Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261114

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

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 12Z TAFs...

Primarily VFR conditions and light winds expected through the
evening. Shower activity appears like it will be confined south
of most of the area as a stationary front remains positioned
across Central Illinois today though some weak waves could trigger
a shower or two mainly south of the interstate 80 corridor.
Light north winds will shift northeast, then be reinforced by some
weak lake influence this afternoon. Speeds will hold at 7-10 kt,
but lighter winds inland. Forcing appears benign through 6z to
preclude precip mention, though not a zero chance if anything
forms near the stationary front and drifts north.

Mixed signals in the 6z- 12z period for as the low level jet
ramps up in broad moist ascent. Elevated instability is weak, but
could see some showers and storms enter the picture, with higher
likelihoods after 6z near KRFD and after 12z closer to the Chicago
terminals., especially if the storms to our west morph into a
mesocale convective system. SW-NE moving disturbances have a
better chance of affecting the area during this latter time
combined with a northward lifting warm front. Have introduced some
shower mention for RFD closer to the main upper forcing before
12z, and for ORD in the 30 hour cycle, though confidence on
specific timing is not that high in a somewhat muddled pattern.



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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