From shortly after noon until early evening Sunday, temperatures hovered at or over 100° across the Chicago metro area. Strong westerly winds drove a warm tongue of low-level heat from the mid-Mississippi Valley across northern Illinois, but these strong winds did alleviate the potentially deadly effects of a higher heat index by mixing drier air aloft to the surface.
As a result, dew points dropped from the mid 70s into the lower 60s—in effect cutting the potential 115°-120° heat index values back into the 103°-107° degree range. For example, veteran weather observer Frank Wachowski reported at Midway Airport that the dew point at 8 a.m. was 75°—the highest of the summer—but as the strong winds mixed in drier air from aloft, the dew point slowly but steadily fell, reaching 63° by mid-afternoon. This diluting of moisture near the ground significantly reduced reported heat index values by approximately 15 degrees.

Seventy one years ago today, on July 24, 1934, Chicago established its all-time official high temperature of 105º, and today’s triple-digit maximums should not be far behind. A hot air dome that sent temperatures soaring to as high as 110º in eastern South Dakota Saturday will surge into the Midwest today, bringing Chicago its first 100º weather in six years.
A long-lasting line of thunderstorms called a derecho that packed winds as high as 85 m.p.h. formed just north of the area of heat and humidity Saturday, sweeping out a swath of wind damage from northeast South Dakota to southeast Wisconsin. The storms dissipated just before reaching northeast Illinois, depriving this region of much needed rain, but sparing the area from power outages just before the intense heat moves in.
Hot weather will last into Monday before northeast winds bring much more comfortable conditions for the remainder of the week.