Plaincast

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Warm and humid conditions will continue through Saturday.

Smoke persists across portions of northeast Illinois and northwest Indiana through Friday morning.

A few showers and storms are possible this afternoon, with scattered coverage expected on Friday. Additional storms may develop along a cold front Saturday, some of which may be capable of strong to locally damaging wind gusts.

Discussion

Wildfire smoke from fires in Ontario and northern Minnesota descended into portions of northern Illinois and northwest Indiana early this morning resulting in unhealthy to hazardous air quality conditions. Visibility reductions within this smoke layer have been as low as 1/2 to 2 miles, lowest across far northeastern Illinois and along the lakeshore. There has been some slight improvement on the southwest edge of the smoke layer in part due to daily mixing this afternoon. Hi-res near- surface smoke models suggest the smoke layer may slosh back farther inland into this evening though with a reinforcing lake breeze leading to continue hazardous air quality through the overnight hours. Low-level flow will turn southwesterly during the day on Friday which may help lift the smoke layer out of the area and improve air quality gradually through the day.

daily cumulus has developed this afternoon over much of the area south of the area of greatest concentrations of smoke, currently south of a Harvard to Chicago to Valparaiso line. Can't rule out a stray shower or storm within this region of cumulus between now and sunset, though a locally higher potential exists within a low-level convergence axis along a Dixon to Morris to Fowler line. In addition to a few lightning strikes and localized downpours, brief funnel cloud also can't be fully ruled out. Severe weather is not expected and the majority of the area is expected to remain dry.

Looking ahead to Friday, temperatures are forecast to warm into the mid 80s to around 90, warmest in the Chicago metro. Increasing cloud cover and shower/storm coverage, particularly south of I-80 initially, may lead to slightly lower temperatures in those areas. If coverage ends up lower, temperatures and heat indices may end up higher than currently forecast. Hi-res guidance continues to be rather sparse on coverage compared to some of the global guidance. However, forecast soundings are largely uncapped during the afternoon coinciding with a weak mid-level vort pivoting into the area. Have accordingly included "scattered" coverage wording in the latest forecast. Primary hazards with any storms include lightning, gusty winds, and torrential rainfall rates (tropical precipitable water to 2"+) which could lead to localized flooding.

Hot and humid conditions continue on Saturday, potentially slightly warmer than Friday, with highs in upper 80s to lower 90s and peak heat indices in the 95 to 105 range. A mid-upper shortwave and associated surface cold front is expected to sweep across the area in the afternoon-early evening timeframe. Ample instability in place ahead of the front (MLCAPE around 3000 J/kg) will be supportive of scattered thunderstorm development along the front as it moves across the area. Given increasing mid-upper flow with the approaching wave, the strongest storms may be capable of gusty to locally damaging wind gusts (severe threat level 1 of 5).

We will also have to monitor smoke levels in the wake of the front into Saturday evening, as it may bring additional smoke back to the surface.

Sunday looks like a nice day with seasonable temperatures in the mid 80s (a bit cooler near the lake) and more comfortable humidity levels. A stronger synoptic-scale trough will then move across the region Monday afternoon and evening, sending another cold front through the area. Saturday's front will only send the low-level moist axis into Iowa, so this may not have much of a problem surging back east across the forecast area on Monday. Dewpoints are progged to rise back into the mid to upper 70s ahead of the front while a plume of steeper mid-level lapse rates build in aloft. While capping could pose a challenge to convective development, significant flow through the column, coupled with building instability suggests this will be a period to monitor closely for more of a strong to severe weather threat in the region. Depending on timing, the threat for storms could continue into Tuesday.