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KEY MESSAGES

- Cold front passage late this afternoon/evening.

- Cooler with shower chances Sunday.

- Dry Monday with more substantial rain threat Monday night/ Tuesday (and possibly beyond but much uncertainty regarding that).

DISCUSSION

Issued at 302 PM EDT Sat Sep 12 2026

Pattern Synopsis: Broad long wave trough in place across western/ central North America; stronger short wave trough/PV anomaly over northwest Ontario with another digging southeast across the Prairie provinces. Water vapor imagery also shows a weaker short wave trough crossing the Dakotas. Weak upper low over the lower Mississippi/Tennessee River Valleys. Positive tilt/-PNA trough over the Pacific Northwest and adjacent Pacific Ocean...trying to pick up an old cut off low northeast of Hawaii and pull it back into the "PV reservoir" so to speak. This attempt looks like it will be unsuccessful...but the remnants of the Pacific Northwest short wave trough will feature prominently in Great Lakes weather during the first half of the coming week. Strong -EPO ridge south of Alaska (+3 sigma 500mb height anomaly) poking northeast into the Canadian Territories. Narrow axis of 1.25-1.50 inch precipitable water values and steeper mid level lapse rates extends from the upper Lakes southwest into the Central Plains ahead of cold front...with cold/dry air advection behind it across the northern Plains/upper Midwest.

Primary Forecast Concerns:

Cold front passage late this afternoon/evening: Cold front at 1800Z ran from a MQT-GRB-DBQ line...with a pre-frontal wind shift moving into Lake Michigan. Water vapor imagery shows upward motion field with short wave trough north of Lake Superior spreading across Upper Michigan and toward Lake Michigan. Getting an increase in cloud streets bubbling up across Lower Michigan...with a few hundred J/kg of MLCAPE west of the I-75 corridor along with decreasing CINH. So expect the threat for scattered showers/thunderstorms to continue until the cold front and short wave trough depart the area this evening. Narrow band of post-frontal St/Sc will give way to some lake induced Sc overnight as 925mb temperatures sink below +12C.

Cooler/breezy with shower chances Sunday: Next short wave trough in the series...digging southeast out of the Canadian Prairies this afternoon...will swing across the upper Lakes Sunday. 850mb temperatures dropping below 4C across Lake Superior will result in a decent amount of lake-induced instability. Combined with larger scale forcing/moisture will likely get showers going off Lake Superior Sunday morning. For now expect rain chances will be lower south of the Straits Sunday afternoon...won't rule it out given the strength of the forcing but the instability/moisture profiles across northern Lower not as impressive. Wondering how much of the lake component will linger into Sunday evening. High temperatures in the 70s and lower 80s this afternoon will give way to highs mainly in the 60s for Sunday (with highs 70-75 across far southeast portion of the forecast area closer to Saginaw Bay). Winds also expected to be gusty with mixing during the day...west/northwest gusts 20 to 30 mph anticipated.

Dry Monday with more substantial rain threat Monday night/Tuesday (and possibly beyond but much uncertainty regarding that): High pressure centered over Michigan Monday gives way to deep layer southwest flow Monday evening...low level jet directed into the upper Lakes with precipitable water values at or above 1.75 inches. Main focus Monday night will be across Upper/northwest Lower Michigan...then shifting into central and northeast Lower as the day progresses Tuesday. Trends continue to suggest drying across Upper Michigan during the afternoon Tuesday...remains to be seen how aggressively this pushes into northern Lower (more on that below). Highest probabilities exceeding 1 inch rainfall/24 hours ending 0000Z Wednesday remain across eastern Upper and the Tip of the Mitt counties. Probably some much needed rain across eastern Upper where things have been quite dry over the past two weeks (less than 25 percent normal precipitation) with some spots of D1 (moderate) drought showing up.

Evolution beyond Tuesday afternoon is rather interesting...with an upper anticyclone become established over the southeastern U.S. Monday...and then expanding north with 594dam 500mb heights knocking on the door of southern Lower Tuesday. This will set up a baroclinic zone across the upper Lakes during midweek...a synoptically ideal setup for a corridor of heavier rain potential. Exactly where this sets up will determine if our southern zones get clipped by this. For now the forecast is dry for Wednesday...with rain returning Thursday. There are some very small probabilities of >2 inches of rain Wednesday night/Thursday south of M-32. So something to keep an eye on going forward.

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 1239 PM EDT Sat Sep 12 2026

Largely VFR conditions through the forecast period. Gusty S to SSW winds this afternoon flip more NW by evening with a cold frontal passage. Some gusts as high as 25kts possible through the evening. Potential for a passing shower or two generally 19-23z, primarily at PLN and CIU... though could scrape TVC and APN, but not likely. Best shot for any flight category restrictions from showers will be at CIU, gradually decreasing the farther south one goes. Winds taper some overnight with CIGs rising again. Breezy NW flow continues into Sunday, with VFR diurnal cumulus clouds sprouting through the day.

APX WATCHES/WARNINGS/ADVISORIES

MI...Beach Hazards Statement until 8 PM EDT this evening for MIZ025- 031-095-098. MARINE...Small Craft Advisory until 8 PM EDT this evening for LHZ345>347. Small Craft Advisory until 8 PM EDT this evening for LMZ323- 341-342-344>346. Small Craft Advisory until 8 PM EDT this evening for LSZ321- 322.