Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 061002 AAA
AFDLOT

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
402 AM CST Tue Dec 6 2016

.SHORT TERM...
335 AM CST

Through Wednesday night...

Stratus is starting to erode from the southeast as a subtly drier air
mass advects in, with the low cloud largely gone along the I-55
corridor southeastward. Farther west expect the stratus and fog to
hold a bit longer being still removed from the drier air. Vsbys
have largely been  -1 mile in the denser fog and really have
inched upward of late. These areas will remain under the inversion
until a cold front approaches this afternoon, with other area s
east still having a higher cloud deck. There are some indications
that with a weakening wind field in the COL region of the
approaching cyclone that vsbys could come back and become locally
dense at daybreak or shortly thereafter. This is across north
central Illinois including the Rockford area. Most locations in
this area at freezing or so in the lower cloud deck that freezing
fog would not be a huge concern but will need to monitor this
first near term concern.

Several additional features of interest in the near term ahead of
our upcoming arctic air invasion. The first is a band of
precipitation that is lifting northeast ahead of low pressure
across the Arklatex region. This low will continue northeast into
the Ohio and Tennessee valleys today, which its precipitation
shield largely heading with it. Weak frontogenesis and isentropic
lift across our far southeastern counties (east Central IL and
several NW Indiana counties) will likely allow some echoes to
move through in the coming hours. Temperatures have warmed to
around 32 degrees across Iroquois county and into
Benton/Newton/Jasper, with some slightly cooler temps farther
north. Low level profiles suggest precipitation will be in the
liquid form with a low level warm nose, though possibly
transitioning to snow with wet bulbing. Expect there could be a
window of light freezing drizzle/rain here a few hours on either
side of daybreak. Much of this activity is not reaching the
ground yet but radar returns are increasing. Model QPF is very
low such that this does not appear to be a big problem, but could
create some localized slick conditions as model reflectivity
fields slow a slight uptick after 11z for a few hours before low
level wet bulbs get above freezing.

Next on the docket is an area of strengthening low pressure across
the upper Midwest. Extending south of the low is a cold front
where an area of light snow is falling. This front will move into
north Central Illinois later this morning/early afternoon. In our
area the front will become more removed from the upper forcing
that will head north of the area, but model reflectivity showing
some very light echoes across our far northern counties this
morning into the afternoon. This could be a few flurries/drizzle
as it comes through. For the rest of the area, this front will be
the leading edge of a much colder air mass that will ooze in from
the northeast in the coming days. Expect winds to pick up mid to
late this afternoon ahead of and behind the front with gusts 25 to
maybe 30 mph. Otherwise for most areas the front will come in with
some clearing and little precipitation.

The leading edge of the cold front will pass through the remainder
of the area tonight. With some clearing and the cold advection
temperatures will fall into the lower 20s in many areas. The cold
air will continue to funnel in from the northwest on Wednesday as
low pressure slides by well to our north. High temperatures look
to remain in the mid 20s north to near freezing south. Cloud cover
will increase ahead of a southern stream upper wave at the base of
the large upper trough axis across the center of the country
extending through Central Canada. No real surface reflection
develops on the frontal boundary ahead of this wave such that
models have trended toward very little precipitation if any
Wednesday night except possibly in our southeastern areas (per
only a few ensemble members) in addition to across the lake where
the lake effect machine will ramp up.

KMD

&&

.LONG TERM...
400 AM CST

Thursday through Monday...

Very cold and somewhat cloudy conditions will greet us on Thursday
as the cold upper trough migrates across the area. Some flurries
are certainly possible as energy will rotate southeast through the
trough axis. Highs peak in the lower 20s. While the focus of the
lake effect machine will mostly be on the east side of Lake
Michigan, the low level flow turns a bit more northwesterly such
that lake effect snow may enter Porter and possibly Lake County
Indiana later Thursday and Thursday night. More sunshine is
possible Friday, but the core of the cold air will have arrived,
making for another very cold day. Highs may not even reach 20 in
north central IL. The coldest night area wide looks to be Friday
night as a high pressure ridge will move overhead. Some of the
drop off could be tempered as warm advective high clouds arrive,
but still looking at teens. Warm air advection will continue
Saturday ahead of strengthening upper level low pressure in the
west.

Of more significant attention in the extended period is the
potential for another round of snow Saturday night into Sunday.
The 0z GFS is amped up with all the classic signs of a powerful
winter storm (deep upper trough becoming negatively tilted,
strengthening upper jet and associated frontogentical
circulations, strong moisture transport on a low level jet,
deepening surface low) on Sunday while other guidance is a bit
more muted, certainly reflective on how the upper trough ejects
from the west and interacts with moist low level flow. All models
show ascent developing ahead of western CONUS upper level energy
with some lighter snow getting into the Mississippi valley by late
Saturday afternoon then spreading into at least portions of the
area Saturday night, then their evolutions diverge considerably
with the EC some 18-24 hours later with bringing stronger upper
level energy capable of developing a strong surface low. The low
it develops is weaker as well but does still bring a snow/snow-
rain mix in later Sunday/Sunday night. The Canadian model is sort
of in between the two with a single system but more muted than the
GFS. GFS Ensembles support a less amplified pattern not like the
operational GFS, but predictability is somewhat low as expected.
Regardless there are signs for another chance for snow during this
period.

KMD

&&

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

Confidence has increased somewhat that Chicago area terminals will
not see a significant deterioration in conditions tonight.
Clearing line/back edge of stratus is marching steadily northwest
across far southern Chicago suburbs. Expecting the northwest
advance of the clearing to slow overnight, but GYY appears likely
to clear out with decent shot of clearing at MDW and not out of
the question at ORD. A modest easterly wind and slightly drier
conditions upstream don`t seem to favor dense fog development
after skies clear though some light fog is likely to persist.
Farther west at DPA and RFD confidence is lower and conditions
could lower tonight, but at this point the chances of conditions
hitting the deck are about on par with potential for conditions to
improve overnight. Given uncertainty didn`t stray substantially
from previous TAF which represented a reasonable compromise in the
possibilities.

Should see winds becomes light/variable or calm for a time Tuesday
morning before cold front comes through midday. In the wake of the
front look for improving cigs/vsbys, potentially quite quickly. As
cold air streams in look for westerly winds to attain some
gustiness late in the afternoon that will likely persist into
Tuesday night.

Izzi

&&

.MARINE...
233 PM CST

Moderate south flow will remain in place across Lake Michigan today
behind a departing high over the Ohio Valley and ahead of a
developing low over the Plains. The low will lift across northern
Minnesota Tuesday to James Bay Wednesday with a trailing cold front
pushing across Lake Michigan Tuesday afternoon and evening turning
winds westerly. Tuesday night into Wednesday, expect westerly winds
to peak around 30 kt with a few gales possible, mainly over northern
Lake Michigan. Expect west to northwest winds to around 30 kt to
persist through the remainder of the week with another possibility
for gales behind a secondary front Thursday night into early
Friday.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...3 PM Tuesday TO 10 AM Wednesday.

&&

$$

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