Winter Storm and High Wind in Illinois, January 12 to 14, 2024

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1120 PM CST Thu Jan 11 2024

.UPDATE...
Issued at 955 PM CST Thu Jan 11 2024

The winter storm is now on our doorstep this evening as we
continue to monitor upstream trends. Impressive dynamics over a
large footprint across the central CONUS are becoming evident via
radar and satellite imagery with a blossoming baroclinic leaf
across the MO/mid-MS valleys and rapidly developing convection
across OK/AR. The congealing area of both clusters of upstream
convection will align well with substantial moisture transport and
deep isentropic ascent into the CWA concurrent with a contracting
low to mid-level baroclinic zone late tonight into mid-morning
Friday. A resulting impressive F-Gen signal with slowing northward
extent combined with 7C/km 700-400 hPa lapse rates yields some
concern that 1-2"/hr snowfall will become common within the precip
shield well into Friday morning. The embedded convective elements
within the precip shield (and perhaps a stray rumble of thunder
south of the Chicago metro) supports increasing forecast snowfall
totals through at least late morning Friday. The incoming 00Z CAM
suite is in surprisingly good agreement that 1hr QPF rates will
reach 0.1-0.2" per hour for several hours into the Chicago metro
between 4-9am. Even with SLR values potentially as low as 5-7:1,
snowfall totals of 3-5" of very wet snow would not be surprising
in just a few hours.

No changes to headlines were made with the updates as the bulk of
increase in the snowfall forecast is within the ongoing Winter
Storm Warnings. But we will continue to highlight the potentially
very hazardous morning commute across all of northern Illinois and
especially within the Chicago metro.

Kluber

&&

.SHORT TERM...
Issued at 300 PM CST Thu Jan 11 2024

Through Saturday...

Key Messages for the Winter Storm:

- Major winter storm (>80% confidence in warning impacts in warned
  counties) with periods of heavy, primarily wet snow and strong
  easterly winds pre-dawn Friday through Friday evening and
  lighter snow with strong westerly winds and blowing/drifting
  snow impacts lingering through at least Saturday morning.

- Winter Storm Warning issued for northern Illinois near and north
  of the Illinois and Kankakee Rivers, with Winter Weather
  Advisories south and east where confidence is lower in magnitude
  of impacts.

- Winter Weather Advisories issued for a tier of counties south of
  the Winter Storm Warning where snow is expected to change
  entirely to rain, and across our southern tier of counties where
  the combination of snow and strong westerly winds Friday night
  will lead to hazardous conditions. (In other words, all forecast
  zones will be in a winter headline Friday evening).

- A period of heavy (60-70%+ chance of >1"/hour) to briefly
  intense (30-40% chance of >2"/hour) snowfall rates is likely
  just prior to and during the Friday AM commute, centered over
  northern Illinois near/north of the Illinois and Kankakee
  Rivers, paired with easterly wind gusts to 30-35 mph.

- The next round of hazardous to dangerous travel conditions with
  1"+ hour snowfall rates will spread southeast mid Friday
  afternoon through Friday evening. Strong northwest to west winds
  gusting up to 45 to 50 mph Friday evening and night may cause
  blowing and drifting snow and very low visibility, especially on
  north to south oriented roads.

- Strong onshore winds off a still mild Lake Michigan will once
  again likely cut into if not eliminate snow accumulations near
  the immediate shore, especially late Friday morning through the
  early evening, including east of I-90/94 in Illinois, and north
  of I-80/94 in Indiana.

- After the "thump" of snow through mid Friday morning, the
  rain/snow line will push as far north as the I-80/I-88 corridor,
  limiting additional accumulations for several hours before
  changing back to snow in the afternoon and evening.

Additional Important Forecast/Potential Impacts Details:

- Similar to previous storm, the first phase through the day on
  Friday will feature low ratio (8:1 or less), heavy, wet snow
  with the rain/snow line likely spreading to north of the I-80
  corridor (as far north as I-88 corridor) and temperatures in the
  lower to mid 30s. Snowfall rates of 1"/hour to briefly higher
  will become common again Friday afternoon spreading from
  northwest to southeast as the mid-level deformation axis pivots
  across.

- The snow ratio will become drier (10-15:1+) as temperatures
  quickly fall Friday night and NW to W winds gust to 40-50 mph,
  (strongest I-80 and south) resulting in the potential for near
  white out conditions as snow becomes more readily blown around
  atop the wet underlying snow. Moderate snowfall rates appear
  probable for at least a portion of the evening in the comma head.
  The worst conditions may occur on north to south oriented roads
  in open areas due to the westerly direction of the winds.

