Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160926
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
326 AM CST Tue Jan 16 2018

.UPDATE...
909 PM CST

Bands of snow showers continue mainly along and north of
Interstate 88, with lighter snow/flurries south. Reports have
indicated that even these showers are occasionally moderate to
briefly heavy and radar returns are not are that great. MKX radar
indicates an area of moderate to heavy snow across southeast
Wisconsin. The inland extent of this snow extends well west, but
model guidance indicates as this band of snow heads into northern
Illinois that the western extent will diminish some. There is also
some suggestion that the convergence/fgen driving these bands of
snow will weaken some with time also. Still we do expect this
band to rotate through tonight, with briefly moderate to heavy
snow rates, albeit not for too long. This will include the city of
Chicago for a period mainly after midnight. Snow rates in this
band have been roughly three quarters of an inch per hour, and
while it is uncertain how intact it will be here, expect an uptick
in snow overnight. Some areas will see another inch or so, with
Lake/Mchenry/northern Cook the favored near lake locations.
Therefore expect for northern areas and lake adjacent counties
some additional issues for the morning commute with the additional
snow and very cold temperatures.

The synoptic and somewhat mesoscale forcing ongoing across
Wisconsin will shift over the lake later tonight, and the focus
will then be on the lake effect snow development, its strength and
location. RAP/HRRR depict a mesolow lingering over the lake and
keeping the focus of the lake effect snow across Lake County
Indiana, and at least the shoreline areas of Cook County, with the
mesolow allowing convergence to remain a bit farther west than
would normally be expected, with things becoming more progressive
as snow shifts toward Porter county. NAM/NAMNest are still
farther east with Porter County more favored once the better lake
thermodyamics get going.

Have a tough time making any concrete decisions to upgrade a
watch, but we could consider at least an advisory for a broader
area. At this time most model guidance agrees in lake effect band
development, but it will be a tough call as to whether these will
stay just offshore of Cook county, and if it may well be that Lake
County Indiana has more residence time with the band. Near lake
counties should stay tuned for forecast updates given this
uncertainty and potential impacts.

KMD

&&

.SHORT TERM...
309 PM CST

Through Tuesday Night...

Multiple concerns, including ongoing snow and snow showers into
this evening and sub-zero wind chills this evening into Wednesday
morning, shifting to a focus on lake effect snow later tonight
through Tuesday. This includes the potential for heavy lake effect
snow for portions of the lakeside counties, especially Porter
County, where a Winter Storm Watch remains in effect.

Somewhat complex set-up over the region this afternoon with large
upper low (lobe of tropospheric polar vortex) slowly rotating
southeastward across the region. This can clearly be seen on mid-
level GOES-16 W/V imagery. Felt that lingering snow over northern
Illinois pivoting with the upper low had diminished enough in
intensity to let the Winter Weather Advisory go. Based off
observations across the region, outside of this snow, there are
flurries and snow showers and visibility reductions likely owing
to patchy blowing snow in open/rural areas. Difficult to locally
hazardous winter driving conditions will persist due to this.

The snow/snow showers over northern Illinois mainly north of I-80
as of this writing will not truly come to an end this evening.
The steering flow around the ULL will wrap these back southward
through this evening and the overnight hours, likely diminishing
in coverage and intensity with southward extent. Additional
accumulations up to an inch or two will be focused into northeast
Illinois, with minor accumulations elsewhere likely under an inch.
As temperatures cool into the low-mid single digits later tonight
southern CWA, persistent westerly winds will result in wind chill
values bottoming out at 10 to 20 below zero into early Wednesday.
Opted to hold off on Wind Chill Advisory issuance with local
criteria being -20 to -29 F wind chills.

Turning to the lake effect, the banding is currently being driven
into eastern WI by easterly flow around the low pressure over the
southern portion of the lake. It`s current location adds concern
for portions of northeast IL, especially lakeside, getting briefly
impacted by moderate snow showers as they pivot southward and
then southeastward into Tuesday. This is despite boundary layer
flow indicated by most guidance to be from a generally unfavorable
northwest direction. It`s an atypical set-up, also possibly
muddied by meso-low formation on Tuesday morning, which could have
significant impacts on the placement by that time of a
strengthening single band.

The scenario portrayed in the grids is of the lake effect activity
swinging through northeast Illinois in the pre-dawn hours and into
Tuesday morning and then shifting into northwest Indiana.
Thermodynamics and inversion heights will be very favorable for
heavy snow in this snow band, with 850 mb to lake delta T`s of
about 20 Celsius and inversion heights at or slightly above 10kft.
Important questions of where exactly convergence sets up, how
long process takes and transient the likely single band is are
ones that are unfortunately of low confidence at this time. The
regional and global Canadian guidance has been very insistent on
bringing the heavy lake effect snow into portions of the NE IL
shore. Suspect that this is likely too far west, but until
observational trends can be ascertained, it`s not one that can be
fully ruled out. More likely is that after clipping the IL shore,
the band clips Lake County and then pivots into Porter,
especially northeast Porter, before exiting into LaPorte late
day/Tuesday evening.

We`ve had past occurrences of this favored scenario being one in
which the band instead lingered over Lake County Indiana with
heavy snow and then was very transient over Porter County,
limiting impacts there. In addition, the boundary layer flow
reorientation can also result in very short band residence time
over Porter, lessening likelihood of warning criteria amounts.
Given expectation of a well organized heavy snow band with 1-2"+
per hour rates, it will only take on the order of a few hours to
produce 6"+ amounts in northeast Porter. Therefore, with all the
above uncertainty in how things evolve tonight into Tuesday,
including timing of band progression, opted to hold onto the
Winter Storm Watch. Will let evening shift assess trends to
possibly make a headline decision.

