Below-average temperatures give way to a weekend of on-and-off thunderstorms, beginning Friday evening.
Weather models show Friday storms could produce frequent lightning, torrential downpours and wind gusts of up to 55 mph in northwest and north central Illinois, as well as northwest Indiana.
Multiple rounds of storms are expected late in the afternoon and evening, beginning in areas west of McHenry County and spreading south.
Only about 20 to 30 percent of the Chicago metro area likely will see afternoon storms, as high temperatures continue to trend below-average in the mid- to upper-70s. By Friday night, conditions become more humid, and scattered showers and storms are likely at times across the area.
Rain continues into early Saturday before the day turns cloudy, much warmer and more humid by the afternoon.
Showers and thunderstorms are expected to develop by late afternoon. Some storms could be severe with damaging wind, large hail and heavy rain through the evening and at night, mainly along and south of the I-80 corridor.
High temperatures could reach 90 degrees with a heat index in the mid-90s.
Periodic showers and storms are expected to persist through a warm Sunday afternoon with highs in the low 80s.
High waves also mean potentially dangerous swimming conditions along beach areas, especially on Indiana shores, Sunday night into Monday.
Temperatures drop back into the 70s for the first part of the week.