Late-season outbreak of colder arctic air to shave 12 degrees off area temps Friday/Saturday; daytime 20s predicted

Cold, late-season arctic air stages a comeback Thursday night and Friday.  A buckling jet stream over western North America sends steering winds aloft deep into the arctic then plunging south into the Lower 48.  That northerly flow is to tug arctic air south into the weekend—cold air likely to arrive with clouds and sporadic flurries—if not several snow showers over the Friday/Saturday time frame.


Returning chill comes on the heels of Wednesday’s surge into the 40s—only the third day with temps that mild this month!


The resurgent chill follows a mild surge, spawned in part by the return of February sunshine Wednesday. Chicago temperatures surged into the 40s for only the third time this month, peaking at 44-degrees at O’Hare, a reading 9-degrees above normal and 46-degrees at Midway. Northerly Island recorded a 45-degree max!


The warmth wasn’t limited to the city: 159 of the 171 WeatherBug sites across southeast Wisconsin, northern Illinois and northwest Indiana registered highs which topped 40-degrees.


Some of the milder Wednesday readings were highs of 46 at Valparaiso, Ind. and 45-degrees at Winnetka, Rochelle, Morris, Lansing, Kankakee and Hickory Hills in Illinois and East Chicago, Indiana.


Temp downturn to arrive with clouds and flurries starting later today


Temperatures cycle a bit lower later Thursday. Daybreak rain showers may segue to afternoon and nighttime flurries as the decline begins.  But, a more significant temperature downturn Thursday night which continues Friday and Saturday is likely to keep flurries coming from widespread cloudiness. The potential for a few more-substantial snow showers can’t be ruled out.


Scientists poised to monitor a “safe” but close asteroid fly-by Friday; it’s to miss Earth by just 17,000 miles!


An extraordinary astronomical event takes place Friday. An asteroid is to pass surprisingly close to Earth, coming to within 17,150 miles of our planet’s surface—which places it within the orbit of geosynchronous satellites which monitor our weather and relay television and other communications.


NASA scientists are characterizing this pass as “safe”. But you can be sure astronomers and others will have eyes to the sky as the asteroid sweeps by around 1:25 pm CST Friday afternoon.


Two wintry systems threaten next week in what looks like an active pattern


Two potentially stormy weather systems are being monitored by forecasters for next week. The first, which could impact the Chicago area or locations nearby Monday into Monday night, may feature a wintry cocktail of precipitation—including sleet, freezing rain and/or snow. But it’s the second weather system which is looking most potent on model forecasts.


The system is a week away, having just moved out into the Pacific off Japan Wednesday. Still, model forecasts have suggested it’s to be a potentially significant player in Chicago and the Midwest’s weather in the Thursday/Friday time-frame next week. Its track will be critical in determining how much and in what form its precipitation falls.


Snow or a wintry mix with late-week storm?


With a sprawling Canadian high pressure predicted by model runs to be centered to Chicago’s northeast later next week, several scenarios are possible. Such a set-up could draw drier air off the high into Chicago at the low levels. This would keep an adequate supply of cold air available to produce snow rather than rain as the storm’s favored form of precipitation.


But, were the storm to track farther north, the warmer air it would bring up and over the incoming dry air at the low levels could lead to a wintry cocktail capable of creating a variety of precipitation, and potentially, quite a mess. In this scenario, the storm’s precipitation would initially fall as liquid into drier air at the lower levels. Evaporative cooling would follow, lowering temperatures at ground level. This

atmospheric set-up would produce a mixture of freezing rain, sleet and/or snow.


Either scenario—a big snow or a wintry mix—could prove disruptive. Forecasters will be monitoring developments with these systems over the coming week.


Snow would add to lackluster 2012-13 seasonal tally—currently the 10th least snow of the past 128 years and 42% the long-term average


A significant snow, were it to occur, could bolster a seasonal snow tally which is the 10th lowest of any of the past 128 years.  The 10.7” current 2012-13 seasonal total is just 46% normal and 73.3” below Chicago’s biggest snow season back in 1978-79 to date.

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