EXPLAINER: December 2007 Archives

The subject of global warming, bubbling in the meteorological community (and elsewhere in academia) for several years, has burst controversially into the public consciousness. It’s our prediction that it will be an increasingly controversial topic in 2008 and beyond.
The Earth’s climate is changing. All evidence is that it has always done so and always will do so, but a new factor in the “change equation” is man’s role in future climatic shifts.
We have been proceeding headlong and, until now, blindly into a future of “climatic uncertainty” in the sense that there exists an unknown ratio between the benefits and costs of climate change. Decision makers need to weigh and compare the risks/costs of premature or unnecessary actions against the risks of failing to take actions that subsequently prove to be warranted. The issues are complex, but we are finally beginning to address them.

--By Richard Koeneman, WGN Weather Center Meteorologist

Air of Arctic origin swirls into Chicago Monday night and Tuesday, and the resultant temperature crash sends the city's readings from the lower 30s today to near zero, at least in the suburbs, 24 to 36 hours later.
Strong winds and light snow, too, will accompany the temperature dive. Snow flurries and periods of somewhat more sustained light snow are likely to linger through Tuesday, but this is only a nuisance snow event. Two-day accumulations of less than two inches are indicated. Of greater significance: 20-30 m.p.h. northwest winds and subzero wind-chill temperatures on Tuesday.
Elsewhere, beneficial rains spread across the drought-stricken Southeast on Sunday, putting down 1 to 2 inches of desperately needed rain. However, thunderstorm winds damaged eight homes in Liberty, Ga., Sunday afternoon.
--By Richard Koeneman, WGN Weather Center Meteorologist

In a typical December, Chicagoans can expect 9 inches of snow. That's the long-term average based on 79 years (1928-2006) of snowfall data as registered at Midway Airport. This month, however, has been among the city's snowier Decembers.
The month-to-date snow total of 18.5 inches ranks this month as the city's 10th snowiest, and the possibility of some light snow late tonight and on Monday will add a bit more to the month's final tally. This month's snow total, while more than twice the climatological expectation, poses no threat to Chicago’s all-time December snowfall record.
Think back seven years to December 2000. Forty-one inches of snow blanketed the city that month and, coupled with exceptionally low temperatures, the snow did not melt away between storms. It was a commuters' nightmare.
--By Richard Koeneman, WGN Weather Center Meteorologist

As Friday's snow-producing low pressure system moves northeast, clouds associated with the low will persist well to the west, keeping Chicago under a cloud cover today.
Assuming that clouds spread quickly back into northeast Illinois ahead of low pressure approaching from the northwest, cloudy skies will likely close out this December -- normally the cloudiest month of the year.
Because temperatures warmed into the mid 30s, yesterday's snow was wet and heavy. Snow melting upon impact and packing kept snow depths in the 2- to 3-inch range within Chicago's city limits.
This month's snowfall at Midway Airport has now reached 18.5 inches -- making this the second snowiest December of the past 28 years there. Farther to the north and west, snow totals gradually increased, reaching 6 inches along the Illinois-Wisconsin border. In the Milwaukee area, 7.5 to 8 inches were recorded.
--By Paul Dailey, WGN Weather Center Meteorologist

By Chicago’s standards, today’s snowstorm is hardly a blockbuster, but it’s surely enough to please the children and simultaneously chill the hearts of area automobile commuters. Snow is likely to begin about mid-morning, thereby sparing the early morning commute, but three to seven inches of snow will blanket the area by mid afternoon and that promises a messy evening rush hour.
Falling with temperatures near freezing, the snow will have a rather high water content and constitute what is colloquially known as “wet snow.” Snow shovelers take note.
This storm system has had a history of moderate snow production. As of Thursday evening, areas east of Denver reported seven to eight inches.
Looking ahead, genuinely cold air is poised to descend on the city on New Year’s Day, bringing official sub-zero temperatures for the first time since February.
--By Richard Koeneman, WGN Weather Center Meteorologist

