FXUS63 KLOT 181124

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
524 AM CST Fri Jan 18 2019

351 AM CST

Through Saturday...

Main concerns/challenges this forecast period continue to be
focused on the impending winter storm that will impact the region
later today into Saturday.

Main changes this morning were to upgrade the Winter Storm Watch to
a Winter Storm Warning for the entire CWA to detail:

* Accumulating snow starting this afternoon for areas along and
north of I-88, spreading south to I-80 corridor Fri evening and
south of the Kankakee River late Fri evening or after midnight.
* Prolonged period of light to moderate snow with periodic bursts of
heavy snow still appearing likely, with some lake enhancement
appearing possible for mainly Lake IL and Cook counties.
* Increasing winds tonight into Saturday which will result in
blowing and drifting snow, especially in open and outlying areas.
* Lake effect snow showers likely into NE IL Saturday afternoon and
night, and Sat night into Sunday for NW IN. Most significant lake
effect snow accums expected late sat evening through Sunday

After a dry period this morning through midday/early afternoon,
will see chances for snow quickly increase this afternoon for
locations in northern IL, mainly for locations along and north of
I-88. Will see WAA ramp up across the area ahead of this upper
level system and surface low over the central CONUS. However, main
support still appearing to be from increasing FGEN, which will
pivot across the region. Guidance continuing to vary with this
initial band of snow, with some of the more aggressive models from
last evening beginning to back off a little. This is appearing to
be in line with some of the short term guidance, which is
bringing this band of snow across northern IL, but keeping it more
focused in the northern portions of IL. Did try and focus initial
highest pops/snow this afternoon in these locations, but its
possible there could be further refinement. If this were to occur,
then there could be some slight delay to the onset of snow for
remaining locations in north central and northeast IL this
evening. A more northerly position of this initial snow is
expected, but do still expect snow intensity to quickly increase
during this time, impacting the evening commute.

Will see large scale ascent increase ahead of this upper level
system, while isentropic ascent and FGEN increase/spread over much
of the CWA this evening. By mid/late evening, snow will be
ongoing for much of the CWA, with the exception being locations
along/south of the Kankakee River Valley in IL/IN. Don't
anticipate snow to begin here until late in the evening, to around
the midnight time frame. From this evening through early Saturday
morning, expect light to moderate snow, with bursts of heavier
snow across much of the CWA. Thermodynamic conditions still don't
appear to be overly great across the lake, but it's appearing that
a persistent fetch off the lake could still support some lake
enhancement and have included this in the forecast.

Only some small changes to the forecast snowfall for this system
through Saturday afternoon, with the highest amounts still favored
across northern IL along and north of I-88. In this location 5-8
inches are expected. For locations near the lake, Lake IL and
Cook, do think slightly higher amounts of 5-9 inches are likely.
For locations along/south of I-80, think 3-7 inches are likely.
Have included mention of these possible lower amounts, but do
think there is still the possibility for these lower amounts to be
slightly higher in this location. Have decided to upgrade the
Watch to a Warning for the likely heavy snow, along with the
likely strengthening winds which will result in some blowing and
drifting tonight into Saturday. Winds will likely remain elevated
Saturday afternoon after the snow ends, and will need to assess
the possibility for blowing/drifting snow continuing beyond the
current warning end time.



408 AM CST

Saturday Night through Thursday...

Lake effect snow showers will continue to be a concern late in
the day Saturday into Saturday night and Sunday across both far
northeast IL and northwest IN. After the synoptic snow ends on
Saturday, expect lake effect development mainly on the IL side.
Thermodynamic conditions over the lake still not appearing overly
great late in the day Saturday into Saturday evening, with
marginal delta Ts and low inversion heights still expected. So, do
anticipate some development to occur right as the synoptic snow
ends, but don't anticipate this to be too impactful at that time.
However, conditions do change through the evening and early Sunday
morning with both delta Ts and inversion heights increasing along
with the potential for strengthening convergence on the western
side of the lake. This should support higher intense snow, though
confidence is still lower with exact placement and duration for
locations in northeast IL Saturday night. With time, should see
this shift more into northwest Indiana by Sunday morning. Still
expect this to occur, but latest guidance is showing this
development to remain in IL. Not a likely solution at this time,
but will need to monitor this possibility. Did not extend the
warning for Lake and Cook counties to include this additional
snowfall of 2-4, maybe 3-5 inches. This was due to the fact that
there will be a large window between the impactful snow, with
several hours of only light snow likely before the intensity of
the snow increases once again Saturday night.

Cold nights expected Saturday night and Sunday night with wind
chills below zero likely. An active pattern appearing likely next
week with the next system to impact the area arriving Tuesday.
However, at this time, most guidance is indicating a warmer
solution and the possibility for more rain than snow.



For the 12Z TAFs...

MVFR conditions and modest winds (generally under 10kt) will be
in place through the first half of the day, but main forecast
concern is for the snow storm that will overspread the terminals
starting late this afternoon or this evening.

WINDS: Light northwest/north winds are in place early this morning
and are expected to veer to the northeast by mid to late this
morning. Winds should then remain out of the northeast through the
remainder of the forecast period. Winds are expected to gradually
increase in magnitude today. Expect moderately strong gusts by
tonight into Saturday morning, topping out in the 25 to occasionally
30 kt range.

PRECIPITATION: Snow is expected to begin overspreading northern
Illinois late this afternoon or early evening. Initially dry low and
mid level air should saturate around 22-00Z with snow intensity
quickly picking up. This initial band of snow is expected to be
relatively narrow, and there remains some uncertainty where it
sets up. While a good number of models indicate this initial band
will set up over northern Illinois, a few models have trended even
farther north into southern Wisconsin.

More widespread snow will overspread the region late this evening
and overnight. This will coincide with winds becoming gusty. In
addition to periods of moderate to occasionally heavy snow, cooling
temperatures should result in a drier snow which will be prone to
blowing and drifting. Through Saturday evening, it appears that 5
to 9 inches of snow is possible at the terminals. Additional snow
is possible beyond the TAF period as lake effect snow sets up over
northeast Illinois and northwest Indiana into the day Sunday.

CEILINGS AND VISIBILITY: MVFR ceilings should prevail through most
of the day today, then IFR ceilings become more favored with the
steadier snow this evening and overnight. Visibility will likely
quickly fall from VFR this afternoon to IFR/LIFR shortly after the
onset of snow. Anticipate periods of 1/2 to 3/4SM visibility this
evening and tonight.



IL...Winter Storm Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
ILZ011-ILZ012-ILZ013-ILZ014...3 PM Friday to noon Saturday.

Winter Storm Warning...ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-
ILZ032...7 PM Friday to noon Saturday.

Winter Storm Warning...ILZ033-ILZ039...midnight Saturday to 3 PM

IN...Winter Storm Warning...INZ001-INZ002...7 PM Friday to noon

Winter Storm Warning...INZ010-INZ011-INZ019...midnight Saturday
to 3 PM Saturday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
LMZ745...9 PM Friday to 3 PM Sunday.




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