Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 052328

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

Issued at 216 PM CST Tue Dec 5 2023

Key Messages:

- Light lake effect rain showers along the IL shore through early
evening shifting into northwest Indiana and mixing with snow tonight.

- Cloudy and dry conditions Wednesday and Wednesday night.

Synoptically-forced precip has ended across the CWA as the slightly
northwest-tilted upper trough begins to shift eastward. Marginally
favorable 850hPa-SST delta temps of 12-14C within weak CAA and a
developing inversion around 6-7kft has supported an increase in lake
effect rain showers from southeast Wisconsin southward into Cook
County early this afternoon. NNE low-level flow will continue to
focus the showers into the eastern half of the Chicago metro through
early evening before a veering low-level wind profile shifts a
broken band of showers eastward into northwest Indiana this evening
and overnight. While thermal profiles will remain warm enough to
support all rain into early evening, especially for areas near Lake
Michigan, the weak CAA will support rain mixing with or changing
over to all snow farther inland into northwest Indiana. With
inversion heights lowering to around 5kft by this time, precip
intensity should remain light enough to limit any accumulations to
anything but a couple tenths of an inch on grassy surfaces overnight.

A surface ridge extending from the southern Great Plains into the
mid-Mississippi River Valley tonight will drift south and east of
the CWA Wednesday through Wednesday morning, effectively shifting
any remaining lake effect showers well east of the area by sunrise.
Gradual mid-level height rises ahead of a large ridge across the
central CONUS will foster a gradually lowering subsidence inversion
late tonight through Wednesday evening. Widespread stratus
throughout the Upper Mississippi Valley will likely remain trapped
below this inversion while advecting across the area tonight and
into Wednesday afternoon, favoring a mostly cloudy to overcast day
amid dry conditions. With winds backing SW and the inversion
lowering through the residual saturated layer late in the day and
evening, existing stratus will erode from the southwest while mid
and upper-level cloud cover increases.



Issued at 216 PM CST Tue Dec 5 2023

Thursday through Tuesday...

Key Messages...

- Unseasonably warm temperatures Thursday, Friday and likely
  Saturday for part of the area, with highs well into the 50s.

- Breezy southwest winds Thursday through Friday with gusts into
  the 30 mph range.

- A strong storm system will affect the region this weekend. There
  are some increased chances (10%) of severe weather southeast of
  a Paxton to Valparaiso line. This system may also bring snow to
  areas northwest of Interstate 57 but uncertainty on snow
  occurrence and location is still very high 4 days out.

Overall, no significant changes to the forecast for Thursday into
Friday night. Southwest winds will be gusting into the 30 mph
range which will allow much warmer air to spread across the
region. Highs on Thursday are expected to be in the lower/mid 50s
and by Friday, highs in the upper 50s are expected with a few
places possibly tagging 60. For the most part, these expected
highs are below the daily records. See climate section below for
specific high temps.

While there still remains quite a bit of uncertainty for the storm
system this weekend, there are some trends in the models and their
ensembles that are becoming more clear. Just about every ensemble
member is now showing qpf across the local area, adding confidence
for precipitation occurring. The extent of the colder air has
always looked marginal, meaning any changeover to snow, at least
across the local area, would most likely be driven by dynamic
cooling which needs several variables to all come together to
occur and produce snow. There is now a notable shift to the
northwest with the track of this system and one of the main
players for how all this evolves will likely be the expansive
high pressure over the southeast U.S. and how fast or sluggish it
is as it departs over the Atlantic Friday night into Saturday.

A track of the surface low across the cwa, as currently shown by
all the operational models would support another warm day south
of the warm front, with highs possibly pushing 60 in the southeast
cwa, while across the northwest, temps may only reach the lower
40s and then possibly fall during the day on Saturday. This track
of the surface low would be concerning for the potential for
severe weather across the southeast cwa and have added thunder
mention for the southeast cwa based on these trends.

There still is a snow potential on the northwest side of this
system, wherever that ends up going. And given the uncertainty of
its location, which could also be a rather narrow swath of snow,
in addition to multiple variables coming together to produce any
heavier amounts, its simply too early for specifics. The weather
grids from the blended guidance transition to a rain/snow mix
Sunday morning with snow for Sunday afternoon, if precipitation is
still occurring at that time. A period of stronger winds is also
possible Saturday night into Sunday, which will be dependent on
the strength and track of the surface low. Much colder temps,
relative to late this week, are expected early next week, but will
be close to seasonal normals for mid December. cms


Issued at 216 PM CST Tue Dec 5 2023

Unseasonably warm temperatures are expected Thursday through
Saturday and here are the record high temps for Rockford and

Thursday, December 7th...64 in 1916
Friday, December 8th.....60 in 1980
Saturday, December 9th...55 in 2020

Thursday, December 7th...64 in 1916
Friday, December 8th.....64 in 1946
Saturday, December 9th...62 in 1879

The record warmest low temperatures for the calendar day, December
8th are,

Rockford...37 in 1987

Chicago....53 in 1946


For the 00Z TAFs...

Aviation Forecast Concerns:

* Lake effect rain showers at KORD/KMDW/KGYY early-mid evening.

* Chicago metro terminals likely see period of VFR conditions
  overnight, with MVFR ceilings returning Wednesday morning.

* Breezy southwest winds with gusts 20+ kts develop by Wednesday
  evening, beneath a 45 kt low level jet.

Surface low pressure will continue to pull away to the southeast
across the central Appalachians this evening, with surface high
pressure developing across the Mississippi Valley and into the
western Great Lakes region Wednesday morning. North-northwest low
level flow behind the departing low, combined with low level cold
advection, will support a few lake-effect rain showers for KORD,
KMDW and KGYY early this evening, before winds shift more
northwest later this evening and overnight. MVFR ceiling and vis
conditions are expected with these showers.

While a large area of MVFR stratus is noted along/east of the
Mississippi River, portions of eastern WI and northeast IL do look
to have a period of VFR conditions later this evening/overnight.
KRFD, farther to the west, will likely remain MVFR. MVFR
conditions will likely spread back across the Chicago terminals
Wednesday morning however, due to west-northwest low level winds.
MVFR ceilings may even gradually lower during the day, beneath a
lowering subsidence inversion within the high pressure region.
Eventually, surface winds will continue to back southwest, and
erode low clouds from west to east during the afternoon.
Confidence is somewhat low on timing the improvement from MVFR to
VFR. Some guidance indicates KORD/KMDW could go VFR as early as
18Z, while high-res HRRR/RAP indicate later improvement.

A strong (45-50 kt) southwest low level jet is expected to
develop Wednesday evening. This would typically set up LLWS
conditions, though it looks as if the surface pressure gradient
will tighten enough to produce surface wind gusts in the 20-25 kt
range. This would limit the magnitude vertical wind shear to below
TAF LLWS criteria.





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