The Chicago area is in the midst of its most extended break in rainfall of the past three
weeks. More than half of the days since April's open—19 of the past 35 days (54
percent of them)--have registered rain. The past two weeks alone have seen more than
3 inches of rain (3.16 inches) fall. A shift in the jet stream is behind the drop in rainfall.
Pockets of especially strong winds within jet streams--referred to by meteorologists as
jet streaks--act to lift and cool air, igniting thunderstorm development. But the jet
stream itself has sagged south in recent days, which has diverted the most organized
thunderstorms well south of Chicago.
With rainfall and accompanying cloud cover more scarce than in many recent days,
temperatures have been able to surge. Monday’s 70 degree high followed a similar high
on Sunday and marked the ninth time in 2009 the mercury has reached or exceeded the
70-degree mark. Additional 70s are predicted each day through Friday. And it's not out
of the question this year's second 80-degree day may occur Thursday if widespread
thunderstorms hold off until after sunset.
Thunderstorms will flare Wednesday, however, and could tap a northbound flood of Gulf
moisture to produce downpours at some locations.
