Marine Weather and Tide Forecast for Grand Beach, MI

UPDATE
Issued at 735 PM EDT Fri May 15 2026

Several thunderstorms have already developed and rapidly intensified upstream across SW Iowa into SE Minnesota. Models still agree on further development as they move east and the LLJ ramps up. What remains uncertain is what condition they will be in by the time the reach our western counties later tonight. No major changes to forecast for the time being, but trends will be observed into the overnight hours.

DISCUSSION
Issued at 152 PM EDT Fri May 15 2026

An active forecast lies ahead but is one muddied by elements of uncertainty. Low pressure over the Manitoba/Quebec line has a cold front spreading through the Central Plains this afternoon.
This cold front is the focus for showers and thunderstorms later today well to our west. This activity tracks east overnight arriving to southern Lake Michigan just before daybreak Saturday. High resolution guidance varies notably with how this progresses. I have some preference to members that offer a weakening trend as the line reaches southern Michigan. This occurs as the primary 500-mb jet (75 kts) is focused well north across the UP during the early- morning hours. Rising heights are noted at 500mb as well. In contrast, there is a blossoming 40-knot 850mb jet over northern IL. A narrow corridor of 1,000 j/kg MUCAPE paired with 30 knots of bulk shear yields a non-zero chance for early morning isolated severe storms, but the lack of upper-level support may be the greatest limiting factor.

As this area of showers and storms tracks east after daybreak Saturday, or perhaps bypasses north, the aformentioned cold front is now stalled east-west across the forecast area and will be the focus for scattered afternoon thunderstorms. Boundary- parallel flow increases the risk of training (backbuilding) thunderstorms which can result in instances of flooding. Our airmass will be plenty warm and humid, with dew points in the 60s and PWATS in excess of 1.6"; high confidence in rain rates of 1" per hour Saturday afternoon. Because of lingering cloud cover, it will take time for instability to rebuild, struggling to reach 1,000 j/kg paired with about 30 knots of shear. Overall, a marginal risk of isolated damaging wind gusts Saturday afternoon, with heavy rainfall the greater concern.

This front is kicked poleward Saturday night as the warm sector amplifies in response to a trough digging in over the Four Corner region. Increasing confidence in a mainly-dry Sunday as showers and storms become focused elsewhere in the Midwest and Upper Great Lakes, but will hold on to NBM populated 20-30% POPs. Monday still appears to be the "best" day overall for severe thunderstorms, but timing remains uncertain. A prefrontal shortwave could be the focus for afternoon storms, well ahead of the cold front that moves through overnight. In the afternoon, steep low-level lapse rates present a damaging wind gust concern from thunderstorms while a unidirectional wind profile, along with elevated cloud bases, limits the overall tornado concern.
Boundary-parallel flow will continue to pose a risk for heavy rainfall in this warm, humid environment.

The cold front slowly moves through Tuesday and Wednesday, its eastward progress slowed by strong subtropical high pressure. Thus, a rainy and stormy start to the week that lingers into at least Wednesday before high pressure pushes in for the late-week period.

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/
Issued at 735 PM EDT Fri May 15 2026

Several thunderstorms already developing across southwestern IA into southeast MN with some producing large hail. As they move east, the individual cells should congeal into a line of storms and move across IA/IL, reaching KSBN near/after 9Z. Much uncertainty remains as to the strength/coverage of these storms as they outrun the best support. Have left the tempo thunder at KSBN and prob30 at KFWA later to handle what may be left over.
Even more uncertainty exists with evolution of additional showers and storms near the stalling frontal boundary. CAMs have their own ideas, causing concerns on exact impacts. Greatest chances will be at KFWA, but for now removed the predominate thunder mention and opted for prob30 in the afternoon. Could need to go back to predominate or tone down even more depending on trends late tonight into Saturday morning.

IWX WATCHES/WARNINGS/ADVISORIES
IN...None.
OH...None.
MI...None.
MARINE...None.