textproduct: Chicago

KEY MESSAGES

- Late spring to summer-like temperatures will continue through Friday with highs in the 70s and 80s and lows in the 60s.

- Several additional rounds of showers and thunderstorms, including a threat for severe weather and flooding, may occur as early as today-tonight, but especially Tuesday and Wednesday.

- A brief break in the pattern is expected late Thursday into or through the day on Friday before another storm system arrives in the Friday evening to Saturday timeframe.

- In the wake of the Saturday cold front, a brief, potentially sharp, shot of much cooler air could bring frost/freeze concerns into next Sunday and Monday.

DISCUSSION

Issued at 338 AM CDT Mon Apr 13 2026

Synopsis:

The transition season will live up to its billing and then some over the next week. The advertised late spring to summer-like stormy pattern (with some breaks) will be us through Saturday, followed by much cooler to possibly downright chilly conditions Saturday night through Sunday night. The timeframe of greatest concern for severe weather this week is Tuesday afternoon into the evening, for which SPC introduced a threat level of 3/5 (enhanced risk) in the overnight day 2 outlook.

Over the next 48 hours, aggregate troughing will become established across the southwestern United States and reinforce longwave ridging from the Mississippi River Valley toward the East Coast. Northeasterly shortwave shedding will support corresponding episodic surface cyclone development from the Plains through the Upper Great Lakes, altogether reinforcing a broad warm sector with late spring to summer-like temperatures into the region for much of the workweek.

Through Tonight:

The daytime hours today will start out dry but mostly cloudy, with low clouds near and north of I-80 initially limiting the diurnal temperature rise. This afternoon, there will be competing factors regarding whether any convection (likely rooted above the boundary layer) can materialize.

As the aforementioned aggregate troughing strengthens across the southwestern US today, a renewed push of southwesterly mid- level flow will advect a stout EML plume featuring very steep mid-level lapse rates (>8 C/km) into the Great Lakes. The strong capping at the base of the EML plume and expected mid-level height rises typically conceptually point toward dry conditions. However, we can't ignore the signal on some of the overnight CAM guidance for scattered thunderstorms on the nose of the EML this afternoon. Based on an analysis of RAP model fields (which initialize the HRRR), a possible subtle 700 mb wave with a defined wind response and appreciable moisture look to be the key player. Unfortunately, it's a bit too early to tell how realistic this depiction is (satellite and observational trends after sunrise this morning should help).

Should the base of the cap be moistened and cooled enough and updraft attempts overcome the lack of large scale forcing, the modeled scenario on some of the high-res guidance may very well come to fruition. Given the competing factors and inherent uncertainty, opted to introduce 30-40% PoPs translating eastward this afternoon. If a few robust thunderstorms are able to develop, effective shear of 30 to 35kt, weak effective inflow, and steep mid-level lapse rates would support a threat for supercells with both damaging winds and hail. The threat level 1/5 of in the initial day 1 SPC outlook appears appropriate for now in light of the highly conditional nature of this setup. Outside of the uncertain convective potential, expect highs in the 70s to around 80F (warmest south of I-80) this afternoon, with southwest winds gusting up to 25-30 mph.

Tonight, there may be a short window of continued potential for isolated to widely scattered storms this evening (20-30% PoPs), possibly related to outflow from previous storms (if any indeed occur this afternoon). Otherwise, intense thunderstorm development is favored to occur somewhere from central Minnesota into central Wisconsin as a subtle upper-level shortwave riding within broad upper-level southwesterly flow induces a low-level jet atop the stalled warm frontal zone to our north. In the absence of any convection this evening, attention will turn to likely activity to our north.

With the northeasterly terminus of the low-level jet pointing toward the west-to-east oriented instability axis from Wisconsin and eventually central Lower Michigan, any upscale growth into a mesoscale convection system should stay decidedly north of our area. However, should the warm front end up further south than currently in our forecast (as hinted by HRDPS/ECMWF output), part of northern Illinois may get a glancing blow by (likely weakening) storms. Currently don't see this as a particularly likely scenario, so will cap PoPs at 30-40% from the pre-dawn through daybreak hours.

