Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170227

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

925 PM CDT

A shortwave embedded in southwest flow aloft and associated with
larger scale negatively tilted trough over the Plains has resulted
in blossoming of showers and thunderstorms along and behind a cold
front from the central Plains to southeast Minnesota. Ahead of
this wave, a weakening MCV over northern Missouri and southeast
Iowa is gradually lifting north-northeast. With steering flow
aloft mainly toward the north-northeast, the aforementioned shower
and thunderstorm activity is only making very gradual progress
eastward. Warm advection regime with modest southwest low level
jet around 25 kt will sustain around 500 j/kg of MUCAPE, possibly
increasing up to 1000 j/kg toward daybreak.

However, negating this will be plenty of CIN (noted on latest SPC
mesoanalysis) due to a warm nose centered just below 700 mb. With
the slow movement of activity to the west and northwest and more
hostile environment over the local area, weakening showers and
possibly an isolated thunderstorm may try to sneak into parts of
northwest/north central IL toward daybreak. The decaying activity
will then try to shift east during the morning, though many areas
could end up dry, followed by possible regeneration along or more
likely east of I-55 Sunday afternoon into Sunday evening. Overall
trended PoPs slower overnight into Sunday morning, and will brief
midnight shift about possible trends for afternoon/evening



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 00Z TAFs...

Main aviation forecast concerns are coverage and timing of shower
potential later tonight into Sunday, eventual wind shift with cold
front Sunday, and the potential for a period of MVFR cigs Sunday

Surface low pressure over western Lake Superior will lift
northeast into Ontario on Sunday in response to a strong mid-level
disturbance tracking well north of the area. A cold front trailing
from this low will move slowly east-southeast across northern IL
and northwest IN Sunday morning/early afternoon. Southwest flow
ahead of the front will gradually increase moisture across the
forecast area overnight, with the potential for scattered shower
and isolated thunderstorms across the terminals after midnight
into Sunday morning. Model trends indicate weakening/decreasing of
precip as it moves into the area early Sunday, though would
expect some festering low-impact showers especially Chicago
westward during the morning. While some weak elevated instability
could support isolated thunder early Sunday, forecast soundings do
depict some capping above 800 mb which would suggest fairly low
coverage and low probability for point locations. The front
slides across the Chicago terminals by mid-late morning, with
redevelopment of showers and thunderstorms expected to be
primarily east/southeast of the terminals Sunday afternoon, with
GYY most likely to be impacted. Greater thunder potential looks to
be further east of GYY along/east of the surface front.

Moist low levels behind the front may support a brief period of
MVFR cigs in the afternoon, before stronger drying arrives Sunday
evening. Have hinted at this with sct 1500-2000 ft layer in TAFs.
Surface winds south-southwest 10 kt or less until cold fropa
Sunday, with winds turning northwest in the afternoon and
eventually more north late in the day/evening.



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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