WEATHER EVENTS: December 2005 Archives

Snow--this time wet and heavy---threatening season's second significant accumulation Wednesday and a rainy late day mix of precipitation over part of the Chicago area

Wednesday's storm is likely to produce snow quite different than last week's fluffy 10.2" accumulation at Midway Airport--and there's likely to be more wind with it (20+ mph gusts from the southeast appear a good bet). This snowfall is to take place as wet and heavy flakes--not the huge, LIGHT flakes which fell last Thursday in the city as lake moisture enhanced snowfall. Temperatures Wednesday, predicted to hover with a degree or two of freezing, promise more common 10 to 1 snow to water ratios rather than the light 26 to 1 ratio snow of last week---snow which fluffed up to more than twice the volume of conventional Chicago snows because of the cold environment within which it occurred. That's likely to make Wednesday's snow more difficult to shovel. Those planning to shovel would be wise to approach that task with added care.

Another potentially noteworthy twist to Wednesday's snow may be the ultimate transition to a mixture of rain and snow mid and late afternoon Wednesday from Chicago south. This will come after a number of hours of significant snowfall and potentially significant accumulations over much of the area. Such a change would occur after a significant accumulation expected to fall--perhaps in the 4-5" range (heavier north) from the city and north--with lighter amounts more in the 1 to 3" range south. (Note: North suburban locations are far less likely to see any widespread liquid precipitation during the heart of Wednesday snowfall, although mixed drizzle can't be completely ruled out late in the upcoming precipitation event.)

Sections of north-central and northwest Illinois as well as Wisconsin and Minnesota are to bear the brunt of this storm's accumulation with heavier totals than those observed in Chicago. While the predicted snowfall in Chicago appears on track to become the second major accumulation of a season which is already the snowiest since 2000 (December's seen almost 9 times the snow of the ENTIRE month of December last year--and the month's not even half over!!). Totals there may well exceed 6"---and may even approach 10" in locations near LaCrosse and Eau Claire, Wisconsin and in and near Rochester and the Twin Cities in Minnesota if current forecast trends continue. Closer to home, Chicago's south suburban areas may see a complete transition to rain during the mid and late afternoon and evening after a couple inches of slushy accumulation.

Advisories, watches and warning have been hoisted across sections of 9 central states--from the Dakotas east to Michigan and Illinois. The best indication now is that snow may overspread the Chicago area between 8 and 10 am Wednesday morning then continue steadily much of the afternoon. Precipitation may become more sporadic and far lighter as the strongest jet stream winds reach the area late in the day and Wednesday night. (Heaviest storm precipitation occurs on either side of the strongest channel of upper air winds--regions meteorologists refer to as "shear zones" because of changing wind velocities in such zones. Interestingly, those high altitude wind speed variations play an important role in setting up the vertical air motions (the so called "lift") which acts to cool air converting the incoming moisture into clouds and precipitation.

-- Tom Skilling

Preliminary Storm snow totals just before 7 p.m
Midway Airport 8.3"
North Side near Peterson & Western 5.5"
WGN Studios (Addison & Western) 4.5"
South Bend, IN 3.9"
O'Hare 3.2"
Arlington Heights 2.5"
Rockford 1.9"
Romeoville 4.8"
Oakbrook 5.2"
LaGrange 7.0"

National Weather Service has upgraded the Snow Advisory to a Heavy Snow
Warning in Cook County and counties adjoining the lake in northwest
Indiana. Snow falling heavily across the far northwest suburbs (sections
of northwest McHenry, Winneobago and Ogle counties) at 7pm.

MIDWAY STORM SNOWFALL SURGES TO 7.7" AT 6 P.M.
3" NEW SNOW PAST HOUR ALONE!
The South Side of the city and southern suburbs--east across northern
Indiana--are being hammered by lake-enhanced storm snowfall. Snowfall
as of 6 p.m. Thursday evening has reached 7.7" at Midway, three inches
of which has fallen in the past hour alone! That suggests we may be
headed for local 9-10" totals there before this system exits the area
later tonight. By contrast, Steve Kahn, veteran meteorologist and fellow
Tribune/WGN-TV Weather Office colleague, reports just 2" down at in our
northwest suburbs at Arlington Heights. O'Hare's tally is at 2.3"--and
the snow continues to fall. The snow/water ratio at Midway Airport is
running 29 to 1 (rather than the usual 10 to 1—indicating EXTREMELY
fluffy snow and supporting the notion Lake Michigan moisture is
enhancing the snowfall. Snows to the east in Indiana are strictly storm
related and are falling in the area of maximum lift being generated by
the sharp upper atmospheric trough which is helping drive this storm.

SEASON'S BIGGEST SNOW TO DATE HITS AT FRONT END
OF A STORMIER PATTERN WHICH COULD LAST INTO MID-DECEMBER
The season's biggest snowstorm is in progress and likely to last into
the night. Accumulations have reached 3.5" at Midway Airport (as of 5
pm) and exceed that amount in the southern suburbs where snow has
fallen longest (since 1-2 p.m.) Mundelein in the northwest suburbs has
reported just an inch. Snowfall reached the city proper 2-3 pm and is
currently accumulating at 1" an hour, slashing visibilities at times
to a quarter mile. Snow may fall until just after midnight with the
heaviest intensities likely through 10 or 11 p.m. Final accumulations
may range from 2-3" in the Wisconsin border area to 3-5 inches
(locally as much as 6" in spots) in the city proper and closein
suburbs and up to 6-8" in Chicago's southern suburbs, including much
of Indiana. It's the 8th measurable snow of the new cold season and
easily the biggest. It's also a VERY fluffy snow. Measurements by
Frank Wachowski, veteran Midway Airport observer and Chicago
climatologist, indicate this evening snow is a 23 to 1 snow to water
ratio snow versus the more typical 10 to 1 ratio often cited as
"normal".

The evening commute is being described as nightmarish--with
commute times from Chicago's Loop to Schaumburg estimated at over 4
hours. Normally it's less than an hour.
A typical December produces 6-7 instances of measurable snow
here. Of the more than 550 measurable December snows logged at Midway
Airport since 1928, 95% have produced accumulations under 6". And the
snow's not limited to the Chicago area. 20 states from Illinois and
Indiana to much of the Mid-Atlantic and Northeast--including all of
the major East Coast cities from Washington to Philadelphia, New York
and Bosto----are under one form of winter weather advisory or another.

This storm appears, from latest computer projections, to mark
the beginning of a stormier than normal pattern, which could last over
much of the U.S. for at least the the next few weeks. This raises the
possibility several other significant winter storms may be the
country's future during that period. While the coldest temperatures
are pulling back a bit in the Midwest, nothing close to resembling a
real or prolonged thaw is in the cards and December, already among the
coldest on record. The month is likely to remain cold. There are even
signs the cold air in the arctic will recharge in coming weeks,
potentially forcing another round of bitterly cold air into the
nation's heartland and impacting a larger area of the country, in the
1-2 week time range.

Tom Skilling