Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161958

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
258 PM CDT Sat Sep 16 2017

244 PM CDT

Through Sunday...

Early afternoon satellite imagery shows the area on the western
flank of an upper ridge with a large upper trough rotating across
the Rockies and High Plains. At the surface, low pressure over
western Lake Superior has a cold front stretching across western
Iowa into Kansas. South to southwest winds are in place across the
local area with a mix of diurnal cumulus and sun overhead. The
rest of the afternoon and evening will remain dry with rather mild
temperatures. Winds will ease somewhat this evening as clouds

Showers and thunderstorms will continue to develop across central
and eastern Iowa and make their way eastward into tonight. The
main upper trough will begin to lift northeast taking the surface
low with it which will stretch the front and weaken its
convergence with time through Sunday. The evening convection will
likely weaken as it moves into northern Illinois but expect that
some showers will linger into the Rockford/I-39 corridor late
tonight with some thunder possible if better organization can
continue. Some of this activity could linger through mid/late
morning into the Chicago area but prospects are not great for this
to occur. The next question becomes how much development occurs
along/ahead of the front Sunday? Low end instability is expected
ahead of the front roughly from Chicago to Pontiac and points east
by early afternoon. Residual cloud cover may limit development
through early afternoon. Chances improve through the afternoon to
the east and it may end up being that any new development is
across northwest Indiana back into Ford/Iroquois Counties. For now
favor some development closer to the Chicago-Pontiac I-55 corridor
with increased coverage further east. Severe potential is on the
low end and heavy downpours and lightning would be the main
threats. Winds shift northwesterly behind the front from northwest
to southeast into early evening.



251 PM CDT

Sunday night through Saturday...

Little change to the longer term forecast as the guidance for
Sunday night through mid week remains consistent with previous
trends for decreasing amplitude and faster progression to the
upper level pattern, with a broad band of fast wly flow aloft,
with a series of weak southern stream shortwaves bringing periodic
chances for some scattered showers or thunderstorms. By mid week
into next weekend, the pattern will trend back toward a more
highly amplified pattern aloft, with broad troughing over the
cntrl plains and wrn CONUS with ridging over the ern CONUS, which
would the local area under swly flow aloft. There are significant
differences, however, in the trends of shortwave energy rippling
through the larger scale pattern, so, confidence is moderate in
the temperature trends driven by the longwave pattern, but
confidence is low in the PoP trends driven by the shortwave
trends. In general, expect temperatures on Monday and Tuesday to
be near seasonal normal levels with highs in the mid to upper 70s
north of the I-80 corridor and low 80s to the south. The exception
will be the lakefront where onshore flow will keep temps a bit
lower, in the low to middle 70s. By midweek and into next weekend,
temperatures should trend back up into the middle 80s as upper
level ridging builds over the midwest. Periodic low chances for
some showers or thunderstorms will continue through the week,
though confidence is low in timing/coverage.


For the 18Z TAFs...

South to southwest winds continue early this afternoon with mixing
bringing occasional gusts to near 20 kt. Scattered VFR cumulus
will continue through the afternoon with bases rising with time.
Wind speeds will ease slightly this evening with the direction
turning south to southeast before turning more south to southwest
overnight. Showers and thunderstorms are expected to move into
northwest Illinois ahead of a cold front overnight but be on a
decreasing trend. Have added shower mention at RFD late tonight as
some activity does look to hold together. Cannot rule out some
embedded thunderstorms. Less confident in precipitation holding
together toward the Chicago area but did include a mention mid

The cold front will cross the terminals into the late afternoon
turning winds northwesterly. New storm development is possible
along the front from roughly ORD to PNT and points east early to
mid afternoon but it is possible that development holds off a
little longer pushing the threat east of the Illinois terminals.
GYY may have the best threat for thunder of any site by late
afternoon. Expect a mix of clouds and sun and some MVFR stratocu
may build in behind the front during the afternoon.



255 PM CDT

High pressure remaining parked across the Appalachians and New
England as a trough and associated cold front push across the
central and northern plains will keep brisk sly-slwy winds over
the lake through tonight and into tomorrow. A cold front will push
across the lake on Sunday. As high pressure builds in behind the
front, winds will turn through nnwly to nely by for Monday and
Monday night and easterly by Tuesday. Another trough and cold
front are expected to push across the plains and to the
Mississippi Valley by midweek, allowing winds to continue to veer
to sely-sly.





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