textproduct: Marquette

KEY MESSAGES

- Showers and thunderstorms are possible starting around 00z this evening. There is a marginal risk (~%5 chance) of severe thunderstorms over the southcentral and central UP during this time. The main severe threats with these storms are large hail, damaging wind, and heavy rain.

- Warm and dry Tuesday with highs in the 80s. The rest of the week will not be as warm, but still above normal.

DISCUSSION

Issued at 219 PM EDT Mon Aug 31 2026

The upper level low pressure over southern Saskatchewan will move east to western Ontario by late tonight, weakening as it continues to occlude. Deep moisture is expected to remain in place as mid- level lapse rates steepen as instability increases to 2500-3000 J/kg over the south-central U.P. by the most likely time of initiation around 00z. Surface-500mb shear is expected to increase to 40-45kts by this time as well. Thus, the instability and shear are present for the potential development of a severe thunderstorm, if initiation occurs. However, given that most models indicate a capping inversion over much of the central U.P. through this evening and that only minimal lift is expected as the boundary continues to occlude as it passes through, thunderstorm development could be limited to only those storms that are strong enough to break the cap. The 1630z update of the SPC outlook continues the Marginal (5%) chance for sever thunderstorms across the central U.P. The main hazards with any storms that develop will be large hail, damaging wind, and heavy rainfall.

Lows tonight are expected to be in the upper 50s inland and low 60s near the lakeshore. Some patchy fog is possible for the central and eastern U.P. after the passage of any thunderstorm, potentially lingering into the early morning hours Tuesday. The western U.P. can expect clearing skies and falling humidities overnight.

The upper low will shift east on Tuesday into central Ontario. Significant drying noted on Advanced Layer Precipitable Water imagery will spread into the area, with PWATs falling to around 0.75 inches. Deep mixing into this dry atmosphere is forecast, with relative humidity values falling into the upper 20s/low 30s (%) over the west half, with wind gusts of around 20 kt in the afternoon. In consultation with fire weather partners, will hold off from issuing an SPS at this time due to unsupportive fuel indices. It will be warm with highs ranging through the 80s, 10-15 degrees above normal in most spots. It will also continue to be fairly humid for the east half with wind flow off of Lake Michigan.

Weak warm frontogenesis will occur Tuesday night/Wednesday morning to our south as PWATs increase again to 1.25-1.50 inches, with guidance mixed on whether associated rainfall will reach far enough north into the UP. Best chances (up to 60%) over the south.

For late week into the Labor Day weekend, ridging builds over the northern Plains, while a deep trough evolves over the Canadian Maritimes. While this pattern is generally favorable for subsidence over the local area, as evidenced by a high pressure ridge building in from the northeast, western parts of the UP may be close enough to the moisture/instability reservoir to see convection at times. Currently capping out at 20-30% PoPs though, so the evolution is far from certain.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 140 PM EDT Mon Aug 31 2026

Conditions have been slow to improve with flight restrictions lingering at all TAF sites this afternoon. Therefore, have now trended toward MVFR at CMX and SAW by late afternoon with IWD right at the MVFR/VFR mark. Anticipate a period of VFR by this evening, but scattered showers and thunderstorms followed by fog tonight could make that a short recovery. Best chances for any convection will be concentrated over SAW, so will continue to highlight with a PROB30 at that site. Then, probabilities for IFR/LIFR vsby have increased at all TAF sites due to fog formation late tonight. So, TEMPOs have been inserted to capture lowest visibilities.

MARINE

Issued at 219 PM EDT Mon Aug 31 2026

Lake Superior: There is the potential for dense fog at times through tonight and will continue to monitor conditions for potential marine dense fog headlines. Winds will be SE to 15 to 20 kts over the eastern half of the lake today. Expect light winds of 20 kts or less to return to Lake Superior tonight. However, SW winds of 20 to 25 kts could return between Isle Royale and the Keweenaw on Tuesday before becoming 20 kts or less again Tuesday evening. The light winds remain until potentially late on Thursday when NE winds of 20 to 25 kts possibly return to the western arm.

Northern Green Bay and northwestern Lake Michigan: Winds will generally be southerly 5-10kt today. A few strong to severe thunderstorms are possible this afternoon and evening which could product large hail and strong winds, however the probability is only around a 5% chance. Expect mainly light winds to continue through the rest of the week, although more showers and storms could return on Wednesday.

MQT WATCHES/WARNINGS/ADVISORIES

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