Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 021533

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
933 AM CST Sat Dec 2 2023

Issued at 933 AM CST Sat Dec 2 2023

The region remains shrouded in a thick cloud deck. The higher
moisture content, supported by cirrus clouds above the stratus
deck, is mainly south of I-80. With weak forcing present, it would
not be surprising if a spit of drizzle occurred this afternoon,
but there is higher confidence that the drier air near the surface
will win out. So, PoPs were adjusted down to below 10 percent
north of I-80, confining the official mention of drizzle to the
southern part of the region. Otherwise, no major changes were made
compared to the forecast listed in the discussion below.



Issued at 334 AM CST Sat Dec 2 2023

Through Sunday...

Key Messages:

* Lingering rain/snow showers this morning will give way to
  overcast skies and periods of drizzle this afternoon.

* Another storm system arrives tonight bringing more widespread
  rain for most and a rain/snow mix to portions of northwestern
  Illinois. Some slushy accumulations will be possible, especially
  if surface temperatures are cold enough.

* Persistent overcast skies and seasonable temperatures through
  Sunday afternoon

A robust shortwave disturbance continues to generate an area of
rain and snow showers across portions of northern Illinois this
morning. The shortwave is expected to exit the area around
daybreak which should allow any lingering showers to diminish in
coverage through mid-morning leaving most areas with dry, but
cloudy skies through this afternoon. However, the area will be
positioned in the diffluent upper flow between this departing wave
and the trough that is currently ejecting out of the Four Corners
Region. The combination of this modest forcing with the steep low-
level lapse rates and 3500 to 4000 ft saturated layer at surface
should allow for periods of drizzle to develop during the morning
and linger through this afternoon.

The aforementioned trough will begin to move into the Mississippi
Valley this evening and then track overhead through the night. As
this occurs another area of rain is expected to develop over
Missouri and spread into northeast Illinois and northwest Indiana
late this evening. Similar to the last system, surface
temperatures are expected to gradually cool as the rain moves in
with most areas forecast to remain in the mid to upper 30s but
with temperatures aloft expected to be well below freezing. This
should once again allow for some snow to mix in with the rain late
tonight into Sunday morning particularly for areas north of I-88
where temperatures look to be coldest. Though unlike the last
system, forecast soundings do show mid-level lapse rates to be
steeping as the system moves overhead which may allow for some
upright instability to be realized in the 500 mb to 600 mb layer.
Coincidentally this layer just so happens to be within the
saturated DGZ which means that if this upright instability can be
realized then more robust snowfall rates should develop and may be
enough to overcome the marginal surface temperatures. Should this
materialize as some guidance suggests, then areas within this
convective snow band could see some slushy accumulations
potentially in excess of an inch.

Given that a lot of conditions which have to come together
tonight for these slushy accumulations to occur confidence in the
location and amounts are somewhat low. Though, guidance is in
decent agreement that areas along and northwest of a Sterling, IL
to a McHenry, IL line look to be the most favored area for any
accumulating snowfall. Since yesterday`s system was able to over
perform with snow despite the lack of instability, I have decided
to add a couple tenths of accumulation to the forecast with this
system for the far northwest portion of our forecast area. While
potential impacts should be minimal due to the generally above
freezing temperatures, we will need to carefully monitor trends as
the system approaches before we will know for certain so be sure
to check back for updates this afternoon.

Tonight`s system will exit the area Sunday morning which will
allow conditions to dry out once again as a weak upper-level ridge
begins to build into the area. Though lingering mid-level
moisture does look to keep skies overcast through Sunday afternoon
which will hold high temperatures in the upper 30s and lower 40s
for Sunday.



Issued at 334 AM CST Sat Dec 2 2023

Sunday night through Friday...

Key Messages:

* Another round of precip Monday night-Tuesday. Low confidence on
  p-types with marginal surface temperatures at this point, but
  have added snow chances to the forecast Monday night.

* Period of dry and unseasonably mild weather late next week

Precipitation will largely have ended across the region Sunday
evening although there is a small signal that a little drizzle may
linger ahead of a weak reinforcing front which will be dropping
out of Wisconsin. Temperatures are expected to be at or above
freezing, so not concerned with this signal at this time. Some
disorganized lake effect rain shower activity will likely develop
across parts of lower Michigan, but with generally W/WNW surface
flow, expecting the bulk of this activity to remain relegated to
the east of our forecast area.

