Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151959

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
259 PM CDT Sun Apr 15 2018

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.



318 AM CDT

Monday through Saturday...

Overview...Snow ends Monday, but another system may bring a wintry
mix to the region Tuesday night and mid week.  Temps warm to near
normal late this week.

Vorticity streamers on the upper level trough`s backside rotate
through Monday morning leading to more light snow showers. Little to
no accumulation is expected.

High pressure builds overhead Monday night/Tuesday allowing temps to
rise into the 40s-50s.  The next system moves into Southeast Iowa
Wednesday morning. Warm air aloft will allow snowflakes to fully
melt, but surface temps below freezing along the IL/WI state line
will be favorable for freezing rain. Surface temps warm Wednesday
morning as the warm sector shifts north turning all precip to rain.
Gusty southwest winds accompany max temps in the upper 40s along the
IL/WI state line to low 60s south of I-80.

Another cold air push follows the low Wednesday night, and rain will
either mix or completely turn over to snow. High pressure late this
week brings another stretch of drier and near normal temperature
days. A low passes over the Southern Mississippi Valley over the
weekend, and its precip shield may extend over the forecast area.



For the 18Z TAFs...

Multiple forecast issues for the terminals through the period.

The main upper low is moving across the Middle Mississippi Valley
while the sfc low is tracking just south of Lake Michigan. While
east flow has covered the region at the sfc, winds have been
diminishing has the sfc low approaches the area. A mid-level dry
slow is overspreading the region, with pcpn transitioning to more
drizzle or a drizzle/light snow mix. While the dry slot moves over
the region pcpn will likely remain remain more drizzle than
anything else. In the sfc east flow, cigs have been mainly at ifr
levels and expect ifr cigs to continue through the period. There
is a chance for cigs to dip into lifr levels when the upper low is
right overhead. There is also a chance for pcpn to become more
convective showery when the upper low is right overhead. As the
sfc and upper low move to the east of the area, sfc winds will
back through northerly to northwesterly and deep layer cold
advection will set up and the pcpn will change over to all snow.
However, by this point, the main forcing will also be well to the
east, so snowfall rates should be light, with only a dusting of
snow likely and vis only in the 3-5 sm range. By late morning,
snow should become more sct snow showers and diminish in coverage
through the day. In the cold advection, nwly winds should increase
and become more gusty through the late morning and into the
afternoon. Do not expect an significant improvement in cigs until
at least tomorrow evening.


328 AM CDT

Strong winds will persist across much of the lake today into
tonight, as the lake remains in between strong low pressure to the
southeast and strong high pressure over southern Canada. While
the strongest winds will be across the northern portions of the
lake, gales are still anticipated over much of the lake this
morning, including the Illinois nearshore. Winds do diminish over
the southern portions of the lake through midday, but remain
elevated over the north half. Have maintained strongest winds of
gales to 45 kt over the northern parts of the lake, however,
continue to monitor the possibility for occasional storm force
winds later this morning into early afternoon. Have not issued a
storm warning for this location as expect these higher winds to be
sporadic, and should not last too long in anyone location. Did
include occasional wording in the open waters forecast though. In
this location, do also expect gales to last longer into tonight,
and have extended the gale warning. Will need to monitor later
trends and guidance, as it`s possible that these gales could last
slightly longer.



IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006 until 7 PM

LM...Small Craft Advisory...IL nearshore waters until 11 PM Sunday.

     Small Craft Advisory...IN nearshore waters until 5 PM Sunday.




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