266 FXUS63 KGRR 210717 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 317 AM EDT Fri Aug 21 2026 .KEY MESSAGES... - Scattered showers/thunderstorms today into Saturday - Cooler and dry Sunday through Tuesday, midweek rain chances && .DISCUSSION... Issued at 317 AM EDT Fri Aug 21 2026 - Scattered showers/thunderstorms today into Saturday The only change in thinking from 24 hours ago is that Convection Allowing Models (CAMs) show widely dispersed convective cells developing this afternoon over much of southwest Lower Michigan. Please see the AVIATION section of this forecast discussion for more specific details. We expect precipitation coverage to be a bit more widespread starting early tonight with additional convection approaching from Lake MI. The latest SPC SWODY1 even has a marginal risk for wind along our lakeshore. This would be predicated on already existing organized multicell clusters making it into the area. Some Machine Learning guidance, such as the NCAR suite, supports this idea. Neither the thermodynamic environment nor the kinematics over western Lower MI look particularly impressive for convective maintenance, so would expect a fairly quick demise of any organized convection that makes it this far east. Models have been fairly consistent with a gradual west to east shift of precipitation Saturday that mostly exits the area by evening. It appears that the severe threat will remain east of our area during this time. - Cooler and dry Sunday through Tuesday, midweek rain chances By late Sunday, the forecast area will be under deep, almost due northerly flow with surface ridging. This is a dry pattern, but as noted previously, can`t completely rule out isolated pop up diurnal showers due to colder air aloft. From Tuesday onward, flow aloft becomes more northwesterly and an upper PV max in this flow will provide enough forcing for upward vertical motion to reintroduce precipitation chances Tuesday night into Wednesday. After this, mean troughing aloft continues with unsettled conditions the rest of the work week. At this point, there will be limited predictability regarding the timing of subsequent precipitation chances. && .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 317 AM EDT Fri Aug 21 2026 Although conditions will admittedly be marginally, we still opted to issue a Small Craft Advisory and Beach Hazards Statement for our northern two marine zones this afternoon into early evening. There will be a risk for longshore currents and locally higher waves, even though the overall wave field may be better described by sub-advisory criteria. Nonetheless, this messaging lines up well with APX`s immediately to the north of these zones. There is another reason for erring on the side of caution here that is not meteorologically based. We are looking at a Friday in the summertime that may be the last Friday before many people return to work or school. Consequently, there could be greater than normal willingness to be exposed to a marginal risk; this gives a net result of significant vulnerability. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Beach Hazards Statement from 2 PM EDT this afternoon through this evening for MIZ037-043. MARINE...Small Craft Advisory from 2 PM this afternoon to 8 PM EDT this evening for LMZ848-849. && $$ DISCUSSION...TJT AVIATION...TJT MARINE...TJT

NWS GRR Office Area Forecast Discussion