textproduct: Chicago

KEY MESSAGES

- Thunderstorms possible through early this evening, any storms that occur could be severe with damaging winds and potentially some large hail.

- Heat and absolutely stifling humidity will result in dangerous heat indices outside of any storms today.

- Dangerous swimming conditions due to large waves and strong currents are expected late tonight through early Wednesday morning.

UPDATE

Issued at 705 AM CDT Mon Jul 27 2026

A Dense Fog Advisory remains in effect until 8 AM for areas near/west of the Fox Valley in Illinois, along with all of northwest Indiana, where dense fog filled in a bit later. Have noted some improvement at a few sites as of this writing, so the hope is that the advisory can be expired as planned at the top of the next hour.

Capping has proven too prohibitive for widespread severe storm development across the northwestern Great Lakes region overnight. This was the area to watch for the modeled scenario of a damaging wind producing MCS surging south across the area this afternoon. Can't completely rule out an elevated (likely severe) storm or two this morning until midday (15-20% chance). However, the seemingly more likely scenario today is for strong capping at the base of the EML to preclude storms until the approach of the cold front from the north late this afternoon into this evening. This timing (6-10 PM or so) would be more in line from approximate extrapolation from the strong to severe convection over/near western Lake Superior this morning. Also arguing against earlier convective initiation (CI) would be winds veering to west-southwest south of a surface low over Wisconsin, reducing low-level convergence amidst only modest forcing (stronger forcing farther north and east).

As such, have reworked PoPs and lowered them with a focus on the early to mid evening (best chance/~30-40% over northeast 1/3 to 1/2 of the CWA). Unless an upstream MCS is finally able to take hold today, very dry air at the mid-levels arriving from the north late this afternoon with the cold frontal trough suggest isolated to perhaps widely scattered convective coverage. With this being said, the conditionally explosive environment in place (as noted in full discussion below) would easily support intense severe storms (even if there's only a couple of storms).

Castro

DISCUSSION

Issued at 334 AM CDT Mon Jul 27 2026

Dense Fog This Morning:

The exceptionally moist low-level air mass and and southwesterly warm/moist advection led to the development of (rare for late July) fairly widespread dense fog. To this point, dense fog coverage has been highest generally west of the Fox Valley (ie. western half of the CWA). Opted to issue a Dense Fog Advisory until 8 AM and assess observations and webcams for any eastward expansion.

Castro

Thunderstorms and Associated Severe Threat Today:

While Mother Nature may finally be starting to tip her hand with regards to convective trends, there remains a good deal of uncertainties today into this evening. Early this morning there is a band of widely scattered thunderstorms from north of Milwaukee to arcing northwest to around Duluth and then northwest to extreme southeast Manitoba. Convective coverage has been somewhat limited thus far, and even the size of the more intense supercells over and near western Lake Superior haven't been that large. This has prevented any meaningful clustering and amalgamation of cold pools up to this point. The 12km NAM and RGEM both had and MCV having already developed by 06z, which clearly hasn't happened yet.

There appears to be a shortwave trough on water vapor imagery from northeast North Dakota into northwest Minnesota, which seems to be driving or at least aiding the convective development across northern Minnesota. Not seeing any guidance that has much of a reflection of this shortwave seen in water vapor imagery, though to the east of this feature is where the RGEM and NAM have already blown up a strong MCV. The million dollar question is will a MCS organize, and if so, when and where?

Really not much of a low level jet out there with 20-30kt westerly flow noted from on DMX and MSP VWPs. The 00z GRB sounding depicted quite a bit of convective inhibition, even for elevated parcels, which would match what SPC mesoanalysis page shows across MN and WI. This is probably why coverage of convection thus far has been as minimal as it has been. Suppose it can't be ruled out that there will be an uptick in convective coverage yet this morning, given the apparent wave noted in water vapor imagery. RAP also shows a modest low level jet developing yet this morning across northern MN aiming into northwest Wisconsin. That is the area that seems most prone to potential clustering and MCS development, so we will be watching that area closely in the coming hours.

Most CAMs do develop an MCS, so if an MCS doesn't evolve out this scattered convection, that pretty much renders most CAM guidance useless for convective trends today. Plan to maintain some low chance pops this morning, especially northeast IL/northeast IN in case any of this scattered elevated convection were to build southward. Do ramp pops upward by early afternoon, especially eastern CWA, in deference to the model guidance. Extrapolation would also suggest that the subtle shortwave noted on water vapor would likely be near southern WI to western lower Michigan 18-21z this afternoon. So perhaps that could be an impetus for convective initiation.

More formidable shortwave is noted farther northwest across south central Manitoba. This is the primary shortwave that will carve out a deep trough over the Great Lakes region this evening and lead to sfc cyclogenesis over lower Michigan. If more widespread convection with this lead shortwave or an MCS from WI rolls across the area this afternoon, that should greatly limit if not eliminate the threat of storms this evening. If there isn't earlier convection in our area, then that ups the odds a bit for convection along the cold front and associated with the more pronounced shortwave this evening. However, the strongest large scale ascent with this shortwave looks to remain more to our east and with considerable dry air aloft, it is possible that storms may struggle to form or remain isolated along the cold front.