Vigorous weather, initially as snow and then as sharply lower temperatures, will be factors of concern to Chicagoans in the near future. The passage of a strong low pressure system across the Midwest suggests we can expect several inches of snow on Friday followed by sub-zero temperatures on New Year’s day.
The federal Bureau of Transportation Statistics (BTS) has released surprising statistics concerning the causes of passenger slowdowns in U.S. airports—including such factors as flight delays, cancelled flights, air traffic congestion and weather. BTS statistics challenge the popular notion that adverse weather is the main cause of commercial flight problems.
Surprisingly, airline issues (such as staffing shortages and mechanical problems) were, in total, greater contributors to passenger slowdowns than bad weather.
--By Richard Koeneman, WGN Weather Center Meteorologist

Sunshine, relatively mild temperatures and light winds on Christmas Day offered a welcome demonstration that interludes of decent weather, at least on a relative basis, do occur here in December. While noting that December is our cloudiest month, Chicago weather historian Frank Wachowski reports that the sun detector at the Midway Airport weather observation station has logged only 3,492 minutes of sunshine this month (through Dec. 25) versus a normal of 13,825 minutes for that period.
Santa Ana winds, an ever-recurring problem for southern California, roared through the Los Angeles area during the Christmas holiday. Los Angeles County's Whitaker Peak registered sustained winds of 55 m.p.h. with gusts to 102 m.p.h. on Christmas Eve. Nearby Malibu was buffeted by gusts to 78 m.p.h. early Christmas morning.
--By Richard Koeneman, WGN Weather Center Meteorologist

Not since 1994, when Chicago’s Dec. 25 temperature jumped to 52º, has a Christmas been milder than this afternoon’s expected high of 40º. Sunshine, too, finally returns to the city after nearly a week of cloudy days.
The sunny interlude will be brief, however, because the weather pattern remains active and another storm system, putting snow across the Rocky Mountains today, sweeps into the Midwest on Wednesday. Snow associated with that weather disturbance arrives here Wednesday night, accumulating an inch or less before heading rapidly off to the east by early Thursday.
Computer models indicate a more vigorous disturbance arrives on Friday with the potential to deliver a few inches of snow. Temperatures, though, will be critical, and a rain/snow mix is not out of the question.
--By Richard Koeneman, WGN Weather Center Meteorologist

Clouds, light snow and gusty west winds remain today, but the low pressure system that gave Chicago its lowest pressure in 20 years is moving rapidly northeast away from the western Great Lakes. A band of heavy snow lies to the west and northwest along the Mississippi River. Portions of the city proper as well as northern Indiana, lower Michigan and western Ohio are cleaning up and recovering from damage inflicted by fast-moving severe thunderstorms early Sunday. Winds in excess of 60 m.p.h. downed trees and power lines in many locations. The National Weather Service collected at least 57 severe storm reports on Sunday.
A wind gust of 88 m.p.h. was recorded Sunday at the Harrison Street Crib off the Chicago shore in Lake Michigan. The official 63 m.p.h. gust recorded at O'Hare Airport was the second highest ever at Chicago in December.
A calmer week lies ahead with clearing skies tonight leading into a sunny Christmas Day.
--By Paul Dailey, WGN Weather Center Meteorologist

By the time most Chicagoans arise, temperatures will be in the 20s and falling with snow caught up and blowing horizontally by 30-40 m.p.h. west winds. Low pressure will have passed over NE Illinois overnight with its associated cold front sweeping through the metro area between 3 and 5 a.m. Prior to the cold frontal passage, despite steady rain, south winds pushed temperatures to their highest Saturday levels just before midnight. Sunday's highs, probably near 50 degrees, will be recorded shortly after midnight. By noon Sunday, readings will most likely sink below 20 degrees -- an approximately 30-degree drop in less than 12 hours. While heaviest snow will occur well to the northwest in Wisconsin, at least 1 to 2 inch accumulations will occur here, most likely in far west and north portions of the area. Outdoor activities and travel will be hazardous while wind chills are expected to fall into single digits before noon and below zero later this afternoon and tonight.
--By Paul Dailey, WGN Weather Center Meteorologist

