Last Update: 6/22/2025 6:00 PM CDT
It's quite a hot day out there, with mostly sunny skies (save for a few diurnal cumulus developing), very dry vertical profiles allowing for plentiful diurnal heating, and thermal ridging overhead (+20-25ºC 850 mb temperatures), highs have easily soared into the mid 90's this afternoon. Deep boundary layer mixing up to around 850 mb has brought down breezy sustained southwesterly 20-25 mph winds to the surface, and gusts up to 30 mph. Lingering low-mid 70's dewpoints, in tandem with mid 90's surface temperatures have allowed for peak heat indices in the 100-105ºF range, thus justifying the extreme heat warning issued by the NWS for Cook county through Monday night.
Breezy southwesterly winds will ease slightly to 15-20 mph after sunset as vertical mixing shuts off, but gusts up to 30 mph will still persist, thanks to a tight pressure gradient caused by low pressure building in the central plains. Gusts will be most pronounced in the mid-morning hours as a low level jet noses into the northwest of the city. Skies will remain clear tonight, with the massive upper ridge still set up over the eastern half of the CONUS. Despite the clear skies, overnight lows will only make it down to the mid-upper 70's areawide, owing to persistent southwesterly flow, and the hot mid 90's highs we had today. With dew points still lingering in the upper 60's and low 70's, it'll be another muggy night with peak relative humidities approaching 80%.
Tomorrow's conditions look to be pretty much the same as today's with sunny skies, though with slightly more dry air in the vertical profiles, dew points will likely mix down to the mid 60's throughout the afternoon, particularly in the northwest suburbs. This should allow highs to make a run into the upper 90's (possibly 100 in some locales), but lower surface relative humidities will top out peak heat indices in the low 100's. Deep boundary layer mixing, along with a tightened surface pressure gradient will once again allow for 15-20 mph southwesterly winds, gusting up to 30 mph.
The upper ridge responsible for our current heat wave will gradually deamplify, and dip southwards throughout the work week, which will send a cold frontal boundary in the northern plains southeastward, eventually hitting northwestern Illinois tomorrow evening. This looks to be the impetus for multiple shower/storm chances throughout the rest of the week, primarily focused along the frontal boundary in northwestern Illinois, at least until Tuesday evening when it sags further south. Lingering 850 mph temperatures near 20ºC should provide sufficient capping to advert any thunderstorm chances until at least Tuesday evening as well.
Weak mid level (3-6 km AGL) lapse rates ~6ºC/km, along with relatively weak upper level flow providing little deep layer shear should limit the overall organization of any storms that do form after Tuesday evening. With low-mid level flow still remaining southwesterly, providing plenty of moisture that will pool together along the frontal boundary, along with peak daily CAPE greater than 2,000 J/kg, and PWATs in the 1.5"-2" range, heavy downpours and overall soaking rainfall appear to be the primary concern with this system. While showers and storms should provide some relief from this weekend's scorching heat, the still very warm air mass in place at the surface, particularly south of the cold front, will result in daily highs in the low-mid 90's, and lows in the low-mid 70's.