textproduct: Chicago

KEY MESSAGES

- There is a threat for thunderstorms mainly late tonight into Monday morning and again Monday afternoon and evening. If storms do occur, there would be a potential for severe weather.

- Heat indices could approach or locally exceed 105 F in parts of the area on Monday, although there is low confidence in this due to the potential for morning thunderstorms.

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

DISCUSSION

Issued at 214 PM CDT Sun Jul 26 2026

Tonight and Monday Morning...

The forecast through Monday remains complex and highly uncertain, although some of the main key players are coming a bit better into focus. Still, the forecast evolution tonight will hinge on several low-predictability aspects, including whether storms congeal into a forward-propagating complex and do so early enough in the MCS lifecyle to direct a southward- moving cold pool into our region (as opposed to shifting off to the east).

The main feature of interest is a 700-500 mb shortwave currently straddling the International Border in southern Manitoba. Elevated convection has been ongoing since earlier this morning with this feature. Guidance has been all over the place regarding convective evolution, and presumably some of the variability has to do with the magnitude of inhibition from the Minnesota Arrowhead, eastward into northern Wisconsin and the western Upper Peninsula. It's unclear if wildfire smoke might be playing a role in slightly stunting insolation a bit, but given the limited extent and vertical depth of any Cu corroborates the significant analyzed MLCIN at this hour. Earlier runs of the HRRR, which aggressively convected in this area mid-late afternoon have notably backed off on this outcome, likely due to increased inhibition.

If this initial activity does not materialize upstream, there's been a secondary signal for storm development across central/eastern Iowa early this evening within the heart of the MUCAPE axis. Should this materialize, steering flow and the instability gradient orientation should generally keep activity to our west and south although this may impinge on our far southwest locales late in the evening.

There's perhaps a better chance for storms to develop later this evening well to the north as the main body of the aforementioned shortwave and attendant forcing shifts across the Arrowhead, eventually congealing into a forward-propagating MCS. Exactly when and how far east this takes place will play a critical role in our storm chances overnight and into Monday morning, as a more disorganized complex would shift far enough east to largely miss our area, while a more quickly-organizing complex would tend to dive southward along the eastern fringes of the main MUCAPE gradient and along the 850-300 mb thickness contours. Given the progged orientation of the instability gradient and upper jet nosing off just to our east, suspect the chances for thunderstorm activity will maximize during the late overnight and early Monday morning timeframe. Any MCS would encounter increasing nocturnal CIN, but sufficient deep layer shear would support sustenance to some degree into our area, mainly with a threat for localized damaging wind gusts.

Monday afternoon and evening...

Assuming convection departs the region through midday, some airmass recovery would be likely during the afternoon as anvil debris is carried away from the region and breezy southwesterly boundary layer flow develops. With sufficient insolation, temperatures could crawl back towards 90, and dewpoints would likely surge back into the mid to upper 70s. This would set the stage for another round of potentially severe convection very late in the afternoon and into Monday evening as an elongated surface low drags a cold front into and eventually through the region.

While a robust shortwave trough will precede the front, it remains unclear if thunderstorms will manage to re-develop during this period as a plume of incredibly dry mid-level air (700 mb dewpoints progged near -30 C) will rapidly surge eastward during the early evening. This stronger forcing may lag just enough to preclude CI prior to the loss of daytime heating and quickly-increasing MLCIN. However, if a storm or two manages to fire, the kinematic environment is concerning, particularly across northeast Illinois and northwest Indiana where deep layer shear will be maximized (near 50 knots) along with notable clockwise low-level hodograph curvature. In this scenario, convective coverage might not be particularly high, but the ceiling on an individual cell or two would be significant, including the threat for tornadoes. Finally, there would be a potential for a supercell to anchor on the incoming boundary or composite outflow/lake breeze which could pose a localized flash flood threat. All of this remains low confidence though, and will depend to a large degree on how overnight and morning convective episodes play out.

