Area Forecast Discussion
Issued by NWS Chicago, IL

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554
FXUS63 KLOT 200206
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
806 PM CST Sat Jan 19 2019

.UPDATE...
806 PM CST

For Evening Update...

No significant changes to going forecast this evening. Light lake
effect snow showers showing increasing coverage as expected, with
high-res guidance remaining similar in depiction of transition to
a more intense single-band structure late this evening. Going
forecast has this detailed nicely, with headlines structured
appropriately.

Evening surface analysis depicts 996 mb low pressure south of the
Ohio River over east central Kentucky. Tight surface pressure
gradient extends into the southern Great Lakes and mid-Mississippi
Valley regions, producing gusty north-northeast winds and strong
low-level cold advection across the region. While snow associated
with the low pressure system has moved out of the forecast area,
the cold low-level air traversing Lake Michigan was resulting in
increasingly organized lake effect snow showers into parts of
northeast IL and far northwest IN early this evening. High-res
model guidance continues to be in very good agreement in depicting
increasing low-level convergent focus developing along the west
shore of the lake later this evening, with a transition from
current loosely organized multi-band snow showers into a stronger
largely single band LES plume later tonight. While inversion
heights remain generally in the 6500-7500 ft AGL range tonight
into Sunday, all other parameters favor the potential for strong
LES with periods of heavy snowfall rates within the single-band
plume. Lake surface to 850 mb delta T`s around 20C, large positive
area/CAPE below the inversion, and cloud layer temps within the
favored -12C to -18C dendritic growth zone all support strong
convective snow elements. 4km NAM/NMM/HRRR are all generally
similar in depiction of heavier snow developing into far northeast
IL after 9-10 pm this evening, with the best convergent axis
slowly shifting from Lake Co. IL southeast across Cook Co. IL
overnight, and eventually into northwest IN by early Sunday
morning. Lighter snow showers or flurries will likely persist
farther inland, though the greatest potential for accumulations of
substance appear likely to be limited to the lake-adjacent
counties of Lake (IL), Cook, Lake (IN) and Porter. This is exactly
as indicated in forecast from previous shift, and see no need for
any changes to the details or headlines at this time.

Farther west, skies have begin to clear across parts of north
central IL, allowing better radiational cooling in those
locations. This has helped the boundary layer to decouple, and
winds to ease with the loss of more frequent gusts. Temps will
slide into the single digits in many areas away from the lake-
effect cloud cover, with lows of zero to 5 above expected over new
snow cover across our western counties where cloud cover will be
minimal. Have lowered temps another degree or so in the Rockford,
Sterling, Rochelle and Belvidere areas, but otherwise going
forecast again remains in good shape.

Ratzer

&&

.SHORT TERM...
318 PM CST

Through Sunday Night...

The first wave of snow associated with the Winter Storm Warning
has ended, though blowing and drifting snow in open areas
continues to be a concern into early this evening. Focus now
shifts to lake effect snow showers, with the high likelihood of at
least temporary heavy rates for tonight and very early Sunday in
northeast Illinois and for northwest Indiana mainly Sunday. A
Winter Weather Advisory has been issued for this.

The strong upper low to our south is entering the Ohio River Valley
and will progress northeast and strengthen tonight. The surface
low around 996 mb will deepen several millibars tonight as it
lifts from Kentucky to the Mid-Atlantic. This will keep winds
elevated through early to mid evening area wide and near Lake
Michigan throughout the night. Regional radar VWP data has sampled
30 kt at the top of the boundary layer and this should remain
steady through that time, so gusts of 30-35 mph look to be
consistent, along with a few gusts to 40 mph especially near the
lake. The reports of blowing and drifting snow have for the most
part not been too bad, or if worse then not that widespread. For
this reason we continue a Special Weather Statement noting areas
of blowing snow into the evening.

