Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220544

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1144 PM CST Wed Feb 21 2018

915 PM CST

The main weather concern is the potential for a period of FRDZ
across southern portions of the area late tonight into very early
Thursday morning.

Fast southwesterly mid and upper level flow across the central
CONUS will drive a series of weather disturbances northeast across
Illinois late tonight into Thursday. We are currently monitoring
an area of freezing precipitation across southwestern through
east central Missouri this evening. This area of precipitation is
likely to remain south of the area. However, additional very
light drizzle and freezing drizzle may develop farther northward
across portions of central IL overnight within a region of of
increasing low-level moistening from isentropic upglide (warm air
advection). The main uncertainties are how far north does this
potential light precipitation make it into Illinois, and also
will temperatures be below freezing if it does arrive? Currently
there is a good deal of dry low-level air across the area as
sampled by the DVN and ILX evening RAOBs, so there is certainly
some work to do to get conditions favorable for light
precipitation, and this adds some uncertainty to the northern
extent of any precipitation.

Nonetheless, it does appear that isentropic upglide may be enough
to moistening the low-levels overnight, especially over far
southern portions of the area. Therefore, there are some chances
for a period of very light precipitation (likely drizzle or
freezing drizzle) across this area by, or a bit before daybreak
Thursday. While any freezing precip could cause issues, I don`t
have the confidence at this point to justify issuing a winter
weather advisory for my far southern counties, but I will
highlight the area in an SPS.

While some drizzle or very light rain may continue across portions
of the area during the day Thursday, it does appear that any
freezing precipitation will end just by mid-morning Thursday as
temperatures begin to warm above freezing.



320 PM CST

Through Thursday...

For the remainder of this afternoon and into the evening, conditions
will be relatively quiet across the region, with only a canopy of
cirrus overhead.  As high pressure builds across the Upper
Mississippi Valley, colder air will continue to filer across the
region.  While the high pressure should continue to build east
across the Great Lakes region, skies should remain mostly cloudy as
winds turn more easterly from off of Lake Michigan.  Broad southwest
flow will remain in place aloft, as an upper high continues to
slowly build east off the south Atlantic coast and longwave troughing
deepens over the western CONUS.  a series of weak shortwaves are
expected to lift through the swly flow aloft, bringing an increased
chance for some precipitation late tonight.  Latest guidance would
suggest that much or all of the pcpn should be confined to the far
southern portions of the CWA, mostly south of the Illinois and
Kankakee Rivers.  With the ongoing river flooding on the Illinois,
Kankakee and Iroquois River, any additional precipitation will only
negatively impact the situation.  At this point, it looks like any
pcpn should be very light.  A greater concern with this
precipitation will be more focused on precipitation type.  While
surface temperatures will be below freezing, some warm advection
aloft could set up thermal profiles that might be conducive to
either light freezing rain or sleet mixing with the snow. Confidence
in the northward extend of the freezing or frozen precipitation is
low at this time, but there is moderate confidence that any wintry
mix pcpn should be of a relatively short duration.

For Thursday, weak upper ridging building across the area should
help allow temperatures to rebound back into the upper 30s to lower
40s and any lingering pcpn tomorrow morning would likely change over
to all rain.  Since precipitation amounts are expected to remain
light, any pcpn should not have an impact on forecast river


228 PM CST

Thursday night through Wednesday...

Main forecast concerns/challenges are with precip chances
Thursday night into Friday with a small chance of freezing rain
mainly for locations north of I-88 near the IL/WI border. Also
monitoring a potentially more robust system over the weekend that
could bring both a period of winter weather and a period of
showers and thunderstorms.

Initial dry conditions in place should be overcome area wide
during the early evening hours on Thursday ahead of approaching
shortwave energy. With strong WAA continuing Thursday night,
thermal profiles will support all liquid across the entire area.
In this setup, temps should remain just above freezing. However,
it is possible for locations north of I-88 near the IL/WI border
to see temps right around freezing for a time in the evening. With
this possibility, did convey this in the min temp/hourly temp
forecast as well as leaving chance freezing rain for this
location. It is possible though that the higher threat of freezing
rain will be further to the north in WI. Will continue to monitor
this possibility though. Rainfall amounts Thursday night into
Friday should be on the lighter side, under one half inch.

Expect precip to depart quickly Friday morning with high pressure
briefly building across the region through Friday evening.
Quickly turn attention to mid/upper level trough expected to push
through the central CONUS into the weekend. Low pressure expected
to develop over the central Plains Friday night into Saturday and
then lift northeast throughout the day. As this occurs should see
precip develop along and ahead of an associated warm front that
should push into the southern portions of the area late Friday
night into Saturday morning. Some uncertainty as to the placement
of this precip shield but given the potential strength of this
system, there is a decent chance of this precip to at least reach
the southern half of the CWA by Saturday morning. Will need to
monitor this potential as another period of a wintry mix will be
possible across parts of the area, and it is also possible for a
period of all snow with initial cooling/saturation. A lull in the
precip may occur but as this highly amplified trough and deepening
surface low move into the region late Saturday into Saturday
night, additional periods of all rain will be possible. A couple
of items to monitor with this time frame will be with potentially
higher amounts of rainfall, as well as the threat of
thunderstorms. Have not included thunder quite yet, but with the
potential strength of this system and for higher dewpoint air to
be in place, it is appearing that thunder could be possible.



For the 06Z TAFs...

High pressure is in place north of the terminals overnight
resulting in northeast winds and VFR conditions. Meanwhile,
across downstate Illinois, widespread IFR/MVFR conditions are in
place long with areas of rain. Some of the precipitation will
spread north through the morning and afternoon Thursday as an
elevated warm front lifts across the region. Couldn`t rule out a
few light showers or sprinkles at the terminals during the
afternoon Thursday, but better chances come Thursday evening/night
as a disorganized area of low pressure lifts across the region
transporting some of the lower moisture northward. Conditions will
likely fall to MVFR and eventually IFR overnight with periods of
rain possible Thursday night. Northeast winds will veer to the
east Thursday afternoon and to the southeast Thursday



325 PM CST

With high pressure pushing across the region, have seen winds
generally diminish today. This high will continue to push east
through the Great Lakes region tonight into Thursday and although
winds may further slightly diminish across the north half this
evening, don`t anticipate this to be too much. This high will
generally push through the area just north of the lake and allow
for a tighter gradient to remain in place across much of the lake.
Expect more of an increase to the winds by early Thursday evening
as low pressure approaches the Great Lakes region, with winds to
30 kt likely. At the start of the weekend, high pressure will move
through the region and expect winds to be on the lighter side.
This quickly changes though as deepening low pressure is expected
to move across the region Saturday into Sunday. Will need to
monitor this period as gales with varying direction will be
possible over much of the lake.






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