textproduct: Marquette

KEY MESSAGES

- Isolated showers and possibly a thunderstorm late this afternoon. - Another round of showers and thunderstorms is expected tonight. Some of the storms could be severe, mainly over the southern half of the U.P. Heavy rainfall is also possible.

- Cooler than normal temperatures Tuesday through the rest of the work week.

DISCUSSION

Issued at 331 PM EDT Mon May 18 2026

The primary focus of the forecast is on the next 24 hours as a dynamic low pressure system lifts through the region. Quiet and benign weather follows for most of the remainder of the week.

Latest GOES water vapor imagery place a deep trough over the Rockies and Plains which is moving east. An embedded shortwave over CO is emerging where a sfc low is organizing. These two features lift to northwest WI by tonight, supported by a right entrance region of the upper level jet and strong LLJ, bringing another round of showers and storms to Upper MI for tonight. Afternoon visible satellite imagery has shown a brief window of clearing passing over the western UP allowing for temps to warm into the 60s to mid 70s interior west. Cooler temps are noted near Lake Superior in the mid 40s to mid 50s where a lake breeze is moving onshore. Where transient showers lingered this morning in the east and cloud cover has been stubborn, temps are in the mid 50s to mid 60s. Not much further warming is anticipated given the brief window of clearing, lake breeze influences, and stationary boundary pressing in from the west. Latest SPC meso-analysis indicates some surface instability developing, which is anticipated to increase up to ~1250 J/kg by this evening per 12Z HREF mean as Tds rise into the upper 50s to low 60s. While marginal instability builds this afternoon, some isolated showers and storms are possible (20-40% chance). That said, ~30 kts of shear and little forcing will leave convection short lived and weak.

The LLJ moving in tonight significantly increases shear after sunset while instability begins diminishing. With cold front convection well to the south, decreasing SBCAPE/low level lapse rates, and a cap strengthening, it's no surprise the UP (and most of WI) has been completely removed from the SPC tornado outlook. MUCAPE and mid level lapse rates approaching 7 C/km linger into tonight, providing a low chance for strong to severe storms with large hail and damaging winds. The downward trend is also reflected in the latest SPC hail/wind outlooks, downgrading the northern tier of the UP to just general thunderstorms with a marginal risk (~5%) over the south. PWATs ~1.5 inch with the main wave tonight will yield rainfall totals of a few hundreths to around 0.5 inches, highest amounts over the west where there is a 25-50% chance for amounts up to 1 inch. This is captured by the WPC marginal risk of excessive rainfall (~5% chance of exceeding Flash Flood Guidance). Otherwise temps fall into the 40s and 50s tonight, coldest northwest.

Isolated to scattered showers linger into Tuesday, however instability will likely be elevated and only briefly uncapped in the east/south-central for a few hours at best. Thunder is in the forecast, but strong to severe storms are not expected. Highs in the east half on Tuesday will be in the upper 50s to upper 60s, lower in the northwestern UP where mid 40s to mid 50s are expected.

From there, the low departs to the northeast and high pressure over the Plains replaces it. This maintains over the Great Lakes through most of the work week yielding dry weather until the weekend. Lingering troughing aloft keeps colder northwest flow through Wednesday, resulting in temps between 5-15 degrees below normal; highs in the 40s and 50s and lows in the 30s. A warming pattern is then expected toward next weekend as mid level ridging slowly builds over the southeast and extends up into the Great Lakes. Another trough over the Rockies on Friday looks to move toward the Upper Great Lakes during the weekend, bringing back precip to the UP. Instability with this is lacking, so impactful strong to severe storms are not anticipated.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 152 PM EDT Mon May 18 2026

Visible satellite reveals partial clearing taking place over the western half of Upper Michigan as of 18z Mon, with surface observations showing a corresponding upward trend in flight categories at area terminals. Expect an eventual trend back down to low end MVFR or IFR roughly 21-00z this evening at KIWD and KSAW as the next round of showers moves into the region. Probabilities for strong to severe thunderstorms have decreased into tonight, but will still carry TSRA at KIWD and PROB30s at KCMX and KSAW through ~10z Tue. Will see gradual improvement from west to east as the system exits Tue morning, but cigs will likely hold at MVFR if not IFR until after 18z Tue.

MARINE

Issued at 331 PM EDT Mon May 18 2026

A low pressure moving northeast over Upper Michigan and Lake Superior tonight into Tuesday. This tightens the pressure gradient and increases northeast winds over the west and north-central portions of the lake to 20-30 kts tonight. As the low moves overhead, winds become northwest and strongest winds shift from the west half to the east as colder air moves in. Probabilities of gales to 35 kts are 25% or less, highest in the east Tuesday afternoon. Showers and storms are expected with this system into Tuesday morning, however severe storms are no longer expected. This rainfall will prolong fog over the east into Tuesday morning. High pressure returns winds to 20 kts or less by Wednesday afternoon, winds likely maintain light into the weekend.

MQT WATCHES/WARNINGS/ADVISORIES

Upper Michigan... None. Lake Superior... Dense Fog Advisory until 11 AM EDT Tuesday for LSZ244-264>266.

Dense Fog Advisory from 1 AM to 8 AM EDT Tuesday for LSZ245>247.

Dense Fog Advisory from 8 PM this evening to 8 AM EDT Tuesday for LSZ248>251.

Dense Fog Advisory until 2 PM EDT Tuesday for LSZ267.

Lake Michigan... None.