Were the ground completely free of snow, much of the Chicago area would be headed for 50 degrees Saturday. Mild weather's staging a temporary comeback: Not since a 45 degree high nearly three weeks ago on Dec. 2 has weather approaching Saturday's "warmth" been observed here. A winter storm, expected to intensify Saturday night on its trek from Texas to Wisconsin, sets the stage for the surging temperatures as the system's front-side SE winds pump Chicago's readings into the 40s.
The intensification predicted overnight supports strengthening winds and even several embedded thunderstorms. A potent cold front swings across the area between 3-6 a.m. Sunday, the point at which rain transitions to snow. The passage of a powerful upper storm Sunday -- which by all appearances is being inadequately handled by computer models -- could bring accumulating snow for a time. The addition of falling temperatures and howling west winds could make for a challenging day.
--By Tom Skilling, WGN-TV Chief Meteorologist

Chicago’s suddenly milder temperatures and decidedly stronger southeast winds Friday can’t hide the fact the clock is ticking toward winter’s official (astronomically determined) open. Winter Solstice takes place just after midnight at 12:08 a.m. Saturday morning—the point at which the sun’s most direct rays fall as far south on the planet as at any time of the year. Winter begins with the year’s shortest day in the northern hemisphere. Sunlight is just one-fifth the strength of June sun in Chicago, with just over 9.1 hours of daylight. The good news is days here begin to lengthen gradually and more than 5 hours of daylight will be added by the time summer begins in June.
A rare December Air Pollution Action Day has been declared by the Illinois EPA Friday. Air quality slipped to unhealthful levels at a number of Chicago area sites Thursday.
--By Tom Skilling, WGN-TV Chief Meteorologist

Dense fog has slashed visibilities to fractions of a mile west and south of the city since late Wednesday. The tiny, supercooled (sub-freezing but still in liquid form) droplets which comprise the fog can freeze on contact with colder outdoor surfaces and care is urged while walking or driving west and south of the city where icy patches have developed overnight. The fog is trapped by a layer of warm air and resides within a shallow layer of cold, dense air not much more than a thousand feet deep. Melting snow is behind the haze and fog of recent days and nights. This atmospheric set-up, which meteorologists refer to as a temperature inversion because the normal decline of temperature with height has been inverted, is having another effect beside the haze and fog build-up. Air quality, as measured by Illinois EPA, neared the unhealthful range in Chicago Wednesday and actually moved into the unhealthful range in the Quad Cities.
--By Tom Skilling, WGN-TV Chief Meteorologist

Starting between 6 and 7 a.m. last Thursday morning and ending at noon yesterday, Chicagoans experienced 126 consecutive hours with below freezing temperatures. This caps off a cold start to December that has seen temperatures average 4 to 5° below normal over a good portion of central and northern Illinois. A warm-up has started that should peak with highs in the 40s Friday and Saturday and even nighttime lows Thursday and Friday staying above freezing. These warmer readings along with light rain or drizzle Friday and Saturday could see much of the snow cover over NE Illinois vanish. However, low pressure moving out of the southern plains is forecast to track through northern Illinois Saturday. This could produce new snow accumulations later Saturday night. An arctic high pressure air mass will surge into the Midwest Sunday with Chicago’s afternoon temps probably falling into the teens.
-By Paul Dailey, WGN Weather Center Meteorologist

The same storm system that brought up to a foot of snow to portions of the Chicago area this weekend buried areas from northern Indiana and Michigan to New England with up to 15" of snow. Some of the heaviest totals included 15" at Walden, Vt., and Saranac Lake, N.Y., and 14" at Bronson, Mich., and Columbia City, Ind. (just northwest of Fort Wayne). Severe weather erupted on the storm's southern flank Saturday night, producing a swarm of tornadoes in southern Georgia that resulted in one fatality.
Warmer air will try to make inroads into Chicago this week, but the cold snow-covered ground here will not only retard the warm-up but also create a lot of low clouds and fog.
This week will start out on the cold and quiet side, but two storms are expected to affect this area -- one by midweek and the other next Saturday, raising the specter of more wintry precipitation for the fourth weekend in a row at Chicago.
--By Steve Kahn, WGN Weather Center Meteorologist