Regarding any heat headlines for Monday: in a totally undisturbed environment, peak heat indices would easily push into Advisory criteria across a large chunk of the forecast area. However, given the increasing signal for convective influences to play a role in the region--either via outflow- affected temperatures and dewpoints or persistent anvil debris cloud cover--there is very low confidence in whether we'll manage to meaningfully reach criteria (105F) in our forecast area. The greatest chances exist roughly west of the Fox Valley where earlier clearing could allow temperatures to warm back into the low/mid 90s as late-afternoon dewpoints surge back into the upper 70s. However, since this is also not all that certain, there just isn't enough confidence to hoist an Advisory product at this time. This unfortunately will need to be a last-minute decision based on convective trends.

Monday night and beyond...

The cold front will shift across the region through Monday night. Some elevated instability will remain atop the frontal inversion prior to the arrival of a plume of much drier mid- level air, so can't totally rule out a few cells festering for a bit, but any lingering precip will end prior to daybreak. Tuesday will feature temperatures in the 80s except only in the upper 70s near the lake. Stout north/northeasterly winds will result in large breaking wave action and a high swim/rip current threat which will persist through Wednesday morning.

With the increasing northeasterly winds, there is some potential for smoke that's been drifting around the region aloft to work its way to the surface across the region Monday night into Tuesday. Just a bit too far out to judge smoke concentrations with a good degree of confidence, so have left this out of the forecast for the time being but this will need to be monitored.

High pressure will largely remain in control of our sensible weather through much of the work week yielding dry and more comfortable conditions, particularly Wednesday and Thursday. The moist and instability axis will start to build back towards the region Thursday afternoon into Friday, and there's a signal for one or several disturbances embedded within the northwest flow aloft to pivot across the Great Lakes region. This may yield the next chances for showers and thunderstorms, which may linger into Saturday before another cold front arrives during the Saturday night/Sunday timeframe.

Carlaw

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 113 PM CDT Sun Jul 26 2026

Key Messages:

- There is a potential for thunderstorms tonight into Monday morning, but confidence in exactly when storms may affect some or all of the terminals remains low.

Conditions are likely to remain dry through this afternoon. MVFR ceilings may linger for a few more hours at RFD, while VFR conditions will likely continue into this evening at the Chicago metro terminals with diurnal cumulus bases there expected to remain above 3000 ft AGL. Winds should generally remain out of the southwest through sunset with the exception of GYY, where a slow-moving/stalled lake breeze may result in variable winds and/or a northeasterly wind shift this afternoon.

There remains a potential for thunderstorms to affect the terminals tonight into Monday morning. Confidence in forecast specifics for this thunderstorm potential unfortunately remains low at this time with persisting uncertainties over whether/where sustained, robust convection will become established upstream of our area in Minnesota and Wisconsin this afternoon/evening. The main time window of concern for storms at our TAF sites still generally appears to be between 09Z and 16Z, so have largely maintained the inherited TEMPO groups for TSRA in the TAFs. Will note, though, that storms could still potentially affect the terminals as early as 03Z-04Z this evening, so upstream trends will need to be monitored closely. If any organized convective clusters manage to develop and plow across the terminals, then strong to intense winds may be observed for a brief period of time as these storms pass through.

Confidence in the prevailing wind direction from late tonight through tomorrow also remains low, owing to the thunderstorm- related uncertainties. If an organized storm complex were to pass through or even just northeast of the area, then a gusty northerly to east-northeasterly wind shift may occur, and a northerly to east-northeasterly wind direction may then prevail for several hours longer. Conversely, if little to no storms occur in the area through tomorrow morning, then southerly to southwesterly winds may persist through the remainder of the TAF period. Even in a scenario with greater morning storm coverage, model guidance generally does favor winds eventually returning to a southwesterly direction tomorrow afternoon after storms clear the area, but wouldn't necessarily say that this is a sure thing yet.

There will likely be another window for potential thunderstorms during the late afternoon and evening tomorrow as a cold front drops through the area. The bulk of this window should be focused just beyond the 00Z end time of the ORD and MDW TAFs, so will let this be addressed in forthcoming TAF issuances.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory until 8 PM CDT this evening for ILZ003-ILZ004- ILZ008-ILZ010-ILZ019-ILZ032.

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