Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240751

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

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 06Z TAFs...

The main concerns/challenges during the forecast period will be
the timing of TSRA, especially this morning and again tonight.

CIGS continue to lower into the MVFR range as lower-level moisture
continues to increase over the area. Expect MVFR CIGS around 1500
FT AGL to persist through much of the morning before gradually
improving this afternoon.

The main focus for thunderstorms this morning still looks to be
complements of a mid-level disturbance, currently over Iowa. This
disturbance will push eastward over northern Illinois by around
daybreak. Therefore, it still appears the best window for thunder
at KRFD will be in the 10-14Z, and more in the 12 to 15Z timeframe
for the Chicago area terminals. So no real changes have been made
to the timing for the 06Z TAFs. However, some small timing tweaks
may be needed as the thunderstorms near the airspace. Thunderstorm
chances then look to diminish over much of the area by midday.
Although some additional widely scattered thunderstorms could
develop during the afternoon within the moist unstable environment,
the chances are too low to include in the latest TAF.

Better chances for a second round of thunderstorms looks to enter
the picture this evening as another mid-level disturbance tracks
towards the area. Although chances are good there will be more
thunderstorms around this evening and tonight, confidence on the
exact timing is still on the low side. The potential is certainly
there though for a several hour period of showers and storms, with
heavy rainfall. Because of this, I have continued with the PROB30
mention late in the TAF period.



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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