Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 050511

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1111 PM CST Mon Dec 4 2023

Issued at 230 PM CST Mon Dec 4 2023

Through Tuesday night...

Key Messages:

- Widespread snow overnight into Tuesday morning before
  transitioning to rain/drizzle by the afternoon.

- Snow totals around 1-1.5" on mostly grassy surfaces, with a
  chance (30%) for a narrow corridor of amounts to around 2".

- Minor travel impacts from slushy spots on roadways and low
  visibility during the morning commute Tuesday. Temperatures
  expected to be at or above freezing.

A stacked low/wave through the troposphere combined with
isentropic ascent and a transient f-gen signal, primarily in the
800-600 hPa layer, will provide ample forcing for a widespread
QPF event late this evening through Tuesday morning. While
moisture will not be all that anomalous for this event, the
arrival of a narrow ribbon of Pacific moisture feeding into the
low will be well-timed to maximize precip efficiency across the
forecast area late tonight through daybreak Tuesday. CAM and
ensemble guidance has displayed an overwhelming increase in QPF
over the past 24 hours, with consensus QPF amounts nearly doubling
from earlier runs to around 0.2". Thermo profiles will support
primarily all snow for most of the area overnight into mid-
morning before slight diurnal warming transitions precip over to
light rain or drizzle by late morning. Low-level temps will be
quite marginal (at or above freezing at the surface) for more
widespread impacts from snow, but locally modest rates and a
favorable diurnal period may support some slushy or slippery spots
on less-traveled surfaces.

Overall, the latest forecast is for around 1-1.5" of slushy snow
on grassy surfaces overnight into mid-morning Tuesday. Amounts
will be greatly reduced along Lake Michigan and into Chicago as
winds begin backing onshore Tuesday morning. One exception for
higher amounts up to around 2" will be if the f-gen signal noted
above becomes more focused from the northwest CWA toward the
southern Chicago metro just north of the low-level low track
around daybreak.

The passing mid to upper-level low will result in persistent deep
stratus and light rain or areas of drizzle through Tuesday
afternoon before precip gradually wanes with a strengthening
inversion through Tuesday night. Meanwhile, 850hPa-SST delta temps
will become marginal for lake effect production with steeper low-
level lapse rates late Tuesday afternoon and evening into far
northwest Indiana and perhaps as far west as the Chicago
shoreline. Precip types will favor rain, but may transition to a
mix of snow well inland across northwest Indiana before precip
shifts east and ends overnight.



Issued at 230 PM CST Mon Dec 4 2023

Wednesday through Monday...

Key Messages:
* Unseasonably mild temperatures expected Thursday and Friday
* Weekend forecast will need to be monitored as a potentially
  vigorous storm system could impact portions of the area

Wednesday will be a day of transition as the trough moves east of
the region and heights rapidly rise as mid-upper level ridging
builds east toward the region. Strong warm air advection aloft
should result in fairly extensive mid-high level cloudiness on
Wednesday, so didn`t make any upward adjustments to NBM temps
despite some recent tendencies to be running a bit on the cool

Thursday still looks to be unseasonably warm with gusty
southwesterly winds and partly to mostly sunny skies. NBM temps
trended down a couple degrees from yesterday, however the NAM and
it`s substantial 2m temp cold bias due to phantom snow cover is
part of the NBM now. Continued to trend a good 5F+ above NBM highs
for Thursday with mid to perhaps upper 50s expected.

Thursday night into Friday, modified Gulf of Mexico moisture is
progged to overspread the area, which is often accompanied by
stratus this time of year. The progged 925mb temps are a couple
degrees Celsius warmer Friday vs Thursday, but there are questions
about how much cloud cover will impact high temps. After
collaboration with neighboring offices, we did adjust Friday`s
high temps up slightly, but if stratus is less extensive than
progged then 60F would certainly be attainable.

There are uncertainties abound with next weekend`s forecast.
Cooler temperatures are expected as the upper ridge is beat down
by a shortwave trough passing north of the region. This will allow
a cold front to move across the area and then likely stall south
of the area over the weekend. Operational medium range models
continue to eject a vigorous southern stream shortwave
northeastward toward the region this weekend. There is significant
spread in guidance in timing and track of this system, with the
12z GFS a full 24 hours faster than the 12z ECMWF. Also worth
noting that models show favorable interaction and some phasing
with this trough and the northern stream jet. Guidance is
notorious for being too aggressive in depicting these types of
scenarios which can often result in large forecast errors. At this
point, made no changes to the NBM, but the forecast for this
weekend will need to be monitored the next few days as there is a
threat for an impactful cyclone affecting the region.

- Izzi


For the 06Z TAFs...

Aviation concerns:

 * Snow moves in tonight and continues after daybreak with
   associated IFR vis/cigs (potentially LIFR) ending by mid-late

 * Lake effect showers develop Tuesday afternoon into early

An area of snow is beginning to move across northwest Illinois
late this evening and will continue to shift east the next couple
of hours. Ceilings and visibility will accordingly steadily
decrease to IFR as the column saturates, earliest at RFD. Snow
is expected to persist after daybreak as the system pivots over
northern Illinois. Continue to hold off on a formal LIFR mention
in the TAFs due to lower confidence on timing and placement of a
locally heavier snow band expected to set up after daybreak
Tuesday morning, currently favored to be just south of the
terminals. If confidence increases in such a band impacting the
terminals, a targeted TEMPO for reduced vis would become
warranted. Forecast snowfall amounts are generally up to around
half an inch for the Chicago area terminals and up to around an
inch at RFD.

Snow will begin to taper from west to east mid-late morning,
transitioning to a light rain and/or drizzle. While RFD/DPA mainly
clear out by early afternoon, isolated to scattered rain/snow
showers develop near Lake Michigan that could persist into the early
evening, especially at GYY. This is accounted for with VCSH at
ORD/MDW and -SHRA at GYY (where confidence is highest) where
snow may mixed in at times. Ceilings remain IFR to low-end MVFR
through Tuesday afternoon, then gradually lifting in the evening.

Winds will be light overnight through Tuesday morning as winds back
from SW to SE to NE by late morning. Winds pick up out of the N to
NNW in the wake of the system in the afternoon around 10 kts.





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