Area Forecast Discussion

KEY MESSAGES

- Shower and thunderstorm threat this afternoon-evening will focus primarily south of I-80, though showers and a few storms will also be possible near the lake breeze pushing inland.

- While there will be daily shower and thunderstorm chances through the upcoming week, expect plenty of dry hours. Outside of the convective chances, it will be very warm and humid (focused away from the lake until Tuesday).

DISCUSSION

Issued at 326 AM CDT Sat Jun 6 2026

Early This Morning:

Scattered thunderstorm development overhead in the pre-dawn hours (as of this writing) is associated with convectively modulated impulses and a glancing blow from a stronger short- wave pushing into the northern Lakes. It's unclear to what extent additional upstream convection from western WI to western/west central IL will push into the area over the next few hours. We'll need to continue to monitor for a few strong to potentially severe storms (wind and hail threat) given fairly steep mid-level lapse rates and 30-40 kt of effective bulk shear.

Today through Sunday:

In the wake of the early morning convection, aside from a few lingering isolated to widely scattered storms south of I-80, conditions should dry out areawide for a time. Fairly strong mid-level subsidence and drying, particularly north of I-80, casts doubt on much in the way of renewed PM thunderstorm development with northward extent in the CWA. The possible exception will be near a lake breeze boundary as it pushes inland across the Chicago metro this afternoon. Farther south, primarily south of I-80 (possibly well south), uncapped, moderately to strongly unstable air mass should yield corridors of scattered "airmass" type thunderstorms. With mid-level lapse rates decreasing to 6C/km or less and deep layer shear down to 25 kt or less, overall severe threat appears limited, likely relegated to localized strong to perhaps severe downburst winds. If a corridor of robust convection is able to persist into or through the evening despite modest at best forcing, high PWATs and high freezing levels could support localized flooding (albeit in our typically less flood prone southern areas).

Given expectation of increasing sunshine this afternoon, especially with northward extent, look for highs inland of the lake breeze in the mid to upper 80s amidst dew points in the 60s to around 70F. Near the lake, highs will reach the mid 70s to around 80F ahead of the lake breeze and then slowly but steadily fall thereafter.

After festering evening storms primarily south of I-80 erode diurnally, there should be a relative lull overnight into Sunday. The lake shadow will extend farther inland on Sunday, confining isolated to widely scattered pulse airmass convection at peak heating to the southwestern 1/3 to 1/2 or so of the CWA. Sub- marginal wind shear and lapse rates suggest severe weather is unlikely. Highs will reach the lower to mid 80s inland, but only upper 60s to lower 70s near the lake.

Sunday Night Onward:

Weakening mid-upper low/short-wave will shear out on Sunday night into Monday as it encounters robust ridging centered over the eastern Lakes by then. Nonetheless, weakly capped tropics- like airmass should support fairly widespread showers and at least scattered storms developing as early as midday Monday (unless debris cloudiness proves too detrimental to sufficient destabilization). Southeasterly synoptic flow and lake breeze reinforcement will keep IL shoreline locations in the 70s on Monday, while the rest of the area reaches the low-mid 80s.

Looking ahead at the rest of next workweek, there have been some guidance members with sufficiently strong mid-level ridging and low-level thermal ridging poking into the Great Lakes region for a few days of very warm (locally hot) and humid conditions. With that said, the more likely scenario is our area being on the precarious northwestern periphery of ridging centered near the East Coast. This should entail less capping and a continuation of shower and thunderstorm chances. By later in the week (Thursday or thereabouts), seasonably strong troughing from the north central US to central Canada could plausibly yield a pattern supportive of organized strong to severe convection into the region.

Note that the NBM depicted temperatures centered Tuesday through Friday (most notably on Wednesday and Thursday) are well outside of the ensemble spectrum due to likely spurious upward bias correction. Thus we will withhold from any heat messaging locally. It looks to be very warm and humid, but not hazardously so, per the current global ensemble guidance consensus.

Castro

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1231 PM CDT Sat Jun 6 2026

Key Aviation Messages for the current TAF period:

- A lake breeze arrival this afternoon will shift winds to the east-northeast

- Isolated thunderstorms near the lake breeze this afternoon, but better chances for thunder to the south of the Chicago Metro

Currently, conditions are VFR with light southerly winds. However, A lake breeze has already started to move inland away from the lake. Winds will switch to the northeast behind it with winds 5 to 10 knots.

Isolated showers and storms are expected this afternoon. However, the better coverage of thunderstorms is expected south of a KSQI to KVPZ line. That being said, the previously mentioned lake breeze could be enough forcing to trigger convection that could develop into at least a shower if not a thunderstorm near the terminals. While confidence is lower in any one cell directly passing over a terminal, the PROB30 was converted the -SHRA mention to the -TSRA given the amount of instability present on an already warm June afternoon. Chances for showers and thunder diminish after 23Z. Light northeast winds will slowly become more easterly tomorrow morning.

DK

LOT WATCHES / WARNINGS / ADVISORIES

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