Marine Weather and Tide Forecast for Superior, WI

DISCUSSION
Issued at 317 PM CDT Thu May 15 2025

Rest of Today - This Evening:

Deep low pressure centered over southeastern North Dakota will largely spin in place through this evening as an occluded front lifts northeast through the Northland. Along and north of this front, a narrow warm sector has developed in central Minnesota down to the Twin Cities metro area, with MLCAPE of 1000-2000 J/kg and effective bulk shear of 35 to 45 kt. The 18Z MPX sounding depicts a favorable hodograph for development of low- level rotation, with 0-1km SRH of 211 m^2/s^2 and 0-3km SRH of 306 m^2/s^2. Scattered pre-frontal discreet storms have developed north of the front in central MN where this environment will be favorable for locally damaging wind gusts, large hail (up to 1.5 inches), and a tornado or two through the afternoon.

Eventually as the front moves northward, hi-res model guidance favors storms in the Twin Cities metro area growing upscale into a line of storms (QLCS) and tracking northeast through the Northland, first in central and east-central MN by late afternoon and then through north-central MN, northwest WI, and the Arrowhead this evening. As the afternoon progresses, the MLCAPE should build to 1800-2800 J/kg from the Brainerd Lakes to northwest WI by late afternoon/early evening while shear remains strong, as well as 0-1km and 0-3km SRH continuing to remain favorable for low-level rotation and potential for a couple tornadoes with any mesovortices that develop in the lines of storms that move through. Damaging wind gusts of 60-70 mph become increasingly likely with the line of storms later this afternoon into evening in this strongly sheared and buoyant environment, particularly in bowing segments of the storms.
Large hail up to half-dollar size is also possible, particularly in northwest WI through early evening before better lapse rates aloft diminish. The steeper lapse rates in central/northeast MN tend to remain ahead of the line of storms, so while large hail (quarters) are still possible there, the threat is lesser than in NW WI.

By mid-late evening, instability should begin to wane as storms move into the Iron Range, northern MN, and the Arrowhead, so the severe threat should gradually drop off towards the 8-10pm timeframe as storms move into those locations. Therefore, expect the tornado potential to drop off by the time storms reach the Iron Range, with the severe wind and hail threats dropping off as storms near the International Border.

Tonight - Friday night:

Expect a lull in precipitation behind the line of storms later this evening and tonight before more widespread, lighter rainfall overspreads the Northland as the low-pressure treks east across northern MN and Lake Superior. Steeper low-level lapse rates in central MN could even be sufficient enough to produce limited instability and a few thunderstorms embedded in the rain there. Rainfall totals for this afternoon through Friday night range from around 0.4" to around 1.25", with the highest amounts (0.75-1.25") extending from St. Cloud to the Brainerd Lakes, into the Twin Ports and parts of the North Shore up to Silver Bay.

This Weekend - Next Week:

Spottier light rain lingers on the back side of the departing low pressure for Saturday into Saturday night, with the potential (20-40%) for light snow to mix in over the Arrowhead Saturday night as colder air works in, though no snow impacts are expected.

Seasonally cool temperatures, with highs in the 50s to mid 60s and lows largely in the 30s persist Saturday through early next week as a prolonged period of easterly surface winds persist.
There are frost and freeze concerns for Sunday and Monday mornings, possibly the Arrowhead Tuesday morning as temperatures drop into the low to mid 30s.

Another rain system is possible Tuesday - Tuesday night, but the track of the low could trend just south enough that the Northland could miss out on more widespread rain chances. Rain chances range from 30-60%, highest in the southern half of the Northland. Beyond early next week, expect a gradual warm up again as the week progresses.

AVIATION /06Z TAFS THROUGH 06Z SATURDAY/
Issued at 1212 AM CDT Fri May 16 2025

A deep surface low is moving into western MN this morning. A dry slot slices through NW WI and wraps back into northern MN but additional rainfall will fill in as the low moves east through the day. Cloud cover is largely VFR right now but will also see ceilings drop to MVFR and IFR through the period. Additionally, there is a small corridor of strong winds in the low levels leading to some shear. Surface winds will increase in the morning and remain enhanced through most of the day. Expect direction to gradually be changing through the day as the low moves west to east.

MARINE /FOR NEAR SHORE WATERS OF WESTERN LAKE SUPERIOR/
Issued at 800 PM CDT Thu May 15 2025

A line of showers and storms around Grand Marais will continue moving northeast through around 10 PM before exiting western Lake Superior. Lightning, small hail, and gusty winds up to 30 knots. Dense fog has diminished as the storms have moved through, so the Dense Fog Advisory was cancelled. The Gale Warning was also cancelled as winds have fallen below gale levels along the North Shore. Wind gusts up 25 to 30 kt continue for the western Lake Superior nearshore waters through early tonight, with waves lingering along the North Shore before diminishing.

Winds remain easterly to southeasterly into Friday and then gradually veer to northwesterly Friday night into Saturday.

For the open water discussion, refer to the NWS Marquette Area Forecast Discussion at weather.gov/mqt.

DLH WATCHES/WARNINGS/ADVISORIES
MN...None.
WI...None.
MARINE...Small Craft Advisory until 5 AM CDT early this morning for LSZ121-140>148-150.