Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180754

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
254 AM CDT Wed Oct 18 2017

200 AM CDT

Through tonight...

High pressure is still stretched from central Texas northeast
through the Ohio valley and into the mid Atlantic. A second and
stronger low pressure system will slide by north of the Canadian
border. Therefore, we can expect a near repeat of Tuesday, with
mostly clear skies, breezy southwest winds (approaching 30-35
mph), and highs in low 70s. The stronger northern low will send a
weak cold front toward the area Wednesday night into Thursday.
Impacts will be minor, but we will see a small cooldown, slightly
more noticeable in far northeast Illinois. Winds will slacken and
veer to WNW, before weak high pressure spreads east of Lake
Michigan on Thursday afternoon, bringing brief onshore flow.



200 AM CDT

Thursday through Tuesday...

The pattern Friday reverts to that of earlier in the week, with the
high over the Ohio valley/mid Atlantic region expanding northward,
and the upstream low will now be ejecting from a deep/broad western
upper level trough axis. With seasonally slightly mild overnight
lows, Friday highs should easily cruise into the mid 70s.

The first half of the weekend still looks to be alright for most
locations as we await the arrival of the upper trough, but expect
clouds to thicken. Friday night will be quite mild for the latter
portion of October, and thus in spite of the clouds highs should
still hit the lower to maybe mid 70s. There are a few guidance
sources that paint a few lighter pre-frontal showers during the
day, but these don`t look to be a big deal as moisture return
behind the departing high will be focused west of the area

Better rain chances return Saturday night into Sunday as the
upper trough will move through. 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, and there is still
a strong signal in a favorable ramp up in precipitation chances
by Saturday night or more so Sunday morning. The system does show
signs of splitting between a more intact low to the south and a
broad northern stream trough, with lend to the idea that this will
not a be a huge rain maker, but still a decent chance of some
rain. With plentiful clouds, instability will be tempered, but
there could be a few t-storms.

With the post frontal air mass being of the pacific origin,
temperatures now look to initially fall back to more seasonal levels
Sunday/Monday, but the upper jet will shift more meridional in the
Tues/Wed time frame which would bring a bit more significant cold
front in during this time.



For the 06Z TAFs...

Winds will be the only aviation concern today as the surface
gradient continues to support gusty south-southwesterly winds.
Expect winds to become gusty again up into the lower 20 KT range
by mid morning, then up around 25 KT during the afternoon.
Otherwise, similar to Tuesday, expect these gusty winds to ease
with sunset.

VFR conditions continue to be expected through the period.



254 AM CDT

The main weather concern over Lake Michigan through the rest of
the week continues to be with the strong gales expected today into
tonight, especially over northern sections of the lake. However,
another period of near gale force winds looks likely Friday night
into Saturday.

An expansive area of high will remain dominate from the Lower
Ohio Valley to the Mid-Atlantic region through the rest of the
week. Meanwhile, a strong storm system over central Manitoba
early this morning, will shift eastward across northern Ontario by
this evening. The associated surface pressure gradient over the
lake will support a rapid uptake in the south-southwesterly winds
over the lake today. Gales of 40 to 45 kt are expected by mid to
late this afternoon into the early evening over the northern half
of the lake. The gales should gradually subside after midnight as
the winds veer westerly.

Yet another period of enhanced southerly winds look possible late
Friday into Saturday as another storm system shifts northeastward
across south central Canada. Wind speeds with this event could
again approach gale force.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...9 AM Wednesday to 3 AM Thursday.




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