- Hazardous conditions with periods of snow showers and blowing
  snow will continue into or through the day on Saturday with
  temperatures in the teens-20s and wind chills of 0-10F.

Important Items of Lingering Forecast Uncertainty:

- Exact Storm Track: Consistent global operational/ensemble mean
  (especially ECMWF/EPS and GFS/GEFS) guidance consensus tracks a
  deepening (down into the 970s mb) low pressure from southern
  Illinois into lower MI Friday-Friday night. Such a dynamic
  system can be expected to have near term wobbles, affecting
  important forecast details. A farther north (south) low track
  would entail farther north (south) 700 mb and 850 mb lows and
  the rain/snow line and dry-slot issues farther north (south),
  affecting temperatures, snow totals, wind magnitudes etc.
  Warning counties reflect areas with highest confidence in 6"+
  amounts and warning level impacts, including during the Friday
  morning commute. Given the likelihood of the rain/snow line
  (aside from marine influence snow/rain mix) surging north to and
  even a bit north of the I-80 corridor, there will likely be a
  several hour period of rain or a non-accumulating mix. This cut
  into the snowfall forecast, including in far northwest Indiana,
  resulting in confidence being too low for a warning in Lake and
  Porter Counties and Kankakee County.

- Onset Timing of Heavy, Wet Snow Early Friday: Guidance varies a
  bit on the specifics of top-down saturation and heavy
  WAA/isentropic ascent/f-gen driven thump of snow beginning, but
  unsurprisingly with such strong forcing and anomalous moisture
  transport, the start time generally moved up to the pre-dawn
  hours of Friday, so the winter headlines were started at 3 AM
  across the area. The evening shift will assess potential for
  snow onset to be any earlier than current expectations. As noted
  in key messages, the very high moisture, lift, steep lapse rates
  through and above the DGZ, and pockets of upright instability
  point toward a distinct potential for briefly intense (2"+ hour)
  snowfall rates across portions of northern Illinois pre-dawn
  through mid morning Friday.

- Potential for Near Blizzard Conditions Friday Night: True 3-hour
  blizzard criteria (visibility <1/4 mile and wind gusts >35 mph)
  is very rare to meet and usually in our area involves an overlap
  with heavily falling drier snow and the strongest winds. The
  initially wet snow will take some time to become fluffier/higher
  ratio and by then (mid-late Friday evening), snow rates will
  likely be on the downswing. Make no mistake, dangerous if not
  life-threatening travel is likely in the warned areas, but
  confidence in needing a Blizzard Warning is low (less than 30%).

- Hazardous Conditions South of I-80 Friday Night: Where primarily
  rain is expected for several hours or more, the precipitation
  will quickly change back to snow Friday evening as the surface
  low tracks into lower Michigan and colder air surges in. The
  strongest westerly wind gusts, up to or even briefly in excess
  of 50 mph, are expected to occur south of I-80 late Friday
  evening and early overnight. Despite only 1-3/2-4" of new snow
  amounts, warning-type impacts are a distinct possibility,
  particularly from blowing/drifting snow issues on north to
  south roads. With the lower snow amounts forecast, confidence is
  lower in the magnitude of impacts, so opted to issue an
  advisory for these areas. An upgrade to a warning remains
  possible, with the best chance of this being in Lake and Porter
  Counties.

- Extent of Limits to Accums Near Lake on Friday: Strong east-
  northeast winds again expected to keep temps above freezing,
  though uncertainty on specifics of how far inland accums will be
  cut down and how deep into Friday evening snow accums ramp up.
  Thinking for the immediate shore, including Chicago, is that the
  best chance for pavement accumulations will be during the burst
  of heavy, wet snow early Friday, with quite possibly little/no
  additional accumulation until temperatures cool down Fri.
  evening. The gradient between snow amounts along the lake and a
  bit inland and several miles will likely be sharper than shown
  in the official forecast.

- Specifics on Snow Amounts Inland: This dynamic and deepening
  system with an ample moisture feed has the likelihood to produce
  8"+ (per warning text) to even 12"+ amounts in spots across
  portions of interior northern Illinois. However, the low ratios
  and temperatures near to above freezing for a time on Friday
  increases uncertainty in the snowfall forecast more than the
  typical uncertainty in a winter storm, similar to the previous
  storm. For the latest snow amount forecast and probabilities of
  exceedence, go to the probabilistic winter page or view our
  latest DSSPacket.

Castro

&&

.LONG TERM...
Issued at 318 PM CST Thu Jan 11 2024

Saturday night through Thursday...

Key Message: Dangerously cold temperatures and wind chills for most
of next week, especially Monday and Tuesday.

