Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 010901

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
301 AM 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 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

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



For the 06Z TAFs...

Forecast concerns include...

Lifr cigs Friday/Friday night.
Periods of rain through Friday night.
Rain/snow mix at RFD Friday evening.

Main change for this forecast is to remove snow mention at RFD for
Friday morning. Much of the latest guidance is warmer with rain as
the main precip type. Rain is still expected to spread across the
terminals during the predawn hours and become moderate, perhaps
briefly heavy, after sunrise and then should quickly taper off to
light rain by late morning. Light rain or drizzle will likely
continue in the afternoon with another period of light rain Friday
evening. It`s during this time that a rain/snow mix will be
possible across northwest IL and at RFD but confidence remains low
for any possible snow accumulations. This precipitation is
expected to end by late Friday evening or very early Saturday

Mvfr cigs have become prevailing across much of the area and will
lower through low mvfr and then quickly drop into ifr as the rain
arrives. Cigs are expected to settle into lifr by mid morning and
will likely remain lifr through early Saturday morning.
Visibilities may remain in the 2-3sm for much of Friday, perhaps
improving a bit by early afternoon and then lowering again early
Friday evening.

Light and variable winds will turn light northeasterly over the
next few hours and then slowly increase in speed to 10-15kts by
daybreak as directions turn more easterly. Winds may become
southeast for several hours at MDW/GYY through mid morning, before
turning back easterly Friday afternoon. Directions will slowly
turn northeasterly Friday evening and more northerly by early
Saturday morning. cms


LM...Small Craft Advisory...Winthrop Harbor to Wilmette Harbor IL
     until 10 PM Friday.



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