Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 042030

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
230 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 trough moves east of the
region and heights rapidly rise as mid-upper level ridge 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 side.

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 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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