119
FXUS63 KGRR 210548
AFDGRR
Area Forecast Discussion
National Weather Service Grand Rapids MI
148 AM EDT Fri Aug 21 2026
.KEY MESSAGES...
- Scattered showers/thunderstorms late Friday into Saturday
- Cooler and comfortable Sun-Tue then midweek rain chance
&&
.DISCUSSION...
Issued at 204 PM EDT Thu Aug 20 2026
- Scattered showers/thunderstorms late Friday into Saturday
Increasing low-level moisture returns Friday in a weak warm-air
advection zone, ahead of a cold front that is expected to pass
through with a deepening upper-level trough Friday night into
Saturday morning. Models generally have CAPE values around
500-1500 J/kg with about 20 knots of deep layer shear. Scattered
showers and thunderstorms may develop from late in the day Friday
and through the night into part of the day Saturday. Severe
weather potential appears low at this time, though isolated and
temporary intersections of better low-level shear and CAPE can`t
be ruled out. Average QPF is approximately 0.1 to 0.25 inches, but
realistically rain totals could be more hit-or-miss, with
isolated spots of over an inch possible.
- Cooler and comfortable Sun-Tue then midweek rain chance
Ensemble agreement in the general pattern from the weekend into
mid next week is fairly good, with troughing over the Great Lakes
evolving into a cutoff low over northeastern parts of the region.
This would likely put Lower Michigan into a northwest flow of
somewhat cooler and drier, more comfortable air Sunday and Monday.
There is a slight chance of daytime showers popping up inland on
Sunday during peak heating underneath the upper-level trough.
Somewhat warmer and more moist air then returns Tuesday ahead of
the next cold front with a chance of showers/storms from late Tue
into Wed.
&&
.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 148 AM EDT Fri Aug 21 2026
As noted previously, there is a non-zero (10-20 percent) chance of
MVFR visibility restrictions in fog towards sunrise at AZO, BTL,
JXN.
The only other change was to introduce a PROB30 thunder group at
all of the terminals. The relative timing for this among the
terminals is atypical and is explained below.
Earlier thinking was that the threat for showers or thunderstorms
would gradually spread from west to east today. Now, however, high
resolution convection-allowing model guidance suggest there will
be a separate north-axis of scattered shower and thunderstorm
development commencing this afternoon centered somewhere between
US-131 and US-127. This axis of precipitation is displaced east
of the main area of showers and thunderstorms expected across far
western Lower MI and much of Lake MI. That is why we currently
have GRR, located between these two regions, with the overall
latest onset of a PROB30 group.
There is some question regarding the amount of conditional
instability farther east and it could be that SHRA is the end
result instead of TSRA, especially at LAN. We will definitely be
revisiting this for the 12Z package. Regardless, it should be
noted that there will be a continued chance for both SHRA and TSRA
after 00Z at the eastern terminals. This detail was excluded in
this update at those terminals for the sake of brevity.
&&
.MARINE...
Issued at 204 PM EDT Thu Aug 20 2026
Low waves and low swim risk through early Friday morning.
Southwest winds increase on Friday 10-15 knots gusting to 20-25
knots, particularly north of Whitehall, where a small craft
advisory and beach hazards statement may be needed for the
afternoon. Then behind a cold front and amid northwest flow of
cooler air during Saturday and Sunday, hazardous waves and
currents are likely for swimmers and small craft.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.
&&
$$
DISCUSSION...CAS
AVIATION...TJT
MARINE...CAS
NWS GRR Office Area Forecast Discussion
