Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 150719

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
119 AM CST Mon Jan 15 2018

753 PM CST

The initial warm advective wing of the system has allowed for
widespread snow to develop, nowhere particularly heavy, with snow
rates generally around one quarter of an inch per hour. With a
deep dendritic growth region and some modest omega in there, the
snow production/snow ratios are such that the snow is quite
fluffy. The snow will keep going all night into Monday morning,
with this leading shot of snow generally holding with visibility
ranging from 1 1/2 miles to 3 miles. After some weaker snow across
western Illinois arrives, the main shortwave across SE MN will
shift into our area, leading to a secondary uptick in snowfall
rates again. Model visibility forecasts of 1-2SM type snows again
with this next batch suggest that any heavier rates would be
minimized. There are some 3/4SM vsby snows upstream which briefly
suggest snow rates could inch up to a brief period of 1/2" per
hour rates. Still expect drier snow through the event, though
ratios may come down towards daybreak into morning. No changes to
the forecast message for continued light and occasionally moderate
snow that will gradually pile up and slick road conditions for the
morning commute.



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.



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



For the 06Z TAFs...

1146 pm...Main forecast concern remains light snow through late
Monday morning.

Light snow continues across northern IL and as the associated low
pressure moves closer to the region...expected prevailing vis in
the 1 to 1.5 mile range for the next several hours with vis
dropping below 1sm at times in heavier bursts of snow. The light
snow will slowly begin to taper off by sunrise...but vis is still
expected in the 1-2sm through mid/late morning. Eventually...the
light snow will taper to flurries. Cigs will remain low mvfr into
Monday afternoon with ifr possible Monday morning...generally
after sunrise but confidence remains low.

There will be another chance of light snow or snow showers late
Monday evening into early Tuesday morning as another weak wave
moves across the area. This snow may be a combination of lake
effect or lake enhanced. While confidence is low from this
distance...potential is high enough for prob mention with the 30
hour ord and mdw tafs.

South/southeast winds will slowly diminish to 10kts by sunrise and
likely under 10kts by mid morning. Winds will begin to shift to
the southwest and then shift westerly as a cold front moves across
the area late Monday morning into early Monday afternoon. West/
northwest winds 10-15kts are then expected for the rest of the
period with some higher gusts possible. cms



119 am...Low pressure over northeast IA early this morning will
move to south central Lake Michigan by late this afternoon and
then begin to weaken as it moves to the eastern lakes region on
Tuesday. Southeast winds to 30 kts with a few higher gusts early
this morning will slowly diminish this morning and turn easterly
this afternoon and then northeasterly tonight. However...based on
the current expected track of this low...winds will likely remain
westerly on the far southern portion of the lake tonight. Strong
high pressure will build across the Plains tonight and Tuesday as
low pressure passes well north of the lakes region midweek. This
will tighten the pressure gradient over Lake Michigan and a period
of gale force winds are possible Wednesday and Wednesday night.
Temperatures will still be below freezing during this time so
freezing spray is possible. cms


IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     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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