Temperatures will drop into the single digits in parts of north
central Illinois by midnight with lows in that area of 0 to 5, and
elsewhere upper single digits to lower to mid teens under lake
effect clouds/snow showers. With winds easing later tonight, wind
chills Sunday morning will not be to the level of headlines, but
some wind chill readings just into the negative double digits are
favored for parts of northern Illinois.

Lake effect snow showers have shown some strengthening signs
upstream in Wisconsin, a sign colder low-level air is creeping
southward behind the departing cyclone. As it continues to do so,
more widespread light to moderate snow showers should be seen
developing into northeast Illinois early to mid evening. Guidance
continues to indicate a well-defined convergence area and
resultant moderate to heavy lake effect snow band across the
western part of the lake into Wisconsin early this evening and
shifting southward into northeast Illinois mid-late evening.
Forecast lake effect parameters remain generally similar to what
has been seen over the past 24 hours, though for far northwest
Indiana on Sunday morning the magnitude and residence time does
look better.

For this lake effect band, it is not a question of whether the
snow will be heavy, it is more so how long the residence time of
the band in any one place will be. That tends to be a very
challenging characteristic to determine greater than 12 hours in
advance. The lake effect parameters that are solid are the low-
level lapse rates and the convective cloud depths collocated with
the dendritic growth zone of -12C to -18C. This likely will make
up for the overall shallower cloud depths of 4-5 kft. The
inversion heights / equilibrium levels are lower and not what we
typically see for concerns of a six plus inch event. So that
battle of parameters continues. But given consistent high-
resolution model trends and applying lake effect banded snow
conceptual model points toward heavy snow (1+ inch per hour)
likely underneath that band. So a quick several inches along with
very low visibility, both from falling snow and some blowing
snow, is enough to warrant an Advisory. Have different start and
end times around the lake to account for the expected migration of
the band. In Indiana the time window has lower confidence hence
why it is longer, but some of the higher impacts are likely in the
morning into early afternoon.

For total snowfall with lake effect the residence time is the key
factor, so messaging 2 to 6 inches with the higher end a bit more
favored in northwest Indiana. Given the strong advection of the
lake effect parameters, lake effect snow is likely all the way
inland to the southern CWA border of Ford and Iroquois Counties!
The snow should not be that heavy that far inland, but some
accumulation including possibly over an inch is possible in parts
of eastern DuPage, Will, Kankakee, Newton, and Jasper Counties.
Some lake effect snow showers are likely to shift back westward
and possibly back into Illinois Sunday evening/overnight, but
parameters should be weakening as the wind field diminishes.

Otherwise Sunday will see the coldest high temperatures of the
winter to this point, with a cold air mass giving a glancing blow
to the region. Highs look to be in the lower to middle single
digits. Lows Sunday Night are primed to drop hard with potential
clearing inland, although high clouds in the west and lake effect
clouds near the lake could alter that. If not, lows to 5 below
zero or colder are certainly possible in outlying areas.

MTF

&&

.LONG TERM...
352 PM CST

Monday through Saturday...

For the long term forecast period, winter will be firmly entrenched
across the region.  The long-wave upper level pattern will
transition from a quickly progressive, high amplitude pattern at the
beginning of the period to lower amplitude, slowly progressive, with
a broad upper trough over the entire CONUS by mid-week.

Monday will start out the long term period cold and dry, with max
temps only reaching into the teens. Upper ridging is then expected
to build across the region Monday night and then, the pattern will
continue to amplify into Tuesday as a southern stream shortwave
lifts out of the southern Rockies and into the Central Plains.
Temperatures should reflect a brief warming trend, with highs back
into the lower to middle 30s as broad swly upper flow and deep warm
advection sets up in advance of the deepening trough to the west.
While the global models start out the period relatively consistent,
they begin to quickly diverge in how they handle the southern stream
shortwave in both timing and intensity.  While the models all
generally advertise a closed upper low associated with the srn
stream system, they all then evolve into a positively tilted open
wave.

