textproduct: Marquette

KEY MESSAGES

- Showers and thunderstorms exit the U.P. this morning with isolated to scattered showers and possibly a thunderstorm this afternoon. - Another round of showers and thunderstorms is expected tonight. Some of the storms could be severe, especially over the southern half of the U.P. Heavy rainfall is also possible.

- Warmer than normal temperatures Monday, followed by cooler than normal temperatures for the rest of the week.

DISCUSSION

Issued at 317 AM EDT Mon May 18 2026

Early this morning satellite water vapor imagery and RAP upper level analysis showed deep troughing over the western U.S. with southwesterly flow over the plains into the Upper Great Lakes. Satellite and radar showed widespread convection from KS up to northern WI ahead of frontal boundary. The storms were clustered over NW WI and the western U.P., close to a sfc low pressure that was tracking northeast into the area. Convection remained elevated, above the cooler easterly flow near the sfc and strong stable layer. SPC Mesoanalysis showed mucape of 500-1000 J/kg with effective shear of 20-30kt over the western U.P. which was helping to maintain thunderstorms pushing into the area but overall there was a weakening trend as cells crossed into the U.P. Temperatures were warming up a bit overnight as southeasterly flow strengthened with most locations in the upper 40s to low 50s.

Showers and thunderstorms will continue to move through the western U.P. through this morning. Conditions will remain borderline for strong to severe thunderstorms, mucape ~1000 J/kg and effective bulk shear 20-30kt. The most likely hazard is hail but cant rule out isolated strong winds. These storms will move through parts of the north-central U.P. with more isolated to scattered showers and thunderstorms over the rest of the U.P. PWATs are in the 1.25-1.5" range, exceeding the 95th percentile from climo, thunderstorms will be capable of producing heavy rainfall.

While scattered showers and an isolated thunderstorms will be possible the afternoon, mainly for the central and eastern U.P., most of the day is expected to be fairly quite with respects to thunderstorm activity due to strong capping and little in the way of large scale ascent. Should a storm manage to form and root itself at the surface it could become severe with cape to around 1500 J/kg and shear of ~40kt but the probability is very low. Southerly flow will increase and despite cloud cover expect temperatures to warm into the 60s and 70s. A lake breeze (Lake Superior) will form by early afternoon bringing falling to the lakeshore areas. The lake breeze / cold pool will expand inland through the remainder of the afternoon, further limiting the prospects for thunderstorm development.

Tonight, a second low pressure will track north into the U.P. bringing another round of showers and thunderstorms. Models continue to trend downwards with respects to instability with href mean mucape of 500-1000 J/kg over much of the U.P. though areas in the south will see a bit more robust instability to around 1500 J/kg. Notable is also the downward trend in mid-level lapse rates in the models over the last several runs. 0-6km bulk shear is around 40kt so there is still the potential for strong to severe thunderstorms, especially over the southern U.P. SPC continues to include the CWA within their 15% risk region (slight risk), with primary hazards being large hail and damaging winds. WPC also continues to maintain a marginal risk for flash flooding with this second wave.

Showers and gusty winds will linger Tuesday as the surface low and trough aloft push through the region. Thunderstorm potential looks to be isolated to the east in the morning. Sprawling high pressure builds in by evening, keeping the region dry through at least Friday morning. Daytime highs Wednesday look to peak in the 50s south and west, and 40s north-central and east. Thursday and Friday will trend back toward normal 60s, but the east may only top out in the 50s both days.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 741 AM EDT Mon May 18 2026

Multiple rounds of SHRA (including up to 35 percent chances of strong-to-severe TSRA) will impact the TAF sites today, with remnant low level moisture from overnight storms leading to LIFR conditions at all sites and airport minimums at CMX. These conditions will continue for the next few hours before a slight improvement at most sites this afternoon, though IWD is 50% likely to not improve above IFR today. The next round of SHRA/TSRA arrives around 00Z and lasts through the overnight period, with a return to IFR and/or LIFR conditions around 06Z, though confidence in the precise timing of specific storms is low. With multiple low pressure systems in the region this period, expect more wind shifts than the TAF forecast allows.

MARINE

Issued at 224 PM EDT Sun May 17 2026

Guidance continues to bring a low pressure into the Great Lakes Monday night, resulting in a tightening pressure gradient today and cold air advection Tuesday behind the low. The former will support a period of northeasterly gales in the western portions of Lake Superior this afternoon and evening. Shower and thunderstorm chances increase tonight, then linger until the low passes Tuesday morning. The first wave comes tonight across the east and then the west late tonight, both potentially lingering into Monday morning. Strong to severe storms can't be ruled out, the lack of effective shear within the cloud layer suggests the risk is marginal. The next organized cluster lifts through Monday night preceded by a warm and moist airmass. This will again support strong to severe storm potential and rainfall may be the trigger needed for overnight fog. In the wake of the low Tuesday, a second period of gales looks possible across eastern Lake Superior; although, ensemble guidance suggests the probability in the east being 25% or less. Once the winds settle Tuesday night/Wednesday, light winds are favored through at least Friday morning.

MQT WATCHES/WARNINGS/ADVISORIES

Upper Michigan... None. Lake Superior... Dense Fog Advisory until 5 PM EDT this afternoon for LSZ244- 264>266.

Dense Fog Advisory until 9 AM EDT this morning for LSZ245-246.

Dense Fog Advisory until 8 PM EDT this evening for LSZ267.

Lake Michigan... Dense Fog Advisory until 11 AM EDT this morning for LMZ221-248- 250.