textproduct: Grand Rapids

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.