The abnormally cool temperatures predicted this final weekend of the month may flirt with records and end up the coolest Saturday/Sunday period in a July since 1984---29 years ago.
The large pool of cool air expected to position itself across a huge swath of the Midwest sets the stage for a good deal of cloudiness here. A few scattered “instability” showers---i.e. spells of rain produced by the steep rate at which temperatures fall with height---can't be ruled out.
Clouds and showers form in this sort of weather regime as warmer air near the surface becomes buoyant and ascends. The cooling which results leads to extensive cloud formation.
Friday’s t-storm clusters precede the unseasonable weekend chill
Friday's scattered morning showers are to segue into a period of more numerous showers and thunderstorms in the afternoon as a cold front approaches. Rainfall is likely to impact up to 60% of the metro area.
These potentially thundery rains
are to fall in a month which, unlike its recent predecessors, has grown dry. O'Hare's rain tally of 1.29" amounts to just 45% of normal. It’s a sub-normal precipitation trend shared by the majority of observation sites across the metro area.
Saturday temps may not break above 70 and could flirt with or set a new record low daytime maximum for the date
Saturday temperatures, which reached the upper 80s just a week ago, may struggle this weekend just to reach the mid or upper 60s. A reading below 69-degrees would set a new record for the lowest maximum on the books for July 27. The previous record this date was set in 1981.