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.