Winter Storm Watch 4, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
357 AM CST Sun Feb 14 2021

.SHORT TERM...
357 AM CST

Through Monday...

There are a number of headlines to discuss early this morning.

Wind Chill Headlines:

We have elected to issue/extend the wind chill advisory for all
of our IL counties through midday Monday. The ongoing wind chill
warning continues through midday today, then will transition to a
wind chill advisory.

Winter Weather Advisory and Winter storm watch:

A winter storm watch has been issued for Cook county and Lake
county IN for tonight through early Tuesday. This was done due to
the increasing potential for a significant and persistent lake
effect snow event.

Outside of the watch area, a winter weather advisory has been
issued for tonight through late Monday night for all counties
along and east of the I-57 corridor. This was done due to the
expectation of 2 to 5 inch snowfall amounts from the next
approaching weather system.

More details:

The 1042 mb surface high over the Dakotas continues to bring us
northwesterly winds and cold air. Additionally we lost our cloud
cover, leading to sub-zero temperatures this morning, with winds
gusting a bit into the 15 mph range, creating wind chills as low as
-20 presently. Temperatures will continue to drop this morning,
dropping wind chills into the -30s, especially in our western
counties. Clear skies will be short-lived as the next short-wave
approaches, moving into our region Sunday afternoon. This system
has several snow creation mechanisms, starting with the right
entry region of an upper level jet max, that eventually becomes
somewhat coupled with another approaching jet max's left exit
region. Additionally we see a region of isentropic ascent
starting Sunday mid-afternoon that continues through Tuesday
early morning, in addition to occasional areas of vorticity that
rotate through ahead of the main upper-level wave. With such a
cold atmosphere, we do not need strong dynamics to create snow,
so expect this possibility starting as early as midday today.

Light snow chances will increase across most of the area tonight and
continue on Monday. Most notable, is the strong signal in forecast
guidance supporting the development of a lake induced boundary layer
convergence band on the southern end of the lake this evening. This
looks to support the development of a prominent single band of
heavy lake effect snow this evening, likely initially impacting Porter
IN. Thermodyamics over the lake look sufficient to support very
heavy snow rates in this developing band of snow, with lake induced
equilibrium levels looking to top out around 6-7,000 feet late tonight
and on Monday. While this band of lake effect snow looks to impact
mainly Porter county early this evening, signs point at it shifting
westward over Lake county IN late in the evening and overnight
tonight, before finally shifting over parts of Cook county on Monday.

Concerns continue to increase that parts of Cook county, including
much of the city of Chicago could be in the line of fire from this
potential band of heavy lake effect snow for most of the day Monday.
With the potential for 1 to 2 inch per hour rates in this band of
snow, we could see some amounts total up well over 6" in parts of
the watch area through Monday night. Also would not be surprised to
see some areas see in excess of a foot.

Outside of the lake effect area, an approach weather disturbance
will result in periods of accumulating snow tonight through Monday
night. The strongest dynamics and better moisture look to favor
east central IL and into IN for the highest snow amounts in the area
(2 to 5"). This is the general area in which we issued the winter
weather advisory. Lighter snow amounts of 1 to 3 inches look probable
farther north, for inland areas of Chicago metro. Drier air potential
will result in snow amounts tapering off to a dusting over far
north central IL.

Throughout this entire forecast period temperatures will remain very
cold, with negative wind chills to -25 remaining probable through
Monday morning. For this reason, the wind chill advisory was extended
through this period.

BKL/KJB

&&

.LONG TERM...
215 AM CST

Tuesday through Saturday...

Extended focus includes lingering lake effect snow into northeast
Illinois Tuesday into Tuesday evening, then prospects for another
synoptic snow system Wednesday night and Thursday. Temperatures
look to remain seasonably cold through the period, though not as
extreme as in the short term portion of the forecast.

Guidance depicts upper troughing remaining across the upper
Midwest Tuesday, with some indications of another embedded short
wave tracking east across the area during the day. Synoptic
precipitation with the early week system looks to be east of the
region, though north-northeast low level gradient flow remains in
place downstream of surface high pressure building across the
upper Midwest. With a reinforcing push of arctic air, lake effect
snow looks to persist across northeast IL given the north-
northeasterly convergent flow, though winds are progged to
diminish through the day likely resulting in an overall decrease
in snowfall intensity. Once the aforementioned short wave moves
east of the region late in the day, the development of short wave
ridging aloft and associated subsidence should allow for further
weakening of any lingering lake effect bands. Surface high
pressure moves in Tuesday night, with light winds shifting
southeast and inversion heights lowering quickly overnight.

Global guidance depicts another upper trough developing across
central NOAM Wednesday, with the forecast area just downstream int
confluent flow east of the trough axis. Models hint at a sheared
mid- level short wave rippling through the area during the day,
though there is considerable disagreement in light precip
development with this feature. Forecast soundings do indicate
rather dry air below 700-750 mb initially, with gradual top-down
saturation depicted later in the day/Wednesday night as better
large scale ascent starts to take shape. Precipitation looks more
likely Wednesday night and especially Thursday into Thursday
night, as the large scale trough propagates eastward across the
Midwest. This is a complex evolution however, with phasing of
northern and southern stream short waves, and guidance shows some
spread with how this occurs. Heaviest QPF remains well off to the
southeast of the forecast area, which looks appropriate given the
location of the lower level baroclinic zone and a forecast surface
low track originating from the Mississippi Delta. Model tracks
diverge however (per handling of the phasing of upper level
energy), with the GEFS and Canadian ensembles and operational runs
lifting across OH and Lake Erie through Thursday night, with the
EPS/ECMWF depict a farther east track into the Appalachians and
mid-Atlantic. Interesting to note that even the drier ensemble
clusters from WPC analysis bring around a tenth of an inch of QPF
into our southeastern cwa with this system, indicating the
potential for some significant snowfall accumulation well
northwest of the low track. NBM blended pops generally highlight
areas along/east of the I-55 corridor for late Wednesday night
through Thursday at this point, and this seems appropriate given
the ensemble solutions at this time. Will continue to monitor this
period.

