Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171944
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
244 PM CDT Sun Sep 17 2017

.SHORT TERM...
211 PM CDT

Through Monday...

Main concern for the short term forecast period will be pcpn chances
along the cold front moving through nern/ecntrl IL-nwrn IN.

In the near term, around 2 PM CDT this afternoon, a cold front
extends generally along the I-57 corridor and is quickly moving to
the east.  Sct shra-isold ts have developed just in advance of the
cold front from north-central Kankakee County to southeastern
Livingston County.  This activity appears to have initiated along a
weak mesoscale boundary just in advance of the synoptic cold front.
With moderate sfc based instability in the pre-frontal airmass, sfc-
based CAPE arnd 1000 J/kg, temps in the mid-upper 80s and dewpoints
in the mid 60s, development has been very fast.  The air mass is
also unseasonably moist with pwats up to 1.7 inches, so the initial
showers have been heavy although there is little lightning, which is
confined to a cell near Bourbonnais.  A lake breeze boundary that
had also developed early this afternoon had pushed a short distance
inland, initiating a few showers near downtown Chicago, but the lake
breeze has been forced back east by the advancing cold front.  For
the remainder of the afternoon hours, expect that there should be
continued shra/tsra along or just in advance of the cold front as it
quickly tracks east through the area.  Expect that the front should
push east of the CWA, into ncntrl IN-ecntrl IL by arnd 00Z.

The remainder of the overnight hours should be relatively quiet as
high pressure builds across the upper Great Lakes.  The sfc front is
then expected to go stationary across cntrl IL/IN overnight and
through the morning hours tomorrow.  A srn stream shortwave lifting
northeast through the Lower Missouri Valley and Middle Mississippi
Valley tomorrow should help the boundary push back north.  The
combination of the upper forcing from the shortwave and the sfc
front moving back north should bring a renewed chance for showers
and thunderstorms to the area tomorrow afternoon.  Latest guidance
would suggest that the activity will likely be west if the I-55
corridor through the afternoon, though there is a chance for some
pcpn to spread east to the Chicago Metro area by late afternoon
depending mainly on how fast the shortwave lifts newd.  Temperatures
tomorrow should be back closer to seasonal normal levels after the
period of unseasonable warmth with highs mainly in the middle to
upper 70s.  With east to northeast sfc flow off of Lake Michigan,
lakefront locations should be a bit cooler, with highs only in the
upper 60s to low 70s.

&&

.LONG TERM...
243 PM CDT

Monday night through Sunday...

A -PNA type Upper-Level pattern is expected to dominate much of
the period. Overall, this pattern should result in a mainly dry
and warm extended period (aside from early Tuesday).

Our best chance for a period of SCT showers/storms over the area
looks to be Monday night into Tuesday. This looks to occur as a
mid-level disturbance over the western CONUS gets pushed eastward
over the area in response a digging upper trough moving into the
Pacific Northwest. This certainly does not look like a big rain
event for the area, but some SCT activity will possible over the
area into Tuesday.

Following this chance of showers/storms the chances for anything
more than an isolated to widely scattered thunderstorm through
next weekend looks very low. This is because the main storm track
in this evolving upper-level pattern is likely to remain to the
northwest of the area along the western periphery of as a summer-
like ~590 DAM 500 MB ridge building over the Great Lakes Region
by mid to late week. In addition hurricane/tropical storm Jose is
expected to be impacting areas near the East coast during this
period, and this should help stall any eastward progression to the
upper pattern. So, mainly dry conditions are expected from late
Tuesday through the upcoming weekend.

The main story will be with the late season heat over the area.
Temperatures are expected to jump into the upper 80s by Wednesday
as a lower-level thermal ridge gets advected northward over the
area in response to a surface warm frontal passage. With the
airmass likely remaining similar over the area into the weekend,
expect mid to even some upper 80s for highs through the period.
We can`t even rule out the possibility for some areas touching 90
degrees, which would be near the record high for this time of
year.

KJB

&&

.AVIATION...
For the 18Z TAFs...

A cold front is moving through nern IL at 1730Z and should push
through the Chicago area terminals by 18Z, turning winds to nwly.
While the front is moving quickly, there is not much of a
temperature drop-off behind the front, so do not expect much more
than the wind shift to nwly, with wind speeds arnd 10-12 kt.
Latest radar and satellite imagery shows no pcpn along the
boundary and just some flat cu development as the air mass is not
very unstable at the time of fropa. There is still a chance that
there may be some shra/tsra development along the front this
afternoon, but will likely not be until the front is well east of
the terminals and will carry dry TAFs. There is some cooler air
with a secondary surge or secondary cold front moving through RFD
at 1730Z. Cigs are a bit lower in the true cooler air mass and
there still is some potential for mvfr cigs for a few hours
following the passage of the secondary boundary, but confidence is
low that the mvfr cigs will make it all the way to ORD/MDW/GYY,
so will only canny a tempo mvfr at RFD/DPA. As high pressure
builds across the upper Great Lakes tonight, winds will continue
to veer through nely overnight to sely by tomorrow morning.

&&

.MARINE...
249 AM CDT

Early this morning, low pressure is over Lake Superior with a
trailing cold front draped across portions of the Upper Midwest.
Moderate southerly flow of 15 to 20 kt will be in place today then
the cold front will begin to move across the lake this evening and
overnight with winds veering to the north and eventually northeast.
High pressure will build across the northern Great Lakes region
Monday and Tuesday resulting in a period of easterly flow across
Lake Michigan. A warm front will lift across Lake Michigan Wednesday
with winds veering back to southerly which is expected to last
through the end of the week.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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