A very robust shortwave is forecast to scoot rapidly eastward
across Missouri and into central/southern Illinois late Sunday
night. Guidance is in pretty good agreement with this feature`s
track at this point, holding the best overlap of deeper moisture
and forcing for ascent south of the Kankakee River and moreso into
central Illinois overnight. Have held onto some low PoPs for these
areas with just rain advertised. Quite the impressive look from a
forcing/Q-vector convergence and upright instability perspective
with this feature...just meandering into a relatively mild

Shortwave ridging will (very briefly) build in overhead on
Monday. It`s always difficult determining stratus staying power
this time of year, and the typically under-mixed NAM solution
depicts stratus remaining entrenched across the region through
Monday afternoon. With some increase in west/southwesterly flow
late in the day, however, seems like there may be some potential
for some degree of clearing which would further help boost
temperatures, perhaps into the low to even mid 40s. More stubborn
stratus, however, would hold temps in the mid and upper 30s.
Latest thinking is headed more towards the former, and showing
highs either side of about 40 on Monday.

The next intense disturbance arrives in the region Monday evening
and overnight. Guidance trends appear to be upwards regarding both
the degree of moisture/saturation and ascent, particularly within
the preceding warm advection wing. Have boosted PoPs a bit above
the NBM offering with good multi-model support at this time.
Thermal profiles remain on the cusp of supporting either just a
cold rain or a changeover to a wet snow, and surface temperature
trends coming out of Monday afternoon will likely play a big role
in the regard. With WAA intensifying through the evening, suspect
temperatures really may not fall all that much and may even slowly
rise through the late evening and overnight hours. For this
forecast package, have generally limited the snow wording to
slight chance/chance in favor of both milder temps on Monday and a
muted temperature fall in the evening with no accumulations
formally depicted, but there`s plenty of guidance now showing the
lead WAA wing arriving as mainly snow. Something we`ll continue to
monitor. Deeper saturation will be lost rapidly through daybreak
on Tuesday turning most lingering precipitation over to a non-
freezing drizzle.

After a brief period of lake effect precip (likely rain), we`ll
begin a moderating trend as mid-level heights rise and the main
active northern stream temporarily heads north of the region.



For the 12Z TAFs...

Aviation weather concerns are:

* Non-impactful -DZ possibly with some flurries at RFD this
  morning. Potential for intermittent -DZ into the Chicago-area
  sites after 15z, but low confidence.

* Increasing coverage of rain tonight at all terminals. Period of
  mixed precip, potentially briefly turning to all snow at RFD.

* IFR to LIFR cigs late tonight into Sunday morning.

At the Chicago-area terminals:
Largely low-MVFR cigs are expected to prevail today, although some
improvement towards 2 kft is possible today. Surface winds will
trend NE this morning. While some very light precip/-dz is
possible later this morning, this would not be overly impactful
and confidence in this is too low to carry a mention in the TAFs.
Spotty showers may develop later this afternoon, but the main
period of rainfall will be after about 06z. Widespread soaking
rain may be pretty brief, with loss of deeper saturation through
about 10-11z suggesting a transition to showers/drizzle. Cigs will
trend down through IFR to potentially LIFR during this time. Winds
will become southwesterly into Sunday morning.

At Rockford:
An area of light radar returns which has been producing
intermittent drizzle/light rain farther to the west will move
across RFD through mid-morning. No impacts anticipated with above-
freezing temperatures. Once this passes, cigs should lift towards
low-MVFR through the day. While some showers may develop later
this afternoon (not presently advertised in the TAFs), the main
window for rainfall is after about 05z tonight. Thermal profiles
should become cold enough to support a rain/snow mix late tonight.
A full changeover to snow is also possible, but confidence in this
is less than 30% right now with no mention in the TAF.

Cigs will build back down through IFR and LIFR this evening.
Signal in guidance exists for conditions to even approach VLIFR
cigs/vsbys towards daybreak Sunday.





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