textproduct: Chicago

KEY MESSAGES

- Hot and humid conditions are expected today, especially western CWA, then potentially area wide Monday.

- There is a chance of thunderstorms tonight, Monday, and into Monday evening. If storms do occur, there would be a potential for severe weather.

- Dangerous swimming conditions due to large waves and strong currents are expected Tuesday and Tuesday night.

DISCUSSION

Issued at 223 AM CDT Sun Jul 26 2026

Synopsis:

A strong mid-upper level ridge over the central-southern Plains into the southwest U.S. is progged to weaken slightly and retrograde west a bit through the middle of the upcoming work week. Meanwhile, mid-upper level troughing will amplifying over the northeast U.S. through midweek, with our area locked in northwest flow much of the upcoming work week. Our CWA will be on the edge of the heat and humidity and strong instability, leaving us prone to convection until a cold front moves across the area Monday evening, ushering in a much less humid and more stable air mass for much of the upcoming work week.

Today:

While a much more humid air mass is expected to expand/spread northeast into our area today, there is some uncertainty regarding temperatures. Tossing out the too deeply mixed HRRR and RRFS, that leaves most guidance suggesting a stout low level inversion will persist today. Despite the high July sun angle, the stratus beneath the low level inversion may be a bit slow to break up this morning. In addition to the potential for lingering low cloudiness, considerable mid-high level cloudiness (especially this morning) is expected as remnants of the earlier SD MCS move across the area. Assuming we get some mixed sunshine this afternoon, which seems to be a reasonable assumption, then high temps reaching the upper 80s to lower 90s looks reasonable. Dewpoints climbing into the mid to perhaps upper 70s over our western/southwestern CWA this afternoon should push heat indices above 100 to perhaps locally around 105 degrees, so no changes planned to the heat advisory.

Other than the models that are too "mixy" there is a pretty good consensus that atmosphere will remain capped today. There's likely a weak remnant MCV with the decayed MCS over IA/MN, but this feature is likely to pass to our west and likely be south of our area before it would have a better chance of sparking renewed convection this afternoon. Overall, precip chances during the daytime hours look to be less than 15% and plan to stick with a dry forecast for today.

Tonight through Monday:

The early evening hours should be dry (precipitation-wise), but quite warm and humid. Focus for convective potential tonight is on shortwave trough evident on water vapor imagery from eastern MT north into southern Saskatchewan. Initialization of this fairly subtle shortwave varies among the various models and unsurprisingly leads to a wide array of possible solutions. Extrapolation would favor this wave cresting the upper ridge this morning and probably resulting in convective initiation over northeastern MN or northern WI later this afternoon. Certainly possible that this convection could congeal into an MCS this evening, but there is uncertainty whether that evolution into an MCS takes place and if it does, exactly where.

If there is an MCS over northern WI early this evening, forward propagating Corfidi Vectors, 850-300 mb thickness, and the orientation of the instability axis would all support an MCS moving southeast and then southward overnight. A 40kt low level jet could help maintain an MCS southward into northern IL or northern IN late tonight or early Monday morning. While there is considerable spread in the timing, the 00z GFS, RGEM, and ECMWF all have a nice convective QPF signal into northern IL tonight or Monday morning. The stronger effective deep layer shear tonight should be north of our CWA across the upper Great Lakes, but there should be enough mid level flow to support an MCS getting into our area, possibly in a weakening phase absent the development of a parent MCV to augment shear. Given the magnitude of instability, potential would certainly exist for damaging winds, should a well developed MCS make it into our area tonight or Monday morning.

Already low forecast confidence gets even lower (if that is possible) for Monday afternoon and evening. If we dodge having an MCS roll through here Monday morning, then all systems would be go for a oppressively hot and humid day Monday with heat indices 100F to locally over 110F in the areas with the most extreme evapotranspiration and likely lower 80 degree dewpoints. If there is an MCS, then temps Monday will hinge on timing of it moving out, how big of a cold pool footprint is left behind, how much cloudiness lingers. With confidence low, but increasing a bit in a potential MCS in the neighborhood late tonight or Monday morning, confidence in dangerous heat Monday is a bit lower. Given this, nudged forecast high temps down a bit from the NBM, but potentially still way too hot if an MCS does significantly alter the thermal fields. If confidence increases that convection won't prohibit the heat, then Monday may well end up needed heat headlines, but for now, a wait and see approach seems to be the most prudent way to go.

Monday evening:

Synoptic shortwave trough is progged to dive southeast into lower Michigan and the eastern Great Lakes. In response, sfc low pressure is expected to develop Monday over lower Michigan with a cold front trailing west into northern IL by Monday evening. Shear profiles would certainly support severe thunderstorms and perhaps even supercells if convection develops along this cold front. That is a big IF though, as atmosphere would need to recover from any potential earlier MCS and sufficiently destabilize. In addition, the strongest large scale ascent is expected to be east of our CWA, which would beg the question as to whether modest shallow frontal convergence alone would be enough to ignite convection. The thermodynamic profile will likely play a big role in answering that question as even modest capping could prevent much or even any convection associated with the cold front given shallow forcing. Conversely, if atmosphere does become very unstable with minimal inhibition, then severe thunderstorms could explosively develop along the front late Monday afternoon into Monday evening. Given all of these uncertainties, did pare back the NBM likely pops to high end chance. The highest chances for storms with the cold front Monday evening look to be over our eastern CWA closer to the stronger synoptic forcing with the shortwave.

- Izzi

Tuesday and beyond:

In the wake of the cold front Monday night, look for fairly gusty northerly winds Tuesday. The onshore flow will hold lakeside temperatures in the upper 70s to near 80 degrees Tuesday through Thursday, and conditions will turn much less humid. Looks like a strong push of northerly winds will accompany the front, which will lead to building wave action and likely a high swim/rip current risk Tuesday through Wednesday. Generally dry/precip-free conditions are expected through the end of the week with high pressure back in control.

Carlaw/- Izzi

AVIATION /06Z TAFS THROUGH 12Z MONDAY/

Issued at 1224 AM CDT Sun Jul 26 2026

Main Concerns:

- Lower CIGs (and temporary reduced VSBY) through this morning

- Potential for TS late tonight/early Monday

MVFR to low VFR CIGs should finally scatter by midday today. Maintained temporary MVFR VSBY at RFD, DPA, and GYY in the pre- dawn through early AM. Any SHRA and embedded TS beyond sunrise should pass safely southwest of the terminals. A weak lake breeze may flirt with GYY this afternoon, resulting in VRB winds, with otherwise southwest winds elsewhere.

Regarding TS potential tonight into early Monday, there remains a wide range of plausible outcomes (including no TS through 12z Monday), and don't have higher confidence in any particular solution. For this reason, deferred TS mention to a later TAF issuance. If a (possibly weakening) TS complex shifts southward across the terminals tonight, a period of gusty northerly winds may occur, probably followed by VRB winds.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory from noon today to 8 PM CDT this evening for ILZ003-ILZ004-ILZ008-ILZ010-ILZ019-ILZ032.

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