Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 012023

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
223 PM CST Fri Dec 1 2023

Issued at 301 AM CST Fri Dec 1 2023

Through Saturday...

Key Messages:

* Storm system will bring widespread rain this morning with
  periods of light showers and drizzle this afternoon

* Rain/snow mix develops across portions of far north and
  northwestern Illinois late this afternoon and evening. Small
  chance (around 20% to 30%) of some slushy accumulations if
  surface temperatures can become cold enough

* Otherwise cloudy and seasonable temperatures through Saturday

A negatively tilted shortwave trough continues to advance
northeastward across southern Missouri this morning with 1003 mb
surface low in tow. Ahead of this system a broad area of light to
moderate rain is advancing across Missouri and central Illinois
with even some periods of drizzle being noted over the southern
1/3 of our forecast area. The expectation is for this area of rain
to continue to spread across northeastern Illinois and northwest
Indiana through the morning as the shortwave slides across the
area. While the majority of any robust rainfall is expected to
occur between daybreak and noon, periods of lighter showers and
drizzle will persist through the afternoon even as the shortwave
pushes east of our area. Given that surface temperatures have
remained in the upper 30s and lower 40s overnight and will remain
nearly constant through this afternoon, all of the precipitation
with this initial shortwave will be in the form of rain.

As this initial wave exits the surface low is expected to lag
behind and be slowly propagating across central Illinois through
the afternoon. A second shortwave, currently moving across the
southwest CONUS, is expected to catch up to the low as we head
into the evening hours which will allow another wave of widespread
rain to redevelop across the area, but especially northern
Illinois. As this occurs winds should become northerly and begin
to advect in colder air from Wisconsin which may allow
temperatures to fall near if not just above freezing. Forecast
soundings do show temperatures aloft to be well below freezing
which should allow some snow to mix in with the rain primarily for
areas along and north of a line from Sterling, IL to Woodstock,
IL. Given that surface temperatures and resultant wet bulb
temperatures are expected to remain in the mid-30s as the snow
begins I suspect that snow accumulations should be very limited
with maybe just some slushy amounts of a tenth or two at best.

However, some recent guidance is starting to indicate that
upright instability may develop in the mid-levels for a period
late this evening which may aid in a brief period of increased
snowfall rates if sufficient moisture in the DGZ can linger long
enough. If this was to occur, the briefly more robust snowfall
rates could overcome the marginal surface temperatures and lead to
some slushy accumulations upwards of an inch or possibly a little
higher. Given that a lot of ingredients need to come together
and the fact that most guidance keeps the bulk of any accumulating
snow more into far northwest IL and southern WI, I have decided
to maintain a forecast for a rain/snow mix with no accumulation
for now. But will need to keep a close eye on trends this
afternoon especially across portions of northern Winnebago County
for signs of this scenario establishing.

Regardless, as the second wave and surface low exit the area
tonight, rain and snow will gradually taper with dry conditions
expected to return by Saturday morning. Unfortunately lingering
mid-level moisture will maintain overcast skies through the day on
Saturday. Though light southeasterly winds should still advect in
enough warm air to keep highs in the low to mid-40s.



Issued at 223 PM CST Fri Dec 1 2023

Saturday through Friday...

Key Messages:

* Several chance of light precip, mostly rain, Saturday night
  through Tuesday morning
* Period of dry and unseasonably mild weather late next


We`ll be between weather systems Saturday, but with no push of
drier air, stratus is expected to linger through the day. Model
guidance varies with respect to the depth of the saturated (ie
stratus) layer, generally between 3-5kft thick during the
morning. Given the expected depth of the stratus, it wouldn`t take
much for some drizzle to occur and have introduced a chance of
drizzle during the morning. During the afternoon, ceilings may
lift a bit and stratus is expected to thin a bit, so held off on
adding drizzle to the forecast during the afternoon, but wouldn`t
be hard to envision drizzle lingering later. The thick cloud cover
generally neutral to slight cold air advection should result in
temps basically flatlining all day Saturday.

Saturday night-Sunday night:

Progressive shortwave trough is progged to ripple across the area
bringing a period of mostly rain, starting late Saturday evening
and continuing into Sunday morning. Thick stratus and warm air
advection in advance of the system should result in temps
generally remaining steady Saturday night. Thermal profiles
support all rain over most of the area, though far northwest CWA
could could see rain mix with or change to wet snow as slightly
drier low levels could allowing for a bit of wet-bulb cooling.
Temps should generally remain just above freezing at the surface,
so at this point the threat of any accums on roads appears
minimal (<25%).

Rain should taper off to drizzle or end during the day as the
trough axis shifts east of the area. Fast on the heels of this
wave, another shortwave trough is progged to track mainly just
south of the CWA Sunday night into early Monday morning. Stronger
forcing should remain mostly south of our CWA, but did hang onto
the slight chance pops southern CWA offered up by the NBM.

Monday-Tuesday night:

Monday should be dry with seasonable temps followed by another
quick moving shortwave trough, an Alberta Clipper, moving quickly
across the region Monday night into Tuesday morning. There could
be some showers Monday night associated with the warm air
advection wing of this clipper. Once again, cloudiness and warm
air advection should result in a precip type of primarily rain
with any showers that do occur. Better precip chances with this
wave look to stay north of our area with sfc low progged to track
north of our CWA.


Medium range guidance is in reasonably good agreement on pattern
transitioning to more of a zonal flow late next week. Zonal flow
this time of year tends to be a mild pattern.  and with storm
track associated with the northern stream jet expected to remain
well north of our area, we could be headed toward a period of dry
and unseasonably mild weather.

- Izzi


For the 18Z TAFs...

Aviation forecast concerns for the current TAF period include:
* IFR and LIFR ceilings through tomorrow morning
* Periods of rain/drizzle through tonight with visibilities as low
  as IFR
* Rain/snow mix likely at RFD late this afternoon through tonight
  with a changeover to all snow possible at some point

This morning`s widespread rain has since departed the area, but
lingering low-level moisture has permitted areas of rain and fine
drizzle to hang around into this afternoon. IFR ceilings and
MVFR/IFR visibilities are generally expected to be the rule as
this light rain/drizzle continues over the next few hours.

Later this afternoon into this evening, widespread light
stratiform precipitation will overspread the terminals again. This
precipitation will fall as rain at the Chicago metro terminals,
but will likely feature at least mixed in snowflakes at RFD. There
remains a chance that precipitation will change over to all snow
at RFD at some point tonight. Confidence in that occurring over
the RFD airfield is low-medium, but is much higher confidence in
that occurring as little as 5-15 miles to the north/west of RFD,
so held onto the inherited TEMPO group with prevailing snow and
IFR visibility. Ceilings tonight are expected to remain IFR for
the most part, but it`s a good bet that some LIFR ceilings will
be observed as well, especially at RFD and DPA.

Precipitation should eventually end during the overnight/early
morning hours, quite possibly becoming more of a true drizzle at
the Chicago metro TAF sites before ceasing entirely. IFR ceilings
should then gradually lift to MVFR after sunrise, though
confidence in the exact timing of when that transition will occur
is low. There is some chance that drizzle could redevelop at some
point tomorrow, but the probability of that occurring appears to
be low to justify a formal mention of that in the TAFs at this





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