Tornadoes and Damaging Winds in Illinois, December 10, 2021

Area Forecast Discussion
National Weather Service Lincoln IL
644 PM CST Fri Dec 10 2021

.MESOSCALE DISCUSSION...
Issued at 643 PM CST Fri Dec 10 2021

Deep low pressure is analyzed over NW MO this hour with a warm
front draped to the east to near/north of Peoria to Bloomington.
Within the warm sector, dew points have increased into the upper
50s to lower 60s which is contributing to seasonably strong
instability moving into the region. Latest RAP indicates that
500-1000 J/kg overspreading most of central Illinois in the next
few hours. Currently monitoring a broken line of storms moving
across the Mississippi River. Strong winds aloft will result in
bulk shear in excess of 60kt much of which is falling in the
lowest 2-3 km. Backed surface flow, very strong effective SRH, and
low LCLs of 500-750m all point to a favorable tornado environment
over central Illinois. Do anticipate that additional weather
watches will be needed downstream of Tornado Watch 553 which
remains in effect until 11 pm this evening.

&&

.SYNOPSIS...
Issued at 255 PM CST Fri Dec 10 2021

A strong low pressure system is moving across the Midwest. Across
central Illinois, fog, light rain, and scattered showers are
expected into the evening. Late tonight into the overnight, severe
thunderstorms are expected as a cold front moves through the
area, with damaging winds and tornadoes the primary hazards.
Behind the cold front, strong winds gusting up to 50 mph are
expected.

&&

.SHORT TERM...(This evening through Saturday)
ISSUED AT 255 PM CST Fri Dec 10 2021

A potent mid-latitude cyclone is bringing widespread impactful
weather to the central CONUS. The surface low pressure system was
located near Kansas City at 20z/2pm, with a cold front extending
southwest through Kansas into western Oklahoma and a warm front
extending eastward through northern Missouri and central Illinois.
There are several areas of focus with this system including (1)
continuing widespread fog, (2) non-severe precipitation Friday
afternoon, (3) severe weather Friday night, and (4) strong
synoptic winds behind the cold front.

(1) Dense fog developed east of the Illinois river this morning,
and has been slow to dissipate. While visibilities have gradually
improved, nearly all of central Illinois remains socked in with
fog this afternoon. Visibilities are generally 1/2 to 2 miles, but
a few areas of patchy fog remain. Gradual improvement should
continue into the evening.

(2) A couple areas of non-severe precipitation are ongoing across
central Illinois this afternoon. Elevated showers developed north
of the warm front, near the I-74 corridor as of 2 PM, at the
leading edge of a 700-mb jet. Further south, isentropic ascent has
resulted in a band of light rain. Both of these precipitation
zones will continue to lift northeast through central Illinois
over the next few hours.

(3) As warm air and moisture continue to advect northward, a
concerning severe weather setup will exist across the warm sector
into the overnight hours. This system is quite dynamic, with an
expansive, strong wind field above the warm sector, including a
widespread area of 50-60 knot SSW winds at 850 mb. With southerly
winds at the surface, there will be clockwise turning of the wind
with height. Model soundings indicate deep layer shear in excess
of 70 knots, with effective SRH of 200-300 m2/s2 and 0-1 km shear
of 35-40 knots. These wind profiles are capable of supporting
organized, rotating convection, and when combined with a thermal
profile that appears increasingly favorable (more on that below)
damaging winds and tornadoes will be possible.

As noted in this morning's AFD update, the thermal profiles have
trended more favorable. Dewpoints near 60 are expected to advect
as far north as the I-74 corridor, with 1000 J/kg of MUCAPE
possible southward from there. In addition to moisture advecting
further northward, the trend has been towards steeper mid-level
lapse rates. While damaging winds and tornadoes remain the primary
hazards, the hail threat should not be ignored. Forecast values
of the significant hail parameter have trended upward, from less
than 1 in previous model runs to 1-2 in the most recent guidance.

In terms of event evolution, into the evening hours model
soundings show an elevated mixed layer/capping inversion atop a
very saturated low-level layer. The capping should help limit
surface-based thunderstorm development across central Illinois
through the evening. Into the overnight hours, strong warm air
advection will keep temperatures in the 60s across the warm
sector. Soundings show the cap eroding across central Illinois
between 02-05z (8pm- 11pm), at which point surface-based
convection will be able to develop and pose a threat of all severe
hazards. Storms may initially be discrete as they develop, but
with deep layer shear vectors nearly parallel to the cold front
(which will be the primary forcing mechanism), upscale growth is
favored, resulting in a linear or quasi-linear storm mode. The
peak severe weather threat across central Illinois will be between
02z-09z (8pm-3am). Given the dynamic nature of the system, storm
motion is expected to be quite fast (60 mph or greater). With
fast-moving, nocturnal severe thunderstorms expected, it is
imperative to have a way to receive severe weather alerts
overnight!

