Winter Storm in Illinois, December 22, 2022

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
539 AM CST Thu Dec 22 2022

.KEY MESSAGES...

1) The forecast for the winter storm remains on track. A very rare
combination of accumulating snowfall (generally 1-3") combined
with very strong northwest winds (20-30 mph sustained, 35- 45 mph
gusts) are expected to yield sporadic near-blizzard conditions
during the heaviest snowfall on Thursday. Travel will be extremely
hazardous if not impossible during this time due to the falling
(and blowing) snow. The bottom line is while snowfall amounts are
not too substantial, the focus is on the high impacts to travel
and the extremely dangerous cold/wind chills.

2) Minimum wind chills Thursday night into Friday morning are
forecast to be in the -25 to - 35F range. For reference, these
numbers would approach the top 10 coldest observed at all three
of our official climate stations (KSTL, KCOU, KUIN). Temperature
wise, widespread below zero lows are forecast Friday morning and
should be the coldest December temperatures seen in our area since
December 23, 1989 (though about 10 degrees warmer).

.SHORT TERM...  (Through Late This Afternoon)
Issued at 401 AM CST Thu Dec 22 2022

The much anticipated intense cold front is moving through northwestern
Missouri early this morning. At upstream observation sites, the
arrival of the front resulted in initially light northwesterly
winds, before gusting to around to 30-40 mph about an hour later.
A combination of very light rain or freezing rain has also been
observed at several locations in the first few hours after the
frontal passage. More intense snowfall is occurring roughly 100
miles behind the front, with widespread observations of moderate
snow, gusty winds, and sub-one mile visibilities.

The front will continue to surge to the southeast today, reaching NE
MO around 3am, the St. Louis metro around 8am, and southern Illinois
around noon. Like in the upstream observations, I anticipate the
gusty winds and light precipitation to lag the surface front by an
hour or so, before the heavier snow arrives an hour or two after
that. Virtually all guidance keeps QPF between 0.15 and 0.25" across
the CWA, which is very similar to previous forecasts. Snow-to-liquid
ratios remain a bit of a question mark due to rapid changes in
dendritic growth zone depth and saturation, the level of the
strongest lift, and the effect of the strong winds on dendrites.
Nevertheless, I think we'll see at least an hour or two where
everything lines up to support SLRs above climatology (13-14:1),
yielding instantaneous snowfall rates up to 1" per hour and snow
totals in the 1-3" range across the forecast area.

The light to moderate snow and strong winds are still expected to
result in at least sporadic near-blizzard conditions, along with
reduced visibilites in blowing snow. There is also some flash-freeze
potential for any areas that see light rain or melting snow prior to
the arrival of the very cold air behind the front. This combination
is still anticipated to result in dangerous travel conditions across
the area, especially on untreated or congested roadways. Therefore
the Winter Storm Warning will remain in place, ending at midnight
tonight.

BSH

.LONG TERM...  (Tonight through Wednesday)
Issued at 401 AM CST Thu Dec 22 2022

Extremely cold temperatures on strong northwest winds will chase the
snow out of the region tonight. If today's snow can remain light and
porous after after reaching the ground, we may see continued
reduced visibilites from blowing snow. This may necessitate a
Winter Weather Advisory after the Winter Storm Warning ends, but
confidence in these conditions are low at this time. I am however
confident that overnight lows will plunge below 0F area-wide, with
some guidance highlighting the potential for -10F toward NE MO.
The extreme cold and strong winds will drive wind chills into the
-35F to -25F range area wide, which more than justifies the Wind
Chill Warning currently in place.

Northwest winds will remain strong on Friday in the wake of
Thursday's cyclone, with gusts in the 35-45 mph range. These winds
will continue to pump cold air into the region, limiting afternoon
highs to the low-to-mid single digits. Winds will begin to abate on
Friday night as the low moves further away, limiting additional
cold air advection. Lingering clouds are also expected to keep
temperatures from falling much overnight, though we're again
expecting lows near 0 on Saturday morning. Slightly warmer temperatures
and lighter winds will result in wind chills in the -25F to -15F
range. This is borderline Wind Chill Warning conditions in our
north and likely Wind Chill Advisory conditions across the rest of
the area. We'll very likely need an advisory south of Saturday
morning's warning, but will hold off for now so we can sort out
the ongoing headlines after the snow ends.

