Chicagoans anxious for a return of summer’s heat will have to keep waiting. The consensus of computer models is that a major weather pattern change is in the works, and though it will take several days for the entire realignment to occur, its implications for Chicago are significant. Area temperatures that have been running near or below seasonal normals in recent days will be heading into the surplus category by next weekend.
It will be warmer—readings into the 80s—but it certainly won’t be a heat wave.
For most of us, 90º is the definition of a hot day. Midway Airport logs 24 of them in an average year, the first one typically arriving on May 29. With no 90s yet in sight, this year’s first occurrence will be delayed.
Midway’s temperature climatology suggests patience in that regard. Every summer has delivered 90° temperatures, but in 1928 the season’s first 90° day did not arrive until July 2.

Seattle on Friday was under a Heat Advisory with highs in the lower 90s expected—some 20 degrees higher than normal. At the same time, northern Illinois experienced readings 20 degrees cooler. It will take the first half of the week ahead, but the cold upper-level low that has dominated Great Lakes weather will slowly drift east, allowing a ridge of high pressure to build out of the southwest into the Midwest. The resulting southwest flow will push 80° readings into northern Illinois by next Thursday with the first 90s of 2005 possible next weekend. However, the immediate interest is in this Memorial Day weekend with latest indications of a significant rainfall Sunday night as low pressure tracks just to south and east of Chicago. Memorial Day may start off with rain, but as the low moves east, clouds should thin as sun breaks through in the afternoon.

Inland locations of California, Oregon, and Washington experienced highs well into the 80s and 90s Thursday; meanwhile, New England shivered in the 40s and lower 50s with freezing rain and wet snow observed in the mountains of Maine and New Hampshire. Chicagoans located in between, ended up with readings in the lower 70s, about half-way between the two extremes. Today could see more of the same although skies over northeast Illinois will start out sunny but quickly see popcorn-like cumulus clouds develop during the forenoon with scattered showers even thunderstorms during the afternoon. Cold air aloft and heating at the surface will create very unstable atmospheric conditions over northern Illinois today. By early next week the upper low should be well to east, still dominating New England weather, while Illinois appears to be establishing a slow warming trend culminating in 80° temps Thursday.