WGN Weather Center Blog

When damp and mild air rolls across Chicagoland at this time of the year, it is usually attended by extensive cloudiness, fog and rain. It’s the price we pay when winter temperatures climb 10-15 degrees above normal.
History tells us that Chicago’s readings climb into the 40s on only about one day in four during the first week of January, and so Sunday’s and Monday’s highs in the low and mid 40s constitute an early-January bargain. But it won’t last.
Cold air returns in earnest later this week, and temperatures plunge 30 degrees to levels that will run as far below normal by Saturday as they are above normal today.
Elsewhere, the weather is causing life-threatening troubles ranging from flooding rains (and massive mountain snows) across California, Oregon and Nevada to warm, desiccating winds across Texas and Oklahoma that have set the stage for grassland fire emergencies.

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Chicago’s ongoing mild spell was interrupted Friday afternoon as rain changed to snow for a few hours. By 8:00 p.m., O’Hare Airport had measured 0.2 inches, and Midway 0.3 inches. These meager sums did not stick as temperatures held above freezing.
Farther north, nearly a foot was expected in Minnesota and Wisconsin around Lake Superior. The storm causing the weather passed just north of Chicago. In its wake, Saturday’s temperatures will plunge across Chicagoland but remain above normal—especially the overnight lows.
Mild weather returns Sunday to greet the new year, but the fast-track of frontal passage every two days continues, with rain returning by Sunday night into Monday, and again Tuesday night into Wednesday. Thereafter, the jet stream buckles and divides the country in two. In the West, the series of pounding Pacific storms ends, while more normal winter temperatures return east of the Rockies.

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The next in this series of fast-moving weather systems of Pacific origin arrives Friday afternoon with temperatures in the upper 30s and a good chance of rain.
Unlike its predecessors which developed after they passed east of Chicago, this system gains strength as it exits Iowa and tracks across northern Illinois. Updated computer models suggest upper-level dynamics will be strong enough to generate colder air in the lower 5,000 feet of the atmosphere—enough to change rain into snow here Friday night despite above-freezing temperatures at the surface.
As a result, wet snow may accumulate an inch of two on grassy or colder surfaces. The best of the snow crosses central Wisconsin with 6 inches of snow possible.
Thereafter, rain is likely with future frontal passages. By next Thursday, the mild pattern will begin to break down, and temperatures will plummet back toward normal.

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During the first three weeks of December, the temperature averaged a frigid 11 degrees below normal, including one night below zero. Then the weather pattern switched: In the past week, temperatures have averaged 13 degrees above normal, during which time the low temperature has not fallen below freezing. Over the United States, the upper flow has been predominantly west-to-east with a powerful jet stream driving huge moisture-laden storms into the West Coast. Any remaining moisture and energy with these winter storms is dissipated over the Rockies, resulting in rather tame systems here with little rain or temperature contrast with frontal passage, now on a fast two- to three-day cycle.
Halfway around the globe, an upper low has settled over Europe, resulting in a steady diet of storm systems originating just east of Greenland. Wednesday’s storm brought central Europe to a standstill with significant snow, arctic cold and strong winds.

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Abundant sunshine over Chicago in a mild pattern dating to last week teamed up to produce the highest temperatures of the month. O’Hare Airport topped out at 47°, while Midway reached 46°—the warmest readings this December. Not far to the north, clouds and fog lingered all day; Rockford reached only 36°.
The strong zonal jet stream responsible for the respite from the cold continues past New Year’s Day and into next week. High temperatures remain above normal, but the real mild air is measured by the minimums, which should remain 10 to 15 degrees above the current 17° normal low.
Upcoming weather systems in this pattern deliver respectable precipitation dynamics in the upper atmosphere, but not much moisture or contrast on either side of each passing cold front. Most precipitation will fall as rain, though flurries are possible on Saturday—the coldest day in the next seven days.

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The string of above freezing hours ended at 106 hours when the temperature at O’Hare dipped to 32° at 9:00 p.m. Monday. Nonetheless, the ongoing mild trend continues, with today’s high expected to reach into the mid 40s, nearly 15° above normal and perhaps the warmest of the month. The high for this month stands at 44° recorded on the Dec. 23.—on the cool side since 84% of all previous Decembers have reported at least one 50° reading.
The current spate of cloudy days lingers until at least the weekend as a stubborn layer of moisture remains trapped in the lower layers of the atmosphere. In addition to the gloomy cloudiness, weak systems of Pacific origin will traverse the country, touching off light rain from time to time, and even a snow shower to close out the year on Saturday.

