Williamston, Michigan Weather Forecast Discussion

519
FXUS63 KGRR 150712
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
312 AM EDT Mon Sep 15 2025
.KEY MESSAGES...
- Dry with summer-like conditions this week
- Rain chances return by this weekend and persist into next week
&&
.DISCUSSION...
Issued at 312 AM EDT Mon Sep 15 2025
- Dry with summer-like conditions this week
Currently we are seeing pronounced subsidence over the western Great
Lakes per GOES water vapor channels and local soundings. Patchy
radiation fog should burn off rather quickly after sunrise, yielding
clear skies for much of the day. As suggested in the observed
soundings from 00Z Monday, we could be looking some continued weak
moist advection off Lake Huron beneath the subsidence inversion,
which could result in some flat cumulus this afternoon, chiefly over
our eastern forecast area.
We continue to expect very warm maximum temperatures through
Thursday. As noted previously, we have a low (~10 percent) chance of
reaching 90 in spots. It should be noted that deterministic GFS has
had a warm bias during this transitional "shoulder" season and
should not be considered in isolation.
Another thing to look for will be particularly warm temperatures
near the lakeshore. Muskegon already reached 86 degrees on
Sunday. Whenever there is nominal offshore flow with robust
insolation (as we will be expecting over the next few days), this
often keeps the lake breeze pinned near the lakeshore. This in
turn can yield additional mesoscale subsidence/warming immediately
inland from the lake breeze circulation. It is not out of the
question that some of our warmest readings in the coming days
could be immediately inland from Lake MI if the strength of the
offshore wind component is just right.
- Rain chances return by this weekend and persist into next week
The flow aloft remains weak locally with the polar jet displaced far
poleward over northern Canada, indicative of the upper ridging
across the eastern CONUS. Some of the deterministic models are
suggesting that the mean ridge position will retrograde with a
cutoff low forming later this week over the Northern
Plains/Midwest vicinity. The approach of the low is what starts to
relax temperatures as early as Friday and introduces PoPs this
weekend across Lower MI. The majority report among the models
suite is for a slightly later onset of precipitation during the
day Saturday (instead of Friday). The deterministic models suggest
this is tied to the slow expected approach of the cutoff low.
ECMWF ensemble guidance is actually more bullish on QPF in the
Mon/Tue timeframe next week, although the AIFS is more in line
with other guidance in terms of starting QPF this weekend.
To say that predictability is limited for the latter part of this
forecast would be an understatement. The key driver for this
scenario (as currently described) is a weak cutoff low that still
is days away from forming. Its development, let alone evolution,
are highly in question given the anticipated very weak steering
flow aloft. So Friday could just as easily end up being wet again
with subsequent model runs. And, any QPF is still expected to be
on the light side due to little or no contribution of Gulf
moisture.
&&
.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 121 AM EDT Mon Sep 15 2025
No changes to previous thinking. Satellite shows some fog over
northeast Lower MI and expect this to gradually spread south-
southwestward. This gives LAN the best chance of any of the
terminals to experience any visibility restrictions, with MVFR
being the most likely outcome.
Otherwise, mostly clear skies will prevail with a possibility for
a scattered cumulus layer around 5000 ft AGL this afternoon. Have
not mentioned this in the TAFs at this time for the sake of
brevity.
&&
.MARINE...
Issued at 312 AM EDT Mon Sep 15 2025
We will remain in a very loose pressure gradient with light winds
that will predominantly be in an offshore direction. So, as noted
previously, we are likely looking at waves of less than 2 feet right
through the work week.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.
&&
$$
DISCUSSION...TJT
AVIATION...TJT
MARINE...TJT

NWS GRR Office Area Forecast Discussion