Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211114

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
614 AM CDT Mon Sep 21 2020

232 AM CDT

Through Tuesday...

Near term weather continues to look dry with a gradual warming
trend to start the week. Smoke from western wildfires will persist
across the area as mid-level flow remains westerly into Tuesday.

Surface high pressure was centered across New England and far
southern Quebec early this morning, with the western periphery of
the ridge axis extending back across northern IL. The ridge is
forecast to develop southward across the Ohio Valley through
Tuesday, as new high pressure develops across Kentucky and
Tennessee between tropical circulations associated with Beta along
the western Gulf coast and hurricane Teddy off the East Coast.
This will allow dry south-southeast low level flow to continue
across the forecast area today, to veer southwesterly on Tuesday.
Low level moisture will increase a bit Tuesday with the southwest
winds, though the main axis of higher dew point air will be
focused west and north of the forecast area as the closed upper
low over southern Canada and a trailing mid-level short wave
trough axis transit the upper Midwest and northern Great Lakes

Cloud cover will be limited across the region through tonight,
though some increase in high clouds is expected into central IL
and IN Tuesday as moisture from Beta interacts with a weak mid-
level trough to our southwest. Westerly flow with the short wave
passing to our north will keep western wildfire smoke across the
area however, as depicted by RAP/HRRR experimental smoke
concentration forecasts. Even in the absence of cloud cover, have
maintained 30-35 percent sky cover to represent this non-opaque
smoke layer.

Temp-wise, expecting continued gradual moderation. Progged 925 mb
temps from +15/16 C today and +17/18 C Tuesday should support
afternoon highs in the mid-70s most areas today and upper 70s on
Tuesday. Slight southeast wind component will support limited lake
cooling this afternoon, mainly along the IL shore north of
Chicago. Southwest winds Tuesday should allow warmer temps up to
the shore.



232 AM CDT

Tuesday night through Sunday...

Warm and dry weather is expected to continue through the end of
the work week, before an upper trough amplifies across the region
and brings precip chances next weekend.

Remnants of tropical storm Beta are progged to track across the
southeastern states mid-late week, while the primary jet stream
and storm track remain north of the forecast area. This keeps
precip potential along/south of the Ohio River and across the
upper Midwest through Friday. Cloud cover is expected to increase
in the Wednesday through Thursday period as a mid-level short wave
trough propagates through the region, though model soundings and
time/height sections indicate relatively dry lower and middle
levels during this period, with no real precipitation threat and
mainly just high level cloud cover expected. The lack of organized
thermal advection looks to support mild but fairly steady temps
mid-week, with daytime highs in the mid-upper 70s and overnight
lows in the 50s.

Medium-range guidance is in good agreement in bringing a broad
upper trough across western Canada, the Pacific Northwest and the
northern Rockies Friday, inducing the development of a surface low
pressure trough across the Plains and upper Midwest. Low level
southerly flow develops through the Mississippi Valley and Great
Lakes region ahead of this trough, and increases thermal and
moisture advection into the Midwest ahead of the developing cold
frontal trough Friday into the weekend. Mid-level height falls,
indicative of large scale ascent with the amplifying upper trough,
develop across the region particularly late Saturday into Sunday.
This ascent occurring atop the low level moisture axis ahead of
the approaching front should ramp precip chances up by Saturday
night, which then persist through Sunday when the front moves
through. Warmer and more humid conditions ahead of the cold front
Friday-Saturday will then give way to much cooler weather Sunday
and beyond, as a deep long wave trough evolves across the region.



For the 12Z TAFs...

VFR conditions with generally cloud-free skies are expected
through the TAF period. Smoke from wildfires across the western
U.S. will continue to drift over the region into Tuesday. Per
HRRR forecast guidance and various PIREPs over the past 24 hours,
the layer of smoke ranges from 5-15kft, with a more concentrated
layer centered around 8-12kft/700 hPa.

Winds will be S between 170-190 degrees for most of the day,
favoring 190 in the morning and 170 in the afternoon. Winds should
remain 10 knots or less, but may briefly gust in excess of 15
knots from 14-16Z when the onset of daytime mixing taps into 20-25
knot winds at 500-1000ft.



LM...Small Craft Advisory...Winthrop Harbor to Wilmette Harbor
until 10 AM Monday.



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