150 FXUS63 KGRR 091737 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 137 PM EDT Sun Aug 9 2026 .KEY MESSAGES... - Strong to severe storms most likely through this evening - Residual small shower/storm chances Monday through Wednesday - Mainly dry and seasonable Thursday and into next weekend && .DISCUSSION... Issued at 137 PM EDT Sun Aug 9 2026 - Strong to severe storms most likely through this evening After above average uncertainty over the last couple of days, it seems that the atmosphere is finally showing its hand more now. A line of strong to marginally severe storms has organized over Wisconsin, and is starting to move over the lake just before 1 pm EDT. This line of storms is associated with a MCV from earlier storms, that is riding the instability gradient aloft ahead of a stalled out sfc front over Northern portions of Illinois and Indiana as of early this afternoon. The favored area for the strongest storms with this line will be over the far SW corner of the forecast area toward Van Buren and Kalamazoo counties. This is where the best instability resides over the area where 2000 J/kg of MUCAPE sneaks up. Deep layer effective shear is also sufficient for some organized storms sitting at around 30 knots this afternoon. Mid level lapse rates and CAPE thicknesses are not too exciting for large hail potential. Gusty winds look like the biggest threat with unidirectional wind profiles and the linear nature of the line. Areas across the far SW corner of the forecast area do have a little better chance of an isolated tornado being closer to the warm front and the better low level wind shear. What boundaries that are left behind from this afternoon are likely to dictate the evolution of the storms for tonight. The reason this is important is that the larger scale model solutions all have a low level jet ramping up to our SW by mid evening, helping to fire more convection along the stalled out front/outflow boundary. We are expecting that most of any severe threat with the next round of storms will likely stay south of our forecast area. We do remain concerned however that heavy rainfall will remain possible. There are signals in the various model solutions and suites that there could end up being a heavier localized band somewhere over our far southern counties, or potentially into Northern Indiana. If the confidence goes up it will be in our area, we may try to issue a Flood Watch. - Residual small shower/storm chances Monday through Wednesday We do expect that most of the widespread convection after anything tonight will likely stay farther south across Indiana and Ohio. That is where the effective front and baroclinic boundary ends up. We do not see the degree of heat or instability south of the boundary, but enough moisture and weak short waves in the steady mid level flow will keep shower/storm chances present for the area for mainly afternoon and evenings Monday through Wednesday. Lake breezes and any other mesoscale boundaries will be the main focus for any diurnal development. - Mainly dry and seasonable Thursday and into next weekend The models have been showing that there is a good chance we dry out for at least Thursday and Friday, and maybe even into next weekend. We will see a upper ridge across Canada to our NW, that will support sfc ridging to our north. This will bring in a drier air mass from Canada with seasonable temperatures. Eventually a SW flow aloft tries to build over the country, which would push the warmer air mass toward the area, and maybe convection along the periphery around it. There is too much uncertainty at this time to try and get too detailed with that at this time. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 137 PM EDT Sun Aug 9 2026 Main change this update was to be a bit more specific about thunderstorm timing prior to 22Z now that we see the line of storms bearing down. Main concern is strong/marginally severe convective wind gusts, which have been mentioned in a PROB30 group. Model guidance shows another round of showers and storms possible after this line of storms. Thus, there remain multiple a PROB30 groups in the TAFs due to significant uncertainty about future storms because their characteristics depend on what this initial round of storms does. Many of the TAFs end at 10/1200Z IFR/LIFR ceiling restrictions. However, this is for the sake of brevity; in reality, ceilings should gradually improve from 12Z to 18Z on Monday. && .MARINE... Issued at 137 PM EDT Sun Aug 9 2026 The best chance for marine headlines is actually early in the forecast period from now through tonight. This is the case as we have a low level jet aloft moving overhead. This wind is not expected to be realized at the surface with it being warm air advection over the relatively cooler waters of Lake Michigan. We have some 3 ft waves being mentioned in the northern zones, which should suffice. We expect winds then to stay below 20 knots through most of, if not all of this coming week. There looks to be some wind cores around 20 knots at night, associated with a weak low level jet. These winds should not make it to the sfc either. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...None. && $$ DISCUSSION...NJJ AVIATION...TJT MARINE...NJJ

NWS GRR Office Area Forecast Discussion