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000
FXUS63 KLOT 030404 AAA
AFDLOT

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
1004 PM CST Sat Dec 2 2023

.UPDATE...
Issued at 538 PM CST Sat Dec 2 2023

The next weather system is approaching from the southwest with a
growing area of radar returns across southern Iowa and much of
Missouri. With nearly neutral low-level temperature advection
tendencies and limited frontogenetical forcing, nearly all
forcing will be tied to the diffluent right exit region of an
approaching 150kt 250mb jet streak and a coherent region of mid-
level DCVA. As a result, our expectation is for radar returns to
further expand while moving into Illinois this evening, reaching
I-39 perhaps as early as 10 PM and the Chicago metropolitan area
around or just after midnight.

With heavy cloud cover (and even signs of developing drizzle) as
well as light winds, current temperatures in the lower 40s may
not move much over the next 12 hours. The exception will be along
and north of I-88 where efficient evaporative cooling into a dry
layer from roughly 850 to 600mb (sampled recently by AMDAR
soundings from MDW) will support temperatures falling into the low
to mid 30s. As a result, a transition from rain to snow appears
increasingly probable across far northern Illinois (again mainly
north of I-88). Such a transition from rain to snow is already
occurring across far northern Missouri, per recent AWOS and ASOS
station reports. Even with the majority of lift above the DGZ,
gradually steepening mid-level lapse rates will support
increasing convective augmentation to snow rates, which may
approach or even exceed 0.5"/hr at times overnight. As a result,
slushy snow accumulations appear increasingly probable on grass
and decks, as well as potentially bridges and overpasses. Such a
threat is a modest departure from our earlier forecasts and
matches increasingly snowier trends in model guidance. Elsewhere,
a cold rain is expected.

Peak forcing for snow will occur from about midnight to daybreak,
which is a somewhat ideal time of relatively low travel.
Regardless, opted to issue a targeted Special Weather Statement
for Lee, Ogle, Winnebago, Boone, DeKalb, Kane, and McHenry
counties to highlight the threat for patchy slick spots tomorrow
morning. An associated targeted impact-based graphic was also
shared on our social media feeds.

Updated products have been sent.

Borchardt

&&

.SHORT TERM...
Issued at 200 PM CST Sat Dec 2 2023

Through Sunday...

Key Messages:

* Rain likely overnight and into Sunday morning for most of the
  region with a rain/snow mix expected in portions of NW Illinois.

* With persistent cloud cover expected through the night, surface
  temperatures are expected to be warm enough to limit the threat of
  snow accumulations.

What a grey Saturday! Widespread cloud cover remains over the area
with seasonal temperatures. The higher moisture content has
gradually moved away from the forecast area and sources for lift
have diminished allowing PoPs to be trimmed down even farther during
the morning update. There is still a non zero (less than 10 percent
chance) chance for an isolated drizzle/sprinkles occur the rest of
this afternoon, but confidence is low and impacts would be non-
existent.

The next weather system moves in overnight. An upper level trough
will move northeastward from the Southern Plains. Chances for
precipitation increase through the night, though the main rainfall
will happen after midnight and into Sunday morning. Most of the area
south of a Waukegan to Peru line will likely have rain given the
warmer temperatures. For areas north of that line, colder
temperatures through the column will result in better chances for a
wet snow to mix. However, due to the continued cloud cover, limited
evaporational cool and additional warm air advection from the system
moving in, the main update with this forecast package was surface
temperatures were increased above freezing. There is a chance that
isolated communities around the Rockford Metro area receives a
trace, but snow accums have otherwise been removed from the
forecast.

There is still a lot of uncertainty with the timing of the exit of
the system. Higher res models are trending quicker with the ending
of precipitation earlier on Sunday morning. While precipitation may
lighten, higher PoPs were maintained through at least noon given the
saturated column profiles.

DK

&&

.LONG TERM...
Issued at 200 PM CST Sat Dec 2 2023

Monday through Saturday...

Key Messages...

