textproduct: Chicago

KEY MESSAGES

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

UPDATE

Issued at 815 PM CDT Sun Jul 12 2026

A recent surface analyses revealed the center of a 1020 mb surface high positioned directly over the Lower Great lakes. Such a pressure pattern is leading to a gorgeous mid-summer evening with clear skies and warm temperatures in the upper 70s to lower 80s. Meanwhile, a strong upper-level ridge is building over the northern United States with 500mb heights sampled around 593dm by the 00Z BIS, ABR, and UNR RAOBs.

Over the next 48 hours, the surface pattern will more or less stay the same with the center of the surface high pressure system centered over the Lower Great Lakes. Even with agricultural evapotranspiration nearing climatological peak rates and mid-level subsidence associated with the surface high, am skeptical that dew points will surge into the mid 70s over the next 48 hours given otherwise full sunshine, drying soils, and blocked low-level moisture trajectories. (Will note that current dew points across the area in the upper 50s to mid 60s are already running 3 to 6 degrees lower than those advertised by NAM and GFS-based guidance). As a result, opted to lower dew points to largely remain in the upper 60s to locally 70 through Tuesday much in line with RAP/HRRR/RRFS guidance. Given temperature and dew points have an inverse relationship, did increase temperatures by a few degrees with forecast highs now in the low to mid 90s tomorrow and mid to upper 90s on Tuesday. Of course, daily lake breezes should hold locations within a few miles of the Lake Michigan shore to the low to mid 80s.

Updated products will be sent soon.

Borchardt

DISCUSSION

Issued at 0600 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. While southern Lake Michigan water temps are currently running close to average, they should warm to solidly above average in the coming days given the light winds, warmth, and full sun during the day. 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 /00Z TAFS THROUGH 06Z TUESDAY/

Issued at 559 PM CDT Sun Jul 12 2026

Only Concern:

- Potential lake breeze wind shift to easterly at ORD late Monday afternoon

Quiet, VFR flying conditions are in store through the period. Easterly winds early this evening will become light/VRB to calm overnight and then light west-northwest after sunrise Monday. A lake breeze will push inland, crossing MDW in the early afternoon, but then slow on its approach to ORD. Plenty of model guidance does not feature a wind shift, but given how light the flow aloft is, still think an easterly shift is plausible enough to maintain mention in the TAF. Main change was to push the wind shift timing back a few hours. Confidence is on the lower side.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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