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December 2007 dawns much as the month did a year ago -- with the season's first significant winter storm in the area. While a 6-8 hour period of sleet and freezing rain starting this afternoon is the major threat with this year's storm, snow was coming down hard as Dec. 1 got underway a year ago. The thundersnow which accompanied the wintry system was a product of cloud tops scanned to heights of 30,000 feet -- unusual in a cold-season storm -- and so loud, it set off car alarms. By the time that system exited, 6.2" had been measured at O'Hare and 3.3" at Midway; some northwest suburbs were covered by nearly a foot of snow.
The scope of Saturday's storm is remarkable. Sections of 24 states were under winter storm advisories Friday night. Ice may accumulate as sleet falls, then shifts to freezing rain. Supercooled droplets falling into the subfreezing air will freeze on contact with colder outdoor surfaces until temps rise above freezing late tonight.
--By Tom Skilling, WGN-TV Chief Meteorologist
The books close on Chicago’s chilliest November of the past five years Friday night at midnight. The month’s 38.6° estimated average temp -- 4 degrees cooler than November a year ago and nearly 0.5° below normal—counters the mild temperature trend which has dominated the three month meteorological autumn 2007 period. The season is to finish 20th warmest since records began here 137 years ago.
Friday’s predicted high of 34° becomes the eighth of the past 9 days with highs which have failed to break out of the 30s. Thursday’s 33° high was the second coolest of this fall and a stunning 30° cooler than the 60° on the same date a year ago.
December, typically the area’s cloudiest, 2nd snowiest and 3rd coldest month of the year, gets underway Saturday with 14 central U.S. states under winter storm watches. Chicago is in for its second major round of precipitation since just before Thanksgiving.
--By Tom Skilling, WGN-TV Chief Meteorologist
Though skies were dark and dreary Wednesday as light showers and sprinkles developed late in the day, it was mild as high temperatures surged to their warmest levels in more than a week. The city’s official 48º high at O’Hare was the mildest here in more than a week since it reached a balmy 55º on Nov. 20. Some areas were even warmer with a 50º high in the Loop at Northerly Island and a 54º maximum at Kankakee.
Wednesday’s warmth will be a distant memory as strong west winds knock temperatures down nearly 15º as today’s highs struggle to reach the middle 30s. And by Saturday the city will be facing the prospects of the season’s first major episode of wintry precipitation as an approaching storm takes aim at the area. Ironically, this storm comes on the anniversary of one of last winter’s biggest storms that produced 6-12+ inches of snow from the city north on Nov. 30 - Dec. 1.
--By Steve Kahn, WGN-TV Weather Center Meteorologist
A reinforcing surge of cold air limited Chicago’s high temperature to 35º Tuesday, the third chilliest of autumn to date. The reading extended to six the number of consecutive daytime 30s—the area’s longest late November sub-40º string in 30 years. Arctic air is notoriously dry, a characteristic on display Tuesday. Dew points, a measure of atmospheric moisture, dropped to just -1º at Midway and +2º at O’Hare. Those are readings rarely found outside the arctic, and when they have occurred here, they’ve generally done so in the midst of bitterly cold mid-winter arctic outbreaks. The push of dry air was also reflected in Tuesday’s relative humidity, which plummeted to just 22%—the lowest November level here in 13 years.
Powerful southerly winds interrupt the string of 30s Wednesday. The winds are occurring beneath a 180 m.p.h. jet stream. Wind speeds of 80 m.p.h. just 3,000 ft. above the city will produce surface gusts in excess of 40 m.p.h.
--By Tom Skilling, WGN-TV Chief Meteorologist
The Chicago area is in the midst of its chilliest weather of the past nine months. For a fifth consecutive day Monday, temperatures failed to reach 40 degrees -- a sixth is predicted Tuesday. Not since March 2-7 have so many sub-40-degree highs occurred.
The sun set last week (Nov. 18) at Barrow, Alaska, and won't rise again there for two months until Jan. 23 at 1:06 p.m. It's a situation indigenous to the arctic, and the resulting lack of sunlight is a major reason cold air collects so quickly there and is able to penetrate into the Midwest with increasing frequency. Although it's chilly in Chicago as Tuesday dawns, bitterly cold early season arctic air is in control along the U.S./Canadian border from northern Montana to Lake Superior. There, temperatures dropped below 0 degrees overnight.
The clock is running on meteorological Fall, 2007. The season has just four days to go, ending with December's arrival at midnight Friday night.
--By Tom Skilling, WGN-TV Chief Meteorologist
The month of December is climatologically the cloudiest month of the year, followed closely by November, and the week ahead in Chicago will live up to those benchmarks.
While there will be periods of sunshine, daytime as well as nighttime skies look to be predominantly cloudy the next seven days. A far-reaching overcast extending north from a low pressure system headed east up the Ohio Valley will hold over Chicago today, with a good chance of light rain also reaching into northeast Illinois. Some sunshine is possible Tuesday, but low pressure to the north will give longer periods of cloudiness and even a chance of light rain or wet snow Wednesday.
Friday may see the first vestiges of a low pressure system in the southern Plains that could have a major impact on the Midwest over the coming weekend. Some computer models project a colder Ohio Valley storm track that could result in accumulating snows in Chicago.
--By Paul Dailey, WGN Weather Center Meteorologist
Morning sun today in Chicago belies the approach of a strengthening storm system. Low pressure is forecast to move north out of the Gulf of Mexico into the Ohio Valley Monday spreading rain over northeast Illinois. Cooler high pressure will follow Tuesday before the next storm system moves through from the west Wednesday. Precipitation may start as rain Wednesday but should change over to snow as much colder Canadian air sinks south into the Midwest. Thursday is expected to be much colder with temperatures 10 to as much as 15 degrees below late November normals. Gusty northwest winds combined with the cold air will likely create a strong lake effect snow event over northern Indiana.