After the snow tapers off Saturday, bitterly cold air will move into
the region Saturday night and persist through most of next week.
Subzero low temps are expected each morning Sunday through Thursday
with single digit lows continuing into at least Friday.  Monday and
Tuesday continue to look like the coldest days, with overnight lows
from -10 to -15 for much of the area and wind chills dropping toward
warning criteria levels of -30. Even during the daytime, wind chills
may remain in the -10 to -20 range through Wednesday before
"warming" into the single digits above zero for the remainder of the
week.

At this time we are not seeing additional major storms on the
horizon next week, but periods of flurries or light snow are
possible as weaker disturbances move through the area.

Lenning

&&

.HYDROLOGY...
Issued at 129 PM CST Thu Jan 11 2024

Key Messages:

* Increasing threat for rivers to reach bankfull mainly southeast of
  Interstate 55 (Vermilion, Kankakee, Iroquois rivers and Sugar
  Creek.

* Increased threat for ice jams and associated flooding on all area
  rivers from next weekend onward.

Discussion:

The hydrologic forecast for the next several days is complicated for
the area due to uncertainty in precipitation types, precipitation
amounts, and river levels during the upcoming period of very cold
weather. Rainfall over the last several days, generally southeast of
I-55, has increased soil moisture and river levels. With the next
system tonight through and Saturday, a period of rainfall is also
expected, with the highest liquid precipitation totals likely
occurring again across to the southeast. As we enter the period of
very cold temperatures Saturday into Sunday, attention will shift
toward concerns for river ice and frozen soils. Although a period of
temperatures averaging <0F for several days is often followed by
widespread river ice and at least a few ice jams, elevated river
conditions can increase the production of river ice further. With
cold weather expected to remain in place for most of next week, a
prolonged period of river ice generation will exist, along with the
potential for significant frost depth penetration in area soils.

Accumulated precipitation on Friday prior to the changeover back to
snow on Friday afternoon will be the most important factor relating
to crests on rivers and streams. The HREF LPMM indicates 0.75-1.25
inches, with the heaviest axis from generally Ottawa to Chicago. The
NBM mean is slightly higher with 1.25-1.50 and a 10th to 90th
percentile range from 0.9 to 1.75 inches. The NBM's heaviest axis is
slightly different, from generally Paxton to Chicago. The highest
overlap in QPF locations includes the Vermilion River Basin, the
lower Kankakee River Basin, and the Thorn Creek Basin. To the south,
in the headwaters of the Kankakee River Basin, precipitation is
likely to remain liquid for a longer period of time, but more
uncertainty exists in the QPF. River forecasts from NCRFC currently
assume approximately 1.0 inches averaged across the Kankakee and
Iroquois river basins, which yields multiple locations nearing or
exceeding bankfull. This assumption appears reasonable based upon
the mentioned model uncertainties. If some of the high rainfall
totals occur, a few rivers may see minor flooding, but the chance
for more significant flooding (moderate or major) appears very low
(<5%) at this time.

By late weekend into early next week, temperatures will quickly
become favorable for the rapid formation of river ice area-wide.
Mild temperatures during December and early January may initially
slow the development of ice due to river temperatures of 35-40F.
Water temperatures will likely cool quickly, with rapid river ice
formation likely by Monday. Isolated freeze-up ice jams will be
possible across the area, but may be more likely southeast of I-55.
Elevated river levels from recent rain and forecasted rain will
likely increase frazil ice production. Ice-prone areas in the
Kankakee River Basin will need to be watched carefully next week as
even small increases in water level due ice jams will cause flooding
due to elevated river levels. River ice flowing from tributaries
into the Illinois River, in additional to river ice that forms
locally, may cause impacts to shipping. Ensemble temperature
forecasts suggest a low chance of high temperatures consistently
exceeding 20F through next week, which may signal an extended period
of river ice problems.

WSL

&&

.AVIATION...
For the 06Z TAFs...

Key Aviation Messages:

- Rapidly deteriorating conditions from SN early this morning
  through mid-morning, with a period of visibility nearing
  airfield mins in the 10-14Z window
- Slight decrease in SN rates late morning and early afternoon
  before another period of SN/+SN at or below airfield mins
  22Z-01Z.
- Increasing WNW winds to 30 knots with BLSN/DRSN overnight into
  Saturday morning

An initial round of SN with 1-2"/hr rates and visibility at 1/2SM
is expected in the 10-14Z window in tandem with increasing E
gusts to around 25 knots. 1/4SM visibility is possible during this
period, but the expected wet nature of the SN (i.e. larger
snowflake aggregates) and minimal BLSN may limit the lowest
visibility potential.

