesday to the lower/mid 80s Friday.
This is indicative of a persistent airmass that experiences
cumulative deepening of the boundary layer with each successive
day. Lows, on the other hand will be a bit more consistent,
staying generally in the upper 50s.

- Cooler with a Few Showers Possible This Weekend

Confidence is relatively good for this time range that an upper
shortwave trough associated with a closed low over Canada will cross
the western Great Lakes on Saturday. As noted previously, the air
will be dry, but at least some chance PoPs (20 to 30 percent) will
be warranted, especially north where PV forcing will strongest.
What`s more certain is a substantial drop in temperatures beginning
Sunday associated with reduced thicknesses in the vicinity of the
upper low. Highs in the lower 70s and even upper 60s can be
expected during this time. Both Monday and Tuesday mornings could
see temperatures drop into the upper 40s in spots.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 143 AM EDT Tue Aug 19 2025

Changes this update were to refine timing of thunderstorms and to
increase southeasterly winds in the short term ahead of the
approaching surface low. Thunderstorms have been having a difficult
time reaching our eastern terminals before dissipating but
chances for this happening should increase later this morning as
the surface low and associated isentropic ascent shift eastward
into Lower Michigan. We expect showery precipitation to redevelop
across much of Lower MI as this happens. Forcing for upward
vertical motion will be greatest at the I-94 terminals (AZO-BTL-
JXN) and at LAN. This will foster convective destabilization and
be supportive of thunderstorm development later today. Model
guidance suggests that showers and thunderstorms should be quite
unstructured and popcorn-like in coverage; this is common in
these types of isentropic ascent regimes. This does make timing at
terminals difficult, however, so have elected to use PROB30 quite
extensively after 12Z.

Another complicating factor is lowering ceilings in the vicinity
of the approaching low. IFR seems likely areawide later today.
Western terminals such as MKG...AZO...GRR could see a transition
to MVFR or even VFR around or after 00Z tonight, but did not
include this for the sake of brevity.

&&

.MARINE...
Issued at 316 AM EDT Tue Aug 19 2025

We saw brisk southeasterly winds earlier this morning that were
likely affecting the marine area with offshore wave growth that was
even depicted in the WW3 guidance. Winds have since decreased a bit,
so no concerns in the immediate term. However, as noted previously,
we do see a signal for a burst of winds down Lake Michigan beginning
this afternoon/evening. A couple of factors will mitigate the
strength of this surge. First, we will probably have extensive
cloudiness over Lower MI which will inhibit insolation and
associated surface pressure falls over land that are needed to
locally enhance the nearshore gradient. Second, the strength of the
high itself is a bit in question.

The period of concern is late afternoon/early evening so would want
to message accordingly no later than this morning. Will take a look
at some of the 06Z high resolution (and other) guidance as it
rolls in and re-visit this shortly. Neighboring WFOs are
similarly adopting this strategy.

&&

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

&&

$$

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