fxus63 klot 212004 

Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois 
204 PM CST Mon Jan 21 2019 

Short term... 
154 PM CST 

Tonight through Tuesday night... 

A rather complex forecast ahead as low pressure lifts from the 
central Great Plains into the upper Midwest Tuesday. This 
afternoon, high pressure stretches from the eastern Great Lakes 
region across the upper Ohio Valley. Meanwhile, low pressure is 
analyzed over Front Range of the Colorado rockies. For the rest of 
this evening and tonight, expect increasing and thickening cloud 
cover in advance of this approaching low, as well as 
strengthening southeast winds. Anticipate non-diurnal temperature 
trends overnight with temps very gradually rising into the upper 
teens or lower 20s by Tuesday morning. 

Precipitation associated with the low is expected to overspread 
the forecast area from the west from mid morning into the early 
afternoon. There are a number of factors on Tuesday that result in 
lower than average confidence in details of the forecast, 
particularly with respect to precip type. For one, there are a 
number of hours on both the front end and back end of this 
precipitation event where saturation is not deep enough and cloud 
top temperatures not cold enough for heterogeneous ice nucleation 
and resulting in drizzle (freezing or not depending on surface 
temps). Next, temperatures both aloft and at the surface indicate 
that we may see the full range of precip types at various points 
in the day. A modest 850mb warm nose with temps around +1-2c will 
spread across northern Illinois through the afternoon atop the 
bitterly cold air mass still in place. Sleet and freezing rain 
both appear possible as temperatures slowly warm aloft. 
Eventually, as the boundary layer warms through the day and into 
the evening, precipitation should change over to rain from the 
south. Anticipate the rain/snow line to set up near the Illinois/WI 
state line Tuesday evening. Pavement temperatures are an 
additional wild card with respect to the impacts of any freezing 
rain/drizzle. Given the expected cloud cover, think that pavement 
temperatures may be slow to respond to any advection driven 
warming. Alternatively, if skies do have partial clearing, 
insolation should help pavement temperatures quickly respond and 
will mitigate impacts. 

Surface low is progged to lift across north central Illinois around 
midnight Tuesday night, with a trailing cold front sweeping across 
the County Warning Area overnight. Temperatures do cool back down for a transition 
to snow, but a narrow dry slot punching across the area may end up 
resulting in some additional freezing drizzle before precip ends. 
That said, forcing drops off pretty quickly as well, so precip may 
just end before additional impacts are realized. 

Will be hoisting a Winter Weather Advisory to cover the freezing 
rain and possible sleet threat for Livingston, Kankakee counties and 
counties to the north in north central and northeast Illinois. This 
area represents the highest confidence in hazardous conditions from 
freezing drizzle/rain and sleet. The advisory will cover the initial 
freezing drizzle/rain threat from mid morning through the afternoon. 
While the forecast will mention some potential an icy mix in the 
remaining counties, confidence remains too low at this time to issue 
headlines for the remaining counties. 



Long term... 
200 PM CST 

Wednesday through Monday... 

The message for the mid to latter part of the week into the 
weekend is a much colder than normal pattern, with the cold air 
mass reinforcing over the central and eastern U.S. During this 
time. The pattern will also remain active around the periphery of 
the eastern North America long wave trough and polar lobe centered 
over Hudson Bay and into Ontario. In the northwest flow through 
Saturday, any snow makers look to be low amplitude clippers or 
driven by strong cold advection and not particularly robust in 
snow magnitude, at least at this distance. 

Wrapparound light snow will be possible Wednesday morning with the 
departing system. Temperatures will struggle to climb much from 
where they start the day in the mid 20s with northwest winds 
likely to gust at times over 20 mph. A potent short wave will drop 
over the Great Lakes Thursday into Thursday night, swinging a 
strong cold front (slightly modified Arctic front) through the 
area Thursday afternoon. Forcing along and behind this front may 
be enough to compensate for very limited moisture to produce 
scattered snow showers. The 850mb temperatures are forecast to 
drop quickly to -20c by Thursday evening. Blustery conditions and 
falling temperatures will occur in the afternoon and night, with 
below zero temperatures forecast prior to midnight. Wind chills 
presently look to drop to -20 to -25 later Thursday night into 
early Friday morning, which could warrant the first Wind Chill 
Advisory of the season. 

