Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS63 KLOT 020103
AFDLOT

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

.UPDATE...
Issued at 703 PM CST Fri Dec 1 2023

It`s a soggy night with pockets of drizzle, scattered showers, and
fog. Much of the same is expected overnight albeit with gradually
decreasing coverage of showers in favor of fog and drizzle. As a
result, temperatures overnight will be pretty similar to where
they stand now and in the upper 30s (northwest) to lower 40s
(elsewhere).

Tomorrow, the signal remains that a thick (2500-3000 feet deep)
layer of low-level stratus will hold tight beneath a strong
inversion rooted near 800mb. Temperatures will, accordingly, hold
pretty steady in the lower 40s. With steep lapse rates through
the stratus layer, areas of drizzle may prevail particularly
during the evening hours as the next system approaches from the
southwest. For this reason, opted to explicitly mention a slight
chance of drizzle in our official forecast all day tomorrow
carried by 20% chances for measurable rain though some (many?)
hours may end up completely dry.

Updated products will be sent soon.

Borchardt

&&

.SHORT TERM...
Issued at 328 PM CST Fri Dec 1 2023

Through Tonight...

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
rates.

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.

Ogorek

&&

.LONG TERM...
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

Saturday:

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.

Wednesday-Friday:

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

&&

.AVIATION...
For the 00Z TAFs...

Aviation concerns:

 * -RADZ with associated IFR vis and IFR/LIFR cigs tonight
 * -SNRA at RFD this evening

Rain and drizzle continues across the area this evening along
with associated IFR visibility and ceilings and LIFR at GYY. This
will gradually transition to mainly drizzle late evening into the
early overnight hours. There is a corridor of improved vis and
cigs approaching the Chicago area terminals that could result in
at least a brief period of improved vis and cigs. Will continue to
monitor trends, though model guidance still supports a return to
drizzle and reduced visibility. The better signal for LIFR/VLIFR
visibility in fog remains mainly south of I-80 and accordingly
have have held onto 3SM for the Chicago area terminals. It is
possible that light drizzle persists into the morning hours on
Saturday (20 percent), though cigs will gradually lift to MVFR
after daybreak into the afternoon.

The earlier band of snow has since shifted east of RFD with light
snow/flurries and even a period of VFR visibility. There is
another band near the Freeport area shifting east that could bring
another round of brief vis/cig reductions in snow (potentially
mixed with rain) this evening.

Northeasterly winds will trend northerly overnight then return to
a northeast to easterly direction during on Saturday.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

Visit us at weather.gov/chicago

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/NWSChicago
www.twitter.com/NWSChicago
www.youtube.com/NWSChicago


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.