Daytime temps only hit 28 Friday—though the calendar day high of 33 occurred just after midnight. The last time daytime temperatures here failed to break above 30 was over 10 months ago in February!
Temperatures rebound in Saturday’s generous sunshine and the weekend as a whole is to be meteorologically quiet while continuing the above normal temperature trend which has dominated the month.
Barometer reading sinks to its lowest level in two years here as Thursday's storm passed overhead
Thursday's intense storm passed right over Chicago producing the city's lowest barometric reading in 2 years: 29.01". The last time the barometer was so low here was Oct. 26, 2010.
Barometric pressure readings offer meteorologists insights into whether the air in a particular region is rising or sinking. Storm systems are regions of tremendous upward vertical motion in the air. When air is rising on a broad scale, air pressures drop and surface winds increase to replace the ascending air.
It goes without saying that the faster the upward motion of the air into a storm, the lower the resulting barometric pressure and the stronger the winds converging on the system in order to compensate for the air being lifted through the atmosphere.
Thursday’s barometric pressure reading was only 0.31” of mercury above the city’s all time low pressure of 28.70" recorded in Chicago March 12, 1923.
Mammoth Thursday storm contributed to exponential growth in Lower 48 snowpack; the U.S. snow cover expanded 8-fold in just 2 weeks!
NOAA, parent agency of the National Weather Service, reported Friday that nearly half the Lower 48—46.9% to be precise—sat beneath a layer of snow early Friday. That’s an area nearly eight times what it was just two weeks ago on December 6 when just 6% of the 48 contiguous states reported a cover of snow.
Active pattern to set-up across the U.S. over December’s closing days; multiple shots at snow appear a reasonable bet
With such an expansive snow shield now in place, frigid air can now seep into the Lower 48 more easily. This is, in time, to lead to a colder pattern in December 2012’s closing days in Chicago.
A jet stream will set up along the southern flank of the cold air permitting a series of disturbances off the Pacific Ocean to race eastward at regular intervals across the country. Any of these disturbances may intensify into a storm and each will be monitored carefully.
It appears at this distance that there may be 3 chances for snow across the Midwest with these systems—the first Monday, a second Wednesday, potentially a "lake-effect" event, and a final shot at snow late Friday into Saturday.
December 2012’s running 9th warmest on record; 2012 just days away from closing as Chicago's warmest year on record
2012 comes to an end in a little more than a week and it appears on a path to become Chicago's warmest year of the past 142.
Contributing to that average has been Dec. 2012’s stunningly mild average temperature of 40-degrees—the 9th warmest on the books here since 1871.
Saturday is to be the 25th consecutive day at/above normal.
Despite all the snow around us, the lack of snowfall here has been matched or exceeded only twice before in 128 years
Only 2 snow seasons have produced as little or less snow than 2012-13's paltry 0.3" to date: 1943 (0.3") & 1993 (0.2"). But, the snow scene here can change quickly. An average of 80% of Chicago's snow falls from this date forward!