textproduct: Chicago

KEY MESSAGES

- Warming trend through Thursday (except near the lake on Wednesday).

- Next chances for thunderstorms as early as Tuesday afternoon, but mainly Tuesday evening and overnight near/south of I-80. - More widespread showers and thunderstorms with another cold front towards the end of the week.

DISCUSSION

Issued at 118 AM CDT Mon Apr 20 2026

The core of a mid 1020s mb surface high will shift east of the region through the morning with tranquil conditions to start the work week. Synoptic flow will promote the development of a lake breeze later this morning and afternoon which will focus primarily across Illinois lakeside locales and perhaps the immediate lakefront in northwest Indiana. Away from the lake breeze influence, temperatures will warm into the mid to upper 50s. Shaved a few degrees off the blended dewpoints this afternoon, particularly across our southwest where deeper mixing should occur into an 850 mb dry layer.

A southwesterly LLJ will develop tonight along with a gradual increase in occasional 20-25 mph surface gusts, mainly near the urban Chicago core. This LLJ will also transport the leading edge of another stout EML plume into the region through the day on Tuesday. The base of this EML plume is forecast to be quite warm, with 800 mb temperatures near +10 C, and this will keep our area decidedly capped (at least to surface-based convection) through the morning and likely much of the afternoon. Given the very steep nature of lapse rates within the EML, however, can't rule out some elevated/high based convection developing during the afternoon north of I-80, but this potential presently looks too low to justify an inclusion in the gridded forecast. Otherwise, Tuesday will feature breezy southwesterly winds (strongest south of I-80 where gusts to of 35+ mph will be possible) and warm conditions with temperatures in the 70s.

A low-amplitude shortwave embedded within broad northwesterly flow aloft is forecast to push across the region late in the day as a weak frontal boundary pushes out of Iowa and Wisconsin. Forecast soundings now generally depict more lifting/cooling occurring at the base of the EML through late in the day and into Tuesday evening, with a commensurate increase in the number of ensemble members convecting in our vicinity during this time. However, chances look to increase through the evening as a southwesterly LLJ nocturnally intensifies. Blended PoPs have increased into the 30-40 percent range near/south of I-80 during this period which looks appropriate at this point. Instability looks quite sparse, with generally limited moisture return. Still, stout northwesterly winds aloft will promote at least modest deep layer shear within the cloud-bearing layer, with a low potential for some updraft organization and a small hail threat Tuesday night. If storms do manage to fire and sustain (which still remains unclear at this time), we would also need to keep an eye on a very localized training/heavy rainfall/flash flood potential given the flow orientation and likely slow storm motions.

Wednesday generally looks quiet as shortwave ridging will build in overhead. Onshore flow, along with the potential for marine fog/stratus will hold immediate lakeside high temperatures in the 50s. The GFS--which is an outlier at this point--holds onto fairly extensive boundary layer moisture and stratus on Wednesday with resultant notably cooler temperatures compared to the rest of the guidance suite. Given its outlier nature at this point, made no changes to the blended forecast which advertises highs in the mid and upper 70s away from the lake.

The next large scale/synoptic trough is forecast to arrive in the region during the Thursday afternoon into Friday morning timeframe. Strong large scale forcing will promote the development of widespread showers and some thunderstorms. At this time, the progged late arrive of the system's cold front, overall dearth of instability, and limited deeper layer shear suggests a muted severe threat into our region. A faster (or slower) arrival closer to peak heating Thursday or Friday afternoons would increase the severe threat. High pressure is expected to build back into the region through next weekend.

Carlaw

AVIATION /06Z TAFS THROUGH 12Z TUESDAY/

Issued at 1213 AM CDT Mon Apr 20 2026

Light and variable winds will gradually become southeasterly through Monday morning and then south-southwesterly into the afternoon. A lake breeze will develop, but it remains unclear if it will manage to push through ORD, MDW, or GYY prior to a general synoptic SE wind shift during the evening. At this time, the chances for a NE shift in the afternoon are the highest at GYY but still too low to add a mention in the outgoing 06z TAFs. After about 00-01z, winds should turn SSE/SE at all terminals.

Through late Monday evening and into Tuesday morning, winds will return out of the S to SSW, with occasional gusts developing, mainly at ORD, MDW, and possibly GYY. A mention of LLWS will probably be needed at RFD and possibly DPA after about 06z, just beyond the end of the current TAFs.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...Freeze Warning until 9 AM CDT this morning for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ105- ILZ106-ILZ107-ILZ108.

Frost Advisory until 9 AM CDT this morning for ILZ104.

IN...Freeze Warning until 9 AM CDT /10 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.