Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 222324

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
624 PM CDT Fri Sep 22 2017

211 PM CDT

Through Saturday...

Continued record or near-record warmth remains the focus of the
near term forecast as fall arrives this afternoon. Stronger lake
breeze likely Saturday afternoon.

Upper level pattern continues to feature a large amplitude trough
across the western CONUS and a corresponding downstream ridge
across the Midwest and eastern US. Low level thermal ridge, with
temps of +20C sampled by both DVN and ILX 12Z RAOBs, continues to
produce record warmth across the area this afternoon, with Chicago
O`Hare already breaking the old record high for this date. Similar
to last night, temps will be slow to fall off into the mid-upper
70`s (lower 80`s in the heart of Chicago away from the lake) this
evening, with overnight mins in the mid 60`s to lower 70`s nearly
as warm as our normal high temps. May see a bit of light patchy
fog again south and west of the Chicago area toward sunrise.

As upper ridge continues to build across the southern Great Lakes
region into Saturday, a slight retrogression of the surface high
to the east will weaken the surface pressure gradient and will
result in a gradual backing of surface winds to the southeast
during the day. This will allow more of a lake breeze push to
develop across the Chicago metro area, with a more pronounced
cooling affect further inland than over the past couple of days.
While areas near and a bit inland from the lake will settle back
into the 80`s during the afternoon, there should be a quick
climb in temps during the morning and midday hours before the lake
breeze develops. Model 925 mb temps do show a bit of a decrease,
ranging from around +25C in the western cwa to the +23/24C range
in the east. This would support high around 90/lower 90`s in most
areas away from the immediate lakeshore, at or just below records
for the 23rd. Cloud cover both tonight and Saturday should be




Saturday night through Friday...

205 pm...Little change to the forecast for early next week.
Models continue to slowly cool the airmass over the region with
highs falling back into the mid/upper 80s. Sunday still looks to
be mainly sunny and expect several areas to still be able to tag
90. With a bit more cloud cover on Monday...highs likely to stay
in the 80s and there is a chance of showers mainly across far
western IL and likely west of the cwa and chances also remain
below mentionable from this distance. A lake breeze is expected
Sunday and Monday and this will likely keep the IL lakeshore
several degrees cooler once it develops. Patchy fog will also be
possible during the overnight/early mornings through Tuesday.

A cold front will move across the area later Tuesday and Tuesday
night and it had appeared that the precip with this front would be
scattered and short in duration and all the models have trended
that direction. Best chance of precip appears to be across the
northwest cwa Tuesday afternoon/evening and then dissipating as it
spreads southeast. While temps will be much cooler behind this
front...for a day or two...just about back to normal for late
September. A second colder airmass settles across the lakes
region late next week and this will likely push high temps into
the lower/mid 60s. cms


For the 00Z TAFs...

Very quiet aviation weather to continue through the next 24-36
hours. Only concern at issuance time is the lake breeze. For much
of the afternoon, the lake breeze had been hugging the shoreline.
Over the past hour, the lake breeze has pushed inland and has
reached PWK at 2320z. Northerly Island and PWK were reporting
winds well under 10 kt east of the lakebreeze, so do not expect
much of a concern. Winds may still shift to sely or ely with the
lake breeze passage, but expect that wind speed should be less
than 8 kt. Winds should becm lgt/vrbl overnight. With high
pressure still in place, there could be some haze in the area in
the stagnant airmass. Do not expect that ORD/MDW will drop below
VFR, but DPA/RFD could see 4-5sm HZ durg the early morning hours.

For Saturday, expect that conditions should be dry and vfr. Winds
will be, synoptically, out of the southeast, with speeds arnd 10
kt developing by late morning.



205 pm...A large trough of low pressure will remain across the
plains through Monday maintaining southerly flow across Lake
Michigan. This trough/front will move across the region Tuesday
night into Wednesday. High pressure will build across southern
Canada and the northern plains late next week and as the gradient
tightens behind this front ahead of the high...a period of
15-25kts from the northwest is expected. cms





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