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Snow is expected to develop over northern IL by late Friday afternoon, or early evening and then overspread much of the area late Friday night. The snow should then gradually tapper off from northwest to southeast Saturday afternoon and evening. While questions remain as to how much snow may fall, there are signs that an area over far northern IL (generally north of I-80), may experience a band of moderate to heavy snow Friday night into Saturday morning. If this occurs, these areas could end up with 6″+ inches of snow by Saturday afternoon. Farther south, over central IL and IN, an additional area of moderate snow is expected during the day Saturday. These two bands of heavier snow may result in the highest snow amounts occurring over far northern IL and central IL, with slightly lighter amounts elsewhere. This is a complex storm system, so amounts may change with later forecast updates. Following this period of snow it appears that lake effect snow will develop over northeastern IL Saturday night, then gradually shift east into northwestern IN on Sunday. Several additional inches of snow accumulation will be possible with the lake effect snow. Stay tuned to the latest forecasts.

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Here’s a look at the weather highlights and hazards for the upcoming weekend into early next week. Accumulating snow is expected across the region beginning late Friday afternoon and continuing into Saturday, with widespread snow amounts of two to five inches. The forecast track could change, so stay tuned for updates and be prepared for winter weather. For late Saturday into Sunday, lake effect snow will be possible with additional accumulations possible near the lake. After the snow, the area will deal with some bitterly cold temperatures, as minimum wind chills could get as low as fifteen degrees below zero.

SHORT TERM…
328 AM CST

Through Friday…

Things starting to look more active on the water vapor early this
morning with a series of shortwaves from the Dakotas to the central
and southern Plains in the generally zonal/weakly amplified upper
flow. These will all translate through the region today. The
southern stream wave is the most organized in terms of having a
surface reflection. This will largely pass through the Ohio valley
this morning, but an inverted trough axis will allow some of the
moisture and forcing to sneak into our area along and mainly south
of Interstate 80. Complicated near term forecasts given the
multitude of waves, mixture of forcing, saturation, and depth to
the low level moisture.

Our primary concern initially is the fairly deep dry layer
between the mid/upper moisture and our continued low level
moisture, which has limited precipitation to FZDZ/DZ. FZRA/FZDZ
has been observed from Lincoln to Bloomington, IL, along with
around the Quad Cities area. Meanwhile, observations from CMI-SFY-
FEP in northwest IL have been reporting snow where the profile is
a tad cooler and where ceilings are a bit higher. While forecast
soundings do suggest that the continued lift will allow for some
additional low-mid level column saturation in our southern
counties, there does appear there is still a window of freezing
precip before we see a shift over to snow. The changeover may be
occurring now with BMI-CMI now reporting -SN/UP as of 330 am CST.
Ceilings are forecast to decrease, so if the column does not
moisten up, we could have FZDZ last a bit longer farther north as
well. And while the column is colder farther north, the forcing is
weaker. Therefore if it precipitates farther north, it is
certainly possible that this could be FZDZ too across the Chicago
area and towards the Wisconsin border, but it appears more likely
that most of the Chicago area would remain either dry or would be
-SN as ceilings will still take some time to drop.

With the transition to snow along and south of I-80, these areas
look to pick up some light accumulations to around an inch or two,
mainly south of US Highway 24, with lighter amounts as you head
toward I-80. By the afternoon the surface trough will shift east
and shortly thereafter the series of upper waves which will bring
an end to precip chances. This should largely end the
precipitation, though there is one last wave in the northwest flow
that will drive a weak cold front across northern portions of the
area tonight into Friday. Light snow will likely remain confined
across Wisconsin.

Our brief quiet period later Thursday into early Friday will be
interrupted later Friday. The front that shifted into northern
Illinois will become the focus our next winter weather system later
in the day/evening. In spite of some clearing of the lower clouds
Friday morning, mid/upper clouds will quickly thicken as the
baroclinic zone sharpens across the southern WI/northern Illinois.
We could start to see some fgen driven snow to begin as early as
Friday afternoon, with northern IL/southern WI the likely focus
area, but these mesoscale band locations are difficult to resolve
quite this early. The better forcing will wait until Friday
evening.

