By Tom Skilling, WGN-TV Chief Meteorologist
It's to evolve into one of those magnificent spring weekends many here contend make Chicago's winter chill and snow worth enduring. The clouds and areas of fog which have developed overnight Friday and have grown dense at some locations--are to give way to sunshine and warmer temperatures Saturday--and to do so expeditiously. Visibilities just north of Chicago had dipped to a half-mile or less late Friday evening from Milwaukee south to Kenosha and Waukegan as nighttime cooling converted airborne moisture in the wake of late-day thunderstorms to fog.
The reappearance of sun by mid and late morning Saturday sends weekend temperatures soaring more than 20 degrees above last weekend's readings and to levels not exceeded since last Aug. 15 and 16 when peak Saturday/Sunday readings hit 89 and 88 degrees respectively.
A huge alteration in high-altitude steering winds across the U.S. is driving the warm-up. And once a new warmer pattern is in place, computer models generate atmospheric blocking which is to lock a warm, rain-free atmospheric set-up in place through the coming week.
Though strengthening southerly winds deliver the year's most humid air to date by Sunday afternoon with dew points predicted to approach 70 degrees--a level common on the Gulf Coast--warmth aloft is predicted to stifle rain development. Referred to as a "cap," its presence will discourage air from rising or cooling as vigorously as might otherwise occur in the presence of the near 90-degree ground level temperatures which are being forecast.
The eight latest runs of the National Weather Service's "GFS" (Global Forecast System) model suggest two-week rainfalls may end up close to the long-term averages. But much of that two-week tally is predicted to fall in Week #2--not the first. If this forecast verifies, the closing days of May could end up wetter than the week immediately ahead.
Thundery rains drench portions of the Chicago area; storm damage and flooding sections of northern Indiana and southern Michigan
Thunderstorms swept the Chicago area in waves Friday, unleashing a barrage of lightning and driving downpours both east and west of the city. At one point Friday afternoon, 1,700 cloud-to-ground lightning flashes occurred in a 10-minute period. Only an hour later, the storm's intensity had mellowed noticeably, and the 10-minute lightning tally had dropped to just 500.
Thursday's driving downpours yielded 1.10" at Aurora, 1.09" at Algonquin, 0.96" Carpentersville, 0.88"at Rensselaer, Ind., 0.77" Palatine, and 0.57" at West Chicago and Valparaiso, Ind.
Rainfall proved so prolific from the late-day thunderheads which towered 35,000 feet above Chicago's west and northwest suburbs late Friday that an urban and small screen flood warning was issued for a time across sections of Du Page, Kane, Lake and Mc Henry counties by the National Weather Service. Minor flooding was reported in the Bartlett area.
Farther east, storms in Indiana and Lower Michigan proved the region's most damaging Friday. Thunderstorm gusts at Colon, Mich., downed trees, while Three Rivers, Mich.,--which also suffered tree damage--was walloped by 2.25" of rain in just half an hour.
Blocking pattern to lock warmth, rain-free weather in place
Not only is this weekend's predicted warm-up expected to deliver May 2010's first 80-degree readings, the warmth is predicted to stick around--tempered only by lake breezes predicted to sweep off cool Lake Michigan waters into shoreline areas from northwest Indiana north to Wisconsin.