Unfortunately, a volatile air mass will be in place across the area through this evening with MLCAPE over 4000 J/kg and unseasonably strong (40-50kt) 0-6km shear. Any mature storms in this environment would likely become severe with primarily a damaging wind threat, but weak low level shear and long hodographs would also support intense supercells which could also pose a large hail threat. This severe threat all hinges on there being storms, which isn't necessarily a lock, given that there are still substantial uncertainties whether storms will occur. The chance of storms increases with eastward extent across the CWA and PoPs reflect that today.

- Izzi

Heat today then beach hazards and smoke:

Outside of storms, the other big headline today will be the oppressive humidity and resultant dangerous heat indices. Widespread 80F+ dewpoints were observed Sunday afternoon within this incredibly moist air mass that is seeping eastward across our CWA. Widespread upper 70 degree dewpoints at this time of day is very impressive and likely portends fairly widespread low to locally middle 80 degrees dewpoints today once evapotranspiration kicks in diurnally. One the fog and low clouds lift and burn off, highs in the lower 90s with dewpoints in the low to mid 80s should support heat indices of 110 to perhaps locally near 120 degrees this afternoon over our western CWA. In collaboration with DVN and ILX, we have issued an Extreme Heat Warning for these areas, despite the 48 hour criteria not quite being met.

Farther east, kept forecast high temps a few degrees lower, "only" in the upper 80s to near 90 degrees. These areas should see heat indices 105-110 degrees. If storms don't materialize until very late in the afternoon or early evening, then heat indices up to around 110F can't be ruled out farther east into the Chicago metro area south down the I-57 corridor. Confidence is lower in these areas, so opted to go with a heat advisory here. Ran all heat headlines until 02z, but where/if storms occur, then portions of advisory/warning can be canceled early.

Behind the cold front Monday night, look for a quick ramp up in northerly winds behind the front. The strong northerly winds down the full fetch of Lake Michigan will result in large waves and dangerous currents. A beach hazard statement has been issued for all southern Lake Michigan beaches through early Wednesday morning.

Only other item of note is the potential for wildfire smoke getting back into the area. Satellite imagery Sunday showed what appeared to be fairly thick smoke across Montana and western North Dakota. The northwesterly flow aloft should advect this smoke into our area later tonight into Tuesday. I'm bumped up sky cover to account for this smoke through Tuesday, but will likely need to be extended out farther. At this point, it doesn't look like there will be high near surface concentrations of smoke, so have not added it into the weather grids. Hopefully the smoke stays mostly above the surface this go around.

- Izzi

Wednesday onward...

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 /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 705 AM CDT Mon Jul 27 2026

Main Concerns:

- Low flight categories (LIFR/IFR CIGs and fog) this morning

- Chance for severe storms, mainly late this afternoon into this evening

- Wind variability in the wake of TS today and then a wind shift to northerly tonight behind a cold front along with a period of lower CIGs and possibly reduced VSBY

Very low CIGs and VSBY this morning (lowest at RFD and DPA) should erode fairly quickly by mid morning, with refinements to TAFS per observational trends. Will also need to monitor the progress of an outflow boundary moving into extreme northeast Illinois as of this writing. Once the flight categories improve, west-southwest winds are expected to quickly increase, with gusts to around 25 kt if not higher this afternoon.

Confidence in the TS forecast has decreased significantly vs. earlier thinking. Parameters today are still strongly supportive of severe storms, conditional upon there being storms, which is highly uncertain at this point. It appears the best chance is late this afternoon into this evening ahead of a cold front, with PROB30s for this time window. Can't completely rule out an isolated thunderstorm prior to that, but confidence is too low.

In the wake of the cold frontal passage this evening, winds will shift to north-northeasterly, with gusts in the 15-20 kt range near the lake through Tuesday morning. In addition, there's a signal for a period of lower CIGs (IFR to low MVFR) into northeast Illinois and northwest Indiana, as well as 3-6SM BR at all terminals early Tuesday morning.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

IL...Extreme Heat Warning until 9 PM CDT this evening for ILZ003- ILZ008-ILZ010-ILZ011-ILZ019-ILZ021-ILZ032-ILZ039.

Dense Fog Advisory until 8 AM CDT this morning for ILZ003- ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-ILZ020- ILZ021-ILZ032.

Heat Advisory until 9 PM CDT this evening for ILZ004-ILZ005- ILZ006-ILZ012-ILZ013-ILZ020-ILZ023-ILZ033-ILZ103-ILZ104- ILZ105-ILZ106-ILZ107-ILZ108.

Beach Hazards Statement from late tonight through Wednesday morning for ILZ006-ILZ103-ILZ104.

IN...Dense Fog Advisory until 8 AM CDT /9 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019.

Heat Advisory until 9 PM CDT /10 PM EDT/ this evening for INZ001-INZ010-INZ011-INZ019.

Beach Hazards Statement from late tonight through Wednesday morning for INZ001-INZ002.

LM...Small Craft Advisory from 1 AM Tuesday to 7 AM CDT Wednesday for the IL and IN nearshore waters.