Tuesday and Wednesday:

Toward the middle of the week, aggregate troughing will shift eastward toward the Four Corners Region leading to a corresponding "nudge" eastward in broad upper-level southwesterly flow into the Great Lakes. The overlap of the plume of steep mid-level lapse rates, stalled frontal zone, a steady feed of low-level moisture advection, and a parade of shortwaves and the eventual ejection of the aggregate trough with associated jet maxima to support deep-layer shear will support a threat for additional waves of severe thunderstorms in the general region.

Storm modes ranging from convective clusters to supercells with a threat for all hazards (including very large hail and tornadoes) will be fair game, particularly on Tuesday. While there is low confidence in exactly how Tuesday PM will play out (and how early storms will initiate), the potential is there for a very favorable setup for damaging to even destructive hail, which drove the SPC day 2 outlook upgrade (favoring near and north of I-80). It remains to be seen if locally backed winds can materialize, with respect to a tornado threat.

Additional, repeated rounds of storms over the same area may support a threat for flash flooding (possibly even locally significant) into Tuesday night. Prior rounds of convection into Wednesday morning may then muddle the picture convectively speaking for Wednesday PM. Ultimately, pinpointing favored locations and time windows will remain challenging until there is better clarity on exact shortwaves and eventual placements of boundaries.

Thursday through Sunday:

There's variance regarding the progression of a cold front across the region, with some of the slower solutions holding onto some potential for storms into Thursday afternoon. Thereafter, into Thursday night and at least the start of Friday, ensemble model guidance advertises a brief break in the stormy pattern as shortwave ridging moves through the Great Lakes ahead of a substantial trough digging into the central US. High temperatures look to rebound into the low to even mid 80s by Friday accompanied by another threat for strong to severe thunderstorms Friday night into Saturday. Thereafter, there is a reasonably strong signal in ensemble guidance for a cooldown to start next week.

The key aspect for the weekend into early next week cooldown is a recent trend towards a much cooler air mass due to trough amplification over the region. If this comes to pass, parts of the area may flirt with sub-freezing temperatures Saturday night into early Sunday, and again Sunday night into next Monday morning. With the summer-like warmth and rain this week supercharging plant growth and leaf out, the magnitude of the weekend cold shot will be one to watch for agricultural interests.

Castro/Borchardt

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 659 AM CDT Mon Apr 13 2026

Main Concerns:

- Chance for TS this afternoon into this evening, a few may be severe with 50+ kt gusts and large GR. In addition, erratic/gusty to ESE winds are possible during and in the wake of any TS at the terminals.

- Lower flight category trends today through Tuesday morning.

- Gusty south-southwest winds through the period, strongest tonight when a period of LLWS appears likely.

The main forecast focus is with possible TS this afternoon into this evening. While some guidance has consistently depicted TS development over the ZAU airspace, there still may be a play for limited TS coverage at most. Maintained PROB30 mention in the TAFs given the uncertainty, and broadened out the time range into this evening. As noted in main concerns, sufficient TS coverage may result in winds taking on a ESE component for a time into this evening before returning to southwesterly in the late evening. Some weakening gusty SHRA/TS may approach from the north overnight, though confidence was too low for any additional TS mention.

While some holes have developed in the OVC over the region, RFD and DPA were still reporting IFR CIGs and MVFR BR as of 12z. General trend should be gradually upward, with VFR conditions probable (aside from in SHRA/TS) into this evening. 1500-2500 ft CIGs are then favored to redevelop overnight into Tuesday morning. Finally, strong low level winds tonight should result in gusts to around 25 kt, while 55-60 kt southwest flow near and above 2kft AGL will likely yield a period of LLWS despite the surface wind gusts.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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