Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281751

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1251 PM CDT THU JUL 28 2016

300 AM CDT

Through Friday...

A mid level shortwave is dropping southeast across the area
this morning leading to scattered showers, with some additional
forcing for upstream activity being provided by an W-E oriented
cold front across southern Wisconsin then extending back into
Central Iowa, where the front is more N-S oriented. Better
coverage from upper level divergence/jet dynamics is found both
north and south of the area. Stabilization has also limited
coverage in NE IL/NW IN.

This leading shortwave will continue its eastward movement though
will down and dampen today, but broad upper troughing will at
least support some weak lift with additional shortwave energy in
northwest flow arriving late this afternoon. Meanwhile, The cold
front will approach far northeast Illinois (and adjacent Lake
Michigan) by daybreak, and then continue inland with onshore flow
today. Meanwhile, the western portion of the frontal boundary,
then get a bit better push southward tonight. This front will
likely be the focus for any redevelopment of showers and
thunderstorms today, with appears to largely be south and west of
Chicago. Decent heating should occur ahead of the stabilizing
influence of the front and incoming shortwave to contribute to
scattered showers and thunderstorms, with the favored area closer
to the arriving shortwave to our west, but with the broad lift in
place will need to maintain some chance pops elsewhere, especially
in the morning and the afternoon. Weak mid level lapse rates and
weak shear suggest that these will be more garden variety showers
and storms, though with the slow storm motion, and seasonally high
PWATs moderate to brief heavy rainfall can always be a concern..
And while not a perfect pattern for this, strong low level lapse
rates and moist low level conditions (Td`s in the low to mid 70s
near PNT) with a slow moving upper wave moving overhead could
support a few funnel clouds in the coming days.

This pattern again favors a diurnal weakening in shower activity
in the late evening and overnight, though cannot go dry given the
multitude of small scale waves embedded in the flow. The next more
notable shortwave in the broad upper trough is currently over
Montana will again approach our area on Friday. Expect another go-
around with an increase in slow moving afternoon showers and



300 AM CDT

Saturday through Wednesday...

Broad upper troughing continues Saturday across the northern tier
of states east of the Rockies with some suggestion that heights
increase, shortwaves appear weaker, and surface high pressure will
build across the area which suggest maybe lower coverage Saturday
but still enough features to support a chance of showers and
storms again Saturday.

Upper level heights get a bit more of a boost on Sunday, which at
this point appears largely dry and a touch warmer than the
previous few days. The mid to upper level ridge axis across the
center of the country will amplify some on Monday. Our region is
on the northeastern fringe of the hot dome associated with this
ridge, and so we can expect some increase in heat and humidity,
but being in that position we will be subject to occasional
periods of convection. There are hints that the upper ridge will
attempt to expand farther east as the week goes on.



For the 18Z TAFs...

Main concern for the afternoon into early evening will focus on TS
potential and associated wind disruptions. On the larger scale, a
weak cold front extends from swrn WI through nrn IL/IN. Scattered
TS have already developed along this corridor and expect activity
to continue through the afternoon, with the main focus invof this
boundary. Prevailing wind direction should be nely north of the
boundary, impacting all TAF sites, but thunderstorms are producing
outflow boundarys all over the area, so wind direction will likely
be quite variable, with ocnl gusts to arnd 25kt or possibly
higher. A weak mid level impulse is expected to cross the region
late this afternoon should allow the area of TS over the RFD are
to become more progressive, with TS ending at RFD arnd 00-01z and
a couple hours later at the Chicago area terminals.

Storms should dissipate quickly through the evening with continued
northeast winds and generally prevailing VFR conditions. Guidance
hints at lower cigs early Friday morning, but do not have enough
confidence to mention low cigs in the TAFs at this time. There is
a greater chance for MVFR vis at RFD/GYY/DPA since those locations
should see residual wet ground combined with high dewpoints and
light and variable winds.


314 AM CDT

The cold front over central Lake Michigan will continue south and
clear the southern end of the lake this afternoon.  Showers and
storms will form along the front.  A weak low over IL will shift
east through tonight.  Northeast winds increase to 15-25 kt late
this afternoon, and waves build to 3 to 5 ft north of Wilmette
Harbor. Waves should remain below small craft criteria so will not
be issuing a small craft advisory.  Winds diminish late tonight into
Friday morning as the low shifts east. However, northeast to north
winds increase to 15-25 kt Friday afternoon and evening as the high
pushes south over the northern Great Lakes.  The high spreads over
the lake this weekend with winds becoming light and variable under
the high. While the western edge of the high will stay over the lake
through mid next week, the center of the high will shift east. Winds
will become south Monday.






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