Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160658

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
158 AM CDT Mon Apr 16 2018

738 PM CDT

Have allowed the Winter Weather Advisory to expire on schedule at
700 PM CDT for the northern tier of counties in Illinois. IR
satellite imagery has shown a gradual cooling trend in cloud tops
over the past few hours indicating greater potential for ice-
nucleation and has corresponded well to a change-over to snow.
Only a couple lingering drizzle or rain/snow mix reports from sfc
obs/mPing this past hour, but these reports were in areas with
surface temps above freezing. Snow overspreading the CWA from the
west thus far this evening has been fairly light, only reducing
visibility into 1-2 mile range, with a few brief pockets under a



258 PM CDT

Through Monday...

The effects of the major low pressure will continue to be felt
across the area through Monday in the form of some mixed
precipitation...mainly snow and well as chilly
temperatures and winds re-increasing into Monday. At this time,
the Winter Weather Advisory will continue until 7 pm. While some
light snow is very likely after that time tonight and even into
Monday morning, impacts should diminish, but this will need to be
watched through the late afternoon (especially if freezing drizzle
were to be a greater threat).

An impressively amplified mid-latitude cyclone is centered in the
mid-levels over the IA/IL/MO border region. The commahead of this
will be moving gradually back over the area tonight, and as such
the combination of deeper moisture and vorticity advection and its
associated lift, will spread increasing light snow coverage back
over the area. This will especially be true for the northwest half
of the forecast area. Re-saturation of the snow growth zone
should support mainly snow with this as it inches west. Minor
additional grassy accumulation is likely in areas north of the
I-88 corridor as surface temperatures inch a few degrees below
freezing by early evening. Patchy dustings are possible further
south including into Chicago late evening into overnight. Again
ahead of this area within the present dry slot, drizzle will
continue, which for the far northern Chicago suburbs through
early-mid evening could continue patchy slick conditions.

The 500mb low should be directly over the area early Monday
morning, with scattered to numerous snow showers supported within
wraparound moisture in the morning, and more isolated to
scattered in the afternoon into early evening. The clouds should
remain widespread with continued cold advection doing well to
offset a large part of any daytime warming. Highs are forecast to
be below 40 for a good part of the area.



310 PM CDT

Monday night through Sunday...

A continued progressive, active flow pattern will remain through
the week. The mid-level wave path is forecast over or near the
area. In simpler terms, this means limited duration of warm-ups as
well as precipitation chances.

Cyclonic flow will continue into Monday night with gradual drying.
It is a challenge to say whether or not skies will clear,
especially east, but temperatures will depend highly on that.
Record lows are possible in some places if clearing were to occur
(Rockford record low for Tue a.m. is 24 set in 1983).

The next closed low, while must less amplified, will move into the
Iowa region by Wednesday. Warm advection aloft Tuesday night is
forecast to move over the area. Moisture looks slower to increase,
and as such we have removed Tuesday night chance of precipitation.
If precipitation were to occur by early Wednesday morning in
northern areas, there could be isolated convection and surface
temperatures may be close to freezing...but do think temperatures
will be quickly rising as winds turn south.

Highs on Wednesday are likely to push into if not well into the
50s if the surface warm front can indeed push through the area.
Scattered showers are possible especially north, and then by
Wednesday night the mid-level low passing over will likely
support precipitation depending on the path. Some of this could
fall as light snow.

Beyond, the next low for the weekend is forecast south of the area
at this time, but confidence is low in this. Worry that blended
temperatures may be a little on the warm side for Thu and Fri
given the wind direction and if clouds stick around behind the
midweek system.



For the 06Z TAFs...

Overall, I didn`t make many changes to the TAFs as the previous
forecast looks really good. A band of light to moderate snow
showers is beginning to weaken. RFD should only see flurries for
the remainder of this morning. The eastern terminals will continue
to see MVFR snow showers that ocnly lower vsbys to IFR through
the early morning hours. MVFR cigs will also continue through much
of the day.

Northwest to west winds will gust to around 25 kt. Additional snow
showers will develop this afternoon with the upper level low
overhead and surface instability in place. Guidance is really
struggling to capture the snow showers, so I kept the previous
forecast`s timing of 17-21 UTC. The stronger snow showers may
produce IFR vsby and cigs.

The upper level low finally shifts east this evening/overnight
allowing cigs to become VFR. Gusty northwest winds will
continue tonight.



158 AM CDT

Winds diminished below gales, so the gale warning for the northern
end of the lake was allowed to expire. No changes were made to the
small craft advisory.

Low pressure over the Ohio Valley will reach southern Quebec through
Tuesday morning. Meanwhile, high pressure is over the Plains, which
will cause north winds to 30 kt today. Some models indicate we could
see ocnl gale force gusts, but my confidence is low, so I will not
include gales in the GLF. Winds back to northwest overnight and
diminish Tuesday as the high shifts east to the Mississippi Valley.
The next low reaches the lake mid week with another broad surface
high late in the week.



LM...Small Craft Advisory...nearshore waters
     until 10 AM Tuesday.




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