Marine Weather and Tide Forecast for Copper Harbor, MI

SHORT TERM /THROUGH SUNDAY/
Issued at 252 AM EDT Sat May 17 2025

Expect light rain showers to dominate the northern tier of the U.P.
today through the rest of this weekend as a vertically-stacked low is moving eastward along the southern shore of Lake Superior early this morning. The wrap-around moisture from the low will continue light rain showers across the area, mainly over the traditional northwest wind lake effect belts. While the light rain showers won't bring too much liquid to even these areas, the wrap around moisture from the low will keep things moist across the area today. In addition, with weak warm air advection moving in via the wrap around moisture, expect high temperatures today to generally get into the 50s, coolest near the Keweenaw and warmest south central and far east.

A secondary shortwave low moves into the area as the initial low leaves for the St. Lawrence Seaway tonight and the Canadian Maritimes/Maine on Sunday. This will keep upslope showers going over the northern tier of the U.P. tonight into Sunday, although liquid amounts from the (mostly) light rain showers will be only a few hundreths. We could see some marginal lake enhancement take effect as 925mb temperatures cool to around -4C over the north wind lake effect belts late tonight into Sunday. In addition, there is a chance that we could see the light rain showers transition over to a very light wintry mix late Sunday as highs over the northern tier are expected to not get above the upper 30s. Regardless, not expecting any impacts if wintry precipitation does occur as the ground should be too warm for accumulations.

LONG TERM /SATURDAY THROUGH THURSDAY/
Issued at 220 PM EDT Fri May 16 2025

Upper level low traversing the area tonight will continue to shift east through the Great Lakes this weekend while mid-level ridging shifts east through middle America. At the surface, low pressure will also progress eastward tonight then exit into Ontario by Saturday afternoon followed by a high moving into Hudson and James Bay. This transition will allow any showers and thunderstorms that develop this afternoon/evening to end tonight for our forecast area, then wrap around showers to press downwind into Upper Michigan's north through the day Saturday while a cooler airmass pours into the area. By afternoon, areas in the south-central should begin drying out. Sunday, showers should begin to diminish in the north as the effects of the ridging over the region becomes more dominant. Dry conditions look to hold fast over the area into at least the middle of next week, at which point a shortwave dropping southeast just east of Lake Superior Monday night begins to slowly phase with an upper level low pressing eastward through the mid-Mississippi River Valley. By Wednesday night/Thursday, the resultant upper level low may provide enough synoptic support for another round of rain.
Confidence is low though given the uncertainty in this feature and it's surface reflection's position.

Temperatures will slide this weekend as the cooler airmass from the north expands into the Upper Great Lakes. Highs Saturday in the 50s to low 60s will feel warm compared to Sunday's widespread 40s. We should warm afterwards back into 50s to low 60s for most of the coming week. Overnight lows will follow the same trend; expect 40s tonight with 30s afterwards, save near 40 by the lakeshores. Some high 20s can't be ruled out in the interior west half Sunday and Monday nights. The effect of the dry air, temperatures, and daytime mixing next Monday-Wednesday should be enough to support afternoon RH values falling into the 30s and 20s, potentially into the teens, in interior locations while gusty northeasterly winds near 20 mph mix down to the surface.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/
Issued at 102 AM EDT Sat May 17 2025

Expect all sites to fall to MVFR/IFR by sunrise, continuing through Sat as a low pressure system tracks E overhead. Some LIFR is possible at CMX and also IWD, but left mention out of the IWD TAF for now as confidence is too low. Periods of showers are likely across the UP with moisture wrapping around this system, but model guidance is not in great agreement on timing.

MARINE
Issued at 220 PM EDT Fri May 16 2025

Low pressure will gradually shift east through Lake Superior through Saturday. Ahead of the low this afternoon/evening, strong low level jet will be capable of producing winds near 30kts. There is some gale potential, but the increasing low level temperature profile over the lake this afternoon will create greater stability and make it difficult for these strong winds to mix down to the surface. That being said though, showers and thunderstorms that develop later this afternoon/evening could provide enough extra support for isolated pockets of stronger winds over eastern Lake Superior, particularly if a stronger storm develops. If a strong storm gets organized, 50+ knot winds will be possible.

Winds shift to the northwest Saturday afternoon/evening in the lee of the low, allowing for a period of cold air advection. This looks to create another opportunity for near 30kt winds, but the profile isn't expected to be cold enough for significant lake-induced instability, so gale potential is low (<25%). Winds look to lighten through the day Sunday, falling back below 20kts lake-wide by afternoon. A tighter pressure gradient looks to develop Monday and Tuesday, mainly in western Lake Superior thanks to a high shifting through Hudson Bay and a low moving east into the middle-Mississippi River Valley. This may produce northeast winds near 30kts going down into Duluth Harbor.

MQT WATCHES/WARNINGS/ADVISORIES
Upper Michigan...
None.
Lake Superior...
None.
Lake Michigan...
None.