SN intensity will then slightly diminish for several hours late
morning into early afternoon, with RA mixing in at MDW and
possibly briefly ORD. E winds will also gust up to 30 knots during
this time. As the surface low nears the area later in the
afternoon, SN intensity is expected to quickly increase and
produce frequent periods of 1/2SM visibility in the 21-03Z window.
Confidence is now high enough to introduce a period of 1/4SM +SN
from 22-01Z when snowfall rates surpass 1"/hr. Winds during this
time are expected to diminish to under 15 knots during this time.

By mid-evening, winds will quickly veer NW around 10 knots behind
the surface low then increase to W gusts to 30 knots by shortly
after 06Z. Intensity of -SN will also begin to diminish, though
BLSN concerns will increase through Saturday morning.

Kluber

&&

.MARINE...
Issued at 420 PM CST Thu Jan 11 2024

A strong area of low pressure will move from south/central IL
Friday morning to central lower MI by Saturday morning. Easterly
winds will steadily increase early Friday morning with gales to 35
to 40 kt likely, especially across the IL nearshore waters. Winds
will shift to the northeast Friday afternoon as the low
approaches and will diminish in strength for a short time into
early Friday evening. As the winds turn northwesterly to westerly
Friday evening and overnight, speeds and gusts will rapidly
increase with gales to 40 to 45 kt expected. The potential for a
few storm force gusts remains for the Indiana nearshore, but
confidence was too low for a storm warning.

The strongest winds will likely diminish by mid morning Saturday
with gusts into the 30kt range persisting into Sunday. A Gale
Warning is in effect from Friday morning through Saturday evening,
though again there will be a lull in the most hazardous conditions
Friday afternoon into early Friday evening before the northwest to
winds quickly strengthen back to gale force.

Much colder air will spread across the region this weekend into
next week and if the winds remain strong enough, freezing spray
will likely develop Saturday night and Sunday.

CMS/Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ103-ILZ104-
     ILZ105-ILZ106-ILZ107-ILZ108...3 AM Friday to noon Saturday.

     Winter Weather Advisory...ILZ023-ILZ032...3 AM Friday to noon
     Saturday.

     Winter Weather Advisory...ILZ033-ILZ039...6 PM Friday to noon
     Saturday.

IN...Winter Weather Advisory...INZ001-INZ002...3 AM Friday to noon
     Saturday.

     Winter Weather Advisory...INZ010-INZ011-INZ019...6 PM Friday to
     noon Saturday.

LM...Gale Warning...IL nearshore waters...3 AM Friday to 6 PM
     Saturday.

     Gale Warning...IN nearshore waters...6 AM Friday to 9 PM
     Saturday.

&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
315 PM CST Thu Jan 11 2024

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-122115-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
315 PM CST Thu Jan 11 2024 /415 PM EST Thu Jan 11 2024/

This Hazardous Weather Outlook is for portions of North Central
Illinois...Northeast Illinois and Northwest Indiana.

.DAY ONE...Tonight.

Weather hazards expected...

  Elevated Snow Risk After Midnight.

DISCUSSION...

Snow will spread across the area primarily north of US 24 after
midnight and continue after daybreak Friday. Snow rates greater
than 1 inch per hour will lead to slippery to hazardous travel
conditions.

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

Friday and Friday Night...
  Elevated Non Thunderstorm Wind Risk.
  Significant Snow Risk.
Saturday...
  Elevated Excessive Cold Risk.
  Elevated Non Thunderstorm Wind Risk.
  Limited Lakeshore Flood Risk.
Sunday through Wednesday...
  Elevated to Significant Excessive Cold Risk.

DISCUSSION...

Additional snow will fall across the region especially north of
Interstate 80 throughout the day on Friday. Friday night, snow
will taper as westerly winds increase and gust 45 to 50 mph
leading to areas of blowing snow and very low visibility.

Bitterly to dangerously cold air will spread into the region
Sunday night through Tuesday night with wind chills as low as 30
degrees below zero.

River ice is expected to rapidly build up on area rivers Sunday
into early next week as a much colder arctic airmass moves into
the area. This will bring a threat for ice jam related flooding
on area rivers into next week.

.SPOTTER INFORMATION STATEMENT...

  Spotter activation will not be necessary through tonight.

LMZ740>745-122115-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
315 PM CST Thu Jan 11 2024

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Tonight.

Weather hazards expected...

  Hazardous conditions for small craft.

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

Friday...
  Gale force winds and or very hazardous waves are likely.
Saturday...
  Gale force winds and or very hazardous waves are likely.
  Limited Freezing Spray Risk.
Sunday through Wednesday...
  Hazardous conditions for small craft are likely.

Borchardt

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