textproduct: Marquette

KEY MESSAGES

- Isolated showers and thunderstorms are possible (20 to 40% chance) this and Labor Day afternoons, mainly over the west half.

- A cold front will move through the region Tuesday night and Wednesday. This will bring widespread rain to Upper Michigan and strong winds to the Keweenaw.

- A cool airmass will build over the Upper Great Lakes late this week.

- There is a 20-40% chance for northwest gales Tuesday night into Wednesday, mainly west and north of the Keweenaw Peninsula.

SHORT TERM /THROUGH TONIGHT/

Issued at 337 AM EDT Sun Aug 31 2025

High pressure ridging continues through the U.P. today through tonight, with mostly clear skies continuing over the area for most of the time. The only two exceptions are as follows: first, some low- level stratus and patchy fog being seen early this morning. While satellite and webcam imagery is just showing low-level stratus for now, with temperatures cooling towards the dewpoint early this morning, a rinse-and-repeat of patchy fog developing over the interior areas (like what happened the previous couple of nights) is expected; some of the fog could be dense in spots. The second time is this afternoon when enough instability develops for some pop-up showers and storms to form, mainly over the interior west (but also up to ~20% in the central and east as well). While we could see some heavy downpours during a 'core dump' of the convective cells, no severe weather is expected given the lack of wind shear with height today. The mostly sunny skies are expected to let temperatures rise into the mid 70s to around 80 for a high this afternoon; meanwhile, mostly clear skies tonight should let the lows drop down into the lower 50s in the interior areas.

LONG TERM /MONDAY THROUGH SATURDAY/

Issued at 337 AM EDT Sun Aug 31 2025

An expansive mid-upper level ridge continues to stretch across the Upper Great Lakes Labor Day to at least Tuesday. With 1020-1025mb ridging over the region thanks to a surface high slowly stretching eastward through the Great Lakes into New England, mostly dry conditions are expected in Upper Michigan. The exception to this is Labor Day afternoon, however, as a weak retrograding vort max alongside daytime instability grows to ~1000j/kg, potentially yielding isolated to scattered showers and thunderstorms along the lake breeze boundaries of north-central and western Upper Michigan. Deep layer shear overall looks to be weak, so strong or severe storms are not anticipated should convection develop. Steady 850mb temps of 11-13C Labor Day and Tuesday will produce temperatures in the mid to upper 70s for most of the region. Although increasing clouds on Tuesday may result in highs only peaking in the low 70s in some locations across the west half. Overnight lows Monday night will fall into the mid to upper 50s near the lakeshores and in the Keweenaw and then to near 50F away from the lakeshores.

A pronounced upper level low is expected to dive southeast through Manitoba and Ontario late Tuesday and Wednesday. 0z and 12z deterministic guidance suggests this feature will park over the Upper Great Lakes Wednesday night into Thursday as it takes on a negative tilt, then lift north and east into Hudson Bay through Friday night. There's more ambiguity in the surface features, but the overall consensus is a deepening low will develop along a descending cold front late Tuesday. The low, or a secondary surface low, will move through or near Upper Michigan/Lake Superior Wednesday or Wednesday night before being pushed northward Wednesday night, potentially retrograding Thursday over northern Ontario, then finally exiting northeast into Hudson Bay Friday night. Despite being 4+ days out, this general evolution is handled well by the deterministic and ensemble guidance suites. This includes the cold front moving through Upper Michigan Wednesday, resulting in a large swath of precip spreading across the area ahead of the boundary, and the stronger wind signal for the Keweenaw, which may result in 30-40 mph wind gusts Wednesday. Additionally, model run to run consistency includes a surge of cooler air filling in behind the front, resulting in below normal temperatures to finish off the work week. Exactly where the coldest air ends up is still a question, but the general idea is for 850mb temps to fall to +1 to -3C. This will yield daytime highs in the 50s or low 60s and overnight lows in the 40s by the lakes and 30s interior. Suspect frost could be a concern toward the latter half of the week, although the cooler temps aloft, continued cyclonic mid-level flow, potential for a secondary surface trough, and lake effect/enhanced rain, may work against significant overnight cooling in some locations.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

Issued at 702 AM EDT Sun Aug 31 2025

VFR conditions look to dominate the TAF sites throughout the period, although some patchy fog could show up late tonight over IWD and SAW (less than 50%). In addition, we could see some isolated showers and storms over the terminals by this afternoon. However, given that the greatest chances are in the western interior (up to 30%) and there is only around a 20% chance at the terminals this afternoon, no mentions of showers or storms are in the TAFs at this time. Winds stay light throughout the period as high pressure ridging still remains overhead.

MARINE

Issued at 337 AM EDT Sun Aug 31 2025

High pressure ridging over the region will, overall, result in winds below 20kts for a majority of the period through Tuesday morning. By afternoon on Tuesday, increasing pressure gradient will allow for increasing southwesterly winds across the western and northern portions to build to near 20kts. A cold front will move move through the region late Tuesday/Tuesday night into Wednesday. Behind the front, winds will quickly become northwest and increase as cooler air builds over Lake Superior. Guidance continues to suggest the potential for gales Wednesday. Latest ensemble guidance packages vary, but generally suggest 20-40% chance of NNW or NW gales Tuesday night and Wednesday. Highest probabilities are located west of the Keweenaw between the peninsula and Isle Royale and lesser amounts northeast and east of the Keweenaw Peninsula. Northwest to westerly flow will persist over the lake for the remainder of the week, slowly falling below 20kts by late Friday.

MQT WATCHES/WARNINGS/ADVISORIES

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