* Rain or snow is likely Monday night, with a small, but
  increased, chance of some snow accumulations (10-20%)

* Above average temperatures likely late in the week

Guidance continues to show a compact, yet fairly vigorous,
shortwave trough tracking east across downstate IL. Given the
current forecast track, it seems likely precip with this wave will
remain south of our CWA. Maintained a slight chance of rain/snow
across the southern CWA just in case guidance were to trend
farther north. While thermal profiles would probably lean more
toward rain at first glance, strong upward vertical motion and
weak static stability suggests at some dynamic cooling could flip
precip to wet snow in spots. Again, this largely looks to be south
of our CWA at this point, so only maintaining slight chances of
precip.

Will stratus clear out Sunday night? Given the lack of any
appreciable dry air advection and likely lingering moist layer
beneath frontal inversion, it seems likely stratus will hang on
through Sunday night. Based on this expectation, have raised
forecast low temps a few degrees. Where and if stratus does manage
to break up, then lows would likely end up cooler than forecast.
As overnight shift discussed in previous AFD, the stratus dilemma
will continue Monday with potential for cloud cover to linger
through the day. Maintained forecast highs closer to 40, leaning
toward the more pessimistic (cloudier) solution, if skies clear
out then highs could be a 5F warmer.

The next (and last) in this series fast moving, vigorous
shortwave troughs is forecast to move quickly across the region
Monday night. There has been a fairly sizable southward trend with
this system in this morning`s 12z guidance. The 12z operational
runs of the GFS and ECMWF and 12z GEFS have all shifted the track
of this system 100 to nearly 150 miles farther south. The more
southward solution increases confidence in precipitation occurring,
so continued to trend pops higher than NBM and a bit higher than
previous forecast. In addition, the farther southward track raises
of a snowier solution and even possible accumulations into our
CWA Monday night. For now, just trended forecast toward rain or
snow, but if subsequent runs remain locked in with this farther
south track then the snow may need to be hit a bit harder in the
forecast.

Transition to a more zonal flow or even low amplitude ridging is
still expected late next week with high temps well into the 40s to
lower 50s. As always this time of year, temp forecast will hinge
on cloud cover, if warm sector doesn`t become covered in stratus
then highs could be at or even above forecast levels. Conversely,
if there is a more rapid development/expansion of stratus then
temps could end up a bit cooler than forecast. Regardless of
exact temperatures, there is high confidence in trend to above
average temperatures.

- Izzi

&&

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

Aviation Concerns:

 * Increasing likelihood for SN at RFD and associated LIFR
   cigs/vis overnight into Sunday AM (potentially VLIFR at times)

 * SN mixing in at ORD/DPA overnight with LIFR cigs and IFR vis
   (potentially LIFR vis at times)

 * Mainly all rain at MDW/GYY with IFR cigs and MVFR vis

Have adjusted 06Z TAFs to account for latest model trends toward
more snow across northern/northwest Illinois. A larger period of
all SN has been introduced at RFD from 7-14Z. Have currently
capped the vis and cig reductions to low-end LIFR though
additional targeted TEMPO groups of VLIFR vis may be warranted
with later updates. Will closely monitor upstream observations
this evening. This could result in a slushy 1 to 2 inches of
snowfall.

Further to the southeast toward DPA and ORD it does look like
snow will mix in at times overnight into Sunday morning and have
introduced a period of RASN from 9-15Z. A targeted TEMPO for SN
may also be needed along with lower vis, especially at DPA. The
rain/snow line looks to reside somewhere near the I-88 corridor.
Accordingly, MDW and GYY TAFs remain all rain with this update.

Ceilings will be slow to increase during the day as the
precipitation transitions to mainly a drizzle/flurries late
morning, eventually returning to MVFR by the afternoon.

Winds will be light southeasterly this evening then becoming
light and variable as the system moves overhead overnight. As the
system shifts to the east late Sunday morning winds prevail
westerly around 10 kts.

Petr

&&

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

$$

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