WE ENTER A SECOND CONSECUTIVE NIGHT in which a majority of the Greater Chicago area is under a FROST ADVISORY.

The only area exempted from the frost threat are, as they were last night, are Lake and Cook counties in Illinois; Kenosha county in Wisconsin and LaPorte county in Indiana. 

This follows the chilliest night of the season across the area. O’Hare’s temp drop to a season’s low of 42 overnight—the chilliest overnight low since early May—5 months ago. 

Area low temps dipped to 30 at Sugar Grove and Rockford; to 31 at Rochelle, Kirkland, South Beloit and Elwood; and to 32 at Warrensville, Bonfield and Chebanse Tuesday morning.

Lows Tuesday night/Wed morning will drip to 40 in the city—but as low as 31 at the coldest spots under clearing skies and amid light winds—the perfect meteorological set-up for maximum cooling.  This puts prospects for frost formation away from the city and warmth of Lake Michigan on the high side again tonight.

A POWERFUL AND WET AUTUMN STORM IS TO DEVELOP over Colorado—and churn eastward bringing waves of significant rainfall to the Midwest and the Chicago area. 

A powerhouse 190 mph jet stream wind max sweeping onto the West Coast from the Pacific is to lift of flow of copious moisture in the system’s warm air sector generating a shield of precipitation which is likely to include thunder at times later Wednesday through Friday. 

And the rains aren’t to end there. THOUGH FAR MORE OCCASIONAL, showery spells are likely to dot the area through the weekend with a strong “NNE” flow sweeping southward the length of Lake Michigan setting the stage for lake-effect rain showers as chilly backside air runs into the city from the warm waters of Lake Michigan.

Model rainfall estimates are impressive—indeed eye-catching—running from 1.50″ to as much as 3.50″ of rain—with the majority of rain tallies likely to fall around 2″ when all the spells of rain are totaled from Wednesday afternoon through the coming weekend.

Rain probabilities will surge to 80% Wed night, fall back to 60 and 50% Thursday and Thursday night—then resurge to 80 to 90% Friday and Friday night. Weekend rain probabilities settle back to 70% Saturday and 60% Sunday.

It’s been the autumn of the ATMOSPHERIC BLOCKING PATTERN ALOFT—which consists of wavy jet stream configurations around pools of warm air aloft, which have the effect of slowing the eastward progression of weather systems. The incoming storm—and the atmosphere within which it will operate—will continue the pattern with a complex OMEGA TYPE BLOCKING PATTERN will slow the eastward movement of the wet storm which is on the way setting the stage for an extended period of gusty north winds over the weekend and linger into early next week. 


FROST ADVISORY from 1 a.m. through 8 a.m. late tonight and Wednesday morning in all but the lakeside counties of Lake and Cook in Illinois: Kenosha county in Wisconsin—and La Porte County, Indiana

TONIGHT: Clearing and chilly with winds dropping to near calm at times away from Lake Michigan.  Frost formation is likely away from the city and Lake Michigan.  Low 40 in the city—but as low as 31 in the chilliest outlying areas west and south of Chicago.

WEDNESDAY: Opens sunny—but clouding over with growing prospects of afternoon rain beginning south of the city in the afternoon and spreading north to Chicago late. Thunder can’t be ruled out in some of the showers.  High 64—but lower lakeshore in the afternoon.

WEDNESDAY NIGHT: Rain and thunderstorms, turning breezy and not as cool. Low 52.

THURSDAY: Showery and windy.  Some thunderstorms likely. High 57.

FRIDAY: Windy and warmer with showers and thunderstorms—the heaviest capable of downpours. Wind gusts approaching 30 mph at times. High 64—but 50s lakeshore.

SATURDAY: Windy with periods of rain likely.  Cooler. Wind gusts to 35 mph. High 59—cooling along Lake Michigan.

SUNDAY AND MONDAY: Extensive cloudiness, windy and cool for the season. Some showery spells—especially close to Lake Michigan. Highs both days 54.

TUESDAY: A good deal of cloudiness, breezy and cool. Some lake effect showers or sprinkles possible—especially morning hours. High 54.