Sleet in Illinois, January 1 to 2, 2022

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
527 AM CST Sat Jan 1 2022

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 340 AM CST Sat Jan 1 2022

***First Significant Winter Storm for Parts of Area Today***

A shortwave trough was evident on water vapor imagery early this
morning across west Texas. This feature is expected to move
northeast through the day today, helping spread precipitation
northeastward well poleward of the departing surface cold front.
Precipitation types will be an issue, with a fairly sharp
changeover (from northwest to southeast) between mostly snow to
sleet and then freezing rain. Deterministic guidance has come into
better agreement with respect to thermal profiles. The NAM had
been a bit of a strong outlier in terms of the warm nose aloft,
but has come more in line with the GFS/ECMWF and rapid update
models including the HRRR. Mostly snow is favored to the left
(i.e., northwest) of the track of the 850-hPa low track which
includes Knox/Lewis and northwestern Shelby counties. Slightly
further southeast, more sleet will mix in across locations such as
Monroe (MO), Marion (MO), and Adams. More freezing rain is
favored from Columbia, MO to Pittsfield, IL as it will take more
time for the cold layer to become sufficiently deep/strong enough
for hydrometeors to refreeze. For this afternoon, a gradual
transition from a wintry mix to snow should occur from northwest
to southeast as the 850-hPa wave slides off to the northeast. For
the remainder of the area, mostly rain is favored during much of
the day today, with a transition to freezing rain once
temperatures fall below 32F late this afternoon/evening. A brief
period of sleet then should occur, followed by some light snow
overnight tonight. This lingering light snow overnight will be due
to a secondary shortwave trough moving toward the mid-Mississippi
Valley in conjunction with weak midlevel frontogenesis.
Deterministic guidance is more uncertain with the coverage and the
exact location of this area of light snow, but generally favors
areas along/northwest of I-44/I-70 late tonight, followed by parts
of southeast Missouri and southwest Illinois early Sunday
morning.

In terms of amounts, the main change was to add about an inch or two
of snow in/around Knox/Lewis counties where precipitation should
fall predominantly as snow. Snowfall totals in this area of 4-7" are
expected. From roughly Paris, MO to Mount Sterling, IL: about 2-4"
of snow (including up to a half inch of sleet) is forecast along
with a light glaze of ice. More freezing rain is expected once again
in the KCOU>>KPPQ axis, with 0.10-0.20" of ice forecast along with 1-
2" of snow/sleet. Further southeast, anywhere from a light glaze to
as much as 0.10" of ice is likely along with a dusting to a half
inch of snow/sleet.

The only changes made to the winter weather advisory and winter
storm warning was to add a few counties to the advisory where
light freezing rain/drizzle is expected to fall for at least a
couple of hours with air temperatures below 30 degrees, and to
extend in time to account for the lingering light snow overnight.
The counties that were added included Franklin/St. Charles
counties in Missouri and Greene in Illinois. I cannot rule out
another expansion further to the southeast potentially being
warranted, but confidence in travel impacts was lower because air
temperatures are likely to fall below 32F toward end of the
period of light freezing rain/drizzle. In addition, some guidance
shows more of a transitory period of freezing rain/drizzle with a
transition to sleet and then light snow. If this occurs, impacts
to travel/roadways would likely be less than if freezing rain is
the more persistent precipitation type.

After the conclusion of any accumulating wintry precipitation late
tonight and perhaps into early Sunday morning, the focus is on the
very cold air mass infiltrating the region. Low temperatures tonight
are forecast to drop to near zero in parts of northeast Missouri and
west-central Illinois. These temperatures combined with sustained
northwest winds of 10-15 mph are expected to yield wind chills of -
15 to -20F. As a result, went ahead and issued a wind chill advisory
from late tonight through much of Sunday morning for
Knox/Lewis/Shelby counties.

High temperatures on Sunday, even with decreasing clouds, are only
expected to reach the low teens to upper 20s from northwest to
southeast. These readings would be about 10 to 20 degrees below
normal for early January. This will be a marked change from much of
the anomalous warmth we experienced during December of 2021.

In terms of actual air temperatures, lows Sunday night should
actually be colder than tonight due to a clear sky and light winds
in/around a seasonably strong (near 90th percentile) surface
anticyclone in eastern Missouri 1200 UTC Monday. Coldest locations
will be in those portions of northeast Missouri and west-central
Illinois which should have a sufficiently deep snow cover (>2") to
enhance the radiational cooling. Lows of 0 to -5F are likely in
these locations, with single digits and low teens further to the
south and east. With little/to wind anticipated due to the surface
high nearby, a subsequent wind chill advisory appears unlikely
Sunday night into Monday morning.

Gosselin

.LONG TERM...  (Sunday Night through Friday)
Issued at 340 AM CST Sat Jan 1 2022

(Monday - Wednesday)

After a very cold start to the week, a quick warmup is anticipated
through Tuesday as the aforementioned surface high departs to the
east/southeast. By Tuesday, temperatures are expected to climb back
into the low to mid 40s which would be slightly above normal.

