Widespread dense fog slowing motorists

Chicago is not alone when it comes to dealing with fog overnight.  Widespread dense fog is blanketing much of the Midwest, and dense fog advisories have been posted for portions of 6 states.   

Long distance travel into or out of Chicago will be slowed, so allow extra time, or perhaps delay your trip until mid-morning Tuesday when the fog will begin to ease.   A survey of area interstates indicates that average speeds of 60 to 70mph have been reduced to 45 to 55mph due to limited visibility.   

Motorists should use extreme caution on bridges, overpasses and elevated highways as the fog is beginning to freeze on some of these surface. 

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Above:  Areas in grey are under a Dense Fog Advisory until mid-morning on Tuesday.   The fog advisory extends as far south as Springfield, as far west as Des Moines, and as far north as Green Bay. 

Longest winter sub-50 streak ending; mildest air in 3+ months coming

Chicago temperatures last surged above 50-degrees—reaching 55-degrees at O’Hare and 56-degrees at Midway—on Dec. 1. In the three months since, no additional 50s have occurred in the city proper. South suburban residents were treated to 50s Monday–including 52-degree highs at Lansing, Kankakee, New Lenox and Valparaiso, Ind. But, a stubborn overcast and light breezes off the lake denied the city readings as warm.

Monday marked Chicago’s 97th consecutive day below 50-degrees, its longest such streak in 31 years—since the winter of 1978-79. The string is to continue Tuesday in the city, but 50s are likely to occur again in Chicago’s south and west suburbs.

Wednesday appears a different story. A break in the sub-50 streak appears on the way, even in the city. More southerly winds are to send peak temperatures well into the 50s in all but north lakeshore communities. There, a slight easterly component to  south winds will bring the flow off Lake Michigan’s chilly waters limiting highs to the low and mid 40s.

Dense fog forms over snowpack and along Lake Michigan
An area of dense fog, responsible for slashing lakeside visibilities under an eighth of a mile late Monday evening, was predicted to coalesce overnight with a second area of fog which had begun forming as post-sunset darkness settled over the Chicago area’s most heavily snow-covered areas to the north and west of the city. The inland-moving, pea-soup-thick fog reached Oak Brook and Arlington Heights by late evening dropping visibilities there to near zero.

Fog forms when the air temperature drops to the dew point. The dew point is the temperature at which the air is holding all the moisture it can or, put a bit differently, the reading at which the relative humidity reaches 100 percent. It’s the perfect threshold for fog development.

With dew points Monday evening which ranged from 33 to 43-degrees across the metro area, surface winds off the 33.5-degree lake waters sent temperatures tumbling to the dew points, laying the foundation for fog formation.

Early season tornado devastates the south side of Hammon, Oklahoma

An eastbound storm system, expected to deliver rain to the Chicago area late Tuesday and Tuesday night, spawned a single but damaging twister in Hammon, Oklahoma just before 6 p.m. Monday.  Damage to four homes and a barn on the town’s south side was extensive.  The tornado took down trees and power lines as well.

1-2-inch rainfalls projected by computer models
 The Chicago area is entering a wetter than normal period. Rainfall occurs this week in two waves—the first due late Tuesday into Wednesday morning and likely to total 0.25 to 0.50 inches. A second, and far more significant rain-generating system hits Thursday then slows to a crawl as a blocking pattern impedes its ability to expeditiously exit the area. The slowdown is expected to keep extensive cloudiness and waves of precipitation a threat through the coming weekend. This may prove troublesome for this weekend’s St. Patrick’s Day parade.
Daylight Saving Time returns this weekend
It’s that time again. Clocks are to be turned ahead an hour this weekend (at 2 a.m. Sunday morning) as Daylight Saving Time returns. Daylight Time is to remain with us until early Sunday, Nov. 7.

Dense fog covers most of the Chicago area-expected to persist through the night

Dense fog shrouds almost the entire Chicago metropolitan area late this evening. Visibilities are generally less than 1/4 mile and as low as 1/8 or 1/16 of a mile in many areas. The fog is expected to persist through the night and linger well into Tuesday morning.  The fog has impacted both road and air travel Monday evening and travelers should check with their airlines for the statues of Tuesday morning flights. 

A dense fog advisory remain in effect for the entire Chicago area through 10 am Tuesday morning.

March and April in Chicago

Dear Tom,
I remember an old weather proverb: “If March acts like April, April acts like March.” Has that been true in Chicago?

–Douglas H. Hanbury, East Peoria

Dear Douglas,
Chicago’s mildest Marches have been followed by milder than normal and,
in most cases, nearly snow-free Aprils. Chicago climatologist Frank
Wachowski tracked the city’s five mildest Marches and found that every
following April was substantially milder than normal, led by April 1878
with an 8.8 degree surplus and April 1921, which was 8.3 degrees warmer
than normal. April snowfall typically averages about 1.5 inches, and
only the 6.9 inches that fell in April 1910 was significant with the
combined snowfall of remaining four Aprils a mere 0.4 inches. On the
flip side, Wachowski noted that four of the city’s five coldest Aprils
came after below-normal Marches.