Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 051153

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
553 AM CST Tue Dec 5 2023

Issued at 256 AM CST Tue Dec 5 2023

Through Wednesday...

Key Messages:

* Snow this morning with slushy accums mainly on grassy and the
  coldest surfaces.

* Transition to drizzle in spots early this morning, although
  spatial extent of any freezing drizzle will remain low and of
  minimal impact owing to temperatures at or above freezing.

* Lingering rain/drizzle through late-morning/early-afternoon

* Scattered lake effect precip (mainly rain) develops later this
  afternoon and tonight.

Have not made significant changes to the inherited forecast for
today. A robust but narrow wing of warm advection-driven
precipitation continues to press across the region early this
morning. The attendant dryslot is also rapidly surging eastward,
with the window for deeper saturation only about 4 hours or less
in any one location. With the entire system gradually
pivoting/acquiring a more southeastward motion, precip will end
quickly south of the Kankakee River this morning, while lingering
through the balance of the morning farther to the north.

Precip/snowfall rates are currently in line with earlier
expectations. As the aforementioned dryslot impinges on our
region through daybreak, a narrow corridor of somewhat enhanced
rates may set up on the steepened lapse rate/deeper moisture
interface, but this continues to look like a tenuous relationship
with a steady loss of deeper saturation area-wide through the
morning hours. This in concert with slowly-rising temperatures
will lead to a transition to poor-quality snow and/or drizzle with
time. With air temperatures at or above freezing, no significant
roadway impacts are expected. Rain looks to be the main p-type for
locales south of a Streator to Rensselaer line.

Starting to see more signs of fog development across eastern Iowa
where the low-level flow is easing. Will continue to monitor this
progression, with some mention of fog possibly needed for later
this morning, particularly south of I-80 as the surface low
trundles overhead.

This afternoon and evening: spotty lake effect precipitation is
expected to develop. The parameter space is pretty modest although
equilibrium levels are forecast to briefly peak near 7 kft. This
should be enough to support disorganized showery lake effect,
initially focusing into NE Illinois before pivoting into NW
Indiana as winds veer through the evening. Looking like
predominantly rain as the p-type, although some mixing or
ephemeral changeovers to all snow are possible this evening under
more convective/deeper activity.

The cloud cover--and hence temperature--forecast is a bit unclear
tonight and into Wednesday. Mix of signals across the guidance
suite from completely scouring stratus out to expansive overcast
conditions lingering through Wednesday afternoon. Suspect that
while we might start to poke holes in things tonight that we`ll
mix back into a strato-cu deck on Wednesday although this is lower
confidence. Another stout upper wave will traverse the region
late in the afternoon which will chuck some increasing mid and
upper cloud cover our way. Currently, looks like the deepest
saturation will remain just north of the region so no precip
chances at this time with this warm advection wing.



Issued at 256 AM CST Tue Dec 5 2023

Wednesday night through Monday...

Key Messages:

* Unseasonably warm temperatures continue to look likely Thursday
  and Friday where highs could flirt with 60 degrees especially on

* Breezy southwest winds develop Thursday into Friday with gusts
  upwards of 25 to 30 mph, though higher gusts are possible.

* Active weather returns looks to return to the area this weekend
  into early next week.

An amplified upper ridge is expected to in place across the
central CONUS at the start of the period and be building into the
Great Lakes during the day on Thursday. As the ridge moves in,
southwesterly flow will develop and advect in much warmer air not
just at the surface but in the mid-levels as well. Guidance
continues to show that 850 mb temperatures of 6 to 10 C will move
overhead which in combination with mostly clear skies should
generate unseasonably warm temperatures for both Thursday and
Friday. Thursday is forecast to be the slightly cooler day of the
two despite the ridge axis being virtually on top of us with highs
in the low to mid-50s, but the strengthening warm advection
should allow temperatures to flirt with the upper 50s and possibly
some 60s Friday afternoon. While our current forecast is below
the daily records for both Chicago (64 for both Thursday and
Friday) and Rockford (64 for Thursday and 60 for Friday) it is not
entirely out of the realm of possibilities that we could get
close especially if temperatures over perform on Friday.

While we are enjoying the warmth, a pair of shortwave troughs are
forecast to be working their way on shore in the Pacific
Northwest. The first of these waves looks to track along the
northern periphery of the ridge as it moves eastward across the
far northern CONUS and southern Canada. As this occurs, guidance
suggests that a rather deep surface low should develop across the
Upper Midwest and northern Great Lakes late Thursday into Friday
which should generate a stout pressure gradient across the rest of
the Great Lakes region. Thus, breezy southwest winds are expected
to materialize late Thursday through the day on Friday where
gusts of 25 to 30 mph look like a safe bet. Though, forecast
soundings do show that a strong low-level jet should be overhead
on Friday that could be tapped into if diurnal mixing is deep
enough which in turn may allow for stronger gusts to be realized.

As for the second shortwave, guidance continues to forecast it
deepening into a broad trough as a 120 kt jet pours into its base.
Continued intensification is forecast as the trough ejects into
the central CONUS on Friday and eventually into the Great Lakes
over the upcoming weekend. In doing so, the trough should overrun
a surface frontal boundary that will be advancing eastward across
the southern Great Lakes during this time which looks to serve as
the breading ground for precipitation and potentially a more
organized storm system. Where exactly where this front will be
when this occurs remains very uncertain among guidance and thus
not much can be said for the coverage and/or type of precipitation
expected locally. Therefore, we will have to keep an eye on this
system through the week as trends evolve but at the very least we
should expect a more active period and seasonable temperatures to
make a return this weekend into next week.



For the 12Z TAFs...

Forecast concerns for the terminals...

* Snow and resultant IFR ceilings/visibilities will continue
  through the morning before transitioning to drizzle this
  afternoon. LIFR conditions possible under heavier snow bands

* Lake effect showers expected to develop this afternoon primarily
  for northwest Indiana

* Gradually improving ceilings overnight into Wednesday morning

An area of snow continues to pivot eastward across northeast
Illinois and northwest Indiana this morning resulting in IFR
ceilings and visibilities at the terminals. A band of heavier snow
is also being observed near LOT and is expected to move over MDW
and GYY by 12z and 13z respectively and produce a brief period of
LIFR ceilings and visibilities around 3/4 SM. Snow is expected to
persist through late morning before transitioning over to drizzle
as cloud ice erodes this afternoon. While most areas will be seeing
precipitation wane by 21z this afternoon, lake effect showers are
forecast to develop around this time initially over northeast
Illinois and then pivot into northwest Indiana through the
evening. Current thinking is that showers should mainly affect
GYY, but there is chance that showers could briefly move over MDW
prior to 00z. Given that this is still lower confidence and the
already busy TAFs, I have decided to maintain the VCSH at MDW for
now but may need to consider a TEMPO with future updates.

Lake effect showers are expected to pivot into northern Indiana by
midnight which will leave our area dry through the remainder of
the forecast period. As this occurs, ceilings should gradually
improve to MVFR this evening and continue to lift towards VFR
Wednesday morning. However, there is still a signal that
lingering moisture could maintain high-end MVFR ceilings through
the day on Wednesday thus have kept ceilings around 030 at the
30-hour TAF sites. Otherwise, winds will remain light through the
period with speeds generally under 10 kts, but directions will go
from SE to NE to NW before finally settling into a westerly
direction Wednesday morning.





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