Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261810

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
110 PM CDT FRI AUG 26 2016

1103 AM CDT

Minimal changes made to the daytime forecast, with only an
addition of a chance of a light shower or sprinkles along the
southern/southwestern periphery of the forecast area. The
likelihood for rain continues to look to be later tonight with the
primary focus west of our area into north central Illinois.

Regional radar mosaic shows a dotting of light echoes across
western/central/northwestern Illinois late this morning. These
continue to be associated with broad lift of a right entrance
region to the upper jet over the Great Lakes and weak isentropic
ascent rooted above 800mb. The primary elevated baroclinic zone
remains draped from west-to-east near I-74 or so. Little change is
expected in the pattern through mid-afternoon before a gradual
moisture transport increase over north central Illinois late in
the day that strengthens through the night. Isolated echoes in
the region should continue with a few showers potentially clipping
the southern forecast area and anything elsewhere likely sprinkles
due to the high cloud bases. Some uptick may be seen just south
of the area, including a few thunderstorms, but again most of the
area during the daylight hours should be dry.

Will continue to assess for tonight, but it is likely the chances
for rain will need to be further increased across parts of north
central Illinois, where rainfall of at least an inch is looking
more likely.



306 AM CDT

Through Saturday Night...

Early this morning, water vapor imagery shows an upper level
shortwave trough lifting across Ontario into Quebec with mid-level
height rises building in the wake of this feature across most of
the Upper Midwest through the day today, providing dry weather
locally. At the surface, ill-defined frontal boundary stretches
from central IN WSW into central/southern IL where dew points are
pooling into the mid 70s. This area is expected to be the focus
for showers and thunderstorms through the day. Farther north, post
frontal conditions and onshore flow through the day today will
result in highs only in the upper 70s north to low 80s south, with
mid to upper 70s expected along the lake front.

Meanwhile, to our west, another shortwave embedded within a deep
trough over the Intermountain West is expected to lift across the
Plains today, and across the mid Missouri Valley late this
evening. Southerly flow will increase across portions of the
midwest in response to this feature, lifting the 850mb baroclinic
zone back across northern Illinois and northwest Indiana
overnight. The best coverage of thunderstorms will remain off to
our west near the LLJ axis through late this evening, but as the
LLJ veers overnight, some of these thunderstorms are expected to
spread east into portions of north central Illinois. Otherwise,
unstable conditions and broad warm air advection could result in
widely scattered thunderstorm activity for the remainder of the

Surface warm front is progged to lift north across the CWA during
the day Saturday ushering 1.9-2.0 inch PWats into the region.
Showers and thunderstorms are likely on Saturday, thought hard to
pin down specifics with what will probably be a messy setup. Jet
stream is oriented from the mid Missouri Valley northeast towards
Lake Superior with deep layer shear very marginal 25-30kt across
the CWA. A severe threat could present itself if there is any
substantial destabilization Saturday afternoon, with strong wind
gusts the primary concern. PoPs will continue into the evening and
overnight hours, but should be on a downward trend as the better
convergence and WAA shift to our north and the boundary layer
stabilizes nocturnally.



306 AM CDT

Sunday through Friday...

East coast upper ridge will gradually flatten Sunday through
Tuesday, but in the meantime, expect active weather pattern to
continue through Tuesday as northern Illinois and northwest
Indiana remain under a moist and unstable airmass. Forcing
becomes rather nebulous through this timeframe, and anticipate
mesoscale features may end up driving diurnally favored
convection. A lake breeze is expected each day and may be one
trigger in particular to keep an eye on. Expect high temperatures
to be in the mid 80s each day, Sunday through Tuesday, except
along the lake front where lake breezes will limit heating.

For the latter half of the week, upper ridge will amplify across
the mid section of the country which will dislodge cooler air
and allow it to spill into portions of the Great Lakes. Strong
high pressure is progged to build into Ontario with northerly flow
bringing a glancing blow of 850 mb temps in the low teens locally.
There are timing differences between the models, with the ECMWF
bringing the cold air down faster resulting in mid to upper 70s
for highs as early as Wednesday while the GFS is a considerably
slower. Did not make any changes from the blended consensus
forecast, but there is certainly potential that the current
forecast may be a several degrees too warm. Will keep an eye on
coming model runs for better agreement before making any big



For the 18Z TAFs...

The challenges with the Chicago area TAFs are timing/duration of
showers and possible storms tonight into Saturday as we return
into a moist and unstable air mass.

High pressure across Minnesota through far northern Illinois will
continue a light northeast to east wind this afternoon with a
potential lake breeze push offering 8-10 kt easterly winds at ORD
and MDW.

Warm and moist advection aloft will steadily increase tonight,
with northward-moving isolated/scattered light showers expected
to develop over northern Illinois this evening. The more
pronounced focus for well as some storms...will
arrive overnight into the early morning hours Saturday as a
weather disturbance lifts northeast into Wisconsin. The confidence
in showers reaching the Chicago airports has grown, as well as the
idea of isolated/scattered TSRA in the northern Illinois region.
Confidence in duration of precip is fairly low, as it could be
just an hour or possibly three or four hours. RFD is the most
likely area to see TSRA nearby, as well as IFR conditions during

In the wake of the warm front lifting northward Saturday morning,
MVFR clouds should linger with medium-high confidence, and cannot
rule out IFR clouds. Even some light fog may be present depending
on how quickly rain ends relative to daybreak. The chance for
scattered storms will return Saturday afternoon as the moist and
unstable warm sector spreads over the region.



239 AM CDT

Fairly benign conditions are expected across the lake outside of
occasional thunderstorm periods. High pressure will build across
the lake today and east the pressure gradient across the north
half. A weak system will cross the north half of Lake Michigan
Saturday night with southeast winds increasing some ahead of it.
High pressure will return to the eastern Great Lakes region Sunday
into early next week. Southerly winds will redevelop as low
pressure slides east across North Central Canada. This low will
send a cold front across the northern half of Lake Michigan.
Modest high pressure will rebuild across Ontario behind the front,
and this will maintain north to northeast flow across Lake






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