Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210805

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

107 AM CST

Through Wednesday...

The story of the next few days will be the temperature swings
along with occasional windy periods. Today we take the ride back
downward as deep low pressure across Lake Superior will steer a
cold front across the region today.

There is a lead shortwave across southern MN and several others
in the NW flow on the south side of the low driving some mid and
upper level cloudiness, but the focus will be on the deeper trough
axis across northern MN. Model guidance is in good agreement in
driving this energy southeastward today. We do not think this
will be a big deal for our area precip-wise, but several short
term guidance sources paint some light QPF with this wave noted by
some 850 mb fgen, compact omega/rh signal as the front shifts
through. The main focus is along and southeast of Chicago into
northwest Indiana and across east/central Illinois, but some virga
or very light precip may fall across northeast IL as well. Low
level positive energy forecast suggests enough low level warm air
lingers for rain to be the dominant p-type, but cannot rule out a
few snowflakes toward the tail end as the colder low level air
arrives. And the cold air will be noted after a mild Monday, as
the colder air will cruise on into north central Illinois not long
after daybreak and spread southeast through the day.

The wave will quickly depart this evening, but the cold northwest
winds will remain in its wake. Lows will fall to near 20 in the
north, mid 20s southeast and in Chicago. The surface ridge will
shift over the area on Wednesday. Winds will slacken some, low
level warm advection will ensue, and there should be plenty of
sun, but highs will struggle to reach 40.



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 06Z TAFs...

The gustiness of the south to southwest winds will likely grow
less frequent with time leading up to sunrise. Recent ACARS
soundings show winds around 2000ft have increased to around 50 kt
and while these winds will slowly ease overnight and veer more
westerly, the corresponding easing of the sfc winds will likely
result in a continued threat of LLWS. The LLWS threat should
diminish after sunrise and end altogether with cold frontal
passage Tuesday morning. Behind the front, winds will shift to
northwest and likely become gusty again as colder air spills into
the area. VFR cigs will likely persist through the period,
gradually lowering, especially behind the front Tuesday afternoon.



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...Gale Warning...IL nearshore waters until 6 AM Tuesday.

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

     Gale Warning...LMZ777-LMZ779 until 3 PM Tuesday.

     Gale Warning...IN nearshore waters until 10 AM Tuesday.




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