Computer models then project the formation of another low pressure system in the southern Rockies Friday that could bring the first significant snow of the season to northern Illinois next weekend.
--By Paul Dailey, WGN Weather Center Meteorologist
Readings failed to break above 32 degrees Friday for the first time this season. Chicagoans haven't shivered through a day as cold in nearly seven and a half months. The chill arrived 10 days earlier than it did last year. The first day in 2006 with a high of only 32 degrees was Dec. 2. Chilly as it was, at least residents on this side of the lake didn't have to deal with the 8.3" of lake-effect snow which accumulated over two days at Benton Harbor, Mich., and finally ended Friday afternoon.
High clouds comprised of ice crystals streaming northeastward off a Plains storm system late Friday evening produced a halo -- a bright circle -- around the moon which caught the attention of a number of area residents. The clouds responsible are to thicken into a fairly solid overcast which may produce a few flurries Saturday before breaking and allowing intervals of sunshine.
Meteorological autumn's final weekend is underway. December -- the start of meteorological winter season -- begins next Saturday.
--By Tom Skilling, WGN-TV Chief Meteorologist
Thanksgiving Day's flurries and the period of light snow the night before have put the first measurable snowfalls into the record books here. Measurable snow -- defined as an accumulation of 0.1" or more -- hadn't occurred since 3" fell 7 months ago on April 11. O'Hare reported 0.3" Wednesday night and Thursday while 0.1" fell at Midway. Lake-effect snows whitened the Indiana/Michigan snowbelt from Porter County to areas east Thursday. By evening, Benton Harbor, Mich., reported 4.5" while Rolling Prairie and South Haven came in with 4"; South Bend, Ind., logged 2.7".
Thursday's 33-degree high was Chicago's coldest since April. A second day of 30s is predicted Friday.
North America's highly "amplified" or wavy jet stream pattern is behind the chill in the Lower 48 -- and unseasonably mild temps in Alaska. It was warmer there than in Chicago on Thanksgiving: Seldovia reported 52 degrees, Homer had 49 degrees, and Anchorage and Fairbanks both recorded 40-degree highs.
--By Tom Skilling, WGN-TV Chief Meteorologist
A light cover of snow from Wednesday night's blustery storm system is evident in north suburban Lindenhurst in this photo sent to us by WGN Weather Intern Patrick Korellis. Many thanks for sharing this and best wishes for a Happy Thanksgiving to you, Patrick, and all of our blog readers!
--Tom Skilling, WGN-TV Chief Meteorologist
Powerful N/NE winds, which roared into Chicago at more than 30 m.p.h. after a 320-mile trek over the length of Lake Michigan, kept precipitation here in liquid form Wednesday. The "warming" effect of 46-degree lake waters on the lower atmosphere is difficult for most storms to overcome. Not until well after sunset did an injection of colder air begin the transition from rain to snow in the far northwest suburbs. Rain changed to snow at Rockford at 6 p.m., and by 8 p.m. had begun accumulating on colder outdoor surfaces near Arlington Heights, Oak Brook and Mundelein. When snow develops at marginally cold temperatures, flakes often stick together. That process in parts of the metro area Wednesday led to reports of 1-inch diameter snowflakes at a few locations. Wind-driven rains Wednesday totaled more than 2" in parts of the city, including 2.20" at Whitney Young High School. The 1.32" (through 8 p.m.) at Midway was the heaviest since Aug. 22 -- three months ago.
--By Tom Skilling, WGN-TV Chief Meteorologist
These photos of dense fog Tuesday morning are courtesy of our intern, Monika Bec. The first one was taken off I-88 near DeKalb and the second was taken in Plainfield. Visibilities continued to be quite limited Tuesday evening with some areas less than a quarter of a mile.
--By Bill Snyder, WGN-TV Weather Producer
A wet, wind-driven snowfall appears a good bet for areas to the west and northwest of Chicago Wednesday night. The transition from a cold rain to wet snow or a mixture of snow, sleet and rain, could begin in far western locations as early as late Wednesday afternoon. Farther east, there is greater uncertainty whether snow will end up accumulating, though there is the expectation the cold rain which is to fall much of the day will eventually change—even if temporarily—to some snow or flurries Wednesday night.
Warm ground and mild lake water temps typically provide an unfriendly environment for late autumn’s first snow systems. In the weather situation predicted to come together Wednesday night, communities in McHenry, Boone, Kane and DeKalb counties appear most likely to experience some accumulation. It’s a development which would lead to this area’s first Thanksgiving in 3 years to boast a snow cover.
--By Tom Skilling, WGN-TV Chief Meteorologist
Drizzle and dense fog greet many as Tuesday dawns across the Chicago metro area. It’s the second day ill-timed fog shrouds the area, a development which threatens additional flight delays. By late Monday evening, visibilities had already plummeted to less than a quarter mile from DeKalb to Aurora and DuPage Airport. Fog forms when nighttime cooling or the arrival of cool air from an adjacent area forces temperatures to fall to the dew point—the temperature of saturation. When the temp and dew point match, air holds as much moisture as it can. The relative humidity soars to 100% and fog often forms. An area from roughly I-80 northward is at greatest risk for fog, while areas south, including Kankakee and Rensselaer, Indiana, are in for springlike low 70s. Monday’s 62° high at Northerly Island and 66° at Kankakee were the area’s mildest readings in nearly a week.
--By Tom Skilling, WGN-TV Chief Meteorologist
