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 1104 AM CDT Mon Jul 27 2026

Now moving into the latter part of the morning, there are a couple of features around the region that we have a close eye on. First, surface analysis depicts a center of low pressure deepening over NW WI along the leading edge of a shortwave disturbance and strengthening cyclonically curved jet max diving SE out of the northern Plains. After forming earlier this morning along the storm's warm front, a cluster of thunderstorms is propagating south- southeastward across northern WI. Meanwhile, the storm's cold front extends southwestward along roughly a Twin Cities to Sioux City line. Next, a confluence axis on the leading edge of the highest MLCAPE is working east across central/southern WI and far northern IL. Thunderstorms just recently began going up along this boundary after 9:00 including over our northern-most counties and have since matured into a severe QLCS over southern WI while remaining a bit more tame down into far northern IL. Lastly, elevated storm coverage recently began to blossom within an accas plume coincident with a subtle 700mb shortwave over eastern IA and into NW IL.

Convective trends through the daytime remain rather uncertain. Storms have matured locally along that confluence axis over our northern-most counties. Those will continue to trek eastward through the end of the morning before pushing out over the lake. So far, these storms out west into NW IL have struggled to climb very high. A 12Z DVN RAOB sampled a subtle, but notable warm nose at 500mb which could be one reason for storms struggling to climb. With MLCIN generally waning into the afternoon, would imagine storms should have an easy time at least maintaining themselves over the next few to several hours on the leading edge of the theta-e ridge as that 700mb wave carries it east across northern IL through late afternoon. Would also imagine that the intensity of storms should also be on an uptick as they encounter less capping as they move downstream. As storms build down closer to the surface, convecting parcels will be exposed to explosive values of instability paving the way for these to potentially become severe in the near term. Whether storms can actually take advantage of most of that instability and become severe is still unclear with at least some CIN still lingering and little synoptic forcing outside of this shortwave. Coverage through the afternoon is also uncertain and may remain isolated to largely scattered simply owing to the lack of convective triggers around. Highest confidence into early afternoon will be along this confluence axis found along the upper Fox Valley as of 1030.

Additional storms may develop this afternoon over western and central WI along the storms cold front which may continue into northern IL and provide and additional shot at strong to severe storms. The front will become more zonally oriented before dropping south into northern IL during the early evening while the surface low is expected to drop south over southern Lake Michigan. Latest guidance (for what it's even worth) has not been too excited about severe convection along the cold front this evening. The leading shortwave will steal away much of our layer moisture, including low level moisture, and push it off to our east prior to the frontal passage shunting LFCs up above 800mb. Forcing along the front appears rather nebulous with little help from features aloft. Models have us largely uncapped immediately ahead of the front, but even so, little low level instability and lots of dry air aloft may still make it difficult for storms to develop and maintain along the front into northern IL. Plus, if storm coverage can expand through the afternoon and gunk up the environment prior to FroPa, that will further work against storm chances this evening. Convective chances appear much better just to our east into southern lower MI and north- central IN where more explosive instability and better kinematic features will focus east of the low track. Depending on the exact track of the low, its possible this more favorable envionrment sneaks into our eastern CWA boosting storm chances. Any storm that does develop this evening will certainly have the potential to become severe with damaging winds and large hail. But with time, convective potential along the cold front is looking more shaky.

Meanwhile, it's been a hot and soupy start to the day. Heat indices around much of the area are already in the 90 with some spots pushing 100F as of 10AM. With the lack of storms and cloud cover that most guidance had been resolving for early this morning, temperatures have begun to outrun our previous forecast. For this reason, made the decision to expand our Extreme Heat Warning for heat indices in excess of 110 this afternoon to include all of our Illinois Counties outside of Cook, DuPage, and Lake. A Heat Advisory remains in effect around the remainder of the CWA. However, these storms now on the scope may end up hindering the extreme heat around parts of the area.

Doom

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- ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-ILZ020- ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-ILZ107-ILZ108.

Heat Advisory until 9 PM CDT this evening for ILZ006-ILZ013- ILZ103-ILZ104-ILZ105.

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

IN...Heat Advisory until 9 PM CDT /10 PM EDT/ this evening for INZ001-INZ002-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.