textproduct: Chicago

KEY MESSAGES

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

DISCUSSION

Issued at 121 AM CDT Mon Jul 13 2026

The core of a nearly 600 dam 500 mb high will start to shift east across the northern Plains/upper Mississippi Valley today and will eventually become nearly stationary across the region through the week. Column thicknesses will commensurately increase today leading to high temperatures taking a roughly 3 to 9 degree jump over those observed yesterday. With a general lack of deeper moisture, cut-off Gulf trajectories, and strong mixing today, have blended some of the lower dewpoints from the drier guidance (HRRR/RAP). This results in peak heat indices generally in the mid 90s away from the lake today.

Locally, 1000-500 mb thickness values are forecast to peak on Tuesday and Wednesday, resulting in the hottest conditions of the week. Could see highs making a run into the upper 90s for locales just west of the inland-advancing lake breeze during peak heating, which includes O'Hare, Midway, and parts of the western Chicago suburbs. Dewpoints remain a lingering significant area of uncertainty during this upcoming stretch, which continues to lead to somewhat lower-than-typical confidence in peak heat indices. Unlike the previous heat wave, moist Gulf trajectories will remain completely blocked off in our region as a persistent surface high will be present just to our west. On the other hand, we're nearing the time of peak evapotranspiration (ET), with crops releasing vast amounts of water vapor into the atmosphere. These two effects remain at odds with one another, as a lack of deep lower-tropospheric moisture combined with strong mixing should allow dewpoints to otherwise mix out during the afternoon.

Based on the latest multi-model consensus, and taking into account the potential for at least some influence of increased ET, will show dewpoints Tuesday afternoon pushing towards 70 away from Chicago, and then a bit more solidly into the low 70s on Wednesday. This results in peak heat indices around 100 on Tuesday and then possibly nearing 105 degrees on a localized basis on Wednesday. Trends today may serve as a bit of a litmus test regarding forthcoming values, as we'll get a chance to see if dewpoints match the typically moist-biased local boundary layer parameterization schemes such as the NAM/NSSL WRF, etc or the drier and stronger mixers like the RAP and HRRR.

Daily lake breezes will afford modest relief, but only right at the lake as the cooling effects will be muted by warm lake temps (mid 70s) and limited inland movement.

On Thursday, the GFS, NAM, and UKMET all weaken the ridge sufficiently to allow a backdoor cold front through the region-- although a notable slowing trend occurred in the latest 00z GFS run. The CMC, while not nearly as substantial with the synoptic boundary, allows of much more robust lake breeze passage during the afternoon. The ECMWF remains a general outlier, holding onto offshore flow through the end of the week with a notably hotter solution even into Saturday. This model spread results in increasing forecast uncertainty at the end of the week.

At this time, there are no plans for heat headlines. If dewpoints end up meaningfully higher than advertised, that could change, although in that scenario, air temperatures would likely be a few degrees lower. Cook County could remain the exception in the event we reach the three-day 100 F heat index criteria, but given the potential for more substantial lake cooling on Thursday, there are also no plans for an Extreme Heat Watch for the city at this time.

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 with chances for showers/storms increasing late in the week into next weekend.

Carlaw

AVIATION /06Z TAFS THROUGH 12Z TUESDAY/

Issued at 1210 AM CDT Mon Jul 13 2026

VFR and light winds are expected through the forecast period. Winds overnight will be light and variable if not outright calm. After sunrise, they are expected to remain below 10 knots but prevail out of the northwest. A lake breeze is expected to develop in the afternoon, which may switch wind directions to the east-northeast for terminals closer to the lake, but there is uncertainty on the inland extent for the wind direction change.

For situational awareness: there is a storm complex presently crossing the Upper Peninsula of Michigan diving southward toward the rest of Michigan. No precipitation is expected, but there is a non-zero chance this complex sends outflow westward that could flip wind directions to the east for a short period later this morning. Confidence is too low for any mention in the TAF.

DK

LOT WATCHES/WARNINGS/ADVISORIES

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