Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170720

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
220 AM CDT Tue Oct 17 2017

220 AM CDT

Through Wednesday...

Mostly sunny and mild fall weather will be
the story for the upcoming days. High pressure stretches from
central Texas northeast through the Ohio valley and into the mid
Atlantic. The high will slowly become centered a bit farther east
each day. Meanwhile, a series low pressure systems will slide by
north of the Canadian border. One today, and a slightly stronger
system on Wednesday. The sensible weather result will be breezy
southwest winds, though plentiful sunshine. Highs will be in the
ballpark of 70 degrees each day after the cooler nights.



220 AM CDT

Thursday through Monday...

The stronger northern low will send a weak cold front toward the
area Wednesday night into Thursday. Impacts will be fairly
negligible, but we will see a minor cooldown, and winds will
slacken and veer to WNW, before weak high pressure spreads east of
Lake Michigan on Thursday afternoon. This will bring some brief
onshore flow, and thus temps may dip a few degrees. We may also
get a few clouds with this too.

The pattern Friday reverts to that of earlier in the week, with the
high over the Ohio valley/mid Atlantic region, but the low will now
be ejecting from a deep/broad western upper level trough axis. With
seasonally mild overnight lows, Friday highs should springboard into
the 70s area wide. Most of Saturday will be dry as we await the
arrival of the upper trough, but expect clouds to thicken. Still, it
will be mild with 70s.

The party will come to an end Saturday night into Sunday as the
upper trough will move through. While the forcing is broad and
stretched out, a feed of 60+ dewpoints will advect in ahead of it on
a strong low level jet. Upper forcing will be augmented by a
strengthening upper jet. There are some timing differences in the
longer range guidance as to how quickly the moisture from the
Gulf of Mexico gets this far north which impact the specific 6
hour PoP forecasts, but all indicate a favorable ramp up in
precipitation chances by Saturday night or more so Sunday morning.
With plentiful clouds, instability will be tempered, but there
could be a few t- storms. Colder air will return behind this
systems surface front as potent northwest flow will setup across
the northern tier of the country. Even colder air may be on the
horizon in far-out model land.



For the 06Z TAFs...

Winds will be the only aviation concern today as the surface
gradient strengthens over the region. Expect winds to become
gusty out of the southwest shortly after daybreak today. Wind
gusts into the low to mid 20 KT range can be expected across the
area by early this afternoon. These gusty winds will abate by
sunset. Otherwise, expect VFR conditions through the period.



224 PM CDT

Despite the quiet weather this week, the persistent and occasionally
strong south to southwesterly flow could produce some challenging
conditions for portions of the lake at times.

The primary window of concern comes late Wednesday as a very tight
pressure gradient and mild lake waters will support a mix down of
winds to around 40 knots and considerable wave growth at the north
end.  A gale watch will be posted for this period, and while the
south end also could see some gale force gusts, winds are not
expected to be quite as strong for as long in this area.  Even so,
it is possible the watch would need to be extended farther south if
things change.

Even before the primary period late Wednesday, waves of 6 ft or
greater will be likely across the northern and eastern portions of
the lake. Conditions then relax toward the end of the week.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...7 AM Tuesday to 7 PM Tuesday.




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