As numerous mid-level jet streaks and associated waves ride from 
western/central Canada into the middle of the U.S. Friday Onward, 
it is challenging at this distance to time snow chances Friday- 
Sunday. Again these generally are clipper systems and do not look 
to have much snow per say, but some snow with blustery and cold 
conditions can still result in minor impacts. Highs look to be in 
the teens with lows dependent on cloud cover and whether in the 
cold advection envelope of any of these systems, but at or below 
zero in outlying areas is favored. At the end of the weekend into 
early next week, most global guidance does indicate a stronger 
clipper/northern Pacific hybrid system cutting into the upper 
Midwest/Great Lakes region, which would steer a tighter 
baroclinic zone and stronger forcing back over the area for 
possibly a more robust snow maker somewhere in the region. 
Obviously at nearly a week out in this active pattern, confidence 
is low at this point. 



for the 18z tafs... 

Fairly steady state conditions expected through the remainder of 
today. Main forecast concern is dealing with the impacts from an 
approaching low that lifts from the central Great Plains early 
Tuesday morning to northwestern Illinois Tuesday evening. 

Winds: modest south to southeast winds will prevail through the 
afternoon, with sporadic gustiness this evening and overnight. 
Winds aloft will pick up sharply late tonight into Tuesday 
morning. Wind speeds around 2000 feet should range between 40 and 
55 kt resulting in low level wind shear. Surface winds are expected to gradually 
veer to the south and south-southwest through the day Tuesday with gusts into 
the mid 20 kt range. 

Precipitation: a few lingering flurries are possible at the 
Chicago area terminals late this morning, but otherwise 
anticipate dry conditions through the rest of today. Precip 
chances increase from the west mid morning through the afternoon 
Tuesday as the low approaches the region. Determining precip type 
is a bit tricky, especially within the first few hours of 
precipitation starting. Forecast soundings are initially below 
zero; however, cloud top temps appear to be too warm for 
heterogeneous ice nucleation, which would favor freezing drizzle 
as a precip type. Meanwhile, just as deeper moisture arrives 
allowing for ice nuclei to form and for snow to occur, 
temperatures aloft are progged to warm enough for a melting layer 
to form. This would favor a window of freezing rain and/or sleet 
during the early afternoon hours. Once surface temps warm above 
freezing around mid afternoon, precipitation should change over to 
rain for the the remainder of the day. Temperatures remain an 
area of uncertainty, and pavement temperatures may lag air temps 
slightly if cloud cover develops early enough in the day. 
Conversely, if sunshine does break out early in the day, that may 
help warm pavement temps enough to mitigate the impacts of any 
falling liquid precip freezing, or at least help to shorten the 

Ceilings and visibility: VFR conditions will prevail today into 
Tuesday morning. As precipitation nears the terminals, anticipate 
ceilings to build down to MVFR. IFR/MVFR visibility is expected with 
precipitation on Tuesday. 



Lot watches/warnings/advisories... 
Illinois...Winter Weather Advisory...ilz003-ilz004-ilz008-ilz010-ilz011- 
ilz019-ilz032...9 am Tuesday to 6 PM Tuesday. 

Winter Weather Advisory...ilz005-ilz006-ilz012-ilz013-ilz014- 
ilz020-ilz021-ilz022-ilz023...noon Tuesday to 6 PM Tuesday. 

Lm...Small Craft Advisory...lmz740-lmz741-lmz742...6 PM Monday to 6 
PM Tuesday. 

Small Craft Advisory...lmz743-lmz744-lmz745...9 PM Monday to 6 
am Wednesday. 



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