textproduct: Marquette

KEY MESSAGES

- Hot conditions continue through Tuesday with highs well into the 90s and heat index values over 100. Extreme Heat Warning now in effect until Tuesday evening as high dewpoints and poor overnight recoveries will increase cumulative impacts. People are encouraged to take precautions to minimize heat related impacts when possible.

- Temperatures moderate from Wednesday onward, with precipitation chances returning late in the week.

DISCUSSION

Issued at 321 PM EDT Mon Jul 13 2026

Afternoon GOES water vapor imagery and RAP analysis reveal a strong upper level ridge of high pressure encompassing much of the CONUS from the Four Corners region to Lake Superior, supporting the hottest air mass so far this season and indeed one of the hottest on record by some metrics for portions of the Upper Great Lakes. RAP analysis shows 500 mb heights rising to 600 dam this afternoon from the Upper Mississippi River Valley into the UP, nearing or in some cases exceeding the climatological max for several locations around the region. Additionally, this morning's 12z atmospheric sounding from International Falls, MN reported an 850 mb temp of 28.4 C, which is the second highest reading on record dating back to 1948. This upstream air mass has continued to advect eastward into the UP this afternoon, corresponding to widespread temperatures in the 90s and heat index values in the 100s across the UP as of mid afternoon. Will have to keep a close eye on temperature readings through the rest of the day as several temperature records will be in jeopardy, including potentially the all time high temperature reading of 99 degrees at the NWS forecast office in Negaunee Township. In addition to the hot weather, strong low pressure deepening over Hudson Bay is helping to squeeze the surface pressure gradient and produce breezy conditions over the UP and Lake Superior into this evening. Fortunately, dewpoints in the 60s are generally keeping relative humidity values above critical fire weather thresholds, though some locations in the western UP may flirt with 30% RH through this afternoon.

Tonight will offer little relief from the heat as most locations around the UP only cool into the mid and in some cases upper 70s, again nearing the climatological max in terms of warm minimum temperatures. Mid and upper level conditions will begin to retreat some from today's historic values on Tuesday, but 500 mb heights above 590 dam and 850 mb temps of 22-25 C will still easily support highs in the 90s on Tuesday, maintaining a major heat risk across the UP for one more day. Given the cumulative impact of warm overnight lows and multiple days of highs in the 90s coupled with 60s dewpoints again pushing the heat index above 100 degrees, have opted to extend the Extreme Heat Warning through Tuesday evening across the area. Bottom line, people should take precautions to mitigate heat related impacts whenever possible through Tuesday. This includes staying hydrated, limiting outdoor time, and checking on those who are vulnerable to the heat including the elderly, children, and pets.

SPC has notably included the UP in the Day 2 general thunderstorm risk for Tuesday. Currently thinking shower and thunderstorm chances are low enough to keep out of the forecast given capping and subsidence that will still be in place under high pressure, but will keep an eye on this potential as there will be plenty of conditional instability available for an cell that does manage to form.

Temperatures will moderate towards more "normal" summer heat from Wednesday onward as the upper ridge is flattened out by persistent strong troughing over eastern Canada. This will yield highs generally in the low 80s near Lake Superior to around 90 over the south-central UP for Wednesday and Thursday, cooling further into mid 70s to mid 80s by Friday. Overnight lows will also return closer to seasonal norms in the 50s by Wednesday night. The forecast remains mostly dry through Wednesday, but the flattening of the upper level flow will pave the way for the next system bringing a higher chance for widespread rain and thunderstorms back to the area by late Thursday into Friday. Do not see signals for any significantly impactful weather in the long term forecast at this time.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 154 PM EDT Mon Jul 13 2026

VFR conditions will prevail for the duration of the TAF period. Main threats will be strong westerly surface winds and LLWS threat. Surface winds will be strongest at CMX where gusts will top off in the 30 to 35 kt range. And, LLWS will be a threat at CMX and SAW after midnight.

MARINE

Issued at 321 PM EDT Mon Jul 13 2026

Southwesterly winds have increased to 20 to 25 kt across the western half of Lake Superior this afternoon as the pressure gradient tightens between surface high pressure to the south and deepening low pressure to the north. Look for winds to increase higher tonight into the 25 to 30 kt range while expanding over the central portions of the lake as well. Both HREF and NBM guidance highlight a 30-50% chance for some gale force gusts between the Keweenaw and Isle Royale this evening, but probabilities and coverage remain low enough to forego any gale headlines at this time. Small craft conditions will impact many of the nearshore zones tonight into early Tuesday as wave build to 4 to 7 feet across western and central portions of the lake. Winds then gradually diminishing through Tuesday and look to remain below 20 kt for most of thew rest of the week.

MQT WATCHES/WARNINGS/ADVISORIES

Upper Michigan... Extreme Heat Warning until 10 PM EDT /9 PM CDT/ Tuesday for MIZ001>007-009>014-084-085.

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