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KEY MESSAGES

- Slight risk for severe thunderstorms this afternoon.

- Marginal risk for severe thunderstorms on Tuesday.

- Sharply cooler weather arrives Wednesday and Thursday, with frost/freeze concerns Tuesday night through Thursday night (especially on Wednesday night).

DISCUSSION

Issued at 1207 PM EDT Mon May 18 2026

Pattern Synopsis/Forecast:

Moisture rich warm sector spreading northeast across the Great Lakes this afternoon as deep layer southwest flow continues between southeast Conus ridging and deep troughing rotating across the Intermountain West. Showers and thunderstorms beginning to percolate across northern Lake Michigan along decaying cold pool convergence (from overnight convection to our west). Surface based warming and attendant northward expansion of over 500 joules/kg of mixed layer cape within marginally supportive convective layer shear should continue to allow intensification of this activity as it races east across our area.

Deep upstream troughing will continue to lift northeast, steadily shearing out as it crosses into southern Canada and the northern Great Lakes later Tuesday into Tuesday night. Attendant cold front expected to cross our area on Tuesday, with additional showers and storms expected to fire in the pre-frontal warm sector until its crossing. Sharply cooler airmass follows this passing wave, setting the stage for additional frost/freeze concerns for the middle of the week.

Primary Forecast Concerns:

Shower/storm evolution and severe potential through Tuesday. Secondary focus on just how cold we become through the middle of the week (those frost/freeze concerns).

Details:

Expect upscale growth to convective activity as it quickly slides northeast across our area the remainder of this afternoon. Combination of instability, those marginal convective layer shear profiles, and expected congealing cold pool dynamics should support some isolated severe potential, with gusty winds and marginally severe size hail the primary concerns. However, slightly backed low level wind fields down near Saginaw Bay will also support a very low risk for tornadoes as well. Loss of surface based instability should end the most aggressive shower/storm concerns quicky early this evening. Focus for most numerous storms and greatest severe potential focuses off to our west tonight in vicinity of primary wave and attendant east advancing cold front. May see some of this decaying activity slide into our area later tonight/early Tuesday morning. Not expecting anything too significant with this activity as surface rooted instability core is lost by then. Cold front will work steadily east across our area on Tuesday...exiting off to our east by early Tuesday evening. Front becomes increasingly detached from any large scale support, though moisture rich pre-frontal environment and forced low level convergence will continue to support shower and storm generation. Severe potential tied to just how much heating/surface rooted instability generation can occur prior to frontal passage. Think ongoing showers and cloud cover may limit this potential, with focus for severe storms centering to our south and southeast. SPC maintains just marginal severe weather potential across northern lower Michigan...themselves focusing on pre-frontal destabilization concerns.

Focus quickly shifts to the arrival of a much cooler airmass as Canada originated high pressure drops into the western Great Lakes Tuesday night into Wednesday. Simple breadth of cold air advection looks to drop temperatures down into the 30s across a good portion of the area Tuesday night, although maintenance of gusty northwest winds will likely prevent much freeze or frost concerns. A chilly Wednesday...when some areas will likely not get out of the 40s...gives way to what is shaping up to be a cold Wednesday night. Pattern recognition definitely supports such, with high pressure centering directly overhead supplying what should be clear skies and light/calm winds...all within an already exceptionally chilly airmass. Widespread lows well down into the lower and middle 30s looks easily attainable, and fully believe some of our traditional colder interior areas will make a run into the 20s. Definitely something to monitor given our recent warm temperatures.

Conditions modify some Thursday and Thursday night, but could still see more frost and potential freeze concerns early Friday morning. Current trends support slow moderation through the weekend, with readings at least approaching more normal levels on Sunday. Other than the chill, extended period looks far less active, with perhaps some better rain chances returning during the weekend.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 216 PM EDT Mon May 18 2026

Showers and thunderstorms to continue through this evening, with occasional +TSRA and wind gusts 25 to 35 KTs possible within the strongest storms. CIGs decrease through the evening as well, with MVFR and IFR tonight (occasional LIFR as well for some). BR possible tonight as well, especially where moist and rainy conditions from this afternoon manifest. Likely turn MVFR for much of Tuesday with isolated thunderstorm chances. Southwest winds will be breezy to gusty tonight and especially through Tuesday.

APX WATCHES/WARNINGS/ADVISORIES

MI...None. MARINE...Small Craft Advisory from 8 AM Tuesday to midnight EDT Tuesday night for LHZ345>349. Small Craft Advisory until midnight EDT Tuesday night for LMZ323-341-342-344>346. Small Craft Advisory from 8 AM Tuesday to midnight EDT Tuesday night for LSZ321-322.