Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 150011
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
611 PM CST Sun Jan 14 2018

.SHORT TERM...
307 PM CST

Through Tuesday...

Main forecast concerns/challenges are with a period of light to
moderate snow this evening through midday Monday. A Winter Weather
Advisory has been issued with 3-5 inches of snow expected across
the area.

In the near term, clouds continue to overspread the region this
afternoon ahead of approaching shortwave energy and WAA. On radar,
weak returns noted across the west/southwest CWA, however, snow
not likely reaching the ground quite yet. This initial forcing is
running into some drier air in place and think there is still
about 1-2 hours until snow develops across the CWA. This start
time will be around the 22-23z time frame across the
west/southwest CWA. Expect snow to then develop soon there after
over remaining areas in northern Illinois. Although some slight
delay to the onset is expected across portions of northwest
Indiana, it shouldn`t be too long as the strongest WAA quickly
arrives early this evening and allows for all areas to saturate.

Still expecting a longer period of light snow from this evening
through midday Monday across the area, however, latest trends and
guidance coming in a little stronger with this system. Initial
snow will be driven with this approaching shortwave and WAA but
will be reinforced with the approaching strong vort lobe and
surface low to the northwest. This coinciding with an area of fgen
moving across the CWA, will see snow persist this evening along
with some increase in intensity likely by around the midnight time
frame through the early morning hours Monday morning. During this
time, expect a light to a times moderate snow to fall. A couple
other changes noted to latest guidance is the location of the
stronger forcing, with it now appearing to be more collocated
within the DGZ. Also, most locations across the CWA will see this
WAA, synoptic forcing tied to the vort, and fgen coincide
together. With all of these features, felt confident continuing
the trend of raising snow totals with 3-5 inches area wide likely.
With the expected snow totals, likely slippery road conditions,
and reduced visibility, issued a Winter Weather Advisory from this
evening through midday Monday.

System snow likely ending by midday Monday, but do expect
additional scattered snow showers to continue Monday afternoon,
primarily north of I-88. However, at this time, don`t expect this
to be enough to warrant extending the advisory into the afternoon.
These additional snow showers will be tied to the arrival of the
upper level low to the north Monday afternoon. With its arrival,
pattern begins to get a little tricky. This now meandering system
will keep the associated surface low in place across the Great
Lakes region. Surface trough axis on the backside of this system
will swing across the area late Monday into Monday evening with
additional snow showers possible with this feature. From the
afternoon into the evening, its possible for an additional half
inch to an inch across the north. Continue to keep an eye on the
lake effect snow potential. As this trough swings through, flow
will become oriented off the lake with now lake effect snow
showers possible spilling into locations in far northeast Illinois
during the overnight hours. This will eventually be focused
across northwest Indiana by early Tuesday morning. Accumulating
lake effect snow is possible across far northeast Illinois Monday
night, however confidence is low on trends. Exact placement of
developing lake effect snow across far northeast Illinois will be
key, along with duration with current trends showing this to be
more progressive. Higher focus will likely occur across northwest
Indiana on Tuesday along with a longer duration, and could provide
higher snow totals in this location.

Rodriguez

&&

.LONG TERM...
307 PM CST

Tuesday night through Sunday...

Any lake effect snow should be winding down, as temps warm and
flow pushes any development east of the CWA. WAA and dry
conditions appear to be the trend through the end of the work
week. Although WAA begins at the start of the period, cool
conditions still anticipated Wednesday and Wednesday night. A
stronger warm push should arrive more towards Thursday, and bring
temps to more normal conditions. Above normal high temps in the
upper 30s to around 40 expected by Friday, and then well into the
40s expected Saturday. Guidance is advertising a system to move
across the region late in the weekend. However, guidance is
struggling with the placement/timing, as is expected this far out.
Could be a system to monitor, given the potential strength of the
system.

Rodriguez

&&

.AVIATION...
For the 00Z TAFs...

611 pm...Primary forecast concern is light snow through late
Monday morning.

A prolonged period of light snow is expected tonight into Monday
morning with a possible short lull in the light snow late this
evening...but confidence is too low to include at this time.
There will likely be some variability to the visibilities
especially early this evening as the low levels slowly saturate.
But vis will likely be between 1-2sm for much of the night. Its
possible there could be some periods of vis under 1sm...mainly
after midnight and before sunrise. End time of the accumulating
snow and lower vis is somewhat uncertain...but likely in the late
Monday morning time period with flurries or very light snow
continuing into the afternoon.

Wind directions are expected to remain south/southeast through
mid morning Monday with speeds/gusts slowly diminishing Monday
morning. A cold front will move through the area during the late
morning...shifting winds westerly 10-15 kts and these westerly
winds will persist into Monday evening.

Cigs expected to remain prevailing mvfr through the period with
ifr cigs possible...especially Monday morning but confidence is
low for ifr cigs. cms

&&

.MARINE...
117 PM CST

Southerly flow across the lake late today will gradually back
southeasterly tonight as a surface low moves from Iowa into
northern Illinois. The center of the low tracks over Chicago
around midday Monday and then very slowly across the south end of
the lake, not really exiting until Tuesday morning. Winds in the
meantime continue backing from southeast late tonight to easterly
Monday afternoon and then northerly Tuesday morning except near
the low at the far south end where winds will be more variable.

Northerly flow backs to westerly by Wednesday morning as high
pressure builds across the Midwest, then to southwesterly
Wednesday afternoon as the high moves into the Tennessee Valley. A
tightening pressure gradient around the high appears to support a
period of gales on Wednesday.

Lenning

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 until noon Monday.

IN...Winter Weather Advisory...INZ001-INZ002-INZ010-INZ011-INZ019
     until noon Monday.

LM...Small Craft Advisory...Winthrop Harbor to Northerly Is. until
     3 AM Monday.

&&

$$

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