The same humid tropical air responsible for more than 2 feet of rain and waves of thunderstorms across sections of Florida last week has made its way to Chicago.
That this air mass is of tropical origin isn't immediately apparent because of winds off a still chilly Lake Michigan, but its moisture content is bountiful. Nearly 2 inches of water is evaporated in a column of air over Chicago -- moisture that is available to be swept into the concentrated downpours of thunderstorms expected to build this afternoon and evening as daytime warming takes place. Converging surface winds along a northbound warm front threaten to enhance the upward motion of air later in the day and into the night. Further complicating the potential for heavy rains are the light winds stacked high in the atmosphere above Chicago. At jet stream levels -- from 18,000 to 40,000 feet where winds can reach 80 m.p.h. or more this time of year -- winds Tuesday should hover near 20 m.p.h. This will slow the forward movement of any thunderstorms that erupt, prolonging downpours at the hardest-hit locations.
That this air mass is of tropical origin isn't immediately apparent because of winds off a still chilly Lake Michigan, but its moisture content is bountiful. Nearly 2 inches of water is evaporated in a column of air over Chicago -- moisture that is available to be swept into the concentrated downpours of thunderstorms expected to build this afternoon and evening as daytime warming takes place. Converging surface winds along a northbound warm front threaten to enhance the upward motion of air later in the day and into the night. Further complicating the potential for heavy rains are the light winds stacked high in the atmosphere above Chicago. At jet stream levels -- from 18,000 to 40,000 feet where winds can reach 80 m.p.h. or more this time of year -- winds Tuesday should hover near 20 m.p.h. This will slow the forward movement of any thunderstorms that erupt, prolonging downpours at the hardest-hit locations.
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