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In a dramatic about-face late last week, the persistent cold weather for the first three weeks of December have flipped to well above-normal readings, including 84-plus straight hours above freezing temperatures beginning last Thursday morning.
This mild spate should continue through the week, tapping air now in the central Plains. On Sunday, Denver reached a record high of 69° with westerly Chinook wind. A modified version of this mild air spreads east early in the week, with 45° possible at Chicago on Tuesday.
The catalyst for the change is the breakdown of the upper ridge in the West, which had forced frigid arctic air southward with a northwesterly jet stream. Now, the jet stream flows zonally west-to-east, bringing mild Pacific air with above-normal temperatures to most of the country. Respectable Pacific weather makers slam into the Northwest, but dry considerably before reaching Chicago.

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The first three weeks of December, characterized by bone chilling cold and continuous snow cover, have yielded to a milder regime as the arctic air retreats north and east. As of midnight Sunday morning, Chicago (Midway) has reported 61 hrs. of continuous above freezing temperatures--since 11:00 a.m. Thursday morning. This air, mild by December standards, has teamed up with south winds, high dew points, and rain to steadily erode the snow cover to less than one inch late Saturday.
Little change in the current mild pattern is expected right through the end of the year, with a good chance that high temperatures will top out above freezing every day with little precipitation. As a result, Chicago should once again report little or now snow cover in the week between the holidays, a trend began in the early 90s.

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The White Christmas which seemed assured just a few days ago is in jeopardy. Friday’s temperatures soared into the 40s across the region for the first time this month. Combined with higher dew points and south winds, the snow cover was cut in half. Midway’s 6 a.m. report of 5 inches was reduced to 2.7 inches by 6 p.m., while O’Hare melted its 3 inches to a scant 1 inch. Further melting is likely Saturday with rain and 40° highs.
Rain is unusual at this time of year, occurring only 10 percent of the days on Christmas Eves past. This rain mixes with snow overnight Saturday, and tapers off to flurries early Christmas morning before ending.
Thereafter, a zonal west-to-east jet stream will keep arctic air locked north in Canada, suggesting temperatures each day will climb above freezing. Any frontal passage during this time should be weak with little precipitation.

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All of Chicagoland recorded the warmest readings since late November, with O’Hare climbing to 38°. For winter-weary Chicagoans, milder above freezing high temperatures should continue right on through the next week. Friday’s high should easily reach into the 40s for the first time this month. On only two occasions since 1870 has December failed to reach 40°, most recently in 2000, when the high for December was only 36° and the monthly snow totaled 41.3 inches. This December, Chicago is currently running 11.2° below normal with 10.1” of snow.
However, amidst this warming trend is a chance for a fresh coating of new snow in the morning hours of Christmas as a developing low passes to our south. With the absence of arctic air and high dew points, most precipitation should occur as rain before changing to snow near the end of the event Christmas morning.

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Strong gusty southwest winds signal the end to one of the coldest starts to December ever. Chicago has experienced only three daytime highs above freezing so far this month (through December 21), eclipsing the previous record covering the 136 years since 1870 of just 4 above freezing days back in December 1-21,1919. High temperature forecasting has been challenging to say the least this week, with a persistent wind flow over hundreds of miles of snow cover to the west and a very dry cold high pressure air mass in place. Cold air at the surface was actually further entrenched by the stabilizing influence of warmer air aloft. Winds are to become more southerly the next 24 to 48 hours and increase in intensity, flowing over a broad expanse of thin or no snow cover to the south. The evaporative effects of the winds as well as warmer temperatures followed by an period of rain should result in a fast snow melt over NE Illinois.

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Warming enters its third day Wednesday, but the process is occurring far too slowly for many area residents, weary of the relentless nature of this month’s chill. Arctic air has dominated, allowing only fleeting respites from the bitter chill. That’s evident from the fact 90% of the month’s daily temperatures have averaged below normal.
December has managed only three days above 32°—one-fifth of last year’s 15 to date and well below the 136 year average of 14 days by the 21st. The month’s mildest single temperature has been 35°, a far cry from the three 50° highs which had occurred by this point in December one year ago.
The month’s 18.2° average maintains December’s rank as Chicago’s 4th coldest, far from the 30.2° average since 1870.
Things are improving. Highs here reached 11° Monday, 21° Tuesday and should hit 31° Wednesday and 37° Thursday.

