Sections of the Chicago area appear headed for a severe weather outbreak late Saturday into Saturday night which warrants careful monitoring.
Unlimited sunshine, intense heat and oppressive humidities will mask the looming storm threat into Saturday afternoon. So will factors all but invisible to surface observers, such as a steepening vertical temperature decline by afternoon and a split in jet stream winds aloft–both developments which encourage air to rise with increasing vigor.
But Doppler radar scans are likely to be tracking proliferating thunderstorms to Chicago’s west and models suggest it wouldn’t be surprising at all to have a squall line–a line of intense thunderstorms–marching across the Mississippi River by mid and late afternoon. It’s that line which could create problems if it holds together in its race toward the Chicago area Saturday evening. Weather watches and warnings may become necessary later today into tonight as these storms approach and pass.
Plains storms, forerunners of this evening’s Chicago-area outbreak, produced damaging 80 mph gusts Friday evening
Storms, which raked the Plains late Friday, offer an insight into what Saturday evening’s storms may be like here in the metro area.
Winds accompanying a series of thunderstorms gusted to 80 mph as they moved across Turton, Redfield and Mellette, South Dakota. 79 mph gusts swept Mitchell and 78 mph gusts hit Huron–both in South Dakota.
Powerful thunderstorm winds produced a trail of downed trees and tree limbs north into Minnesota. Gusts at St. Cloud were clocked at 59 mph. A bow-shaped squall line raced into the Twin Cities at 40 mph late Friday night prompting weather warnings across the region.
Heat contributing to severity of storms by strengthening upper level winds
The intense dome of heat which has broiled the nation’s Heartland in the past week is contributing mightily to the weather pattern which may generate Saturday evening’s storm outbreak in the Chicago area by strengthening upper-level winds. Predicted winds at jet-stream level late Saturday (15,000 to 35,000 feet aloft) are to reach velocities rare for August later Saturday. These high-altitude winds play a role in strengthening thunderstorms and even contribute to the wind gusts they produce at ground level.
Jet stream level winds form at the demarcation between vast pools of cool and hot air in the atmosphere. The bigger the temperature spread between these varied air masses, the stronger the winds aloft.
The extreme heat which has broken records all week up and down the Plains, has significantly strengthened the summer jet stream by increasing the difference in temperature between these air masses.
Friday’s 95-degree O’Hare high the hottest August temperature in 6 years; Saturday’s high headed for 97–isolated, 100-degree readings possible
Friday’s 95-degree high at O’Hare made it the 38th day of 2012 to reach or exceed 90-degrees there. And, the hot readings occurred despite hours of well developed east to northeast winds off the lake. That more cooling didn’t occur is a testament to how warm Lake Michigan has become this summer and the fact we’re so far into the warm season, that hot weather has become so expansive, not even contact with Lake Michigan water drops the temperature all that much.
The weekly report from the U.S. Army Corps of Engineers indicates lake water levels have now dropped 10-inches below a year ago. The shoreline water temperature taken the City of Chicago’s Water Management Department near the water intake at the Jardine Water Purification Plant close to Navy Pier is running 11-degrees warmer than the same time a year ago. The reading was 82-degrees Friday versus 71-degrees last year.
Chicago headed for its 39th official 90+-degree high of 2012
Saturday’s predicted 97-degree high will mark the 39th daytime temperature at or above 90 in 2012 and the 40th 90-degree+ reading at Midway.
Oklahoma City ties its all time record of 113-degrees; the city heads into its 18th consecutive triple digit high Saturday
Oklahoma City has topped 100-degrees 22 times this year. Friday’s all time, record-tying 113-degree high there marked its 17th consecutive triple digit maximum temperature and equaled the city’s highest temperature ever recorded 76 years ago on August 11, 1936.
Wild fires, possibly started by arsonists, sent flames and clouds of smoke high into the air Friday, in the area surrounding Oklahoma City. The state is under an outdoor burning ban because of the extreme heat and serious drought conditions plaguing the area.
Tropical Storm Ernesto churning across the Caribbean; could become Category 1 hurricane in next 3 to 5 days
Tropical Storm Ernest is headed over warmer Caribbean waters in coming days, a development which leads tropical forecasters to believe the storm will strengthen to hurricane intensity (74+ mph sustained winds) in the next three or four days. The storm’s peak sustained winds Friday were 50 mph.