textproduct: Chicago

KEY MESSAGES

- Hot and humid conditions are expected for much of the upcoming week; though somewhat cooler near Lake Michigan.

DISCUSSION

Issued at 1241 PM CDT Sun Jul 12 2026

An already strong mid-upper level ridge of high pressure over the Rockies will strengthen further as it builds eastward across the northern Plains into the western Great Lakes/upper Mississippi Valley through midweek. Most guidance has 500mb heights peaking around 6000m over the upper Mississippi Valley. To put into perspective just how impressive that is, in the nearly 80 year long record of upper air soundings from the Minneapolis area, the highest 500mb height ever record was 5980m.

This near-record strong upper ridge will set the stage for several days of hot and likely humid conditions across much of the region. The core of this heat dome does look to stay just north of our area with the highest temp anomalies likewise expected to remain just north of our area. While the most intense heat should remain to our north, it still looks likely to get quite hot locally, especially Tuesday and Wednesday when high temps are likely to top out in the low to mid 90s across our entire CWA.

There remains considerable uncertainty in the dewpoint forecast, which will dictate just how high heat indices will get. Low level trajectories will be pretty light with the origins of our air mass solidly continental with no contributions from the Gulf. The expected increase in dewpoints over the next few days will be almost exclusively driven by evapotranspiration (ET), a process which isn't explicitly modeled, but rather handled by a parameterization and then fed into the physical model. This certainly opens the door to potential errors, but given we are entering the climatological peak of ET, the model guidance that depicts dewpoints increasing into the 70s by Tuesday and Wednesday seem fairly plausible. Subsidence associated with the very strong upper ridge should result in a fairly low inversion and likely prevent deep mixing which could allow the more shallow low level moisture to mix out. Ultimately, the NAM overall did a good job with temps and dewpoints with the heat wave a couple of weeks ago, so opted to lean heavily on that model for temps/dewpoints through Wednesday. This looks to result in heat indices peaking in the upper 90s to slightly over 100 degrees both Tuesday and Wednesday afternoon.

By Thursday, the GFS is more aggressive with weakening of the upper ridge and for a couple of runs now depicts a back door cold front resulting in some meaningful break in the heat late in the week. The ECMWF and GEM maintain a stronger upper ridge and do not have a backdoor front on Thursday. Given the spread, the NBM seems to offer up a reasonable compromise in solutions, but the spread in potential high temps/peak indices for Thursday and even Friday is fairly large.

Proximity of the surface ridge should result in weak low level flow which should allow or lake breezes to form each of the next several days. Lake Michigan water temps are running well above average and should only warm further in coming days given the light winds and heat. The resultant lake breezes will probably only cool temps slightly into the low/mid 80s, with that more pronounced cooling likely confined to areas very near the shore. The magnitude of cooling as well as the timing and extent of inland penetration of the lake breeze will play a pivotal role in just how hot it will get over the densely populated urban corridor of Chicago. If the higher dewpoints do materialize, then it is possible that portions of Cook County farther inland from the lake could see heat indices peak near or just over 100F Tuesday and Wednesday with the potential for some late afternoon/early evening moderation of the heat depending on the progress and strength of the lake breeze.

Unless dewpoints overperform even the most aggressively moist models, then we will likely generally remain below headline criteria for heat. The one exception could be Cook County away from the lake which could reach its 3 day Extreme Heat Warning criteria Tuesday through Thursday, assuming that the backdoor front doesn't bring more meaningful relief. Given the potential for dewpoints to mix out, lake cooling to be more prominent, and the possibility of a backdoor front on Thursday, the chances of needing a heat headline for Cook County look to only be about 30-40% at this point.

Late this week and into next weekend medium range guidance is in pretty good agreement in breaking down the upper ridge with more of a northwest flow pattern setting up locally. This could result in a more active pattern locally with chances for showers/storms increasing late in the week into next weekend.

- Izzi

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1224 PM CDT Sun Jul 12 2026

Key messages for the 18Z TAF period:

* Uncertainty in the passing of a lake breeze Monday afternoon

NE winds largely below 10 kt this afternoon will go light and variable tonight. Monday will begin at WNW below 10 kt. A lake breeze will protrude inland Monday afternoon, though there is uncertainty in whether it makes it across the Chicago terminals, as well as the timing on when that would be (likely after 18Z). Winds are expected at near or below 10 kt on either side of the lake breeze. Expect VFR throughout the period.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.