All is go for a major snowstorm Friday night into Saturday and
Saturday evening, with steady snow likely to reach the city between
5-7pm this evening---then fall steadily, and at times heavily, much of
the night. Total snow yields from this system appear likely to
challenge those of the winter's previous big snow back on January 4-6
which produced 9.8" of snow at O'Hare. 6-8" of snow is to accumulate
by morning with more occasional snowfall continuing, but at more
varied intensities Saturday--at times falling as light snow and
flurries, at other times--especially in the city and the counties
adjacent to Lake Michigan where lake enhanced snowfall is to occur as
the day proceeds---heavier intensities are possible. And, 8-10" seems
a good bet as the storm system's contribution to accumulation
here---with another 3-5" of lake-effect snowfall not out of the
question Saturday into Saturday night in sections of the counties
closest to Lake Michigan (i.e. Lake, Cook, eastern DuPage and parts of
Will county in Illinois and Lake and Porter Counties in Indiana). The
arrival of lake-effect snow appears likely to begin occurring mid or
late morning Saturday then increase at times in both frequency and
intensity in the afternoon and evening. A lot of lake ice has banked
up against Lake Michigan's western shoreline and this cuts down on the
amount of open lake water able to feed moisture in the atmosphere,
making the amount of lake snow which occurs in a mid-season storm like
this one a little problematic. But, a steep temp decline with
altitude (a condition referred to by meteorologists as "instability")
is to set up later Saturday morning and afternoon just as winds begin
blowing more from the NNE rather than the SE, increasing the "fetch"
(distance) over which they travel before streaming into the city. This
increases the time spent over lake water and the chance to pick up
moisture on the trek into lakeside areas of northeast Illinois and far
northwest Indiana. At the same time all this is happening, a vigorous
upper air disturbance is to pass over the area, increasing the "lift"
which helps generate clouds and snow. The confluence of all these
factors is behind the expectation that lake snow may provide
potentially significant additional accumulations to Saturday's snow.
Another factor likely to take on significance is wind. Fairly
moderate winds will accompany much of this storm's steady "system"
snow. But, as the storm deepens and a 1045 mb (30.86") arctic high
dives with arctic air into the Plains, powerful winds with gusts 30+
mph are likely to take hold Saturday afternoon and evening. The
comparatively fluffy nature of this storm's snow (with a 15 to 1 snow
to water ratio likely rather than the usual 10 to 1 ratio because of
the cold temps at which snow will occur in this system) may more
easily become airborne. This raises the specter of blowing and
drifting, particularly in open areas and areas along Lake Michigan, as
Saturday afternoon and evening proceeds. Single digits temps will
plunge into the area over the fresh snowpack Saturday night--with near
0-degree lows possible in colder west suburban locations (i.e. the Fox
Valley area).
All in all, a fascinating meteorological situation which we'll
continue updating here, on our WGN-TV programs and in the Chicago
Tribune.
