textproduct: Marquette

KEY MESSAGES

- The warmest airmass of the season so far will impact the UP with highs in the 90s today through Tuesday. High dewpoints and poor overnight recoveries will increase cumulative impacts. People sensitive to heat should make preparations for these conditions.

- There is a chance of thunderstorms today with the warm and unstable airmass. The SPC has outlooked a Marginal Risk (category 1 of 5) for severe thunderstorms (5-15% chance).

UPDATE

Issued at 325 AM EDT Sun Jul 12 2026

The going forecast remains mostly on track this morning. GOES-East Nighttime Microphysics RGB imagery shows mostly clear skies over the UP, with some thunderstorms over northern Ontario that so far are tracking to miss the region to the north. The two big forecast impacts for the day are the potential for severe thunderstorms and the oppressive heat, with the heat lasting through at least Tuesday.

Today's Thunderstorms: CAMs continue to be highly spread on the details of timing and storm mode for thunderstorms today. The ingredients are a mixed bag: shear is ok with HREF mean 0-3km SRH of around 250 by 18Z and good curvature in the low levels of the hodograph, though speed shear is somewhat lacking. Lift is questionable at best given approaching synoptic ridging and surface high pressure providing subsidence, but with significant heating today, some places may reach the convective temperature and while thermodynamically speaking, a lake breeze should be possible, southwest winds of around 10 kt may prevent the lake breeze from penetrating inland and serving as a mesoscale frontal lifting mechanism. Lift over the UP may not even be necessary as CAMs show a variety of possible MCS initiation scenarios over northwest Ontario/MN Arrowhead. Instability and moisture will be abundant as surface Tds look to be around the 70 degree mark and PWATs in excess of 1.5 inches this afternoon are over the 90th percentile of climatology, per HREF. The moisture and excessive heat will allow significant destabilization especially in the west half, where the HREF mean SBCAPE balloons to locally over 3,000 (!) J/kg. All this to say, the ingredients will generally be present for severe thunderstorms but an actual lifting mechanism to initiate convection is nebulous at best and lacking at worst. This is a high ceiling, low floor forecast scenario because if thunderstorms form, all severe threats are possible, but there is also a scenario where little to no convection actually occurs (or passes to our north or south). The SPC Outlook remains at a Marginal Risk (category 1 of 5) of severe thunderstorms.

Today's Heat: A Heat Advisory is in effect for the western half of the UP as the warming trend from the last few days continues. The aforementioned dew points near 70 will combine with highs in the 90s for most to give a 3+ hour window this afternoon of locally triple- digit heat indices. The east will miss out on the hottest temperatures today as the aforementioned southwesterly surface flow will advect in Lake Michigan-cooled air. That southwest surface flow will also make for downsloping to even further heat up the shorelines of Marquette and Baraga Counties as well as portions of the far west that downslope from the highlands along the MI/WI state line.

Monday/Tuesday Heat: The heat comes to a head Monday and Tuesday as the upper level ridge extends overhead from the west. Multiple weather balloon sites with periods of record dating back to the middle of the 20th century and 50,000+ launches will be in danger of breaking their all-time 500mb height record, potentially multiple times. As 1000-500mb thickness is proportional to temperature, a record 500mb height would mean record high temperature records are also in play. Monday, the usually cool-biased LREF shows up to 30- 60% chances of exceeding 100 degrees in some interior locations while the NBM (which can be warm biased) shows widespread 50-90% chances of exceeding 100 degrees. These probabilities are similar (maybe slightly lower and shifted east somewhat) Tuesday. These temperatures combined with dew points in the 60s to low 70s will also lead to heat indices climbing into the triple digits for most. Making matters worse is that the LREF shows that the west half may be 20% or less likely to fall below 70 degrees overnight for both Monday and Tuesday mornings. Without air conditioning, vulnerable populations (such as the elderly and immunocompromised, outdoor workers, and outdoor recreationists, among many others) are going to struggle not only to battle the daytime heat, but also may not get relief overnight. Thus, the Heat Advisory may be extended into the overnight hours and into the next day. Alternatively, the NBM does have some isolated downsloping areas in the west half and much of the Lake Michigan shores exceeding 105 heat index Monday, so an Excessive Heat Watch may also be warranted.

Midweek and beyond: Differences in how ensembles handle the troughs that bookend the ridge result in significant spread from Wednesday onwards, with LREF 10th to 90th percentile spread of at least 10-15 degrees (the difference between highs cooling to the upper 70s or low 90s heat lingering). Thus, did not deviate from the NBM in that time period.