KMD

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.LONG TERM…
333 AM CST

Friday night through Wednesday…

The primary concerns during the extended forecast continue to
focus on:
-Snow amounts Friday night and Saturday.
-The threat for a period of moderate to heavy lake effect snow
with additional accumulations over portions of far northeastern
IL and northwest IN Saturday night into Sunday.
-The period of very cold weather expected Sunday and Monday.
-More periods of active weather next week, which could result in
more accumulating snow.

Not much has changed from previous thinking with the storm system
expected to shift eastward across the Ozarks and into the TN
valley on Saturday. While the main track of this storm system will
remain well south of the area, there are good chances for
accumulating snow over our area Friday night into at least
Saturday morning. Model guidance continues to suggest that a
strong band of 850-700 mb frontogenesis will develop somewhere
over northern IL/southern WI beginning late Friday, but really
ramping up Friday evening and night. While the actual placement of
this band of frontogenesis, and the associated heaviest snow
potential, may shift some in the guidance over the next 24 to 36
hours as forecast guidance continues to hone in how this
mesoscale setup will evolve over the area, signs continue to point
at northern IL (mainly north of I-80) Friday night. The before
focus for snow then appears to shift over the southern half of my
CWA into Saturday in association with the main storm system
moving into the TN Valley. Overall, this setup could result in two
maximum areas of snow over the area; one potentially over far
northern IL, with a second over my southern CWA.

There is some increasing concern for heavy snow associated with
the band of snow Friday night over northern IL. Given the good
feed of moisture into the area, with 700 mb mixing ratios
expected to be around 2.8 g/kg,this should result in some effect
snowfall. This should especially be the case given that the
frontogenetic forcing for ascent looks to produce good ascent
right through the favored dendritic growth zone. With all this in
mind, along with the fact that we should have fairly high snow
ratios of 14-16 to 1, we could easily see some areas of northern
IL end up with 6+ inches of snow through Saturday morning.
Thereafter, the main synoptic snow should begin to focus over my
southern CWA by Saturday afternoon, where amounts here in the 4″
to 6″ range appear probable through Saturday evening. Increasing
northeast winds during the day Saturday, may also result in some
blowing and drifting snow issues, especially in open areas.

The next concern will be the lake effect snow following the main
system snow. While some lake enhancement could keep light snow
going over portions of northeastern IL Saturday afternoon, it
appears that the better thermodyamics set up for heavy lake
effect snow may hold off until sometime late Saturday evening or
night. Strong lake induced instability is likely to setup over
southern Lake Michigan Saturday night through Sunday as 850 mb
temps drop to around -18C, essentially setting up ~20C
temperature differences between the lake temperature. Inversion
heights should reach around 6000-6500 feet, which should cut right
through the dendritic growth zone, so this should be sufficient
to support heavy lake effect snow. While the specifics will take
time to iron out, the synoptic pressure pattern of low pressure
moving up the east coast and a strong surface high building over
the Upper Midwest looks favorable for the potential development of
a single band of heavy lake effect snow. This band looks to start
over northeastern IL Saturday night into Sunday morning, before
gradually shifting over northwestern IN during the day Sunday. The
main question that remains is residence time of the band over any
given area, which will have large implications on the amount of
snow that falls.

A winter storm watch may be needed for parts of northern IL, north
of I-80 from Friday night into Saturday, with the watch possibly
being needed for far northeastern IL through Sunday morning, due
to the continued lake effect threat. We have opted to hold off on
any headline at this time though, to let the day shift get another
look at this snow event.

Otherwise, expect cold conditions over the area for Sunday and
Monday. Highs both days in the teens look likely, with overnight
lows below zero in some areas of IL. We could also have some -10
or colder wind chills Sunday night.

The weather pattern looks to remain active into next week. This
looks to result in more storm systems to keep an eye on for the
potential for more accumulating snow across the area.

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