Outside of the lake effect snow concerns, Tuesday will feature
below normal temperatures in the upper teens to mid 20s. Flurries
will remain possible under cyclonic flow regime of exiting upper
low, gradually shifting southward into the afternoon. Skies should
become partly cloudy Tuesday night, with relatively light winds on
northeastern periphery of surface ridge. Temperatures could
potentially fall a bit colder in favored spots than in current
forecast if skies clear enough and winds drop enough. Wind chills
should fall below zero area wide, but not approaching headline
criteria.

Castro

&&

.LONG TERM...
144 PM CST

Wednesday through Monday...

The axis of the upper longwave trough clears the area late Tuesday
as the associated surface low finally starts to move off toward
the eastern Great Lakes. This will bring an end to any lingering
flurries or light snow for the remainder of the work week. The
most significant influence on local temperature and precipitation
trends then becomes a broad ridge building over the Plains.

Deterministic models are in reasonable agreement with the center
of this feature somewhere in the vicinity of Tulsa by early
Wednesday, with a cold start to the day followed by winds backing
from northwest to west locally on the northeast side of the high.
Winds continue backing to southwest by late Wednesday and persist
from this direction through Thursday, Friday, and Saturday as the
ridge moves into the Gulf Coast region and eventually into the
Atlantic.

Along with a steady warming trend and late-week thaw, this would
set up an interesting open Gulf scenario for later in the weekend
across the Midwest, especially given the developing upper trough
across the Rockies and the surface low that models show forming in
the Southern Plains. The period from Sunday into Monday certainly
deserves attention as the time gets closer, and while it is far
too early to start clinging to any deterministic solution, there
is considerable and somewhat surprising agreement between the
latest ECM/GEM/GFS in bringing the deepening sfc low through
northern Illinois in the late Sunday timeframe.

Again, much could and likely will change over the next several
days, but very early signs suggest the potential for convection
ahead of this low and significant winter weather on its north
side. It is too early to say what this means for the local area,
as all depends on the ultimate track and strength of the low, and
whether it even develops as models presently advertise. For
example, a quick look at the GFS ensemble shows a much weaker and
more transient upper wave, as might be expected, rather than a
slower and deeper closed circulation as depicted in deterministic
solutions. Either way, there appears to be a good chance for
active weather late in the weekend, with the potential for both
liquid and frozen precipitation.

Lenning

&&

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

Main forecast concern is snow potential through Tuesday afternoon.

A band of moderate snow is moving into far northern IL with vis at
ugn as low as 1/2sm recently. This band of snow will spread south
across all the terminals over the next few hours but is expected
to slowly weaken. With prevailing vis already in the 2-3sm with
light snow...it is likely that vis will dip below a mile at
ord/dpa but if the band does weaken...confidence at mdw/gyy is
lower. Heaviest part of the band may remain east of rfd but vis at
or just below 1sm still possible there. Behind this band there
may be improvement to mvfr vis...or better. However...there still
appears to be the potential for continued very light snow. Lake
effect snow is expected to develop Tuesday morning and currently
is expected to remain east of ord/mdw. Confidence is too low to
include any mention at ord/mdw but trends will need to be
monitored for possible lake effect snow showers at ord/mdw during
the mid/late morning Tuesday. These lake effect snow showers are
expected to affect gyy but confidence on timing has decreased. Its
possible the lake effect snow persists into Tuesday afternoon at
gyy before shifting east.

West/southwest 10-15kts with a few higher gusts will shift to the
northwest by morning and then more north/northwest later Tuesday
morning. Prevailing mvfr cigs will continue into Tuesday afternoon
with brief ifr possible with any heavier bursts of snow. cms

&&

.MARINE...
324 AM CST

Low pressure is centered just east of Lake Michigan early this
morning with a baggy trough stretching west across the far south
end of the lake. The low will move east today and weaken allowing
northerly flow of 15 to 25 kt to overspread the entirety of Lake
Michigan by late this morning. Strong high pressure over the
northern Plains today will build across the lower Mississippi
Valley by tomorrow while low pressure moves southeast across
Hudson Bay. A westerly gradient will develop over Lake Michigan
between these two features and gales are a concern for Wednesday
through Wednesday night. GFS/NAM soundings both show a strong
inversion off the deck which will limit the depth of mixing and
casts some uncertainty on the strength and duration of any gales.
In addition, there are modest model difference and GFS soundings
do not support gales over northern Lake Michigan at all, and only
marginally so across the south. Meanwhile, NAM soundings show
marginal gales across the lake. Will hold off on any gale
headlines at this point and only mention occasional gales in the
GLF during those periods.

Another low will move across the Canadian Prairies this weekend
bringing another potential period of gales Friday night into
Saturday. Models continue to advertise a deep low late in the
weekend into early next week lifting from the central and southern
Great Plains into the Upper Midwest or Great Lakes. If this
scenario pans out, could be yet another window of gales,
possibly strong gales.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory...ILZ006-ILZ014 until noon Tuesday.

IN...Winter Weather Advisory...INZ001 until 9 PM Tuesday.

     Winter Storm Watch...INZ002...9 AM Tuesday to 6 AM Wednesday.

LM...None.
&&

$$

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