DISCUSSION

Issued at 408 PM EDT Sat Jul 11 2026 Amended at 325 AM EDT Sun Jul 12 2026

Over the next few days, the robust Four Corners high amplifies and shifts northeast toward the Upper Great Plains. As the high approaches the Upper Great Lakes on Monday, its northward progress is deflected eastward as a sub-arctic trough over the Hudson Bay deepens and an upper-level jet streak moves over the Canadian Prairies. Thus, continued anticyclonic flow advects extremely anomalously warm air toward the region for several days before steering flow shifts it south. The NAEFS ESAT is currently showing 500 hPa mean geopotential height at 100% of climatology for most of the UP for Monday afternoon through late Tuesday.

For today's ramp-up period to this extreme heat, a Heat Advisory has been issued for noon through 9PM for the western UP given widespread apparent temperatures (heat indices) reaching 90F-100F. The NBM has over 50% probability of high temps greater than 90F for nearly all of the CWA, with the exception of south shore of eastern UP whose high temps will be influenced by light, onshore southerly flow off Lake Michigan. Portions of far western UP and Keweenaw have highest NBM probabilities of over 90% chances of greater than 90F, and over 50% chance of greater than 95F. NBM MaxT interdecile spread for today tops out at 9F, which puts most of UP in the mid-80s at the low end. Southwesterly surface flow indicates extra warming is possible via downsloping for the central UP coast, from Copper Harbor to western Alger County. With NBM mean dewpoints in the mid to high 60s for the second half of today through Monday morning, this puts apparent temperature on the first day of this heat streak at 90F-100F for the western UP, and in the upper 80s in the eastern UP.

Along with the warm, moist airmass, thunderstorm potential is nonzero for this afternoon. HREF mean SBCAPE ranges from 1500-3000 J/kg tomorrow evening, concentrated in the western UP and including the Keweenaw. HREF bulk shear magnitude values increase to moderate through the day as the jet moves in from the NNW, though directional shear (from HRRR soundings) is largely unidirectional, veering from NW'ly to N'ly through the night, above the low-levels. With scant synoptic forcing and high pressure moving in, any lake breezes which form may be a focus for convection. Though CAMs struggle to arrive at consensus with respect to time and placement, the environment is supportive of isolated severe storms; thus, SPC's Marginal Risk for severe thunderstorms tomorrow.

This evening has poor recoveries from the heat with apparent temps only decreasing down to the high 60s and low 70s by early Monday morning. Should widespread thunderstorms manifest this afternoon and evening, cloud cover and recent rainfall may increase mugginess. With recent NBM runs trending overnight minimum temperatures higher with the latest model runs, this may worsen cumulative heat-related impacts.

The UP is set to experience widespread apparent temperatures in the high 90s and triple digits on Monday and Tuesday, the worst of which will be felt between 1PM and 8PM each day. Between MinT's decreasing only to the low 70s and dewpoints in the high 60s, Monday's overnight period recovers particularly poorly with apparent temps in the mid 70s. As the mid-level ridge begins to flatten out toward midweek, the warmest airmass shifts south of the CWA, though it's possible that above-average heat will remain through the week. The NBM shows widespread, significant interdecile spread (>15F) for temperatures after Wednesday night.

AVIATION /06Z TAFS THROUGH 06Z MONDAY/

Issued at 117 AM EDT Sun Jul 12 2026

Mainly VFR conditions are expected through the TAF period outside of isolated thunderstorms. Instability will increase over the area today introducing the chance for thunderstorms over the U.P. Have dropped any mention of thunderstorms for the morning hours as upstream convection has yet to materialize. That being said, there is still a non-zero chance for a thunderstorms over the western U.P., possibly impacting IWD or CMX, during the early morning hours. Additional isolated thunderstorms are possible in the afternoon. Some of the storms could be strong to severe with large hail and high winds. Any storms could bring a brief period of IFR/MVFR.

Light southerly winds will increase in speed through the day with gusts of 20-25kt in the afternoon, especially at CMX.

MARINE

Issued at 408 PM EDT Sat Jul 11 2026 Amended at 325 AM EDT Sun Jul 12 2026

There will be an uptick in winds starting today as the pressure gradient tightens while a strong low moves along the high pressure ridge. Southwesterly winds will increase to 15-20 kt on today and to 25-30 kt over the central portions of the lake by Monday, persisting through the first half of next week and expanding to include western and eastern portions of the lake as well. Models continue to trend toward a few gale force gusts late Monday into Tuesday across the central portions of the lake.

MQT WATCHES/WARNINGS/ADVISORIES

Upper Michigan... Heat Advisory from noon EDT /11 AM CDT/ today to 9 PM EDT /8 PM CDT/ this evening for MIZ001>005-009>011-084.

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