(4) The cold front and any associated precipitation should exit
the area before daybreak on Saturday. In the cold air advection
regime behind the front, pressure rises will help mix down strong
westerly wind gusts. Soundings from a variety of models
universally support 45+ mph gusts across much of the CWA. At
several locations, the ensemble mean peak wind gust is right at
advisory criteria (45 mph) and several ensemble members show gusts
in excess of 50 mph. The signal for gusty winds becomes a little
weaker west of the Illinois River and south of I-72, so held those
areas out of the wind advisory for now, but additional expansion
may be needed. Even if advisory criteria are not met in these
locations, wind gusts of 35-40 mph are still expected. Winds
should start to weaken by late Saturday morning, with gusts
dropping below 30 mph by mid-afternoon.

Saturday's high temperatures will occur shortly after midnight,
with temperatures falling sharply behind the cold front. Most of
the area will only manage to reach the upper 30s during the
daytime hours on Saturday. Combined with the gusty winds, it will
feel quite cold on Saturday.

.LONG TERM...(Sunday through Friday)
ISSUED AT 255 PM CST Fri Dec 10 2021

As our impactful low-pressure system departs, surface high
pressure establishes over the southeastern US late this weekend.
This sets the stage for a period of well-above average
temperatures as southerly flow persists for much of next week.
Temperatures warm each day Sunday through Wednesday, with
Wednesday looking like the warmest day of next week. Wednesday
high temperatures are currently forecast to be in the mid-60s to
near 70, which is 20-25 degrees above average! There are
indications that another mid-latitude cyclone will sweep a cold
front through the area next Thursday, resulting in precipitation
chances. There will be plenty of time to monitor this system as we
move into next week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 521 PM CST Fri Dec 10 2021

Deep low pressure will lift from northern MO to northern IL this
evening with a trailing cold front sweeping across central
Illinois. Rain and IFR ceilings with variable visibility will be
in place ahead of the low this evening, then a line of strong to
severe thunderstorms will move across the terminals associated
with the cold front. Behind the front, strong west to northwest
winds gusting in excess of 30 kt are expected overnight through
much of the day. Ceilings should improve back to MVFR behind the
front with possible clearing late in the day.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 3 AM to 3 PM CST Saturday for ILZ028>031-037-
038-042>046-053.

&&

MESOSCALE...Deubelbeiss
SYNOPSIS...Erwin
SHORT TERM...Erwin
LONG TERM...Erwin
AVIATION...Deubelbeiss

Hazardous Weather Outlook
National Weather Service Lincoln IL
308 PM CST Fri Dec 10 2021

ILZ027-036-040-041-047>052-111200-
Cass-Christian-Fulton-Knox-Mason-Menard-Morgan-Sangamon-Schuyler-
Scott-
308 PM CST Fri Dec 10 2021

This Hazardous Weather Outlook is for portions of central
Illinois and west central Illinois.

.DAY ONE...TONIGHT...

Strong to severe thunderstorms are possible this evening. The
primary threat will be damaging winds, though a few tornadoes
cannot be ruled out, especially in areas closer to I-72.

West winds will increase after midnight, and will be gusting over
40 mph before sunrise.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY...

West winds will gust from 40 to 45 mph into Saturday morning,
before diminishing. This will cause difficult travel for high
profile vehicles, especially on north to south oriented roadways.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is likely this evening between 7 PM and 11 PM.

ILZ028>031-037-038-042>046-053-111200-
Champaign-De Witt-Logan-Macon-Marshall-McLean-Peoria-Piatt-Stark-
Tazewell-Vermilion-Woodford-
308 PM CST Fri Dec 10 2021

...WIND ADVISORY IN EFFECT FROM 3 AM TO 3 PM CST SATURDAY...

This Hazardous Weather Outlook is for portions of central
Illinois and east central Illinois.

.DAY ONE...TONIGHT...

Strong to severe thunderstorms are possible later this evening,
continuing until midnight close to the Indiana border. Damaging
winds and a few tornadoes are likely, especially from Bloomington
to Danville south.

West winds will increase after midnight, and will be gusting to
around 45 mph before sunrise.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY...

West winds will gust from 40 to 45 mph into Saturday morning,
before diminishing. This will cause difficult travel for high
profile vehicles, especially on north to south oriented roadways.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is likely tonight. The most likely time frame
is between 8 pm and midnight.

ILZ054>057-061>063-066>068-071>073-111200-
Clark-Clay-Coles-Crawford-Cumberland-Douglas-Edgar-Effingham-
Jasper-Lawrence-Moultrie-Richland-Shelby-
308 PM CST Fri Dec 10 2021

This Hazardous Weather Outlook is for portions of east central
Illinois and southeast Illinois.

.DAY ONE...TONIGHT...

There is an enhanced risk of severe storms tonight, threat level
of 3 on a scale of 1 to 5. Damaging winds and scattered tornadoes
are the main concerns. There is potential for a few of the
tornadoes to be on the stronger side.

After the storms exit, west winds will increase and gust to around
40 mph before sunrise.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY...

West winds will gust over 40 mph through Saturday morning before
diminishing. This will cause difficult travel for high profile
vehicles, especially on north to south oriented roadways.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is likely tonight. The most likely time is
between 8 pm and 2 am.

Geelhart