By Meteorologist Tom Skilling

    January 28 is a significant day in the annals of Chicago weather history. It's the city's coldest day of the year on average and also the date beyond which area residents begin to see an upward overall trend in temperatures. This doesn't mean all cold weather is over--far from it. Chicago's weather pattern has been known to produce wintry spells well into March and even April. But the trend is clear. Longer days and strengthening sunlight begin from this point in the season forward to nudge temperatures higher with increased frequency.
    A computer sweep of 141 years of official temperature data reveals Chicago's average January 28 temperature comes out to 22.95745-degrees while the second and third coldest days of the year here work out to be Feb. 2 (which averages 23.12766-degrees) and January 27 with an average temperature of 23.17021-degrees.  
   
 
The list of this winter's warm weather credentials keeps growing!
 
     No matter how it's examined, winter 2011-12 continues to look quite extraordinary.  The period since Dec. 1 has been the warmest here in 78 years, comes in as the 12th warmest of the 141 winter seasons on record, has produced the fewest sub-freezing daytime temperatures--just 11 of them--in four decades and the fewest days in which the ground has been snow covered in 23 years.   
    Friday's 43-degree high became the season's 33rd daytime high at or above 40-degrees, the most in nearly four decades.  By comparison, last winter had produced just three 40-degree or warmer daytime highs. 11 is the long-term average.


Days are getting longer

    Saturday will see 45 additional minutes of daylight compared to the year's shortest day back on Dec. 21st. Lengthening days contribute to warmer temperatures, though the seasonal warming process is slow.


Water levels in the Great Lakes up in part because of the mild winter

    Lakes Michigan and Huron boast water levels 7 inches above those observed a year ago according to this week's report from the U.S. Army Corps of Engineers. A number of factors influence lake levels, but among the most important are evaporation rates from the huge bodies of water. These rates soar in cold winters because temperatures above the Great Lakes decline rapidly with height in periods of bitterly cold arctic air. Such a setup encourages moisture to evaporate at a swifter pace than in periods in which temperatures above the lake are warmer.
    The warmer-than-normal winter this season has greatly diminished the loss of moisture from the lake surfaces while cutting down on lake effect snow production. The result is less water has exited the lakes through evaporation keeping lake levels higher.


The chill is on this weekend but a return to much above normal temperatures looms

    Colder air spilling into the Midwest this weekend won't be around long. A second burst of cold air threatens a new period of light snow late Saturday night and early Sunday in the wake of Saturday's sunshine. But the return of warm air isn't far behind.
    The passage of a warm front and the onset of strong southwest winds Monday and Tuesday are to send temperatures, expected to hold in the 20s Sunday, soaring into the 40s Monday and to near 50 Tuesday--a level just under 20-degrees above normal.

By Meteorologist Paul Merzlock

On Sunday the high temperature at O'hare airport will remain below freezing, marking only the 12th such occurrence this winter season. Normally, Chicago experiences 42 sub-freezing days over the course of the winter. This is just another statistic measuring how mild the winter of 2011-2012 has been. Intrusions of polar air into the continental U.S. have been minor, with the core of arctic air remaining bottled up over northwest Canada and Alaska. Cold spells have been brief as well. The longest stretch of consecutive sub-freezing days was a 4-day period from the 18th through the 21st of January. Our current brush with wintry weather will be no different. Mild air will once again sweep across the Midwest on Monday. Stout south to southwest winds will send temperatures soaring well into the 40s both Monday and Tuesday, with some 50 degree readings not out of the question. A brief cool down is expected later in the week, but the long term trend through February will favors mild temperatures.

By Meteorologist Paul Merzlock

A one-two punch of colder air will sweep across the Chicago area this weekend. The first will arrive in the wake of Friday night's light snowfall as blustery west winds usher more seasonable temperatures into the Midwest. Despite this cool down, Saturday's temperatures will still run a bit above normal.

A second, more potent shot of polar air is due to arrive Saturday night, with the core of coldest air passing across the area during the day Sunday. The arrival of this colder air will be accompanied by another band of light snow or snow showers Saturday night into Sunday morning. This snow will be minor, with accumulations of less than an inch expected. Nonetheless, the combination of subfreezing temperatures, blustery northwest winds and flurries on Sunday will make it seem more wintry.

As has been the case this winter, this brush with polar air will be transitory. Temperatures will begin to moderate Sunday night, leading to a significant surge of mild air on Monday.

Dear Tom,

Chicago's normal daytime high has been stuck at 31 degrees for some time now. I thought it went down to 28 for the normal high at this time of year?

