Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 122044

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
244 PM CST Fri Jan 12 2018

244 PM CST

Through Saturday...

Primary forecast concern in the short term is the lake effect
snow. Lake effect snow flurries and isolated light snow showers
and have been steadily streaming into northeastern IL today and
have shown a bit of an uptick in coverage and a slight uptick in
intensity in the past few hours.

Bufkit soundings from southern Lake Michigan this afternoon show a
sharp frontal inversion just below 5kft, which is likely limiting
the intensity of the snow showers. Some cooling is forecast
between 5-10kft tonight which should allow equilibrium levels to
rise a bit. As the cooling occurs, temperatures in the convective
cloudiness should cool some and probably result in a bit better
flake production. While there looks to be some improvement in
parameters tonight, the set-up overall continues to look marginal
and given the expected transient nature of the band would expect
accumulations in NE IL to be in the coating to half inch range
before it shifts east into IN this evening. NW IN stands a better
chance to seeing amounts fluff up to locally around an inch or so
before the lake effect shifts east of Porter County Sat night.

Elsewhere, look for clearing skies and some relaxation in the
magnitude of the winds tonight. The gradual easing of the winds
should do little to help moderate the wind chills as temps fall
into the single digits over most of the area, the exceptions being
downtown Chicago and northwest IN where lake cloudiness will
provide a bit of a blanket.

- Izzi


244 PM CST

Saturday night through Friday...

Long term forecast focus is on the snow chances Sunday night into
Monday as another lobe of very cold air breaks off the polar
vortex and meanders south into the Great Lakes region early next

Numerical guidances remains in fairly good agreement in phasing a
pair of shortwave troughs over Canada and digging them virtually
due south into the Upper Mississippi Valley and western Great
Lakes region Sunday into early next week. This energy is then
forecast to close off a deep tropospheric circulation that should
slowly move east. A very cold air mass associated with this upper
low should result in another bout of below to much below average
temps much of next week.

A Manitoba Mauler (eastern cousin of the Alberta Clipper) will
dive south into MN then turn east to northern IL or southern WI
Monday. Not a classic track for accumulating snow locally,
however a north-south oriented warm front will extend south from
the sfc low with respectable isentropic ascent to the east of the
front combined with DCVA ahead of the approaching upper low likely
to produce an area of mostly light snow. Forecast time-height
cross sections show a fairly deep DGZ (dendritic growth zone), but
fairly low in the troposphere (below 10kft). The strongest and
majority of the ascent and snow production looks to take place
above the better snow growth zone, which would suggest that
despite the cold temps SLRs (snow:liquid ratios) could end up
lower than would be expected with a system and air mass like the
one in place. Generally ran with about 15:1 SLR in the grids,
which is probably a bit generous given the forecast profiles.
Having said that, if guidance trends lower with the stronger
omega or higher up with the better DGZ, SLRs could certainly end
up higher. At this point, it looks like a longer duration 1-2" to
1-3" snow across the area.

Once the synoptic snow ends, we will probably continue to see
occasional flurries or light snow showers through Tuesday as
steep low level lapse rates beneath the upper low provide a
favorable environment for flurries/light snow showers in the very
cold air mass. In addition to the steep lapse rates (which lake
induced instability will further steeped) the upper low will also
result in high equilibrium levels and a very favorable LES (lake
effect snow) environment early next week in the cold air advection
regime behind the Mauler. Where the lake effect targets will
depend highly on the precise wind regime, which is hard to pin
down this far out. At this point, models and climo would certainly
favor the lake effect event being a IN/MI problem more so than
IL, but certainly bears watching.

Temperatures should remain very cold and well below average Monday
through Wednesday night. If we lay down a respectable few inch
snow cover with the Sunday night/Monday system, then temps could
end up being a bit colder than forecast. But would like to see how
much snow falls, before making changes to the blended model

Cold upper low is forecast to get shoved east as high amplitude
upper ridge builds east into the area mid-late week. While models
are in good agreement in this evolution, it is worth noting that
closed upper lows like this have a habit of being more stubborn
than forecast and moving out more slowly. Once it does move east,
the upper ridge and strong warm air advection should result in a
big moderating trend late in the week. The depth of snow cover
will play a role in temperatures as initially the warmer air will
be dry and not particularly effective in snow melt. Once the snow
melts, the temperatures could once again get unseasonably warm,
though a lot of uncertainty this far out, so again maintained the
model blended temperatures unchanged.

- Izzi


For the 18Z TAFs...

Gusty northerly winds, MVFR ceilings, and periodic lake effect
snow showers will continue to be concerns this afternoon into much
of the evening. Expect much of the cloud cover to depart this
afternoon, however, lake effect MVFR clouds will still impact
ORD/MDW/GYY into tonight. Though I expect ceilings to remain MVFR,
some lowering to around 1kft will be possible later this
afternoon/evening. Gusty northerly winds will persist this
afternoon into much of the evening, with no change to the current
higher gusts. Won`t see any real diminishing trend until late
tonight into early Saturday morning, while the direction becomes
more northwest. Lake effect snow will continue well into Saturday
morning, with some increase in intensity and while the focus
becomes more into northwest Indiana by Saturday morning. In the
near term, flurries or light snow will impact ORD/MDW/GYY with a
slight increase expected this afternoon/evening. However, expect
only MDW and GYY to observe this higher intense snow. Shouldn`t
increase too much, with vis only likely falling to 2-4sm. While
focus becomes more into northwest Indiana later tonight, intensity
may also increase. Given the likely variability with lake effect
snow, don`t have too high of confidence for lowest vis, mainly for
GYY. Will need to monitor this as it could be lower than the
current forecast. Snow should finally end for GYY by early/mid
morning Saturday.



244 PM CST

Northerly gales will subside tonight and no changes were needed to
the gale warning end times tonight. Freezing spray looks to
continue though nomogram suggests the threat of heavy freezing
spray is low so no headlines appear necessary for the freezing
spray. Winds should gradually back over the weekend and lock in to
a period of fairly strong southerlies Sunday night in advance of
the next system. At this point, it looks like the gradient won`t
be terribly strong in the wake of the Monday system, so haven`t
included gales in the grids in the cold air in the wake of the
system, but it will be something to keep an eye on for Monday
into Tuesday.

- Izzi


IL...Lakeshore Flood Advisory...ILZ014 until 4 AM Saturday.

IN...Lakeshore Flood Advisory...INZ001-INZ002 until 4 AM Saturday.

LM...Gale Warning...nearshore waters until 6
     AM Saturday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...6 AM Saturday to 3
     PM Saturday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...6 AM Saturday to 6
     AM Sunday.

     Gale Warning...LMZ777-LMZ779 until 3 AM Saturday.




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