Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231920

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
220 PM CDT Fri Mar 23 2018

220 PM CDT

Through Saturday...

The main concerns continue to focus on the winter storm expected
to impact portions of the area tonight and on Saturday. Overall,
the main changes to the forecast was to transition the remaining
portion of the watch (with the exception of Ford and Grundy
county) to an advisory. We did add Ford and Grundy counties to
the warning.

Our storm system of interest is currently taking shape over the
Plains states. Radar imagery early this afternoon already
indicates a band of precipitation ongoing across the Dakotas
southeastward into Iowa in association with strengthening lower-
mid level frontogenesis. This activity is expected to develop
eastward into western and Central Illinois overnight as the storm
system begins to shifts out towards the Lower Missouri Valley. It
still appears that this band of frontogenesis will support the
development of a band of moderate to heavy snow over my
southwestern CWA tonight, with the heaviest snow expected after 3
am. Farther to the east-northeast towards the Chicago area, an
easterly influx of low- level dry air is expected to fight against
this area of heavy precipitation. This will, as previous
discussions have noted, result in a sharp cutoff between snow and
no snow.

While some snow cannot be ruled out into portions of the
Chicago area towards Sunday morning, it appears that the dry air
may win out resulting in mainly virga over most of the Chicago
area. However, the farther to the southwest you go from the
Chicago area into the current warning area, heavy accumulations of
wet snow (5"+) are likely. This is especially true to the
southwest of a line from near Dixon southeastward through
Marseilles to Paxton. These areas are expected to experience
periods of moderate to heavy wet snow late tonight through at
least mid to late afternoon on Saturday. The snow should begin to
tapper off over the area later Saturday afternoon into the early
evening as the main mid-level wave shifts over the area.

In addition to the usual impacts of heavy snow leading to low
visibility and snow covered roads in the winter storm warning area,
increasing easterly winds on Saturday (gusts up to 35 mph) could
result in some additional impacts. While some blowing snow will
be possible, the wet nature of the snow may limit this threat
some. The main concern is the fact that this snow will likely
efficiently accumulate on elevated surfaces, such as tress and
power lines. Because of this, the extra weight with wind could
cause falling tree limbs and possible isolated power outages.

Given the likelihood of the main snow accumulations and the
associated impacts remaining outside of much of the ongoing watch
area (with the exception of Grundy and Ford counties) we have
decided to transition to a winter weather advisory for the
possibly of some isolated accumulations of a couple inches. I
tightened up the gradient of accumulations, and this results in
little to no accumulations over much of the Chicago area. Only
some small portions of the northern counties in the advisory have
any real accumulations mentioned in the forecast now, and as a
result the advisory may end up being overkill in portions of the
southern Chicago area counties. However, with the possibly of
some last minute shifts in the heaviest snow amounts and some
small forecast errors, we felt it prudent to at least have an
advisory out at this time. As the event begins to unfold tonight,
we may be able to drop some of these counties from the advisory.

Things are expected to quiet down quickly by early Saturday
evening as the snow ends.



126 PM CDT

Saturday night through Friday...

Forecast concerns continue to be on periods of rain Monday night
through midweek. possibly a few thunderstorms.

High pressure will transit the area on Sunday in wake of Saturday`s
storm system. With surface high pressure over northeast Quebec,
northeast flow will maintain seasonally cool conditions in spite of
plentiful late March sunshine as readings hold in the 40s, with low
40s over snowpack covered areas and also near the lake.

We will transition to a moist southwest flow pattern early to mid
week as a deep upper level low currently off the Pacific Northwest
coast line will dig into an expansive trough across the western USA
on Monday. A surface low will organize in lee of the Rockies Monday
and lift toward Lake Michigan on Tuesday. The southwest flow will
advect well above normal precipitable water values toward the region
and will also push a warm frontal boundary from the Arklatex region
Monday to our area on Tuesday. Several periods of rain are
expected as a strong upper level jet lingers to our northwest.
The first will come Monday into early Tuesday with the warm
frontal surge and associated strong low level jet Monday night. A
second round is expected later Tuesday as the northern stream
portion of the deep western low pressure trough will steer a cold
front across the region Tuesday night.

The main concern will be rain on top of any areas that receive snow
this weekend, but this will look to keep basins elevated, especially
across east central Illinois and into northwest Indiana (Illinois,
Kankakee, Iroquois basins).

High pressure will move across the area Wednesday into Wednesday
night with perhaps some lingering showers Wednesday morning. A
cold front will be moving across the midwest/Great Lakes region
Thursday into Thursday night as low pressure moves northeast from
the Ohio Valley to New England. Some differences in the models for
how much precip develops ahead of this slightly more baroclinic
front yields low confidence for precip for our area with chance
pops reasonable for the area. It appears late next week we will
transition to a west-northwest flow regime which will keep things
on the cooler to seasonal side.



For the 18Z TAFs...

Concerns are in increasing easterly winds, and then
pinpointing the expected sharp cutoff in heavy snowfall overnight
into Saturday morning.

Snow will develop from northwest IL through east central IL late tonight
and continue into Saturday, with a band of heavy wet snow expected
mainly well southwest of the terminals. A sharp cutoff to the
main precipitation shield is expected on the northeast side of
and will likely be near a rfd/mdw/vpz line, with current guidance
getting some light snow into rfd but not quite into the Chicago
terminals due to the low level dry east winds. There could be a
window from late tonight through mid Saturday morning (roughly
10z-17z) that will be a period to watch if any shifts in the snow
band occur. Very strong lift will suggest an increased
supercooled mid level cloud deck and at least some virga or some
occasional snow flakes. VFR appears to rule outside of the snow
band were IFR/LIFR will occur. At this time will keep the Chicago
area terminals dry for the above rationale.

The other concern will be the stronger east winds, which will
approach 30 kt, and maybe get slightly higher than that, with the
main ramp up occurring after 10z overnight and continuing through
the day.



1231 PM CDT

A ridge of high pressure across the Great Lakes will
slowly shift east tonight as low pressure moves from the central
Plains today to Tennessee Saturday as it weakens. The gradient
between these two systems will tighten tonight with easterly winds
increasing to 30 kts on the southern end of Lake Michigan. As the
low dissipates Saturday night...the ridge will build back across
the Great lakes region for Sunday and Sunday night. A trough of
low pressure will slowly move across the western lakes region
Monday through Tuesday night. Southeast winds will shift
southerly may increase to 30 kt ahead of this trough. A series of
generally weak cold fronts will follow for the latter half of the



IL...Winter Storm Warning...ILZ010-ILZ019-ILZ021-ILZ032-
     ILZ039...midnight Saturday to 7 PM Saturday.

     Winter Weather Advisory...ILZ008-ILZ011...1 AM Saturday to 4 PM

     Winter Weather Advisory...ILZ020-ILZ022-ILZ023-ILZ033...4 AM
     Saturday to 4 PM Saturday.

IN...Winter Weather Advisory...INZ010-INZ011-INZ019...4 AM Saturday
     to 4 PM Saturday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...1 AM Saturday to 4
     PM Sunday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...4 AM Saturday to
     10 AM Sunday.




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