102 FXUS63 KGRR 031846 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 246 PM EDT Fri Jul 3 2026 .KEY MESSAGES... - Severe Thunderstorm Threat Today into Tonight - Scattered Thunderstorms Possible This Weekend && .DISCUSSION... Issued at 246 PM EDT Fri Jul 3 2026 - Severe Thunderstorm Threat Today into Tonight This morning`s CAMs have been offering the strongest signal I have seen yet this week for developing convection this afternoon amid a destabilizing atmosphere in our CWA (other than the signal they had for US-10 and north storms on Wednesday). Despite the majority of CAMS struggling to initialize and maintain the northern Illinois convection, most ended up eventually developing either scattered convection or a loosely organized line of storms with a severe wind threat anyway (albeit modeled storms arriving later than reality). It is likely that the storms currently over Lake Michigan will be today`s primary threat, followed by a lull in its wake in the evening, then perhaps a secondary development of scattered storms later this evening or overnight. For this afternoon, concern is for destabilization occuring near and south of I-96 as increasing cumulus field is seen developing. Downdraft CAPE, as has been the case this week, is substantial at 1000-1200 J/kg. Deep layer shear is not great, but wind shear 0-3 km is marginally supportive for a wind-driven QLCS at 25 knots, perhaps could assist the development of mesovorts along the line which would enhance wind damage threat in narrower corridors (tornado threat is low but "gustnadoes" may instead be the mode). - Scattered Thunderstorms Possible This Weekend While some uncertainty remains in the details, scattered thunderstorms with an isolated gusty wind threat remain favored for parts of the day and evening Saturday and Sunday. A deepening upper level trough over the Great Lakes this weekend will maintain instability and a couple embedded shortwaves would provide added lift to weaken any cap. This weekend`s storms will be less of a "ring of fire" progressive MCS setup, and more of a pop up, hit or miss, especially inland away from Lake Michigan during the afternoon Saturday. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 128 PM EDT Fri Jul 3 2026 A complex forecast is underway associated with the timing of thunderstorms today. A line of storms across Illinois is ongoing with additional development out ahead of it. Tracking this line of storms brings it to MKG/BIV around 20z and progressing east with time, which the RRFS/RRFS-MPAS seem to be the only guidance handling this well. Additional development is possible ahead of this line which we are already seeing upstream. Have progressed to TEMPO TSRA for AZO/BTL/JXN where the best environment is ahead of this line and PROB30s for MKG/GRR/LAN have been maintained. Amendments will be made as needed. Storms will be capable of winds in excess of 40 knots, as well as MVFR and lower conditions. Depending on how the afternoon storms evolve, one or more additional rounds of storms are possible after 00z, however confidence is low. Other than that there are some signals for fog to develop near and north of MKG overnight. Southwest winds to 15 knots will continue through the afternoon before winds become light and variable overnight. && .MARINE... Issued at 239 PM EDT Fri Jul 3 2026 Severe thunderstorms crossing southern Lake Michigan this afternoon with gusts 50 knots. Additional scattered storms are possible late this evening or overnight. Waves and swim risk expected to be fairly low this weekend in the absence of storms. A couple scattered storms could develop over the Lake this weekend. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Heat Advisory until 8 PM EDT this evening for MIZ046-051-052- 056>059-064>067-071>074. MARINE...None. && $$ DISCUSSION...CAS AVIATION...Thomas MARINE...CAS

NWS GRR Office Area Forecast Discussion