Temperatures will begin to moderate on Saturday and Sunday as a
surface high moves over the area. That said, we'll remain below NW
flow aloft and it looks like we'll see a series of shortwaves move
through the area over the weekend and into next week. Fortunately,
the mid-Mississippi Valley will be fairly moisture-starved through
much of this period, limiting precipitation chances. The exception
will be on Sunday night or Monday morning. Even then, the chance
for measurable precipitation is only around 30%, with a near-zero
chance for anything more that 0.1". Temperatures are expected to
further moderate next week after Monday's system moves out of the
area.

BSH

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 530 AM CST Thu Dec 22 2022

The forecast remains on track for a very strong cold front to move
through the region today. Confidence is high that the front will
initially turn winds to the northwest, before becoming gusty
shortly thereafter. Light snow or drizzle will begin within a few
hours after the fropa, before increasing in intensity. Upstream
observations show a wide swath of of 1 to 1/2 SM visbys and 400 to
500 foot cigs beneath the heaviest snow, and it appears like that
similar conditions will impact all terminals later today.

The heaviest snow will not last more than 3-4 hours at a given
location, but very strong northwest winds may result in blowing
snow and reduced visibilites for several hours after the snow
ends. I'm not confident on whether the BLSN will occur or how low
the visbys may go, so for now, have just gone with MVFR conditions
for a few hours after the snow ends. This may need to be adjusted
as additional observation come in.

BSH

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Storm Warning until midnight CST tonight for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln
     MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-
     Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Shelby MO-Warren MO-Washington MO.

     Wind Chill Warning from 3 PM this afternoon to noon CST Saturday
     for Audrain MO-Boone MO-Callaway MO-Knox MO-Lewis MO-
     Lincoln MO-Marion MO-Monroe MO-Montgomery MO-Pike MO-Ralls
     MO-Shelby MO-Warren MO.

     Wind Chill Warning from 6 PM this evening to noon CST Friday for
     Cole MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-
     Jefferson MO-Madison MO-Moniteau MO-Osage MO-Reynolds MO-
     Saint Charles MO-Saint Francois MO-Saint Louis City MO-
     Saint Louis MO-Sainte Genevieve MO-Washington MO.

IL...Winter Storm Warning until midnight CST tonight for Adams IL-
     Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-
     Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Pike IL-Randolph IL-Saint Clair-Washington IL.

     Wind Chill Warning from 3 PM this afternoon to noon CST Saturday
     for Adams IL-Brown IL-Calhoun IL-Greene IL-Jersey IL-
     Macoupin IL-Montgomery IL-Pike IL.

     Wind Chill Warning from 6 PM this evening to noon CST Friday for
     Bond IL-Clinton IL-Fayette IL-Madison IL-Marion IL-Monroe
     IL-Randolph IL-Saint Clair-Washington IL.

&&

WFO LSX

Hazardous Weather Outlook
National Weather Service St Louis MO
409 AM CST Thu Dec 22 2022

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-231015-
Greene IL-Macoupin IL-Montgomery IL-Bond IL-Fayette IL-Clinton IL-
Marion IL-Washington IL-Randolph IL-Adams IL-Brown IL-Pike IL-
Calhoun IL-Jersey IL-Madison IL-Saint Clair-Monroe IL-Knox MO-
Lewis MO-Shelby MO-Marion MO-Monroe MO-Ralls MO-Pike MO-Boone MO-
Audrain MO-Moniteau MO-Cole MO-Osage MO-Callaway MO-Montgomery MO-
Lincoln MO-Gasconade MO-Warren MO-Saint Charles MO-Franklin MO-
Saint Louis MO-Saint Louis City MO-Jefferson MO-Crawford MO-
Washington MO-Saint Francois MO-Sainte Genevieve MO-Iron MO-
Madison MO-Reynolds MO-
409 AM CST Thu Dec 22 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 strong winter storm with accumulating snow will move through the
area today. Strong winds combined with falling snow may result in
blowing snow and near blizzard conditions. Please see the Winter
Storm Warning product for more information.

Very cold temperatures and strong winds will result in dangerously
cold wind chill values beginning late tonight. Please
see the Wind Chill Warning product for more information.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Very cold temperatures and strong winds will last through Saturday
morning, resulting in dangerously cold wind chill values. Please see
the Wind Chill Warning product for more information.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

BSH