textproduct: Chicago

KEY MESSAGES

- After a summer-like afternoon, showers will move across the area this evening and tonight.

- Summer-like temperatures will continue Monday through Wednesday with highs in the upper 70s to lower 80s and overnight lows in the 60s.

- Waves of severe thunderstorms (all hazards including flash flooding) may occur in the region as early as Monday night but more likely Tuesday and Wednesday.

- A brief break in the pattern is expected on Thursday before another storm system arrives in the Friday to Saturday timeframe.

- A transition toward a cooler and less stormy pattern is favored by early next week.

DISCUSSION

Issued at 1252 PM CDT Sun Apr 12 2026

This Afternoon through Tonight:

A recent hand surface analysis places a 996 mb low pressure system near the North Dakota, South Dakota, and Minnesota borders early this afternoon. A warm front extends eastward away from the center of the low across northern Wisconsin and central Lower Michigan, while a hybrid cold front/dry line extends southward through the southern Plains. Much of the Mississippi River Valley, including our local area, is in the broad warm sector of the deepening cyclone. Aside from southwesterly winds gusting 30 to 40 mph, it is otherwise a beautiful mid-April afternoon with partly cloudy skies and temperatures in the mid 70s to lower 80s.

With continued broad southwesterly flow across the Mississippi River Valley, an upper-level shortwave currently lifting into central Missouri will progress northeastward and into the Lower Great Lakes this evening. Gradually increasing DCVA atop a somewhat moist and largely uncapped mixed layer should support the development of scattered showers as soon as 3 PM primarily northwest of I-55 this afternoon. Coverage of showers should increase steadily thereafter and peak across our area between 6 PM and 10 PM as the heart of the wave passes directly overhead. With the instability profile remaining fairly low-centric (a consequence of meager lapse rates near 6 K/km), the threat for lightning with showers this afternoon and evening remains low (15% or lower). While this evening does not look like a complete washout, keep a rain jacket handy if out and about!

Monday and Monday Night:

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 summer-like temperatures into the middle Mississippi River Valley through midweek. Indeed, highs Monday through Wednesday are favored to remain in the upper 70s to lower 80s while overnight lows will only "fall" into the low to mid 60s.

As the aforementioned aggregate troughing strengthens across the southwestern US tonight into Monday, a renewed push of southwesterly mid-level flow will advect a plume of steep mid- level lapse rates (>8 K/km) into the Great Lakes. The combination of stout capping at the base of the EML plume, neutral to modestly positive mid-level height tendencies, and a lack of robust mechanisms to force ascent collectively suggest that daylight hours on Monday will remain dry. However, locally augmented convergence along the edges of advecting EML plumes usually supports agitated ACCAS that, if clumped enough to locally cool/moisten the base of the outer edge of the cap, can lead to surprise convection. Accordingly, will manually pencil in a northeastward-moving ribbon of 20% PoPs for thunderstorms tomorrow during the afternoon and evening hours keeping in mind the heavily favored (80% chance) outcome is a completely dry day. Should a thunderstorm 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. Again, we heavily favor tomorrow being dry across our area.

Tomorrow night, intense supercellular (and potential GWAC) 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 excites a low-level jet atop the stalled warm frontal zone. 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 storms toward daybreak Tuesday. Currently don't see this as a particularly likely scenario, so will cap PoPs at 30% favoring areas north of I-88 through the overnight hours.

Tuesday into 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 tornadoes) will be fair game. And, repeated rounds of storms over the same area will support a threat for flash flooding (locally significant). With all of that said, pinpointing favored locations and time windows will remain challenging until there is better clarity on exact shortwaves and eventual placements of boundaries. This is all to say, the Tuesday through Wednesday timeframe continues to look quite busy in our neck of the woods.

Thursday onward:

Ensemble model guidance advertises only a brief break in the summer-like and stormy pattern on Thursday 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.

Borchardt

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1252 PM CDT Sun Apr 12 2026

Key messages for the 18Z TAF period:

* A period of showers this evening with associated MVFR vsbys possible

* MVFR cigs moving overhead this evening and remaining into Monday with IFR favored over RFD

A system of showers will lift across the area this evening. Showers may get going at RFD as early as mid-late afternoon, during which an isolated embedded thunderstorm will be possible. An embedded storm cannot be ruled out this evening but thunder coverage, if any, should be scarce. Pockets of moderate to heavy rain will be possible, as will periods of MVFR vsbys. Rain will move away toward the end of the evening.

MVFR cigs will settle overhead this evening and remain into Monday. At RFD, IFR cigs are favored. Probs for IFR are lower across Chicagoland, but it's possible that a period or two of IFR materializes through early Monday. Guidance favors early Monday afternoon for a return to VFR. Expect largely P6SM vsbys overnight, except at RFD where BR looks to keep MVFR vsbys around for much of the night and into early Monday. A period or two of MVFR vsbys cannot be ruled out at the other sites during this period, primarily at DPA and GYY.

Meanwhile, SW winds will continue gusting to 30 to 35 kt through the remainder of the afternoon before subsiding to closer to 25 kt for the evening. SW direction will hold at 10 to 20 kt overnight and during the day on Monday. A period of gustier SW winds may materialize during the late afternoon and early evening.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Gale Warning until 9 PM CDT this evening for the IL and IN nearshore waters.

Small Craft Advisory from 9 PM this evening to 4 AM CDT Monday for the IL and IN nearshore waters.