Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190658

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1258 AM CST Fri Jan 19 2018

120 PM CST

Through Friday...

The clipper system to our north will quickly exit the region
tonight. Southwest winds have ushered in some more seasonal
temperatures getting close to the freezing mark. High pressure
aloft will shift across the Great Lakes tonight. At the surface
high pressure will to the southeast USA, while a second low will
dig into south central Canada. This will maintain breezy southwest
winds through tonight and Friday. Expect lows to hold near or even
above yesterday`s highs, with lows tonight mostly in the 20s.
Continued warm advection on Friday will steer high temperatures up
near 40. We will have some higher clouds due to the low passing by
to the north and from a system well out in the west.



326 PM CST

Friday night through Thursday...

Warm trend continues at the start of the period, with dry
conditions also persisting. Initial zonal flow will transition to
more ridging as upper level low/trough moves through the central
CONUS this weekend. In this pattern, WAA will be the trend with
lows Friday night in the 20s rising to highs around 40 on
Saturday. However, more pronounced moist advection will be delayed
some, with dewpoints not really rising until late Saturday into
Sunday morning. This will result in increasing low clouds and fog,
with drizzle becoming more likely Sunday night. Similar concerns
with the low chance of freezing drizzle continues, and so have
added mention to the forecast primarily for far northern Illinois.
At this time, it does not appear to be overly concerning but
enough to warrant mention.

Fog concerns really increase into Sunday, with the arrival of 40+
dewpoint air. Increasing forcing will help to continue precip
chances throughout the day Sunday, with light rain becoming more
likely. However, strongest forcing associated with the now closed
low and surface low doesn`t arrive until late Sunday night into
Monday. This should help a more widespread precip shield to spread
across the region, with a period of steady rain likely during
this time. Guidance still indicating some elevated instability
through this period, with rumbles of thunder still a possibility.
Will continue to monitor flood issues/concerns, with the current
snow pack and this possible rainfall, especially if there ends up
being more convective type precip. Cold returns on the backside of
this system later in the day Monday, with any lingering precip
likely transitioning to snow.



For the 06Z TAFs...

Conditions will continue to be fairly quiet with VFR conditions
over the next 24 to 30 hours. Southwest winds (200-220 degree
direction) will increase by the late morning or early afternoon
and become gusty. Expect gusts generally in the 20-25 kt range
through Friday evening, with winds and gusts then diminishing
overnight. The low level jet will ramp up Friday evening, causing
southwest winds at FL ~020 to increase to about 50 kt. If wind
gusts are less frequent than expected, LLWS may need to be added
to the TAF.



1258 AM CST

An active pattern continues over the lake into next week. A
quick-moving but fairly strong Alberta Clipper system will cross
Ontario today and this evening. A tightening pressure gradient
will result and enough so for Small Craft Advisory winds over the
Illinois and Indiana nearshores by late morning and for gales over
the open water by mid-afternoon. These conditions will persist
through this evening before easing late tonight.

The associated cold front from this low will dip southward into
the northern part of the lake and weaken on early Saturday. A
stronger Plains low will develop Sunday afternoon and move
northeast, lifting a warm front over the lake on Monday.
Increasing temperatures and dew points over the cooler waters, as
well as added moisture from snow melt to the south of the lake,
may increase the likelihood of fog across the southern part of the
lake late Sunday night into early Monday. Confidence on this is
low at this time. Winds ahead of this system may touch near gales
across the north and central late Sunday night into Monday, but
the better widespread potential for gales it on the system
backside later Monday night into Tuesday. With quite a bit of
variance in model guidance on where this low cross the lake, and
just how strong it will be (though it should be gradually
deepening), confidence in gales at this time is low.



LM...Gale Warning...LMZ777-LMZ779...2 PM Friday to 3 AM Saturday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...9 AM Friday to 3 AM Saturday.




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