Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241751

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1251 PM CDT WED AUG 24 2016

1224 PM CDT

Latest radar imagery depicting larger precip shield steadily
progressing through the eastern portions of the CWA, still
impacting portions of northwest Indiana. Think the potential for
more widespread area of showers and thunderstorms is likely over
for much of the CWA going into the afternoon. However, MCV still
pushing through with weak surface trough in place will allow for
continued widely scattered shower/thunderstorm development over
the next several hours, similar to what is currently occurring
over north central Illinois. Lacking steep low and mid level lapse
rates will limit any stronger development, but with isolated gusty
winds still possible this afternoon. High PWATS around 2 inches
still in place will continue to support heavy downpours with any
shower or thunderstorm.



250 AM CDT

Through tonight...

Early this morning skies have remained mostly cloudy and locked
temps in the lower 70s, but may be able to fall a degree or two
prior to daybreak to around 70. Mid-lvl heights are steadily
falling in advance of a weak elevated trough currently positioned
over Iowa/Missouri. The better forcing has remained displaced to
the northwest of Illinois; however, along the warm air advection
wing a few showers/thunderstorms have developed and are lifting
north/northeast. Behind the warm air advection wing Td`s rise to
around 70 and further upstream Td`s are in the lower 70s within
the moist channel.

Guidance continues to point towards the complex/mid-lvl trough
lifting northeast towards Northern Illinois this morning, and expect
showers/thunderstorms to accompany this feature but should be
progressive and lift northeast of the forecast area by late morning.
In the wake of the elevated wave heights are progged to rise once
again and may provide a lull in convective/precip coverage through
the better part of the day. The moist axis will be positioned from
the Southern Plains stretching northeast through Illinois and Lower
Michigan throughout the day, so the potential for the thick cloud
cover to this appears to be unlikely. So expect temps to generally
warm into the lower 80s but with Td`s rising into the lower 70s the
humidity will become noticeable by early afternoon.

Wind profiles remain uniform from the southwest throughout the
column this aftn/eve with minimal directional shear. Thus with
guidance indicating another wave lifting northeast from Southern
Iowa/Northern Missouri around 00z Thur, and within the moist axis,
that the potential for pockets of heavier rainfall associated with
convection will be likely later this evening/overnight. Presently the
difficulty is on placement of the convection along with timing. Some
of the hi-res solutions are suggesting the more focused
convection/heavy rainfall could setup south of a LaSalle to Chicago
line. The frontal boundary will remain well to the north, which
could allow the coverage for convection/rain to be further north. So
have held onto high chance POPs further north and likely to
categorical along the southern portions of the CWA overnight.



250 AM CDT

Thursday through Tuesday...

Thursday through Thursday Night: Main challenge for Thur will be on
the forward speed of the frontal boundary, and possibly some
lingering pockets of moderate to at times heavy rainfall across the
eastern CWA in the late afternoon hours due to some frontogenetical
forcing ahead of the boundary. With the surface ridge sliding into
the Central Plains, and a downstream ridge over the Mid-Atlantic
region, this could allow the moist axis to buckle slightly and
enhance the forcing ahead of the boundary Thur midday/aftn. But
expect a steady push for dry air to arrive from the northwest in the
aftn/eve, with skies becoming partly cloudy by the evening hours.
The far southern CWA could see some lingering showers prior to
midnight, but then heading towards daybreak Fri anti-cyclonic flow
will likely in control with steady subsidence in the near surface
environment. Highs Thur will be around 80, but the far southern CWA
could still see warm/humid conds with highs in the mid 80s. Then
with cooler/drier air arriving in the evening, lows will fall into
the mid 50s to lower 60s across Northern Illinois/Northwest Indiana.

Friday through Tuesday: Ensembles are in good agreement with the mid-
lvl trough axis pivoting east towards the Great Lakes region
Fri/Sat, and subsequent surface ridge sliding across the region
towards the Northeast CONUS by the second half of the weekend. This
will allow a quasi-stationary frontal boundary to drift north for
and precip chances return. Quasi-zonal flow will try to develop late
in the weekend into early next week, which will bring periodic
chances for showers/thunderstorms throughout the bulk of the
extended periods. Temps will start in the 70s for Fri/Sat, then
return to the low to possibly mid 80s for the remainder of the



For the 18Z TAFs...

The main concern continueS to focus on TSRA timing, tonight.

Following the passage of a mid level impulse, much of the pcpn has
moved east of the terminals and cigs at the terminals have
improved to mvfr, though a few pockets of ifr cigs persist across
the region. An old circulation generated by thunderstorms over the
central plains is moving across nrn IL this afternoon per the
latest radar imagery. This system is generating some scattered
showers and a few thunderstorms, but the chances for any of these
storms to impact the terminals is low. However, given the presence
of this system, will maintain a mention of vcsh through the
remainder of the afternoon.

Of greater concern will be another round of thunderstorms
overnight tonight. Expect that the activity will lift from
southwest to northeast across the region, reaching the terminals
shortly after midnight. Given some uncertainty of the timing of
onset, have only gone with prevailing shra and a tempo tsra, but
expect that prevailing ts will be required at some point durg the
early morning hours. Some extension of the ts chances is possible
as a cold front drops out of the upper MS valley tomorrow
afternoon. The models don`t have a great handle on the timing or
coverage of ts with the front, so have gone with an optimistic
forecast for now, but updates will likely be needed to add in some
more pcpn tomorrow.


131 AM CDT

The combination of a surface ridge of high pressure over the
Atlantic coast and surface low pressure over southeastern
Manitoba, will continue to support stout south-southwesterly
winds (up to 25 to 30 KT at times) over the lake today. This low
will shift eastward across Ontario later today into this evening.
As this occurs, an associated cold frontal trough will shift
over the lake late tonight into Thursday. Expect the winds to
abate and veer westerly during the day following this frontal
passage. Then, a secondary front is expected to push over the
lake Thursday night, and with its passage a short period of
stronger northwest winds (around 20 KT) look probable. However, as
a surface high builds over the western lakes region on Friday,
wind speeds will diminish.






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