Arctic blast to send temperatures crashing

Barometer readings in the Yukon, where Chicago's weekend weather has taken form, have
soared to levels rare -- even by arctic standards. Cold air masses are so dense, it's not
unusual for pressure readings within them to surge. The atmospheric pressure at the core
of this air mass -- strengthened by a frigid cross-polar injection of bitterly cold
Siberian air the past few days -- is predicted to reach 31.40 inches (1063 mb) over northwest
Canada early Friday -- a reading far higher than Chicago's record barometer reading of
30.98 inches (1049 mb). Air masses supporting such lofty central pressures rarely sit
still -- and this one is no exception. It's frigid southern flank invaded Montana and the
Dakotas late Thursday with 40 m.p.h. wind gusts and 24 hour temperature drops of 30+
degrees. The high's move into the Chicago area is to be marked by strengthening winds Friday
that will blow haze and any morning fog out of the area, but also to send temperatures
crashing to single digits Friday night. Though not quite as cold or as windy as the
barbarically chilly air which gripped the area just over a week ago, the Yukon high threatens to
produce Chicago's chilliest weekend temperatures in 54 weeks and more snow in the
Indiana/Michigan snowbelt.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune