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000
FXUS63 KLOT 011141
AFDLOT

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

.SHORT TERM...
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.

Yack

&&

.LONG TERM...
Issued at 301 AM CST Fri Dec 1 2023

Saturday night through Thursday...

Key Messages:

* Another round of widespread showers/rain Saturday night into
  Sunday. Some potential for non-impactful snow/rain mix across
  far NW IL Sunday AM.

* Additional precip (mainly rain) chances Monday evening-Tuesday.

Hemispheric moisture channel loops reveal numerous eddies and
embedded perturbations stretching from the Pacific Northwest,
westward across the Gulf of Alaska and back towards the Kamchatka
Peninsula. This highly active northern stream will result in
repeated disturbances traversing the CONUS through the weekend and
next week.

The main feature of interest for our weather Saturday night into
Sunday looks to be one the many regions of strong cyclonic shear
vorticity currently traversing the Aleutian Islands. This feature
will eject out across the Central Plains with some degree of
amplification indicated across the guidance suite as it sweeps
across the Great Lakes region. Model guidance is in pretty good
agreement regarding the salient large scale features, especially
considering how chaotic and unstable the upstream flow appears.
Forcing will increase Saturday night as jet divergence increases
overhead and anticipating an increase in precipitation chances as
a result. With just a little in the way of elevated instability
present, precipitation may be a bit convective/showery in nature,
and suppose a renegade lightning strike can`t entirely be ruled
out.

With this forecast package, have limited the spatial extent
of snow wording into Sunday with surface wetbulb temperatures
largely expected to remain in the mid 30s and above. Thermal
profiles look like they`ll get close to supporting a changeover to
snow through Sunday morning across northwest Illinois, but by
that point forecast soundings show a rapid loss of deeper
saturation as the upper trough axis swings east of the region.
Eventually, precipitation may transition to spotty drizzle as
saturation above 750 mb is lost entirely through Sunday afternoon.

Most midrange guidance depicts another shortwave following
quickly on the heels of the Sunday system arriving somewhere in
the vicinity of central or northern Illinois Sunday night/Monday
morning. This could be a feature to monitor a little more closely
since thermal profiles with its arrival will be a bit cooler and
more conducive for supporting snow. In addition, while there`s a
fairly large N/S spread in its track across the guidance suite,
very cold H5 temps near -30 C will yield a reservoir of more
meaningful instability aloft. At this point, the multi-model
consensus seems to favor a track perhaps just south of our
forecast area.

Another robust disturbance will then approach the area on Monday
with renewed precip chances Monday night through Tuesday. Once
again, marginal thermal profiles suggest we`ll be right on the
cusp of liquid vs. frozen p-types in the region. Did not make
adjustments to the NBM-delivered p-type output for this period as
a result, which yields just rain at this point. Strong ascent
within the initial WAA-wing Monday night might support a burst of
snow, but it remains unclear if initial sub-cloud dry air will be
eroded in time. Otherwise, after a brief period of lake effect
precipitation into NW Indiana on Tuesday and Tuesday night, dry
conditions are advertised through Thursday with warmer
temperatures.

Carlaw

&&

.AVIATION...
For the 12Z TAFs...

Aviation weather concerns are:

* IFR to LIFR cigs today and tonight
* Periods of rain through tonight
* Rain/snow mix at RFD mainly this evening, perhaps changing
  briefly to all snow

Widespread rain will persist across all of the terminals this
morning with MVFR/IFR vsbys and cigs. Have recently noted a
gradual improvement in cigs from near MDW, southeast towards GYY
and parts of the Kankakee River Valley locales. Plausible that
cigs temporarily improve over the next few hours as this region
lifts northward. However, expectation is for largely IFR cigs to
prevail today. Precipitation will trend a bit more towards
intermittent showers and drizzle this afternoon.

Additional waves of showers will expand later this afternoon and
evening. Cigs should gradually build down, with the greatest
chances for LIFR cigs of the TAF period this evening and
overnight. Potential even exists for VLIFR cigs/vsbys, but the
latest signal for conditions this low is from about GYY and
southward.

Thermal profiles will become more favorable for snow mixing in at
RFD late this afternoon and evening. Latest guidance suggests a
period of all snow will be possible as well, but this may be
short-lived as deeper saturation will be lost through the late
evening/overnight hours. With above-freezing temperatures
forecast, only very slushy accums on grassy and very elevated
surfaces expected.

Precipitation will gradually end area-wide tonight/Saturday
morning, although some lingering pockets of drizzle will likely
remain.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Winthrop Harbor to Wilmette Harbor IL
     until 10 PM Friday.

&&

$$

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