The cold arctic high that is currently centered over Manitoba
Canada, near Lake Winnipeg, and is expected to gradually slide
southeast across the Great Lakes and Ohio Valley by Monday morning
and then continue to progress to the East Coast as the sfc low
associated with the southern stream system lifts into the Central
Plains while quickly deepening.  However, as is frequently the case,
the models are exhibiting differences in the timing and intensity of
the system.

However, there are signals that there may be a significant mixed
pcpn situation with warmer, moist air aloft overspreads a cold sfc
layer.  Currently, will continue to carry the categorical PoPs for
Tuesday afternoon and evening, ramping up quickly Tuesday morning
and then quickly ending from west to east late Tuesday into into
Wednesday morning.  Words of caution...mixed freezing/frozen/liquid
pcpn details are difficult to pin down in the short term and are
very uncertain this far out in the forecast period.  So, with this
caveat, current thinking is that morning snow will transition to a
freezing rain/sleet/snow mix and then transition to mostly rain late
in the day before changing back over to snow Tuesday night into
early Wednesday.  Also, small deviations in the track of the system
could have significant impact on the sensible weather features.  A
shift to a more sly/ely track to the system would favor a greater
chance for more snow than freezing pcpn/rain.  A more nly/westerly
track would suggest a greater chance for more mixed pcpn/rain. There
is also some concern that the associated warm front may not get far
enough north to spread warm, above freezing, surface air across the
local area and that the warm front could be suppressed south by the
cold air associated with arctic high as it slides from Manitoba to
the ern CONUS.  This would set up conditions favorable to an
extended freezing/frozen pcpn mix of sleet/freezing rain/snow for
Tuesday into Tuesday evening.  Readers are advised to follow
forecast updates as details regarding this system are likely to
chance over the next few days.

Following the passage of this system, the longer range guidance is
in generally good agreement on a significant cooling trend with
temperatures for the end of the week into the weekend.  Sub-zero
lows are possible for next weekend wind chill reading approaching 20
below zero.

&&

.AVIATION...
For the 00Z TAFs...

546 PM...Primary forecast concern is lake effect snow late this
evening into Sunday morning. Additional concerns are blowing snow
and winds this evening.

Lake effect snow showers are developing/spreading across southeast
WI with flurries/light snow showers across northeast IL. The lake
effect snow showers will continue to increase in intensity across
northeast IL this evening...likely forming a band by late evening.
Timing is still on track in the 04z-08z period for ord and 07z-11z
at mdw. The duration is likely to be less than 4 hours but not
confident to narrow the time window just yet. If the lake effect
does form into a single band...visibilities 1/2sm or less are
likely though the lowest vis may be east of ord as it begins to
organize overnight. The band is expected to move into northwest IN
by 12z and duration at gyy is uncertain. The models move this band
back west Sunday afternoon...possibly to the IL lakeshore by 00z
as its weakening. Added prob mention to gyy for the end of the
period but confidence for how far west this band may move is low.

Winds will remain northerly this evening with gusts into the mid/
upper 20kt range. Blowing and low drifting snow is expected to
continue and as more fluffy lake effect snow falls...blowing snow
may continue with gusts finally diminishing by sunrise though
speeds will remain 10-15kts Sunday as directions turn more to the
north/northwest. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory...ILZ014 until 9 AM Sunday.

     Lakeshore Flood Advisory...ILZ006-ILZ014 until 9 AM Sunday.

     Winter Weather Advisory...ILZ006 until 4 AM Sunday.

IN...Lakeshore Flood Advisory...INZ001 until 9 AM Sunday.

     Winter Weather Advisory...INZ001-INZ002...midnight Sunday to 4
     PM Sunday.

LM...Gale Warning...nearshore waters until 3
     AM Sunday.

     Heavy Freezing Spray Warning...LMZ740-LMZ741-LMZ742-LMZ743-
     Gary to Michigan City IN until 10 AM Sunday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...3 AM Sunday to 3 PM Sunday.

&&

$$

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