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Chicagoans were greeted by frigid arctic air again Tuesday morning.
Coming days will see quite a temperature inversion develop as incoming
mild Pacific air floods into the area. Thus, the hint of haze captured
in this shot looking southeast at the Metra Tracks in Cary, Illinois and
taken by Anson Mount will become a more obvious feature in the days
ahead--especially as temperatures surge above freezing Thursday and
Friday and melt snow. Temperature inversions--a state of the atmosphere
in which warm air sits atop cold air reducing or eliminating the usual
decline in temperature with height, decreasing or overcoming the usual
vertical mixing of air which occurs when warm, buoyant air is near the
surface and ascends into colder, denser surroundings---often allow the
build-up of haze. As a result, much hazier environment is likely later
this week.

-- Tom Skilling

Photo courtesy of Anson Mount

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There’s nothing unusual about cold winter weather in Chicago. But, the intensity of the chill we’ve endured in recent weeks is quite unusual for this early in a new cold season. The opening 19 days of the month have averaged 18.4° making it the fourth coldest December open since 1870. It’s a temperature 14.3° below a comparable period one year ago and only 1.3° away from than the coldest Dec. 1-19 period on the books in the city: 17.1° in 1958. Monday’s 11° high was the coldest December daytime temperature in 5 years. Only 46 December days have been as cold or colder since 1870.
The area’s first official sub-0° low of the season occurred at 8:30 a.m. Monday when O’Hare’s readings reached -1°. West suburban DeKalb and Rochelle hosted the area’s coldest lows of -7°.
Heating this air without adding moisture Monday produced indoor humidities of just 4-6%, a fraction the 35% considered comfortable by most.
-Tom Skilling

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Melissa Pavlica of Westmont snapped these photos of "sun dogs" Monday
morning. Sun dogs result as incoming sunlight is refracted by ice
crystals, producing a halo, two segments of which are visible, giving
the appearance of a sun on either side of the disc of the sun. Monday's
extreme cold, including -1 at O'Hare, 0-degrees at Midway, -6 at west
suburban Aurora Airport, -1 at Waukegan and -5 at Rockford no doubt
contributed to the presence of the airborne ice crystals which created
this visual phenomenon.

- Tom Skilling

Photo Courtesy of Melissa Pavlica

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Anchorage has had unusually mild weather lately but very little (like
none) sunshine. However, my friend and fellow meteorologist in Anchorage, Tom
MacPhail, reported FINALLY seeing the sun Monday albeit briefly. He
snapped this picture of a spectacular sunrise which occurred at 10AM.

During late December near winter solstice in Anchorage, the sun is only
above the horizon for a brief 5 hours per day and even then never gets
very high in the sky so provides little if any warmth. Tom reports
that..."Thankfully, we're nearing the end of the tunnel and days begin
getting longer after Dec 21; very gradually at first...a few seconds per
day...but accelerating to a gain of about 5 minutes per day by late
January." Anchorage's daylight begins to feel normal again by
mid-February and by late March, they are already getting over 12 hours
of sun above the horizon. By June's summer solstice, the sun is up over
19 hours per day and darkness comes in the form of a short period of
dusk between midnight and 4AM. It is truly a land of extremes!

-Tom Skilling

Photo courtesy: Tom MacPhail

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The week ahead starts off with a cold, dry arctic air mass still holding fast over the western Great Lakes and Midwest. However, jet stream flow aloft will shift from north to west midweek, allowing maritime cool air from the West Coast to finally penetrate into the central Plains. The warm-up should begin Thursday and peak Friday with 40°-plus high temperatures forecast for the first time this month. There have been only two Decembers in 135 years of record that did not have at least one 40° day—1983 and the snowy December of 2000—and two others, 1903 and 1919, only had a single 40° day.
The Northeast Illinois snow cover ranges from around 3 inches south to 6 inches north along the Wisconsin-Illinois border. As winds pick up Wednesday, warm air moves in Thursday and rain spreads over the area later Friday into Saturday. The snow pack will slowly diminish with a good chance of bare spots by Christmas Day.

