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KEY MESSAGES

-Rounds of showers and storms continue across the Great Lakes Region through Tuesday with severe weather possible.

-High pressure returns cool and quiet weather Wednesday through Friday.

-Cool overnight low temperatures lead to widespread frost concerns beyond the midweek.

DISCUSSION

Issued at 336 AM EDT Mon May 18 2026

Pattern Synopsis:

Deep longwave midlevel troughing currently positioned over the North American Rockies with strong flow over the Great Lakes Region will continue to eject multiple energy waves into the northwoods, resulting in rounds of active weather for the next 48-72 hours. Surface low pressure develops over the Minnesota/Wisconsin Border while a warm front extends northward into southern Ontario north of the CWA. A hot and humid air mass remains settled over the region this morning, combined with afternoon diurnal heating processes to fire off convective-driven showers into the evening hours. Another round of showers and storms will persist this Tuesday as the aforementioned low tracks northeast directly over Northern Lower with an attached cold front, providing enough lift for widespread storms with a marginal chance for severe weather (per SPC outlook).

Conditions turn quiet the second half of the workweek as the previously mentioned trough axis progresses northeast. Backside cool and dry air moves into the Great Lakes Region supported by mild ridging pattern building over the midlevels. Subsidence aloft and surface high pressure will support quiet and overall non-impactful weather through the second half of the workweek. Surface temperatures will fall this Wednesday into the 50s for highs with a post-frontal cP air influence, followed by mild ridging slowly building temperatures back to near climatological normals by the end of the work week. The only impacts to monitor will be dry atmospheric conditions that could potentially lead to some fire weather concerns and rapidly falling overnight lows generating widespread frost.

Forecast Details:

Showers and storms today and Tuesday...

Today: A few showers remain possible this morning from a decaying upstream line, but the majority of the CWA will be under the influence of partly/mostly cloudy skies and hot/humid weather. Cloud cover will attempt to clear out in order to build instability in the afternoon before a line develops and sweeps across the region west to east. Hi-rez guidance depicts most convection developing during the afternoon/early evening hours, mainly west of I-75. Model soundings depict a skinny CAPE profile with enough instability to support single-celled thunderstorms along the line potentially creating severe criteria hail stones, wind gusts, and even one or two supercells to develop with rotation. That being said, criteria for all types of severe weather remain on the lower/essential end of values needed for storm development. The most likely outcome will be the widespread support for heavy rain as cloud layer shear is very low along Northern Lower compared to downstate convection.

Tuesday: The aforementioned cold front will begin to sweep across the CWA during the day with enough lift to provide another round of convection Tuesday afternoon. Latest trends hint at more stability compared to Monday's setup as low pressure crosses directly overhead. Again, PWATs near climatological maxs, saturated mid levels, and instability will favor convective-driven rainfall across the majority of the CWA. Higher shear values would require training storms for any type of flooding, but afternoon diurnal heating processes will build enough instability to continue thunderstorms with a marginal risk for severe weather.

High pressure returns cool and quiet weather Wednesday through Friday...Strong cold frontal passage will pull dry, cool cP air into the northwoods. Subsequent departing low pressure and subsidence aloft will return high pressure and quiet weather Tuesday night through the end of the work week. Dry low-level profiles and dew points falling to near freezing will lead to concerning RH levels that could be a threat regarding fire weather. Nevertheless, post- frontal temperatures will drop into the 50s this Wednesday, followed by the aforementioned ridging pattern to slowly warm surface temps throughout the rest of the week until reaching the upper 60s/low 70s.

Frost concerns Tuesday night through Thursday night...Strong cold air advection and efficient radiational cooling processes will likely lead to frost development Tuesday through Thursday night as lows drop into the 30s. The coldest night will likely be the coldest as high pressure is centered over the Midwest, leading to clear skies and lows near freezing. The typically coldest areas of the CWA will likely fall below freezing into the upper 20s while areas closer to the lakes remain in the mid-30s.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 1156 PM EDT Sun May 17 2026

Showers and a few embedded TSRA will tend to lift northward with time overnight. MBL/TVC/APN are done with the more widespread SHRA activity, but CIU/PLN are not. APN/CIU/PLN will both contend with IFR cigs at times overnight into Monday morning, ending from s to n. Conditions otherwise improve into Monday morning, until more showers/storms are possible in the afternoon and evening.

Breezy southerly winds develop Monday.

APX WATCHES/WARNINGS/ADVISORIES

MI...None. MARINE...Small Craft Advisory until 5 AM EDT early this morning for LHZ345-346. Small Craft Advisory until 8 PM EDT this evening for LMZ323- 341-342-344>346. Small Craft Advisory until noon EDT today for LSZ321-322.