Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160040

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
740 PM CDT Sun Apr 15 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 00Z TAFs...

Main aviation concerns are periods of light snow/snow showers and
brief IFR conditions this evening. Snow showers are expected to
persist Monday, with northwest winds becoming gusty around 25 kts

Surface low pressure was pulling away to the east of the region
early this evening, with the upper level trough expected to slowly
cross the forecast area overnight. Low level winds have backed
northwesterly, pulling cooler/drier air into the region, which is
helping to erode much of the extensive IFR ceilings and turn mixed
drizzle/rain/light snow over to all light snow. Thus light snow
and low end MVFR cigs are expected to prevail tonight, though
with occasional IFR cig/vis this evening in more intense snow

The main upper level circulation gradually drifts east of the area
Monday, though several smaller scale disturbances are expected to
rotate through north-northwest flow aloft and maintain the snow
shower threat for much of the day. Low level lapse rates steepen
up as well, suggesting the potential for scattered stronger snow
showers capable of producing IFR vsbys at times. A few snow
showers may persist into Monday evening, though generally drier
air should allow ceilings to trend up to VFR range. The deeply
mixed boundary layer and lingering tight surface pressure gradient
in the wake of the departing low will also support gusty
northwest winds around 25 kts.



300 PM CDT

Gales will persist across the northern 2/3 the lake at least into
this evening and there is a chance that gales may persist over the
far northern portions of the lake overnight. Expect that winds
will diminish below gale force over the whole lake by tomorrow
morning, but as strong cold advection in nwly flow sets up
tomorrow, winds should still remain arnd 30 kt. Small Craft
Advisory conditions will likely persist through Tuesday. Winds and
waves over the lake should diminish Tuesday night as a ridge of
high pressure moves across the western Great Lakes region.


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

     Small Craft Advisory...IL nearshore waters until 11 AM Monday.




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