259 FXUS63 KGRR 102344 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI Issued by National Weather Service Las Vegas NV 744 PM EDT Wed Jun 10 2026 .KEY MESSAGES... - Thunderstorm Chances Increase This Evening into Tonight - Some Significant Severe Weather Possible Thursday Night - Heat Risk May Continue into Thursday && .DISCUSSION... Issued at 307 PM EDT Wed Jun 10 2026 - Thunderstorm Chances Increase This Evening into Tonight SPC continues to include our region in a Slight Risk for severe weather tonight. Prior to the main window of concern (6pm-12am), scattered showers and thunderstorms may pop along the lake shadow boundary this afternoon in the midst of very high SBCAPE values of 3000-4000 J/kg. This mesoscale boundary may act as a localized source of lift along with the fuel in place to pop some thunderstorms and provide isolated gusty winds and possibly hail as well. An isolated severe-level storm is possible but would be the exception to the rule this afternoon. A second mesoscale feature we`re watching is the residual outflow boundary from earlier convection across NE Wisconsin that has now pushed southeast into central Lake Michigan. This feature could act as another localized source of lift to interact with thunderstorms slated to move over the lake this evening. The setup later this evening will feature a high CAPE but low shear environment, with our primary focus centering on the potential for an existing complex of storms moving into northern Illinois to tap robust instability ahead of it and be propelled over Lake Michigan later this evening by a strengthening low level jet of 40-50 kts shown by the RAP13. This complex of storms, while encountering an environment that has deep layer shear values of only 20 kts across our region, conversely will be experiencing DCAPE values exceeding 1000 J/kg. This factor, combined with such high PWAT values (1.75"+), will help compensate the lack of significant shear by at least locally boosting a damaging wind risk (60 to 70 mph gusts possible) for areas especially along and west of a RQB-GRR-BTL line this evening and tonight. Hail may also develop, and a deeper dive into mid level lapse rates reveals that the setup is better than initially expected, with HRRR soundings revealing 700-500 mb lapse rates of 8.0 C/km across SW Lower MI this evening. These same soundings are not overly concerning for a tornado risk tonight, however, given the unimpressive 0-3 km wind fields in place. Subsequent non-severe shower and thunderstorm development late tonight into early Thursday morning is certainly possible as a LLJ remains in place through about 06z-09z Thursday. However, the signal from the 12z HREF 6 hr QPF PMM is not strong, indicating that repetitive storm development getting draped across the region is less likely to occur than not. As such, only localized / nuisance short term flooding is possible tonight mainly across the traditionally susceptible urban and poor drainage areas. This does not look to be a major risk at this time but we will monitor. - Some Significant Severe Weather Possible Thursday Night A greater risk for severe weather will develop later Thursday, especially in the 7pm-12am time frame, when both damaging to destructive wind gusts and tornadoes will be possible. Prior to this window, generally dry conditions are expected during the day Thursday. Upper level jet divergence moves in later in the day along with a robust mid level jet of 50-70 kts moving over Lake Michigan in the 8pm-11pm time frame. A LLJ around 50 kts is likely to develop. 12z HRRR soundings at GRR around 00z Thursday evening show impressive low level shear values with strongly curved hodographs. For example, the 0-3 km shear is shown to be over 35 kts. The HREF fixed layer significant tornado parameter is showing values of 3 to 4 along and west of US 131 tomorrow night, which are quite high. The HRRR is indicating potential for values even higher than that. While the 12z CAMs generally support an organized line of storms as the mode, mesocyclonic portions of the line may develop and the threat for embedded tornadoes is higher than normal compared to most of our events. Subsequent model guidance tonight into tomorrow morning will help provide clarity on the magnitude of this tornado threat, but we are concerned. The broader risk to the region will be damaging wind gusts and some swaths of more destructive gusts of 75 mph or greater possible. SPC has us in the hatched wind gust threat, which is well warranted given the robust wind field in place via the soundings. DCAPE values of 1000-1200 J/kg will be present ahead of the anticipated line of storms moving over the lake late evening. SBCAPE values of 2000-3000 J/kg should help support storm intensity beyond the lake crossing, so a rapid weakening is not expected at this point but a downward trend is likely by the time storms reach US 127. All told, the potential is certainly there for Thursday night to be an active night. Between tonight and Thursday night, power outages are likely to occur in some areas. - Heat Risk May Continue into Thursday Given the delay in storm arrival tomorrow, we may be looking at another day of heat index readings pushing the mid 90s to near 100 especially east of US 131. Will evaluate the need for potential heat headlines for tomorrow. && .AVIATION /00Z TAFS THROUGH 00Z THURSDAY/... Issued at 744 PM EDT Wed Jun 10 2026 Thunderstorms remain the main concern this evening. The line of storms is passing through eastern Michigan now, and will approach KLAN and KJXN between 01 and 02Z. After the main line, there is a 30 percent chance of showers and thunderstorms over the terminals, with best chances between 04 and 06Z for KMKG and between 04 and 11Z for remaining TAF sites. IFR to LIFR expected with these storms along with damaging west winds with gusts between 35 and 45 kts. Showers and thunderstorms should lift out of the region between 10 and 12Z Thursday morning, with MVFR cigs thereafter. Another system will make its way into the region Thursday evening, returning chances of showers and thunderstorms. Otherwise, expect gusty southwest winds with gusts between 20 and 25 kts. && .MARINE... Issued at 307 PM EDT Wed Jun 10 2026 Waves of 2 to 4 feet are likely into this evening and early Thursday, likely just staying shy of SCA criteria. However, Thursday night looks hazardous ahead of a line of storms with SW winds increasing and a SCA looking likely. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Heat Advisory until 8 PM EDT this evening for MIZ071>074. MARINE...None. && $$ DISCUSSION...Hoving AVIATION...Soulat MARINE...Hoving

NWS GRR Office Area Forecast Discussion