As the weekend storm exits the Chicago area Sunday morning, heavy snow totals will be left in its wake. Tallies are likely to range from 5-9” northwest of the city to a foot or more in northwest Indiana.
The first half of December has been a rough one for Chicago, with frequent bouts of freezing precipitation repeatedly glazing the area. A major snowstorm on Dec. 4-5 brought up to 8” of snow to northern portions of the metropolitan area.
Not only has this December been snowy and icy, it has also been cold, averaging nearly 5º below normal. So far, there has been only one day this month with a high in the 40s, and the city recorded its earliest 0º reading in three decades on Dec. 6. More cold weather is on tap for tonight. As skies clear, the fresh snow cover will act as a cold-weather factory allowing readings to plunge into the single-digits with the coldest interior spots headed to near or even below zero.

--By Tom Skilling, WGN-TV Chief Meteorologist

Northern Indiana and Chicago's southern suburbs appear likely to become ground zero for the heaviest snows of this area's third consecutive weekend storm. The system, responsible for 8" of snow near Dodge City, Kan., and as much as 9" in eastern Colorado Friday, is in its developmental stages and could bring the city proper as much as 4-8" of snow. Totals are likely to drop off markedly the farther north and west one travels from the city.
Patches of light snow arrive Saturday morning and grow steadier by Saturday afternoon. But, Saturday night into early Sunday is expected to play host to the storm's most wintry combination of accumulating snow. Winds gust to 20 m.p.h. this afternoon and to 30 m.p.h.-plus Saturday night -- a situation which suggests blowing snow may become a problem in open areas. More snow may fall on some of these areas Saturday and Saturday night in this storm alone than the 8.7" which is considered the December norm.
--By Tom Skilling, WGN-TV Chief Meteorologist

Before 8.4" of wind-driven snow walloped Boston Thursday -- a record for Dec. 13 -- only 1.5" had fallen so far this season. But Boston's snowfall paled in comparison to other New England totals -- including 13" reported at Burrillville, R.I., and Whitman, Mass., and the 9.5" at Binghamton, N.Y. Thursday's storm may be only the opening salvo in a far more active pattern predicted across the region in coming weeks. A mammoth new coastal storm this weekend could bury interior New England under an even heavier layer of snow while blasting the huge coastal cities of the Mid-Atlantic and Northeast with heavy rain.
Chicagoans, who have suffered through December's opening 13 days with only 14 percent of the area's possible sunshine (39 percent is normal), are treated to precipitation-free weather Friday for only the second time this month. The day's predicted 28-degree high could end up the 3rd chilliest of the new cold season.
--By Tom Skilling, WGN-TV Chief Meteorologist

Travel east the next four days will be severely impacted by two low pressure systems forecast to move east up the Ohio Valley into New England, each producing significant winter weather. The forecast track of the storms appears to be just far enough south to give Chicago clouds and snowfall associated with the northern fringe of each storm, although Lake Michigan-aided snow enhancement in NE Illinois and NW Indiana could occur with Saturday’s storm.
Some areas of the Northeast are targeted for a double-barrel hit with heavy snow later today and then again Saturday and early Sunday. Portions of Pennsylvania, New York, Massachusetts, Connecticut, Rhode island, New Hampshire and Vermont appear set to absorb two 5 to 10”+ heavy snow events. Flooding could occur in Kentucky, West Virginia and southern Pennsylvania today with Ice storms also a threat in the latter two states.
--By Paul Dailey, WGN Weather Center Meteorologist

Cold air’s grip on the area has tightened. While small pockets of warm air at cloud level make it possible a little freezing drizzle could linger with Wednesday’s sporadic flurries, a repeat of Tuesday’s widespread 0.25” ice accumulations across most west and northwest suburban areas is unlikely. Tuesday’s 0.67” of precipitation (water equivalent) at O’Hare made it one of the city’s two heaviest precipitation events since the 1”+ deluge back on Aug. 23. With just 13% of it possible sunshine, December’s opening 11 days have been the cloudiest here since 1997.
As icy as it was over sections of the metro area Tuesday, areas west and north were hardest hit. By late Tuesday, 60,000 Iowa households were in the dark as a result of the icestorm there. Snow blanketed much of southern Wisconsin. Accumulations reached 7” at Cottage Grove just east of Madison while to the west of Milwaukee, Oconomowoc was hit by 5.9”.

