Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 242330

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 PM CDT Sat Oct 24 2020

223 PM CDT

Through Sunday night...

Cool weather will be the theme through the remainder of the
weekend. It is more cloudy than not in most locations from a
combination of lingering stratocumulus inland and an additional
feed of lake induced moisture.

At the surface, strong high pressure is centered over southern
Alberta with the ridge axis extending well southeastward into the
lower Great Lakes. A while the cold front pushed through
yesterday, there is an enhanced baroclinic zone farther upstream
across the Upper Midwest. Aloft, an upper trough noses into the
northern States, with the upper jet axis riding ahead of it across
the northern Plains into the upper Great Lakes. Mid level flow is
west-southwesterly, and is steering some vorticity centers and
some mid level isentropic lift across the southeastward spreading
frontal/baroclinic zone across the plains where temperatures
support snow.

Closer to home, the near term forecast issue is largely relegated
to clouds and associated lake effect sprinkles or very light rain.
Low level flow ahead of the surface high will shift more easterly
this evening and overnight. This will steer lake clouds inland.
Lake induced equilibrium levels peak up near 6,500 ft, at that
cusp of sprinkles to maybe some light rain (a hundredth or two)
given some increased moisture depths on forecast soundings, and we
have had some precipitation in NW Indiana today.

The upper jet will steer those weak vorticity centers closer on
Sunday with a corresponding increase in mid and upper level
clouds. The low level flow should maintain a feed of drier air and
limit precipitation during the daytime hours for most locales.
With the combined effects of any lingering lower clouds, Sunday
will again be cool with highs mostly in the 40s.

The forcing overnight does not increase too much, although the
frontal zone does get a bit closer which may support a better
chance of precipitation sneaking in to the area from the
northwest. Thermal profiles support this remaining a cold rain for
most locations, though northwest of the Chicago area, any light
precip could be a mix to even just snow as you shift closer to
the Rockford area and across the I-39 corridor. Precip amounts
appear light through Sunday night.



327 PM CDT

Monday through Saturday...

There are no significant weather impact concerns in the long term
portion of the forecast, with the main item of note being the
ongoing mainly light precipitation into Monday night. Some
locations could see a mix or change to wet snow, but marginal
surface temperatures will probably yield little chance for any
meaningful accums.

On early to mid Monday morning, the air mass aloft is forecast to
be cold enough to support snow, and the boundary layer is also
forecast to be cold enough for wet snow or a mix for roughly the
northwest half of the CWA. There are continued questions on the
magnitude of forcing and available moisture regarding coverage of
precipitation during that timeframe. Some guidance indicates some
transient frontogenesis could contribute to localized enhancement
of precip rates farther north and deeper into the cold air, while
others are more muted and/or farther south with this signal where
the air mass will be more marginal. Therefore, have low to mid
chance PoPs for this period. Even if snow occurs for parts of
northern Illinois, temperatures at or above freezing mean that at
most would expect a very slushy coating on grassy/colder surfaces.

Into Monday afternoon and evening-night, trends favor the f-gen
component shifting southward where the thermal profile will be
more marginal after a chilly light rain in the afternoon. The
guidance also varies on how far south the axis of appreciable
precip will be shunted. Latest NBM showing highest PoPs generally
south of I-80 appears reasonable. Once again, any wet snow mix or
brief changeover Monday evening/night would likely yield
little/no accums (at most slushy coating on grass) and impacts
given surface temperatures above freezing.

Monday and Tuesday will feature well below normal high
temperatures in the lower to mid 40s (some spots NW may not reach
40F on Monday). This will be followed by decent warm advection and
moderation on Wednesday to closer to normal for late October. A
bowling ball type upper low may eject out of the southwest US,
with associated surface low tracking to our south into Thursday.
This is the next chance for precipitation, though confidence is
low in how far north this system will get. The GFS has been
consistently depicting another trough and stronger cold front
bringing a reinforcing shot of chilly air that could keep the
system safely south, while ECMWF suite generally favors a weaker
trough over the northern Great Lakes allowing it to get farther
north. Either way, should the system track farther north as shown
on ECMWF, it will be devoid of cold air, so no p-type issues.

Thursday`s temps will be affected by how much cloud cover and/or
precip makes it northward. Then Friday will have a surface high
settling over the area, so a dry and cool day with highs in the
upper 40s to around 50 and lows in the 30s Friday night. At this
time, Halloween appears it will feature decent weather for this
time of year, with moderate southwesterly winds, a good deal of
sun, and temperatures well into the 50s.



For the 00Z TAFs...

The primary aviation weather concerns this period revolve around
the potential for MVFR cigs and lake-effect (rain) showers at the
Chicago-area terminals this evening and overnight.

Regional TDWRs reveal a gradual expansion to a region of light
lake-effect showers across northwest Indiana early this evening.
As an associated inverted trough pivots westward, the expectation
is for this activity to shuttle into MDW and ORD later this
evening. Lake effect parameters are marginal, but should be
sufficient for a few hours of on-station -SHRA. Latest guidance
and model trends support the introduction of TEMPO groups for VFR
showers in addition to the inherited VCSH mentions. High-end MVFR
cigs will accompany this activity. Surface winds should continue
to back and may occasionally favor a west-of-north orientation at
ORD/MDW/DPA, but speeds will stay under 7-8 kts. The potential for
cloud ice appears minimal, so not anticipating any snow with this
activity even though surface temperatures will be falling into
the mid and upper 30s.

This activity should push west of ORD early Sunday morning. At
this time, the potential for precipitation at RFD seems too low
for a formal mention at this time, but there is some potential
that light activity materializes during the afternoon, potentially
with some flurries as well if the cloud layer remains cool enough.

Just beyond the TAF windows, the next disturbance will approach
the region, and may bring a period of light snow to RFD, as well
as a potential for -RASN at ORD/MDW.





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