By Meteorologist Tom Skilling
Chicagoans haven't shivered through air as cold as Saturday's predicted 26-degree high in the 10 months back on February 10. The arrival of chilly air the past few days represents a sea change in the area's recent weather pattern.
Friday's 32 and 18-degree extremes (through 11 p.m.) produced a 4-degree deficit, making it the first day to finish below normal in 3 weeks. The cold air is whittling away at December, 2011's 4.2-degree surplus.
The chill comes at the end of an extraordinary run of mild weather. 51 of the most recent 70 days--73 per cent of them--managed daily temperature surpluses. While the string of mild days has broken temporarily, milder air is eastbound again from the Rockies, and the gusty southwest winds on which it arrives here in Chicago Sunday are expected to propel temperatures as much as 17-degrees higher, reaching the low 40s. Once here, the milder than normal temperatures are predicted to linger well into next week until a new blast of cold arctic air hits late week, behind a wet, mild storm system expected to hit Wednesday night into Thursday.
Saturday's 26-degree high is to be the first sub-freezing temperature this season; 2011 joins 15 of the past 141 years without a sub-32-degree reading going into Dec 10
The lack of cold air until recently--just like the lack of snow here this season--has been unusual. Friday morning's 0.5 inches of snow at O'Hare marked the first time "measurable" snow (0.1 inches or more) has fallen here this season and ties as the 5th latest date of the past 127 years to record measurable snow.
All indicators point toward Saturday recording the first official daytime high this season which fails to reach 32-degrees. The city's last official 32-degree high occurred earlier this year on April 1.
Underscoring how rare it is to wait until this date to record a season's first sub-freezing daytime temperature, is the fact only 15 of the past 141 years have failed to produce a sub-32-degree peak daytime reading by Dec 9!
Powerful, downsloping Chinook winds dive out of the Rockies into the western Plains Saturday, warming in the process; the milder air is Chicago-bound
Mild air of Pacific origin is wending its way across the Rockies. As this air descends out of the mountains into the Dakotas and Canada's Prairie Provinces, temperatures late Friday had surged as much as 30-degrees over levels 24 hours earlier.
That mild air is to continue its eastward drift putting it into the Chicago area Sunday. Temperatures surge into the low 40s at that time--a 17-degree increase over Saturday afternoon predicted 26-degree high.
Milder temps initially arrive within dry, Pacific air; moister Gulf air is to follow with more clouds Monday
The transition back to milder temperatures predicted later this weekend for Chicago starts with the arrival of dry Pacific air. This air is "dry" because its moisture has precipitated over the West's mountains. Rising air cools, and this process produces precipitation on west-facing mountain slopes. Its moisture depleted, the Pacific air proceeds east toward the Midwest and skies here remain sunny.
By Monday, however, a transition from Pacific to Gulf air begins. Clouds will increase and rain prospects are to raise Monday night into Tuesday.
By mid next week, a vigorous winter storm system is to come on the scene, predicted to develop over Colorado then lift into Wisconsin. That disturbance is to tap the Gulf air's moisture, producing what could be generous rainfall here Wednesday night. The storm's path will keep Chicago in its "warm sector" until Thursday at which time temperatures are to plunge as the next windy arctic surge arrives.
Big snow not yet in sight
As it stands now, there is no consensus among our computer models on a snow system over the next two weeks. This can change and we will continue monitoring developments, letting you know of any signs of snowier weather ahead.
Chicagoans haven't shivered through air as cold as Saturday's predicted 26-degree high in the 10 months back on February 10. The arrival of chilly air the past few days represents a sea change in the area's recent weather pattern.
Friday's 32 and 18-degree extremes (through 11 p.m.) produced a 4-degree deficit, making it the first day to finish below normal in 3 weeks. The cold air is whittling away at December, 2011's 4.2-degree surplus.
The chill comes at the end of an extraordinary run of mild weather. 51 of the most recent 70 days--73 per cent of them--managed daily temperature surpluses. While the string of mild days has broken temporarily, milder air is eastbound again from the Rockies, and the gusty southwest winds on which it arrives here in Chicago Sunday are expected to propel temperatures as much as 17-degrees higher, reaching the low 40s. Once here, the milder than normal temperatures are predicted to linger well into next week until a new blast of cold arctic air hits late week, behind a wet, mild storm system expected to hit Wednesday night into Thursday.
Saturday's 26-degree high is to be the first sub-freezing temperature this season; 2011 joins 15 of the past 141 years without a sub-32-degree reading going into Dec 10
The lack of cold air until recently--just like the lack of snow here this season--has been unusual. Friday morning's 0.5 inches of snow at O'Hare marked the first time "measurable" snow (0.1 inches or more) has fallen here this season and ties as the 5th latest date of the past 127 years to record measurable snow.
All indicators point toward Saturday recording the first official daytime high this season which fails to reach 32-degrees. The city's last official 32-degree high occurred earlier this year on April 1.
Underscoring how rare it is to wait until this date to record a season's first sub-freezing daytime temperature, is the fact only 15 of the past 141 years have failed to produce a sub-32-degree peak daytime reading by Dec 9!
Powerful, downsloping Chinook winds dive out of the Rockies into the western Plains Saturday, warming in the process; the milder air is Chicago-bound
Mild air of Pacific origin is wending its way across the Rockies. As this air descends out of the mountains into the Dakotas and Canada's Prairie Provinces, temperatures late Friday had surged as much as 30-degrees over levels 24 hours earlier.
That mild air is to continue its eastward drift putting it into the Chicago area Sunday. Temperatures surge into the low 40s at that time--a 17-degree increase over Saturday afternoon predicted 26-degree high.
Milder temps initially arrive within dry, Pacific air; moister Gulf air is to follow with more clouds Monday
The transition back to milder temperatures predicted later this weekend for Chicago starts with the arrival of dry Pacific air. This air is "dry" because its moisture has precipitated over the West's mountains. Rising air cools, and this process produces precipitation on west-facing mountain slopes. Its moisture depleted, the Pacific air proceeds east toward the Midwest and skies here remain sunny.
By Monday, however, a transition from Pacific to Gulf air begins. Clouds will increase and rain prospects are to raise Monday night into Tuesday.
By mid next week, a vigorous winter storm system is to come on the scene, predicted to develop over Colorado then lift into Wisconsin. That disturbance is to tap the Gulf air's moisture, producing what could be generous rainfall here Wednesday night. The storm's path will keep Chicago in its "warm sector" until Thursday at which time temperatures are to plunge as the next windy arctic surge arrives.
Big snow not yet in sight
As it stands now, there is no consensus among our computer models on a snow system over the next two weeks. This can change and we will continue monitoring developments, letting you know of any signs of snowier weather ahead.
