Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250839

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
339 AM CDT Mon Sep 25 2017

325 AM CDT

Through Tonight...

Another hot one by not only late September criteria but also mid-
summer standards is in store today. If 90 is reached at Chicago
it would be the first time with six 90 degree days in a row since
July 2012 and the first time in September since 1978.

The longstanding upper ridge continues across the eastern half of
the country today. There is a weak short wave circulation and
associated 700mb cool pocket (+5C) seen on water vapor imagery
over southern Illinois this morning. This feature will drift
north today in the highly amplified pattern, and patches of
high/mid cloud with it as well as some more cumulus will be seen.
Forcing from this wave provides an outside chance of a couple
showers this afternoon mainly in northwest to north central
Illinois. This is where the cap is forecast to be neared or just
barely overcome -- and that is factoring boundary layer mixing of
lower dew points. None of the CAM solutions are overly excited
but have simulated reflectivity "blips" in that area.

Temperatures early this morning are running about 1-3 degrees
cooler than previous mornings thanks to lower dew points and
850-925mb temperatures down about 1C in 24 hours. Taking this into
account, have high temperatures forecast around 90 which would be
near or right at record. The slightly lower dew points of lower
60s keep forecast heat index readings just a degree or two above
the temperature. A lake breeze again will develop by noon and
advance inland in lake adjacent counties in a similar way to
yesterday. Expecting downtown to peak in 80s.



325 AM CDT

Tuesday through Sunday...

A couple small chances for rain -- which has been a rarely used
word locally this month -- present themselves on Tuesday and then
again late in the week. All in all though, dryness should prevail
much of if not all of this week. While some locations especially
east could be up to the 90 mark Tuesday, a noticeable change in
temperatures will occur during this period. The first drop back
to around normal is by Wednesday and the one to 5-10 degrees
below normal on Friday.

The surface cold front over the Plains this morning will make very
gradual progress into the area Tuesday evening. Overall forcing
along this boundary is not impressive, especially for much or any
thunder given marginal lapse rates. The speed of this boundary may
be even slower than what guidance is indicating given the
strength of the inherent ridge as well as the parent surface low
being fairly weak in pressure (1012mb at 00Z Wed). The slightly
better probability of some showers Tuesday afternoon and evening
is west of I-55.

Northwest winds behind the front on Tuesday night into Wednesday
will bring a seasonable air mass into the area, with highs in the
70s for Wednesday and Thursday. Both of those midweek days look to
be mainly sunny.

As the upper ridge collapses, a modestly strong upper trough drops
southeast into the Great Lakes on Friday. The associated cold
front looks to be strengthening as it moves through, meaning
breezy conditions on Friday, possibly with some showers along the
front. Lake effect clouds look likely though do not see a strong
enough signal for lake effect showers at this time. Highs look to
dip to the mid to upper 60s for a good part of the CWA Friday and
Saturday with a slight rebound into early next week.



For the 06Z TAFs...

Similar aviation weather to the past several days is in store
today. There will be some clouds -- few to scattered cumulus and
some cirrus -- but all VFR. A lake breeze will develop again and
likely not arrive to ORD/MDW/GYY until after 21Z. Confidence in a
lake breeze arriving at the airports is medium and is also medium
in specific hour of arrival.



335 AM CDT

A similar wind pattern will be seen over the lake today as in
recent days, with an onshore lake breeze developing in the
Illinois nearshore and to a lesser extent the Indiana nearshore. A
cold front will move across the lake on Tuesday night turning
winds northwest. Small craft criteria waves on Wednesday look
probable for part of and maybe the entire northwest Indiana shore.
A second stronger cold front, more typical of early autumn, will
move southward across the lake late Thursday night into early
Friday. This has potential for some gales over the open water
depending on the low strength and exact track as it dives
southeast over Ontario and Great Lakes region.






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