-By Tom Skilling, WGN-TV Chief Meteorologist

Serious ice accumulations, not unlike those which cut power to 410,000 Plains residents and snapped tree limbs across sections of 12 states Sunday and Monday, threaten sections of the Chicago area Tuesday. The region west and NW of the city—including the Fox Valley, DeKalb, McHenry and Boone counties and areas west toward Rockford and into southern Wisconsin appear at greatest risk for the heaviest ice build-ups. While temps gradually ease above freezing in Chicago and areas south, halting ice formation and the melting which has already occurred, readings may be hard pressed to break above freezing to the west.

Tuesday marks the fourth consecutive day this area has played hot to freezing rain or drizzle. An analysis of Chicago weather records dating back to 1924 by my colleagues Frank Wachowski and Steve Kahn reveals it’s something which hasn’t happened before.

--By Tom Skilling, WGN-TV Chief Meteorologist

A persistent upper-level jet stream pattern flowing from the Southwest to northern New England has allowed a series of low pressure systems to develop in the southern Rockies and Texas Panhandle and move northeast through the Ohio Valley to the East Coast. This pattern is expected to continue into midweek, with two more low pressure surges forecast to impact northern Illinois Tuesday and Thursday. Water-equivalent precipitation with the late Monday night/Tuesday storm could be as much as a half to one inch, so surface temperatures now forecast to warm slightly above freezing will be crucial as to whether northeast Illinois will receive rain or freezing rain.
The following storm looks like it will lean more to the cold side with a rain/wet snow combination beginning later Wednesday. The upper-air pattern will then shift northwest-southeast over Chicago, and a cold Canadian air mass will be steered into the western Great Lakes late in the week.
--By Paul Dailey, WGN Weather Center Meteorologist

Saturday evening numerous accidents were reported on I-80 and areas to the south as freezing rain and freezing drizzle created hazardous conditions on roads as well as streets and sidewalks. With the 20,000-foot steering level winds flowing from the SW to NE, low pressure originating in the Texas Panhandle tracked east up the Ohio Valley, spreading a wintry mix of snow, sleet and cold rain or freezing rain from Missouri into into central and northern Illinois and northern Indiana. No sooner does this sytem’s cloud shield move east than a second look-alike low pressure system is forecast to take a similar track Tuesday and Wednesday. Early indications are this midweek storm might entrain slightly warmer air producing a period of rain in Chicago Wednesday before changing back to possible accumulating wet snow that night. A bitterly cold Canadian air mass is then expected to spread east over the Great Lakes and Midwest.
--By Paul Dailey, WGN Weather Center Meteorologist

It's been a wild opening week for Chicago's meteorological winter 2007-08.
It started with last weekend's icy glaze, followed by the midweek snowstorm and the city's earliest 0-degree reading since the 1970s. And now, the city is facing two more winter storms that threaten a return of adverse weather, first this weekend and then again by the middle of next week.
Latest computer forecast trends hint at an icy mix for most of the city this weekend with more snow to the north and glaze to the south.
Following a brief respite Monday and early Tuesday, the second storm is expected to roll in, once again bringing a threat of rain and/or snow to the city. Very cold air currently building across the northern Plains and southern Canada is forecast to plunge south into the Midwest in the wake of this storm, bringing another round of early season cold by the end of the week.
--By Steve Kahn, WGN Weather Center Meteorologist

Not since 1976—a period of more than three decades—has Chicago’s official temperature dropped to 0° so early in a cold season. For five minutes beginning at 4:04 a.m. Thursday morning, Chicago’s official temperature bottomed out at 0°—a reading which tied the 1972 record for December 6. The frigid benchmark isn’t typically reached here for another three weeks and didn’t occur for another two months a year ago. The first 0° waited until a -6° low was recorded Feb. 3 earlier this year. The combination of clear skies, dry air, a fresh snow cover and light winds was behind the sub-zero readings which occurred at a number of west and NW suburban locations early Thursday.
The week’s third snow, part of a system which left up to 4.5” on the ground in Iowa, arrived in a weakening state Thursday evening. The 3-5 hour snowfall was predicted to deposit 1-2” in the Chicago and end well before sun-up Friday.
--By Tom Skilling, WGN-TV Chief Meteorologist

