Rain-weary Chicago-area residents have just witnessed the longest spell
of precipitation-free weather in nearly a month. But thunderstorm
prospects are to surge later Wednesday afternoon and evening, and again
Thursday afternoon and evening, with an influx of Gulf moisture. A
spotty shower or two isn't out of the realm of possibility into early
Wednesday afternoon. But the primary thunderstorm development is to be
stoked by daytime heating that is to take place beneath a pool of
comparatively cool air aloft. Heating of the lower atmosphere encourages
air to become buoyant and ascend into cooler reaches of the atmosphere.
Computer model vertical-temperature profiles suggest this process could
produce towering cumulonimbus clouds-the suffix "nimbus" always refers
to precipitation falling from clouds-which could extend to heights of
nearly 40,000 feet. Modest winds at the higher steering levels of the
atmosphere could allow any thunderstorms that form to move comparatively
slowly, producing locally heavy rain totals.
70-degree spell to extend to 6 days
The current mild spell that began Sunday is likely to continue through
Friday. If forecasts verify, the six-day string would arrive nearly two
weeks earlier than the average starting date of the season's first
six-day 70-degree spell, (May 15).
