Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 241127
AFDLOT

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

.SHORT TERM...
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.

Beachler

&&

.LONG TERM...
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
extended.

Beachler

&&

.AVIATION...
For the 12Z TAFs...

The main concerns/challenges continue to focus on TSRA timing,
especially this morning and again tonight.

CIGS have temporally improved across the Chicago area. However,
this will be short lived, as low MVFR CIGs down to 1,000 AGL will
return by, or shortly after 12 UTC. These MVFR CIGS look to
persist through much of the morning before gradually improving
this afternoon.

Thunderstorms continue to shift eastward towards the Chicago area
terminals in response to the approach of a decent mid-level
disturbance, currently over eastern Iowa. However, the leading
edge of these storms has been weakening as it tries to move
eastward into more stable conditions over NE IL. As a result,
this will slow the expected onset of storms over the Chicago area
terminals until after 13Z, when better dynamics with the
approaching disturbance should finally drive these storms into NE
IL. Currently it appears that the period of storms will be
favored between about 13 through 15-16 Z this morning. Thereafter,
a few lingering showers could last through midday, but any
additional storms this afternoon would likely be favored to the
south and east of the terminals.

Another round of thunderstorms is likely tonight, especially to
the south of the terminals. However, these storms tonight could
try to develop northeastward into northern Illinois overnight
ahead of an approaching cold front. As a result, I have continued
with a PROB30 mention for TSRA, but I have pushed the timing off
by a couple hours to cover the late evening into the overnight
period. Confidence on timing does remain a bit on the low side,
however.

KJB

&&

.MARINE...
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.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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