Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230459

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1059 PM CST Thu Feb 22 2018

757 PM CST

The main forecast changes were slowing rain timing and adjusting
fog coverage tonight. Rain over much of Iowa and southern IL will
slowly shift east and north. We could see scattered light rain
showers closer to midnight, but steadier rain won`t reach the I-39
corridor until after midnight. The showers will shift east
through early Friday morning. Steady above freezing temperatures
support liquid precipitation. Rainfall amounts of 0.1-0.2 inches
are expected, but the rain will not have a significant impact on
area rivers.

Areas of fog will develop after midnight, but only isolated areas
will see dense fog. Again, temperatures will be above freezing, so
freezing fog is not expected. Fog will dissipate mid Friday



138 PM CST

Through Friday...

Another round of precipitation in this active weather pattern will
arrive gradually through the evening, and more so overnight.
Fortunately for this go around, forecast profiles suggest the
p-type will be rain.

Dry surface easterly flow has kept conditions largely dry across
the area this afternoon. A surface warm front is well south of
the area, but southwest flow over the sloped frontal surface
closer to our neck of the woods is advecting more extensive low
clouds northward. Several disturbances will traverse the frontal
surface lead to occasional rain. The first batch which will be
less organized will come later this evening where moisture will be
a bit more shallow. Precipitation chances ramp up more so
overnight as the lower level frontal zone will get closer to the
area, and more pronounced lower level convergence occurs.
Dewpoints will head above freezing area wide, so p-type will be
rain. Expect rain to generally be on the lighter side, but brief
spurts of moderate rain are possible. QPF totals will peak up to
one quarter of an inch, though most areas will have much less.

After some morning clouds and fog, west-southwest winds will
become breezy with gusts to 20 maybe 25 mph. This should help to
bring some gradual clearing or at least lifting of clouds, though
at this point low level moisture may be stubborn enough to limit
much clearing until late in the day. In spite of that, slightly
above normal highs in the 40s (to near 50 south) will occur.



220 PM CST

Friday night through Thursday...

Main forecast concerns/challenges are with a potentially more
robust system over the weekend that could bring both a period of
winter weather and a period of showers and thunderstorms.

High pressure that builds across the region at the start of the
period quickly departs, and 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 anticipate precip to develop ahead of an
associated warm front. Still some uncertainty as to the placement
of this precip shield but guidance is beginning to indicate this
precip reaching as far north as I-80, or just slightly north, by
early Saturday morning. Still monitoring the potential of a wintry
mix across parts of the area, that does include a period of all
snow. Cold surface temps, widely varying thermal profiles, and the
presence of dry air will support the possibility for a wintry
mix. For locations from around I-80 south to the Kankakee river
valley, a wintry mix of snow, sleet, and freezing rain will be
possible. The period of this chance of a wintry mix does not
appear to be overly long, maybe in the 3-6 hour time frame.
Warming surface temps and column will help transition this to all
rain by mid/late Saturday morning. South of the Kankakee river
valley, all rain is appearing likely. Rainfall in this location
should be on the lighter side, but a steadier rain is possible and
will need to monitor rainfall amounts.

A lull in the precip may occur during the day Saturday 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 area wide. A couple of items to
monitor with this time frame will be with potentially higher
amounts of rainfall that could affect ongoing river flooding, as
well as the threat of thunderstorms. Given the strength of this
system, strong LLJ/WAA, and increasing instability, felt confident
to include slight chance thunder area wide early Saturday
evening. At this time, the threat of strong/severe storms looks to
remain just south of the area. Precip will end during the
overnight hours Saturday night, with the arrival of colder and
drier air. On the backside of this system, winds do quickly
increase with strong winds/gusts possibly becoming a concern late
Saturday night into Sunday morning.



For the 06Z TAFs...

The main forecast concern overnight into the morning continues to
focus on the LIFR CIGS and visibilities across the area.

An area of low pressure over will shift into Wisconsin during the
day. As it does so, expect an associated frontal boundary to move
across the area around daybreak, with winds becoming westerly in
its wake for the remainder of the day. Ample low-level moisture
over the area ahead of this boundary will continue to result in
deteriorating CIGS over the next few hours. Overall, expect the
developing IFR CIGs to to fall into the LIFR category overnight
into early Friday morning ahead of the approaching boundary. There
is also likely going to be a period of low visibilities overnight
in fog and some light rain showers. It still appears that the
lowest visibilities, down around 1/2 mile, will be favored mainly
west of the main Chicago area terminals. For the 06z TAFs I have
changed little in regards to the timing and magnitudes of the
conditions through the morning.

Conditions should gradually improve across the area during the day,
though some MVFR CIGS may hold on over the area into the



356 PM CST

The current east northeast winds will become all easterly early
this evening while winds increase, as low pressure approaches the
Great Lakes region. 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. Continue
to monitor this period, Saturday night through Sunday, as gales
with varying direction will be possible over much of the
lake/nearshore. Included with this are the possibility for higher
end gales to 45 KT. Also will need to monitor the possibility for
storm force winds for a time Sunday morning, mainly for the north
half of the lake. Do have growing confidence for gales, with lower
confidence on storm force winds. Given the time frame of these
gales and possible storm force winds, have not issued a watch yet.






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