textproduct: Marquette

KEY MESSAGES

- A Frost Advisory is in effect this morning for low temperatures falling into the low to mid 30s at interior locations.

- Dry weather persists into the early weekend with high pressure over the area. High temperatures start climbing above normal today toward near 70 this weekend.

DISCUSSION

Issued at 218 AM EDT Wed Sep 23 2026

This morning's RAP analysis shows the Canadian high continuing to strengthen: the surface feature now sits at 1034 mb squarely over the Province of Quebec. The trailing ridge which extends over the Upper Great Lakes has the UP under a pronounced pressure gradient from 1029-1031 mb on the back end of the high. Gradient flow has veered southeasterly and surface winds are calm across the UP. With GOES water vapor imagery displaying a substantial dearth of tropospheric moisture and no clouds, tonight's waxing gibbous is clearly visible on this Autumnal Equinox.

As expected with clear sky radiational cooling, temperature trends so far show interior locations dropping into the low 30s. A Frost Advisory is in effect through 9 AM for most counties of the UP for temperatures reaching into the low to mid 30s before sunrise. Despite dry air aloft, saturation should be realized at the surface at interior locations, leading to frost formation. Today, increasing WAA raises daytime temperatures into the mid to high 60s and overnight temperatures into the high 30s so that the NBM registers minimal chances of temperatures less than 36F (proxy for frost formation) through Wednesday night, despite clear skies. Thursday, as the influence of high pressure wanes slightly through the day, a shot of moisture from the Plains is able to reach northward to Canada before collapsing eastward toward the Upper Great Lakes. While this doesn't generate meaningful QPF within the CWA, it does cause Thursday night to cloud up across much of the UP. This and southerly WAA contribute to well-above freezing temps in the high 30s to 40s Thursday night, as well as 20-40% chances of fog in inland areas. Thus, we appear to be past the frost threat - for now - after this morning.

By Friday, high temperatures reach 5-10 degrees above normal to be very near 70F. Despite this slight anomaly, the EFI ESAT table through next Wednesday is nearly devoid of color, indicating ECMWF forecasts very near the average of historical forecast distributions for a five-week period centered on the forecast date. In other words, near climate-normal conditions can be expected for those variables analyzed by the EFI table (CAPE, CAPE-shear, wind speed and gusts, minT, maxT, QPF, and snow). The cause of these near-normal parameters locally, however, is not normal. The Canadian high which is driving UP conditions this week is quite robust: LREF 500 mb heights shows the mid- level high reaching 100% of climatology for much of Wednesday as it tracks east toward James Bay. Split flow continues through Saturday as Canadian high pressure is mostly maintained, and the conjoining mid-level low deepens off the mid- Atlantic coastline. Saturday evening, a second shot of moisture rides the windward edge of a ridge centered over the Great Lakes north toward Saskatchewan. Saturday night into Sunday still looks to be the next best potential for precipitation, with the highest chances concentrated over the western UP. Sometime Monday, high pressure at last appears to break down over eastern Canada as a trough approaches from the North Pacific.

AVIATION /06Z TAFS THROUGH 06Z THURSDAY/

Issued at 118 AM EDT Wed Sep 23 2026

VFR conditions prevail through at least the daytime hours today. High pressure remains in the region, with winds at the TAF sites more consistently out of the southeast. The first patches of fog will have ~40% chances to develop further into the interior western UP by 06Z tonight, but the terminals are currently expected to remain fog free until closer to 12Z Thursday.

MARINE

Issued at 218 AM EDT Wed Sep 23 2026

For Lake Superior: Easterly winds diminish this morning and today before picking up out of the east to 15-20 knots in late afternoon. Wave heights increase to 2-3 ft by late evening. These elevated winds turn southerly through tonight prior to becoming isolated to the east-central portions of the lake into Thursday afternoon, at which point wave heights subside to 1-2 feet. Winds fall back below 10kts across the entire lake Friday while waves decrease to 1 foot or less. Southerly wind gusts up to 20kts develop again Saturday ahead of a shortwave, then winds fall back into Sunday, diminishing to 5-10 knots on Monday.

For the Bay of Green Bay and Northern Lake Michigan: Easterly winds are currently sustained at 15-20 knots with gusts 25-30 knots, and wave heights are reaching up to 4 feet. Easterly gusts decrease slightly (gusts to 15 knots and sustained to 10 knots) for a brief period this mid-morning before gusts increase again to 20 knots (sustained 10-15 knots) for Wednesday afternoon and night. During this time, wave heights remain at 3-4 feet in Lake Michigan and 2-3 feet in the Bay of Green Bay. Winds shift over to the E/SE Thursday, falling below 15 knots, and then go very light from the south on Friday. Wave heights subside to below 1 foot by Friday morning. Winds remain below 10 knots through Sunday.

MQT WATCHES/WARNINGS/ADVISORIES

Upper Michigan... Frost Advisory until 9 AM EDT /8 AM CDT/ this morning for MIZ002-004>007-009>011-013-014-084-085.

Lake Superior... None. Lake Michigan... None.