Wednesday’s 2-9” area snowfall hasn’t settled and another 2-4” is on the way. It’s part of an extraordinarily active pattern which may result in three additional winter disturbances over the coming week. The first arrives with snow toward nightfall in the Fox Valley then spreads into the city proper an hour either side of 6 p.m. The bulk of this system’s snow appears likely to fall in just 6-8 hours, diminishing in Friday’s predawn hours.
Bursts of lake snow Wednesday sent storm tallies to more than 9” Wednesday. Buffalo Grove’s 9.2” was the area’s heaviest followed closely by Lincolnshire’s 9.0”. Both Waukegan and Arlington Heights logged 8” while city snowfalls included 5.5” at O’Hare and 4.6” at Midway. Amounts tapered to 2” in Kankakee County’s Bonfield.
A 3” or greater snowfall Thursday night would mark only the third time since 1884 that December’s opening week has hosted two 3”+ snows.

--By Tom Skilling, WGN-TV Chief Meteorologist

This was the scene at 10:30 a.m. Wednesday morning as lake snow invaded the Waukegan area. In typical fashion, bursts of lake snow from Milwaukee south to Chicago slashed visibilities at times to a half mile. This photograph from Waukegan Airport comes to us from pilot Anson Mount. Many thanks Anson!

Tom Skilling

Photo courtesy of Anson A. Mount Ill.

The area’s heaviest snowfall of the season isn’t over yet. While the current storm’s large scale “system-wide” snow moves on Wednesday, its “lake-effect” phase is just beginning. Alberta Clippers on tracks as far south as today’s are notorious for tapping lake moisture in their final hours over the Chicago area. Easterly winds, which travel over only 75 miles of water, shift more north-northeast Wednesday morning as the storm’s center races southeastward out of central Illinois. This more than triples the distance over water before rushing ashore. That plus the arrival of colder air on the system’s backside produces an environment in which lake-effect snow bands flourish. Though hardly steady snow-producers, snow can arrive in visibility-reducing bursts capable of a couple of fast inches of snow. Final area snowfalls are likely to wind up in the 3-6” range with a few 7-8 inch totals possible at any location hit by some of the lake snow showers.

The biggest snow of the new season appears to be on the way—potentially the biggest since 8.8” fell earlier this year on Feb. 13. A classic Alberta Clipper, named for the southwest Canadian province through which the storm passes before diving into the Midwest, is responsible for the impending snowfall. An offshoot of a powerhouse Pacific storm which lambasted the Pacific Northwest Monday, the system boasted a central pressure (949 mb. or 28.02”) Sunday night—the equivalent of a Category 3 hurricane.
Near Lake Michigan, flurries may flutter to earth sporadically into early afternoon. But, snow begins falling more steadily from Tuesday afternoon’s thickening overcast, and is likely to fall heaviest at times toward evening and through a good portion of the night. Initially light winds strengthen Tuesday night and gusty NNE winds deliver lake-effect snow showers Wednesday. Snowfalls in the 3-6” range appear a good bet.
--By Tom Skilling, WGN-TV Chief Meteorologist

Winter got off to an auspicious start Saturday as Chicago was battered by a smorgasbord of weather that included snow, sleet and freezing rain. While the storm brought mainly glaze and rain to the Chicago area, the Upper Midwest was buried in snow. Babbitt, located in the Minnesota Arrowhead, received 13", while Bayfield and Niagara in far northern Wisconsin both logged 10".
Brief warming Sunday morning removed most traces of this winter's first shot, but the week ahead will likely bring one and possibly two chances for snowfall to this area. A storm system originating in Alberta will race southeast through the area Tuesday night and Wednesday, bringing a period of accumulating snow. A surge of cold air will follow, setting the stage for a second storm currently projected on a path from the southern Plains up the Ohio Valley Thursday night and Friday. This track is one of the most favorable for a significant snowfall in Chicago.
--By Steve Kahn, WGN Weather Center Meteorologist