Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 212026

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
226 PM CST Tue Nov 21 2017

226 PM CST

Through Wednesday...

The cold front quickly pushing across the region extends from
northern Indiana through central Illinois at 230PM CST.  A band of
pcpn along the front is filling in and some pcpn will continue invof
the front and should exit the far sern portions of the CWA by late
afternoon or early evening.  Temperatures are dropping off in the
strong cold advection behind the front and expect that temperatures
should be in the upper 20s to lower 30s by shortly after sunset.
Skies should clear out from west to east through the remainder of
the afternoon and into the evening with much of nwrn IL already
seeing scattering clouds.  with clearing skies and diminishing winds
overnight, there should be a radiative cooling component to the
downward temperature trend along with the cold advection.
Temperatures tonight should drop into the lower to middle 20s across
area, with some isolated upper teens possible in sheltered low-lying
areas.  With high pressure building in from the west tomorrow, winds
should be relatively light through the day and even with ample
sunshine, temperatures tomorrow will only reach into the lower to
middle 30s.


107 AM CST

Wednesday night through Monday...

Another much weaker wave will slide through Wisconsin Wednesday
night with precip staying north as the ridge axis will stretch out
southeast of the area across the Ohio/Tennessee valleys into the
mid Atlantic. A secondary high will be in place over the area for
Thanksgiving Day, which will allow temps to recover to the lower
40s. This is still below normal, but with the high resulting in
likely our lightest wind day of the week, all in all pleasant
weather conditions for the holiday.

Friday will be a repeat of yesterday (Monday), as gusty southwest
winds will develop ahead of deepening low just north of Lake
Superior. The low level thermal fields support even warmer highs
than Monday, which were in the low to mid 50s area wide. The only
kicker will be there may be a bit more cloudiness as the upper
forcing is a bit farther south with this wave. Models still don`t
paint much precip again and very well could end up dry, but have
some close bay to warrant not removing a model blend of low
chances. The cold air behind this system does not come slamming in
that quickly but gets reinforced through the weekend, thus highs
Saturday will reach the 40s, then only the 30s Sunday. High
pressure will mean weakening winds, dry conditions, and sun.



For the 18Z TAFs...

Main forecast concern for the period will be wind trends with the
passage of a cold front.

A narrow band of pcpn is moving east of the terminals at issuance
time, with only some lingering light rain likely at GYY for
another hour or so. With the pcpn, cloud bases should lower to
lower end vfr, 4-5 kft for the remainder of the afternoon. In the
strong cold advection and pressure rises following the frontal
passage, the gusty swly winds are shifting to nwly and should
remain gusty to 20-25 kt through the remainder of the afternoon.
By arnd 00z, the gradient should relax as high pressure begins to
spread across the Middle Mississippi Valley, allowing winds to
drop off overnight. As the colder, drier air filters across the
region, sky cover should diminish to mostly clear overnight with
no operationally significant cloud cover. As the high pressure
continues to spread across the region tomorrow, conditions should
be dry, with relatively light, westerly winds.



205 am...Deep low pressure just north of Lake Superior early this
morning will move into northern Quebec tonight as it slowly
weakens. A trailing cold front will move across Lake Michigan this
morning shifting southwest winds to the northwest. The gales will
likely diminish toward 30kts with the front but a period of low
end gales is possible behind the front...which may last into early
this evening...but confidence is low. A large area of high
pressure will build across the plains Tuesday night and then
extend east into the Ohio Valley Wednesday night. Strong low
pressure will move across Ontario Friday and Friday night with a
tight southwest pressure gradient developing over the lakes
region. Southwest gales are possible again Friday morning into
Friday evening...with a cold front shifting winds northwest by
Saturday morning with gales still possible behind the front. cms


LM...Small Craft Advisory...IL nearshore waters until 9 PM Tuesday.

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




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