(Wednesday Night - Friday)

A shortwave trough is forecast to move through the Upper Midwest and
Great Lakes Tuesday night into Wednesday, sending a cold front
through the area. While this shortwave trough and associated cold
front should have only little impact on the bi-state area (dry
frontal passage with colder temperatures behind the front), it is a
key feature nonetheless. The reason why is because the
strength/timing of this feature will heavily influence the
upstream shortwave trough. The quicker and less amplified this
first shortwave trough, the longer the wavelength between it and
the upstream wave. This would allow for more amplification which
would produce more precipitation in the form of snowfall (see
yesterday's 12Z ECMWF). Conversely, a stronger/slower preceding
wave (similar to 0Z GFS) would not allow for any amplification and
would likely result in a weaker subsequent wave with little/to
snowfall. Looking at the GEFS/EPS, the most likely result at this
juncture is somewhere in between, with a nod a bit more toward a
lighter QPF/snow event as portrayed by the overnight run's of the
CMC/ECMWF. Probabilities for at least a tenth of an inch of liquid
on both the GEFS/EPS are about 40-50 percent at this range. We
will have to continue to watch this system over the next couple of
days, as there is at least the potential for widespread light
accumulations of snowfall in the Wednesday night/Thursday time
frame.

Regardless of whether or not it snows midweek, the next air mass
looks stronger than what is moving into the region over the next 24-
48 hours. Latest GEFS/EPS mean 850-hPa temperature anomalies are
almost -15C. High temperatures in the teens/twenties are once again
likely Thursday/Friday, with lows Thursday night near or below zero
for many areas. If snow is on the ground, well below zero lows
would become more likely and widespread.

Gosselin

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 522 AM CST Sat Jan 1 2022

IFR ceilings along with a north/northeast wind are forecast
areawide today. A variety of precipitation types are expected
across the area. For KUIN, a mix of freezing rain/sleet this
morning should changeover to a sleet/snow mix before midday.
Eventually, all snow is favored late this afternoon onward. For
KCOU/KJEF, rain should turnover to freezing rain this morning,
followed by sleet and then snow late this afternoon into early
this evening. Metro terminals will have rain most of the day, with
a changeover to freezing rain by early evening followed by sleet
and then light snow late this evening/early overnight. The most
adverse conditions will be at KUIN with a few inches of snow/sleet
and a glaze of ice. For KCOU/KJEF, 0.10-0.20" of freezing rain
accretion is forecast with up to an inch or so of snow/sleet.
Metro terminals are expected to see a light glaze of ice and up to
a half inch of snow/sleet. Precipitation should slowly exit from
northwest to southeast tonight, with dry conditions favored into
the day on Sunday. IFR ceilings are also likely to improve to MVFR
late tonight from northwest to southeast as well.

Gosselin

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory until 6 AM CST Sunday for Boone MO-
     Callaway MO-Cole MO-Franklin MO-Gasconade MO-Lincoln MO-
     Moniteau MO-Montgomery MO-Osage MO-Saint Charles MO-Warren
     MO.

     Winter Storm Warning until 3 AM CST Sunday for Audrain MO-Monroe
     MO-Pike MO-Ralls MO.

     Winter Storm Warning until midnight CST tonight for Knox MO-
     Lewis MO-Marion MO-Shelby MO.

     Wind Chill Advisory from 1 AM to 11 AM CST Sunday for Knox MO-
     Lewis MO-Shelby MO.

IL...Winter Weather Advisory until 6 AM CST Sunday for Calhoun IL-
     Greene IL-Jersey IL.

     Winter Storm Warning until 3 AM CST Sunday for Pike IL.

     Winter Storm Warning until midnight CST tonight for Adams IL-
     Brown IL.

&&

WFO LSX

Hazardous Weather Outlook
National Weather Service Saint Louis MO
356 AM CST Sat Jan 1 2022

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-021000-
Adams IL-Audrain MO-Bond IL-Boone MO-Brown IL-Calhoun IL-Callaway MO-
Clinton IL-Cole MO-Crawford MO-Fayette IL-Franklin MO-Gasconade MO-
Greene IL-Iron MO-Jefferson MO-Jersey IL-Knox MO-Lewis MO-Lincoln MO-
Macoupin IL-Madison IL-Madison MO-Marion IL-Marion MO-Moniteau MO-
Monroe IL-Monroe MO-Montgomery IL-Montgomery MO-Osage MO-Pike IL-
Pike MO-Ralls MO-Randolph IL-Reynolds MO-Saint Charles MO-
Saint Clair-Saint Francois MO-Saint Louis City MO-Saint Louis MO-
Sainte Genevieve MO-Shelby MO-Warren MO-Washington IL-Washington MO-
356 AM CST Sat Jan 1 2022

This Hazardous Weather Outlook is for portions of eastern and
central Missouri as well as west central and southwest Illinois.

.DAY ONE...Today and tonight.

A storm system will bring a combination of accumulating freezing
rain, sleet and snow to parts of the area through tonight. Please
refer to the winter weather advisory and winter storm warning for
more details.

Dangerously cold wind chills of -15 to -20F are likely late tonight
across parts of northeast Missouri. Please see the wind chill
advisory for more details.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Dangerously cold wind chills of -15 to -20F are likely to continue
Sunday morning across parts of northeast Missouri. Please see the
wind chill advisory for more details.

.SPOTTER INFORMATION STATEMENT...

Spotters are encouraged to report precipitation types, changes in
precipitation type, and amounts of snow, sleet, and freezing rain to
the National Weather Service.

Gosselin