System pulls away to the east Friday, with surface high pressure
ridging indicated later in the day and into Friday night. Some
lake effect threat may linger for northwest IN early Friday,
though guidance is generally fairly quick to back low level winds
more west- northwest which would shift the focus east of the WFO
LOT cwa. Generally dry and quiet weather conditions would be
expected Friday into Saturday at this time. There is some
indication of another wave which approaches the area from the west
later Saturday which could bring another round of light snow,
though have rather low confidence at this distance.

Temperature wise, core of colder arctic air centered just north
of the region early in the week is progged to moderate during the
period, with another modest cold push Friday behind the departing
mid-week system. Temps look to remain below average, though not as
cold as early in the week, with daytime highs in the teens and
20s and nighttime lows from the positive single digits to lower
teens.

Ratzer

&&

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

Primary forecast concern is light snow developing Sunday night and
continuing into Monday.

Another storm system will move across the southern Great Lakes
region Sunday night into Monday. Light snow is expected to develop
across the area by early Monday morning. There remains uncertainty
regarding when the snow will develop and continued prob mention
with prevailing light snow likely starting after 06z for the
Chicago terminals. A period of moderate snow may develop before
daybreak Monday but low confidence. There may be a fairly sharp
cutoff to the snow and northwest IL and rfd may end up staying
dry. The snow is expected to transition to lake effect snow on
Monday which may affect ord/mdw/gyy.

Northwest winds around 10kt will continue through the period.
There may be a few gusts at times but these are no longer
expected to become prevailing. Wind directions are expected to
turn more northerly early Monday morning. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Chill Warning...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ019-ILZ020-ILZ021-ILZ032 until noon Sunday.

     Wind Chill Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ019-ILZ020-ILZ021-ILZ032...noon Sunday to noon
     Monday.

     Wind Chill Advisory...ILZ006-ILZ013-ILZ023-ILZ033-ILZ039-ILZ103-
     ILZ104-ILZ105-ILZ106-ILZ107-ILZ108 until noon Monday.

     Winter Weather Advisory...ILZ023-ILZ033-ILZ039-ILZ108...6 PM
     Sunday to 3 AM Tuesday.

     Winter Storm Watch...ILZ103-ILZ104-ILZ105...3 AM Monday to noon
     Tuesday.

IN...Wind Chill Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 until
     noon Sunday.

     Winter Storm Watch...INZ001...6 PM Sunday to 3 AM Tuesday.

     Winter Weather Advisory...INZ002-INZ010-INZ011-INZ019...6 PM
     Sunday to 3 AM Tuesday.

LM...None.
&&

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Hazardous Weather Outlook...UPDATED
National Weather Service Chicago/Romeoville IL
653 PM CST Sat Feb 13 2021

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-150100-
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-
653 PM CST Sat Feb 13 2021 /753 PM EST Sat Feb 13 2021/

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

.DAY ONE...Tonight.

Weather hazards expected...

  Significant Excessive Cold Risk.

DISCUSSION...

Dangerously cold wind chills are expected tonight, with the
coldest wind chills across interior northern and northwest
Illinois.

.DAYS TWO THROUGH SEVEN...Sunday through Thursday.

Sunday...
  Significant Excessive Cold Risk.
  Elevated Snow Risk.
Monday...
  Elevated Excessive Cold Risk.
  Elevated Snow Risk.
Tuesday...
  Limited Excessive Cold Risk.
Wednesday into early Thursday...
  Limited Excessive Cold Risk.
  Limited Snow Risk.

DISCUSSION:

Dangerously cold wind chills are again possible late Sunday night
into Monday morning.

Several rounds of snow are possible Sunday night through Tuesday.
Accumulating lake effect snow will likely impact travel Sunday night
across northwest Indiana and then shifting into portions of
northeast Illinois on Monday.

In addition to lake effect snow, areas south and east of Interstate
55 stand the best chance for hazardous travel due to snow during
this time also.

.SPOTTER INFORMATION STATEMENT...

  Spotter activation will not be necessary through tonight.

LMZ740>745-150100-
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-
653 PM CST Sat Feb 13 2021

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.

  No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Sunday through Thursday.

Sunday night and Monday...
  Hazardous conditions for small craft are likely in an around
  heavier lake effect snow showers due to gusty winds and reduced
  visibility.

Carlaw/KMD

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE CHICAGO IL
1039 PM CST SAT FEB 13 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0600 PM     SNOW             NWS CHICAGO-ROMEOVILLE  41.60N 88.08W
02/13/2021  M1.8 INCH        WILL               IL   OFFICIAL NWS OBS

            STORM TOTAL SNOWFALL. LIQUID EQUIVALENT OF
            0.08 INCH.

&&