--Robert Calhoun

Dear Robert,

You are correct to remark that Chicago's normal daily high temperature has been at 31 degrees for some time -- since Jan. 2, to be exact, and it will remain there through Jan. 29. After Sunday, the normal daily high goes to 32 degrees and begins a gradual ascent to a peak of 85 degrees during the second week of July. Those normals are derived from a smoothed average of temperatures as recorded at O'Hare International Airport during the 30-year period of 1981-2010. The previous normal daily highs, from 1971-2000, ranged from 29 degrees to 84. Reflecting an ongoing warming trend, Chicago's new daily normals are generally 1 or 2 degrees higher than the previous normals.

Two more tornadoes yesterday (1 in Mississippi and 1 in Louisiana) brings the preliminary total for the month of January to 70 already.  The vast majority of those tornadoes struck the deep south.  The last three years the US has averaged 17 tornadoes during the month of January. This is now the 3rd most active start to a year in terms of tornadoes since 1950.

While tornadoes in January are rarer for northern states, Illinois is certainly not immune to twisters during this normally cold month.  The state has seen about 30 tornadoes during the month since 1950.   The bar graph below shows the peak months in Illinois are April, May and June.

One noteworthy January tornado because of its location and video documentation, occurred on January 7th, 2008.  It was the most northernmost tornado to hit Illinois in January, touching down in Harvard.  A camera mounted on the train shows how powerful tornadoes (even January tornadoes) can be as the cars of the train are swept of the track by the ferocious winds.

 

Illinois state climatologist Jim Angel has a summary of Illinois' January tornadoes on his blog.  The map below shows where January tornadoes have touched down across the state.  Notice the lack of tornadoes in the immediate Chicago area but the otherwise random distribution of tornado tracks. 

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By Meteorologist Tom Skilling

    Snow is to follow Friday's sunshine, much of it to come down in a 6 to 8 hour period commencing just an hour or two beyond sunset Friday night. It's the first of two snowy spells likely to occur this weekend. The second hits later Saturday night into Sunday morning.
    Neither spell of snow is to come ANYWHERE close to generating the 4 to 10-inches which fell last weekend, transforming the Friday rush hour into a 5 to 6 hour nightmare for many.
    The clouds which spread back into the area Friday afternoon and evening, could send up to 6 to 8 hours of wet snow into the Chicago area Friday night.
     Snow is to commence within an hour or two of sunset Thursday evening.  A slushy half inch to two inch accumulation appears a good bet in a number of areas near Chicago Friday night, especially on grassy and colder outdoor surfaces.
  
   
This winter has produced the fewest sub-freezing days in nearly four decades

    With just 11 sub-32-degree temperatures so far this year compared to the average since 1928 of 25 at Midway Airport, this winter has seen less than half the typical number of below-freezing days in the city.  By comparison, last winter had logged a phenomenal 43-days below freezing by now.
  
   
Meteorological winter 2011-12 heads into a 47th above-normal day since Dec. 1 Friday
 
     Temperatures peek above normal Friday for the 47th time since the 3-month Meteorological season began on Dec. 1. 82 percent of this season's days have produced a temperature surplus.
  
   
Second snowy spell sweeps in Saturday night ahead of a brief rendezvous with daytime 20s Sunday
 
    A second snowy spell ahead of the weekend's coldest air is to sweep across the Chicago area late Saturday night into early Sunday.  Once again, modest snow accumulations are possible--potentially an inch or two of snow at some locations.
    The passage of that snow will allow daytime readings Sunday to settle back to the low or mid 20s amid powerful northwest wind gusts.
  
   
Strong warming follows next week; a string of 40+-degree days begins Tuesday
 
       The chill won't last long if current forecast trends prove accurate. Daytime readings are expected to bounce back to the mid or upper 40s Tuesday and peak in the 40s Wednesday and Thursday next week.

Dear Tom,

I worked at O'Hare International Airport from 1981-1995 and remember bouts of extremely cold weather two weekends in a row in the early 1980s. Can you help me out?

--Wayne Varnes, Strongsville, Ohio

Dear Wayne,

The early 1980s featured an unprecedented clustering of bitterly cold weather in Chicago. Since late 1870 the city has recorded temperatures of at least minus 20 degrees on just 15 occasions and nine of them occurred between 1982 and 1985. Your frigid weekend memories are of back-to-back Sundays in January 1982 that featured intense cold and snow that combined with strong winds to create blizzard conditions and incredibly low wind chills. On Jan. 10, 1982, the high was minus 4 and the low was minus 26 (the city's record low at the time) with just under an inch of snow. The following Sunday brought 2 inches of snow with a high of 4 above and a low of 23 below zero.

Thanks to Stephen Cacioppo, formerly of New Orleans--a Chicagoan for the last 2 years--for this shot taken Sunday, January 22nd at North Avenue Beach looking east. Stephen finds Chicago's winter weather fascinating, and this shot is one reason why.