textproduct: Marquette

KEY MESSAGES

- Warmer than normal temperatures continue through early next week.

- A strengthening pressure gradient will bring a period of gales to far western Lake Superior this afternoon into the early evening.

- Multiple rounds of showers and thunderstorms are expected Sunday night through Tuesday. Some of the storms could be severe, especially over the southern half of the U.P. on Monday. Heavy rainfall is also possible.

DISCUSSION

Issued at 224 PM EDT Sun May 17 2026

A deep trough is analyzed digging through the Northern Rockies with sharpening southwesterly flow across the Plains. Within this flow, MCV is currently observed moving through Wisconsin with another impulse over North Dakota. For our forecast area, this has resulted in high clouds streaming into the region and light rain lifting into Menominee/Delta counties where proximity to the MCV to the south is smallest. Temperatures have been observed in the 40s and 50s so far, under easterly winds in the west, and northerly central and east.

The main forecasting challenge continues to be the results of the deep trough lifting across the Plains and its subsequent impact on MCS development, the ongoing MCV along and just to the north of a northward lifting warm front, and a surface low lifting northeast through the forecast area Monday night into early Tuesday. The ongoing MCV and a warm front are expected to lift northeast through Wisconsin today, with the front lifting into the forecast area tonight. This will result in showers gradually lifting into the southern and eastern UP associated with the MCV and then thunderstorms this evening/tonight. Instability doesn't increase significantly until around 0z tonight. Elevated MUCAPE eventually builds to 1000-1500j/kg while 0-6km bulk shear builds to 40-50kts late evening and tonight, which should sustain showers and thunderstorms associated with any upstream development closer to the front; however, strong to severe potential will be limited by marginal effective shear. Should a stronger storm occur, primary hazards will be damaging winds and hail. Latest HREF suggests the best timing for stronger storms focuses on this evening in Menominee County and then mainly in the west after midnight tonight. PWATs are also expected to climb to 1-1.5 inches across the region. Model soundings suggest a deep warm layer, with 1-1.2k feet freezing heights. Additionally, MBE velocities suggests some training potential. WPC positions the region within a marginal risk for flash flooding because of the risk posed by these. Early Monday morning, the front will lift through, positioning the forecast area within the warm sector for the day. This will support gradual destabilization as we warm into the upper 70s to low 80s and dewpoints climb into the 60s. Afternoon northwesterly flow and possible lake breeze may limit warming along Lake Superior. Latest CAMs favor a mostly dry day, save for any diurnally, lake breeze, or outflow (from convection well to the south) enhanced convection occuring interior west/south-central by afternoon. Latest guidance suggests timing for the second wave is most likely Monday evening/overnight and will come with a better organized thunderstorm potential and higher risk for severe weather. 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 /18Z TAFS THROUGH 18Z MONDAY/

Issued at 159 PM EDT Sun May 17 2026

VFR conditions persist at Upper Michigan terminals this afternoon as east winds pick up to around 10 kt ahead of the approaching system over WI. Radar shows a few echoes spreading into the southern half of the U.P. as of 18z Sun, but expect these to struggle to reach the ground initially with plenty of dry air to overcome in the low levels of the atmosphere. May see a few sprinkles through around 22z, but do not expect these to have much impact on flight categories for the remainder of the afternoon. Conditions to trend downward more substantially this evening as heavier showers move in after 00z Mon, with cigs dropping to MVFR 00-02z and then IFR by 04-06z. Showers will then keep conditions in the IFR/low end MVFR range through the end of the TAF period. Elevated instability will bring a chance of TSRA which will last through much of the rest of the TAF period as well. Will handle this with PROB30s for the most part. Some storms may be capable of producing large hail or gusty, erratic winds late tonight into Monday. A 45-50 kt low level jet will also move into the area tonight, bringing LLWS to the terminals through early Mon morning.

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... Gale Warning until 11 PM EDT /10 PM CDT/ this evening for LSZ162.

Lake Michigan... None.