Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192327

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
527 PM CST Sun Nov 19 2017

150 PM CST

Through Monday...

High pressure is centered over the southern Great Plains this
afternoon with ridging extending into the Upper Midwest. Ridge
axis will gradually traverse the CWA this evening and tonight with
winds backing to the southwest behind. Low/mid level WAA will
overspread the region after midnight tonight. Temperatures will
fall into the upper 20s outside the Chicago Metro late this
evening, then should hold steady through much of the overnight
hours, and may warm a degree or two in spots as WAA strengthens.
Expect highs right around 50F during the day Monday with mostly
sunny skies and breezy southwest winds on the backside of the
ridge which will settle across the Tennessee Valley.



212 PM CST

Monday night through Sunday...

Still relatively warmer at the start of the period with upper
level trough and surface reflection well to the north. This will
change again on Tuesday as both of these features progress across
the region, and with a colder air mass returning late Tuesday into
Tuesday night. Frontal passage on Tuesday looks to be mainly dry,
however, there is the possibility for some isolated development
later in the day. Then expect cooler and dry weather through the
end of the work week. Approaching system will allow a warmer air
mass to return along with precip chances by late Friday into the
start of the weekend.



For the 00Z TAFs...

Dry weather will prevail over the TAFs through Monday but there
will be gusty winds of note by Monday afternoon, and low-level
wind shear (LLWS) potential Monday evening.

High pressure will pass to the south of the TAF sites this
evening, with winds backing to south-southwest by overnight.
Speeds will start to increase prior to daybreak and show frequent
gusts by mid to late morning. The winds should continue to inch up
through the afternoon with the sustained showing no diminishing
after dark Monday evening...possibly even increasing. Confidence
in the wind trends through Monday is medium-high, though in
specific gust magnitude is medium. A direction of 200-220 degrees
has medium-high confidence.

On Monday evening, a southwest low-level jet of 50-55 kt is
expected to develop with a base somewhere between 1000-2000 ft.
Given that surface winds will be breezy still Monday evening,
there is some uncertainty on whether true low-level wind shear
criteria will develop, but there is likely to be some inversion
so thought it worth including in the ORD TAF.



212 PM CST

Surface ridge axis has been moving across the region, with some
slight diminishing wind trend today. However, don`t anticipate any
additional weakening as the start of the week will once again
remain windy. Gradient picks back up ahead of an approaching low
to the northwest. Speeds will initially pick up over the southern
portions of the lake tonight into Monday, but anticipate this
increasing trend to include the northern portions later in the day
Monday. By late Monday into Monday evening, will see gales
develop over the entire open waters and because of this, have
issued a gale watch. Some lower confidence remains with start and
end times to these gales, and will need to monitor this for a
likely later transition to a warning. Also to monitor, the
likelihood for high end gales to 45 kt and even the slight
possibility for a brief window of storm force winds. Hazardous
conditions for small craft will expand on Monday and likely
continue into mid week. There is the possibility for gales to be
observed for a time Monday night into Tuesday morning. However,
confidence is low at this time. Will include mention of gale force
gusts during this time, and continue to monitor the possibility
of more prevailing gales over the nearshore waters.



LM...Small Craft Advisory...IN nearshore waters until 9 AM Wednesday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...noon Monday to 6
     PM Tuesday.

     Gale Watch...LMZ777-LMZ779...3 PM Monday to 3 PM Tuesday.




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