One of Chicago’s five warmest late March periods in more than a century ended with powerful wind and hail generating t-storms late Wednesday. The day’s official high temperature of 77° was 25° above normal! Readings that warm typically occur three weeks later around April 19.
Late day t-storms took advantage Wednesday of a lower than average freezing level in the atmosphere, generating more than six-dozen reports of hail 0.75” in diameter or greater across the metro area. Will County alone logged 23 such reports followed by Cook County with 12. Firefighters at Engine 127 near Midway Airport reported hail covering the ground around 6:45 p.m. Hailstones in Lemont reached 2.75” in diameter—the size of baseballs. And Joliet, Romeoville and Woodridge were hit by 1.75” hail. It was the first time since 1992 March t-storms in the city have been hail producers. More than 400 cloud-to-ground lightning strokes flashed across the area in one ten minute period around 6 p.m.

Warm weather isn’t over yet. A second day of 70s appears a good bet Wednesday amid strengthening south winds likely to gust over 30 m.p.h. by afternoon. An intense spring storm in the Plains is responsible for the wind and warmth. And, the system could spin up even stronger gusts Wednesday night once a wave of potentially severe late afternoon or evening t-storms sweeps across the area. The powerful, post squall line winds tonight are to be associated with a storm feature known as the dry slot—a region within which air sinks vigorously, producing the narrow cloud-free corridor or indentation in the cloud mass which lends storms their comma shape when viewed on satellite imagery. This wedge of dry air can host particularly powerful winds, because it marks a region in which the air flow through a deep layer of the atmosphere is moving in the same direction. That’s a formula which could lead to overnight gusts of 40 m.p.h. or higher at times.

The year’s first 70° high—the mildest reading here since 72° on Oct. 30—is a good bet Tuesday. Sun-filtering high clouds may well shave a few degrees off temperatures at some locations, restricting highs there to the mid/upper 60s. And, in communities closest to Lake Michigan’s northeast Illinois shoreline, an easterly component to Tuesday’s dominant southerly winds is likely to restrict highs to the low/mid 60s. But, for the rest of the metro area, readings Tuesday will surge 5-10 degrees above Monday’s. An official 70° high Tuesday would nearly match the 134-year average date for the region’s opening 70° reading of the year, which is March 31. Normal highs of 70° don’t occur here for another month and a half (May 16). The 60° high at O’Hare was the city’s highest in three weeks.
Big rains flooded parts of the East Monday, establishing daytime rainfall records at New York’s Central Park (2.94”) and in parts of five states from Florida north to Delaware.