Weakening Alberta Clipper's snows only to swipe the Chicago area Friday



Frustrated area snow enthusiasts, already disheartened by the lackluster winter snow season they’ve been forced to endure,  won’t find much to celebrate in the paltry snows expected with Friday’s passing Alberta Clipper. The system traverses the Midwest in weakening form, as evidenced by the rising central barometric pressure readings.


Barometer readings reflect the rate at which air within a weather system is rising or sinking. When air rises with vigor, barometer readings fall.  But when the rate at which air is rising within a low pressure system actually slows, pressure readings react by rising. When that happens, winds within the weather system weaken, slowing the influx of moisture required to maintain clouds and precipitation.


That’s the situation with Friday’s Alberta Clipper, and while patchy light snow or flurries may occur, totals appear likely to be far from overwhelming.



The Chicago area’s in the grip of its coldest spell of weather in two years; recent chill has January running 3+-degrees behind a year ago



Chicago is entering a 6th consecutive day of below normal temperatures Friday and is in the midst of its chilliest run of weather in nearly two years.



Thanks to the recent arctic outbreak, January 2013 is running more than 3-degrees behind the same period a year ago.



Quiet Saturday to include fair amount of sun—but wintry precip mix is in the offing Sunday



The coming weekend opens cold but with a generous helping of sunshine Saturday.  It’s the period beginning late Saturday night into Sunday night that becomes problematic. That’s when a potentially icy mix of freezing rain and freezing drizzle, possible ice pellets and even a spell of wet snow, takes hold.



Our retreating cold air mass is to be overrun by northbound Gulf “warmth” and moisture.



With a dense, cold air mass in residence at ground-level  here, and slower-moving than the warmer, moister air predicted to run up and over it, the stage is set for “warm” clouds to produce liquid-form precipitation which is to fall into the colder air below.



Such an environment fosters freezing rain which falls into colder air below and can turn to ice upon contact with cold outdoor surfaces.


Pockets of cold air can still survive in the early stages of this pattern, supporting the development of snowflakes or ice pellets, which can mix with the freezing rain at times.



Warming to bring 40s and rain—not snow—-Monday



Continued warming Sunday night suggests a gradual shift in the phase of the area’s precipitation from frozen to liquid-form and Monday’s 40s all but guarantee any moisture which falls will do so as rain.


Strong arctic outbreak sweeps mid-week and to spill over in February’s opening days



Another sprawling mass of arctic air sends temperatures on a slow downward glide Tuesday followed by a plunge to bitterly cold levels from next Wednesday forward as we close the books Thursday on the month of January and transition next Friday to February 2013.