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WGN-TV meteorologist Richard Koeneman sent these images of the Blue Ridge Mountains of western North Carolina, which experienced a 10-hour period of freezing rain Wednesday night into Thursday. The freezing rain left an ice accumulation of 3/8th inch on mountains and ridges above 4,000 feet.
Below are views in the mountains above Fairview, N.C., on Friday. Asheville lies in the valley beyond the ridge in the center of the lower photograph.

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Northerly jet stream winds aloft will continue to steer arctic cold into the western Great Lakes and northeast Illinois, but Wednesday the pattern is expected to change with a more west to southwest flow gradually guiding cool maritime-source air from the west coast into this region. High temperatures should slowly nudge higher, reaching well into the 30s Thursday and 40s Friday as strong southwest winds take over at ground level accelerating snowmelt and the shrinking snow cover. Much of the week ahead low pressure systems will be concentrated along the Canadian border to the north and the Gulf of Mexico to the south, resulting in very little in the way of precipitation for northeast Illinois. Moisture will be on the increase Friday with rain likely Christmas Eve in advance of an approaching Central Plains low pressure system. The late-week warming combined with Saturday’s anticipated rain could eradicate much of the current snow cover by Christmas Day.

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The breadth of December’s chill across the Lower 48 has been stunning, extending from the Pacific Northwest to New England and south to northern Florida. It’s an important reason why snow now covers 43 percent of the country—two and a half times the area a year ago. Another 1.1” was added to Midway’s tally in bursts of snow early Friday, boosting the 2005-06 seasonal total there to 17.3”—5.5 times the amount by this date last year. It’s the city’s sixth snowiest open since 1928.
Chicago’s not alone with snow. Many Midwest cities have measured five or more times the snow on the books by this time last season. Just east of Duluth on western Lake Superior, snow has fallen non-stop since Tuesday night.
While Chicago has shivered, mild and rainy weather has pushed Anchorage, Alaska’s, average December temperature nearly 4 degrees warmer than Chicago’s: 23.7° vs. 19.8°.

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Thunder and lightning accompanied a quick burst of heavy snow at Kankakee late Thursday. Thundersnow was also reported in sections of Indiana and Ohio. Still other snow showers slashed visibilities to a quarter mile late Thursday evening in north suburban Algonquin. The squally snows erupted beneath a frigid pool of cold air aloft which eased across the area and lingers Friday. The huge vertical temperature declines which developed produced what meteorologists refer to as an explosively unstable environment—one in which air is encouraged to ascend and cool to saturation. The result is clusters of clouds with tops to 10,000 to 15,000 feet capable of producing bursts of snow. Tiny upper air impulses embedded within the jet stream aid the process.
Thursday’s 0.2” of snow marked the 9th day with measurable snow of the month. That’s more than the typical number in a full December and triple the three measurable snows which occurred a year ago.
--Tom Skilling

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Thursday dawns foggy and drizzly in the midst of Chicago’s mildest daybreak temperatures in 17 days. Not since a 39° low on Nov. 28 has it been any milder here. The flood of mild, moist air into the area in combination with the natural settling of snow since last Thursday’s snowstorm has trimmed six inches from the area’s still-extensive snowpack at Midway Airport in just the past 5 days—from 11” last Friday to 5” by late Wednesday evening.
A burst of snow early Wednesday produced over an inch in surrounding areas, including 1.5” at Antioch, 1.1” at Arlington Heights and 1.8” at Lake Villa—north and northwest of the city—while 2.1” fell at Plainfield. Downstate Springfield reported 3.5” before a switch to rain occurred.
With Midway’s 0.2” of snow Wednesday, the seasonal tally reached 16”, the 7th snowiest early season tally since 1928.
-Tom Skilling

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Snow has covered the ground on December 14 only 23 of the past 78 years at Midway Airport— less than a third of the time. It’s a revelation likely to surprise many Chicagoans who have come to assume December and a covering of snow go hand in hand. Observations of the snow cover at Midway Airport indicate the ground has been snow-free here on this date more than 70% of the time since 1928.
That’s not the case this year. In fact, the 7” on the ground before Wednesday’s incoming storm has been exceeded only once on this date since records began in the late 1920s at Midway. That occurred five years ago in 2000 when a 17” snowpack was in place.
Wednesday’s predicted snowfall, likely to be the second heaviest since November, is the 12th measurable accumulation of the new season.
Only two other years since 1884 have produced more days of measurable snow by now—1989 with 14 days and 1972 with 17 days.

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