textproduct: Marquette

KEY MESSAGES

- Widespread frost likely (50-80%) in the interior UP Tuesday morning, and a freeze possible (20-40%) across portions of the interior-west. Frost and Freeze headlines are in effect late tonight through Tuesday morning.

- Dry pattern with persistent high pressure sets up through much of this week. Below-average temperatures through Wednesday give way to above-average temperatures by the weekend.

DISCUSSION

Issued at 156 PM EDT Mon Sep 21 2026

Afternoon water vapor imagery reveals quite nicely a compact midlevel low spinning over the Plains, centered near Sioux Falls, though high pressure centered north of Lake Superior is helping to suppress this well south of the UP. Closer to home, we remain under patchy high cloud cover, with temperatures so far only in the mid and upper 50s. With more breaks in the cloud cover developing, most spots should be able to climb slightly higher into the lower 60s. The exception will be over the Keweenaw and the north-central UP, where northeast flow off of Superior should limit temperatures to the mid/upper 50s. Tonight, skies gradually clear from north to south, and with light winds allowing for good radiational cooling, we are looking at another chilly night. Temperatures this past night bottomed out in the mid 30s across much of the interior-UP, with some of our typically cooler spots below freezing. Given clearing skies, we stand a better chance at dropping even further with lows ranging in the lower to mid 30s for most of the UP and possibly flirting with the upper 20s. Will be issuing frost/freeze headlines for most of the UP, excluding the Keweenaw, late tonight into early Tuesday.

The Ontario high pressure center strengthens and tracks east tomorrow, setting up a Hudson Bay High pattern to the northeast with a Plains trough immediately to the southwest. As the surface high, reinforced by mid-level flow, strengthens into Wednesday, the Plains trough moves south toward the Lower Midwest, and clear skies prevail by the second half of tomorrow. Modeled soundings show deep-layer dry air with substantial (>20C) surface dewpoint depressions Tuesday afternoon, yielding minimum RHs in the high 20s in the western interior (widespread 30s elsewhere) tomorrow afternoon. Dry weather persists Wednesday and Thursday, though more moisture should be working into the area as the surface high shifts eastward and southerly flow develops over the UP; expect RH to bottom out around 35-40% in the driest spots Wednesday, and to remain above 40% area- wide Thursday. Still, even Tuesday, critical fire weather conditions will be far from achieved given light winds <10 knots in the driest spots and high temperatures in the low 60s for the next few days.

The split flow pattern slowly moves east of the Great Lakes region into Thursday as high pressure persists over Ontario/Quebec into the weekend. Slight variations in LREF clusters make their appearance beginning on Saturday, general consensus exists that some form of regional ridging is likely through Sunday. Thus, below-average high temperatures gradually warm into above-average high temperatures (5- 10 above on Saturday) by the weekend while conditions remain dry.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 128 PM EDT Mon Sep 21 2026

VFR conditions will continue to prevail through the TAF period. The probability of any brief, shallow fog at IWD/SAW late tonight/early Tuesday is less than 20 percent, so will not mention in the TAFs. East to northeast winds around 10 kt this afternoon will diminish to less than 5 kt this evening into the overnight.

MARINE

Issued at 156 PM EDT Mon Sep 21 2026

Winds continue to diminish as high pressure builds in from the north. The robust Ontario high will lead to slowly veering surface winds generally from the northeast this afternoon to southerly on Thursday. As the high strengthens and tracks east through Thursday morning, brief periods of higher wind speeds will occur as the pressure gradient tightens at times. After Thursday, winds go very light (gusts < 10 knots) and waves diminish to less than 1 foot.

For Lake Superior: NE wind gusts around 20-25 knots become more common over the western lake (just east of the Apostles) where funneling occurs through this evening. Wind gusts up to 15-20kts will be possible over the east-central portions of the lake and the nearshores through the first half of the night. Expect light easterly winds the second half of the night to increase to around 20kts again for the afternoon and evening hours Tuesday and Wednesday. Wave heights fall to around a foot tonight, but pick up to 1-2ft as winds increase during the afternoon and evening hours. Thursday and Friday, winds come in generally from the south to southeast, and while gusts to around 20kts will be possible over the east-central portions of the lake Thursday, winds fall below 10kts across the entire lake Friday.

For the Bay of Green Bay and Northern Lake Michigan: NE wind gusts up to 15-20kts become more common this evening through tonight. While sustained winds remain light (<10 knots) through the next few days, gusts to 15-20 knots remain possible for the majority of Tuesday through Wednesday morning. Winds shift over to the E/SE Thursday and Friday, falling below 15kts. Waves will generally be around 1-2ft, but latest guidance shows a potential for higher wave heights up to 2-3ft for a period Tuesday evening.

MQT WATCHES/WARNINGS/ADVISORIES

Upper Michigan... Frost Advisory from 2 AM EDT /1 AM CDT/ to 9 AM EDT /8 AM CDT/ Tuesday for MIZ002-005>007-009-011-013-014-085.

Freeze Warning from 2 AM EDT /1 AM CDT/ to 9 AM EDT /8 AM CDT/ Tuesday for MIZ004-010-084.

Beach Hazards Statement until 5 PM EDT this afternoon for MIZ005.

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