By Tom Skilling
The books close on warmest meteorological summer in 15 years amid the best chance for soaking rains in more than a month.
Air conditioners have hummed nearly twice as often over the past three months as the same period a year ago, according to estimates based on temperature data. Tuesday's 93-degree highs at O'Hare and Midway marked the 21st and 23rd times respectively that the mercury has topped the 90 mark this season at the city's two major observation sites.
While summer officially has another three weeks to run -- the astronomical end to the season doesn't occur until 10:09 p.m. Sept. 22 -- the 3-month June through August "meteorological" summer period ended at midnight, finishing the warmest summer here since 1995. That was the year in which the city was gripped by a blistering July heat wave which went on to claim 700 lives.
Meteorological summer 2010 averaged 75.2-degrees -- 6-degrees warmer than the same period a year ago and 4.1 degrees above normal. That was warm enough to place the season's temperature eighth warmest of the 140 meteorological summers on the books since records began in 1871.
Thunderstorms wallop Iowa with 3-inch or more rains Tuesday
Thunderstorms, some towering 57,000 feet into the atmosphere, exploded to life in warm, humid air across Iowa and Minnesota late Tuesday. Rainfall at the hardest hit locations in Iowa was stunning: 3.85 inches has fallen by 9:49 p.m. at Sheldahl, Iowa, 22 miles northwest of Des Moines, and 3 inches was reported at nearby Madrid. More than 3,200 cloud-to-ground lightning strokes accompanied the powerful storms, which whipped other areas of the Hawkeye State with winds as high as 50 to 60 mph. Templeton, Iowa, clocked a gust of 64 mph while pea to nickel-sized hail hit Churdan, Iowa.
Thundery rains for Chicago
The same atmospheric set-up will send showers and thunderstorms into the Chicago area a portion of Wednesday, generating at some locations the heaviest rains which have fallen since cloudbursts in July produced flooding at some locations. Predawn and early morning thunderstorms are likely to thin out as Wednesday morning proceeds. But several lingering showers can't be ruled out beyond the opening wave of rainfall and just a modest degree of heating Wednesday afternoon could coax the unstable atmosphere into new thunderstorm development, particularly over scattered locations in the southern suburbs.
New shower and thunderstorm development takes place Wednesday night into Thursday morning. The emergence of mixed sunshine Thursday afternoon and evening might well prompt yet another round of thunderstorms, some potentially severe. Winds aloft will have strengthened by then, possibly offering thunderstorms that can transfer some of that wind energy to the surface.
Extended dry period breaking
Rainfall numbers vary between computer forecast models -- hardly unusual in summer when so much rain is generated by thunderstorms which are capable of concentrating precipitation over comparatively small swaths of real estate. The potentially repetitive nature of some thunderstorm development in the humid, unstable atmospheric environment Wednesday into Thursday night increases the potential for some final rainfall tallies to be quite heavy.
Two-week Chicago rainfall estimates from a suite of model forecasts Tuesday put potential rainfall accumulations between 1.5 and 6 inches -- well above the 1.73 considered normal for a 14-day period this time of year.
Rain will be welcome in parts of the area. John Hazzard, who farms in Will County near Wilmington, reports rain hasn't fallen there in 17 days -- and that rain fell on only three days in August.
Forecasts keep Earl's eye offshore
Hurricane Earl's sustained winds remained near 135 mph Tuesday evening, strong enough to place the storm as a solid and extremely dangerous Category 4. Hurricane watches were hoisted for much of eastern North Carolina, an area whose beaches are likely to be raked by increasing surf as Earl approaches from the southeast.
Though a landfall isn't predicted at this point, Earl's path is likely to carry the storm on a northwest-to-northerly track paralleling the Eastern coastline from North Carolina north to New England. The storm is large and its swells are likely to bombard the East Coast with waves and possibly the storm's outer squalls.
Placed over Chicago, Earl's tropical storm force (39+ mph) winds would extend up to 200 miles from the center while its hurricane force winds (74-plus mph) would reach from Joliet to Beloit to Saugatuck, Mich.
Sharp temperature downturn to be transitional
The cooldown later this week is to be a strong one -- but temporary. While highs Friday may not climb out of the 60s at many locations, the pattern of northwest steering winds aloft is a transitional one. By early next week, temperatures are likely to be in a rebound, surging from 67 Friday to 82 Monday and into the 90s Tuesday.
