Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 012128

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

Issued at 328 PM CST Fri Dec 1 2023

Through Saturday night...

Key messages:

* Precipitation increasing in coverage again this afternoon into
  this evening. Most locations will see all rain, but snow may mix
  in with the rain, or even fall as the outright precipitation type,
  roughly along and northwest of a Dixon to Woodstock, IL line.

* Where snow falls in our forecast area, road accumulations are
  unlikely, but areas north and west of Rockford may receive
  anywhere from a dusting to an inch or so of slushy snow
  accumulation on grassy and elevated surfaces.

The core of the upper-level disturbance that helped foster this
morning`s widespread precipitation has long since departed to our
east. Lingering low-level moisture allowed for some areas of light
rain/drizzle to hang around into this afternoon, and this
precipitation is still being observed across a good chunk of our
forecast area as of this writing. Meanwhile, a residual vort max
hanging out back closer to the center of a surface low in west-
central Illinois has helped an additional area of stratiform
precipitation flare up across the northwestern portion of the state
this afternoon. The expectation is that this area of precipitation
will continue to expand in coverage through the remainder of this
afternoon and into this evening as this vort max shears out and
shifts east and as a trailing shortwave over the lower Missouri
River Valley rides up a southwest-northeast oriented baroclinic zone
into the area.

Surface temperatures predominantly in the 40s across much of our CWA
will support rain remaining as the sole precipitation type at most
locations. However, across our northwestern counties, temperatures
will be cold enough to support snow mixing in at times and even
falling as the outright precipitation type. Surface temperatures
there are still expected to remain at or above freezing, which
should help mitigate road impacts, but webcam imagery just across
CWA lines in Freeport, IL has shown that the snow there has fallen
at a steady enough clip to accumulate on grass at least, and a
similar scene could be seen in extreme northwestern portions of our
forecast area (roughly west and north of Rockford). Any snow
accumulation that occurs within our forecast area would be unlikely
to be much more than a slushy inch due to the marginal surface
temperatures and limited residence time of the steadier snowfall

Most or all of the precipitation is then expected to end during the
overnight hours as the aforementioned trailing shortwave starts to
peel away. Precipitation could acquire a more drizzle-like nature
again before ending as forecast soundings indicate that cloud ice
should be lost before saturation depths within the overhead stratus
deck are slashed by a meaningful amount. Even then, some guidance
indicates the stratus deck could remain deep enough to support
patches of drizzle through and beyond sunrise tomorrow morning,
though confidence in this outcome being realized at this time is
only low-medium.



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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