Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 041822

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1222 PM CST Mon Dec 4 2023

Issued at 300 AM CST Mon Dec 4 2023

Through Tuesday...

Key Messages:

* Round of snow arrives tonight (except a rain/snow mix in our
  far south). Most accumulations look light (less than a half
  inch) and relegated to grass and the coldest surfaces.

* A chance (about 35%) for a very narrow ribbon of locally
  "higher" accums to 1-2" somewhere in the vicinity of I-80,
  although confidence on the precise system track remains
  unusually low.

* Most roads should remain just wet, but if/where enhanced rates
  set up, a dusting is possible on bridge decks.

Our feature of interest appears nicely in simple water vapor RGB
loops emanating out of a zone of intense cyclonic shear vorticity
across southern Alberta and Saskatchewan. This feature will
continue to dig and amplify today as it rapidly traverses the
northern Plains. Intense large scale forcing for ascent will
increase quickly late this evening and overnight yielding an
expanding zone of precipitation across the entire forecast area.
Wobbles in the system track continue, but think we`re seeing
guidance stabilize (finally). 00z HRRR looked like it was too far
south, with the 06z run coming more in line with the global
guidance. Have boosted PoPs to categorical across parts of the
forecast area with this in mind.

While p-types might initially start as rain, Bourgouin melting
energy output suggests a changeover to all snow will likely occur
shortly after onset north of about our Kankakee River/I-80
locales. Do think the NAM continues to look a bit too cold,
however. As is typical with these types of systems, moisture won`t
be in great supply, so still looking like a lighter event for us.
Cobb SLRs look pretty good, and suspect these will average out
around 7-9:1 or thereabouts. With this in mind, hourly snowfall
rates where things do fully changeover to snow will be less than a
quarter inch per hour. The one exception might be late
tonight/Tuesday morning immediately north of the 700 mb westerly
wind max. Ephemeral f-gen forcing might yield a very small
corridor of locally-enhanced precip rates for a brief period.
Uncertain where this will set up, with some guidance as far north
as I-90 and others as far south as a Moline to Kankakee line.

Regardless of the exact track, not really anticipating travel
impacts with air temperatures at or just above freezing. The lone
exception would be where/if aforementioned enhanced rates set up
which could yield a dusting on the very coldest surfaces such as
bridge decks yielding patchy slick travel.

Precip will wane through Tuesday morning, although soundings look
conducive for continue spits of drizzle/poor-quality snow even
into the afternoon.



Issued at 300 AM CST Mon Dec 4 2023

Tuesday night through Sunday...

Key Messages:

* Lake effect rain showers in northwest Indiana Tuesday night into
  early Wednesday morning

* Dry and unseasonably warm conditions likely to develop towards
  the second half of the week

* Active weather pattern and more seasonable temperatures return
  next weekend into the early part of next week

A broad upper trough is forecast to be centered over the Great
Lakes and much of the eastern CONUS Tuesday night, but will
getting shoved eastward during the day on Wednesday. While any
lingering precipitation from earlier in the day Tuesday should be
just about over, lingering low-level moisture and northerly fetch
down Lake Michigan will support some lake effect showers across
portions of northwest Indiana. Forecast soundings do show that the
bulk of the deep (~8000 ft) saturated layer should reside below
the DGZ which should keep the showers mainly in the form of rain,
but there does look to be a brief period Tuesday evening where
some snow flakes could mix in. Though no accumulation is expected.
The better moisture and modest lake induced instability should
begin to wane after midnight Tuesday night as winds become more
northwesterly which should in turn push the lake effect showers
east of our area into northern Indiana and southwest Lower
Michigan prior to daybreak Wednesday morning. Thus, leaving us
with dry conditions for our Wednesday but with some residual cloud
cover and seasonable temperatures.

As the aforementioned trough exits the area on Wednesday, a stout
500 mb ridge is expected to develop over the central CONUS and
advect eastwards into the Great Lakes by Thursday. This ridge will
not only aid in suppressing any cloud cover, but also generate
west-southwesterly mid-level flow which is forecast to warm 850
mb temperatures upwards of 6 to 8 C. These mid-level temperatures
are expected to translate to unseasonably warm temperatures for
Thursday and Friday where highs are forecast to be in low to
mid-50s and mid to upper 50s to near 60 respectively.
Additionally, winds look to become breezy during this period as a
deepening surface low traverses the northern edge of the ridge and
tightens the pressure gradient. Depending on how deep we are able
to mix during the afternoon hours, winds could possibly gust
upwards of 30 mph each day.

However, the unseasonable warmth is not forecast to last long as
a deepening trough begins to take hold across the western and
central CONUS during the day on Friday and push eastward into the
Great Lakes over the upcoming weekend. As the trough pushes east
it will shove a cold front through the area which will serve as
the focal point for more precipitation and seasonable temperatures
in its wake. Though, exactly where this front will be when the
trough ejects into the area remains very uncertain which will
obviously dictate precipitation coverage and type across our area.
So while it is too soon to discuss these specifics, it can be
said that more active weather is on the horizon for next weekend
into early next week.



For the 18Z TAFs...

Aviation forecast concerns for the current TAF period include:

* High MVFR ceilings this afternoon into tonight

* Snow moves in tonight and continues into tomorrow morning,
  bringing IFR ceilings and visibilities (with some chance for
  LIFR conditions as well)

* Lingering snow transitions to rain late tomorrow morning with
  scattered rain showers continuing into the afternoon at/near
  the Chicago metro terminals

Stratocumulus trapped beneath an 850 mb temperature inversion is
expected to persist this afternoon into this evening. Ceilings
should generally remain between 2500 and 3500 ft AGL during this
time frame everywhere except GYY, where they are more likely to
remain closer to 2000 ft AGL. Winds will become more westerly
behind a southward-dropping surface boundary this afternoon
before trending southwesterly tonight as an incoming clipper
system approaches.

This system will reach the area overnight, bringing snow to all of
our TAF sites, though precipitation may initially onset as rain or
a rain/snow mix before becoming all snow no later than 2 hours
after onset. Ceilings will tank to IFR relatively quickly after
the steadier snowfall begins and will likely be accompanied by
IFR visibilities as well. There is about a 30% chance at this time
that LIFR ceilings and/or visibilities will be observed at some
point at any given TAF site, but opted to hold off on introducing
a formal LIFR mention in the TAFs for now.

Most snow will taper off after daybreak tomorrow, though areas of
light precipitation will likely linger around through the
remainder of the morning, when the predominant precipitation type
will eventually transition from snow to rain with time as
temperatures rise slightly. During the afternoon, scattered rain
showers are likely to be observed near Lake Michigan, though
can`t entirely rule out some snowflakes mixing in with these
showers as well. Ceilings should remain IFR through the morning,
though some gradual improvement to MVFR is expected by the
mid/late afternoon.





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