Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160925
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
325 AM CST Sat Dec 16 2017

.SHORT TERM...
116 AM CST

Through tonight...

Low pressure over the Dakotas will move southeast to Iowa this
afternoon before dissipating. A warm front currently stretches
southeast from this low across southern Wisconsin. As the low
drops southeast to Iowa, the warm front should reverse course and
begin to ooze south into northern IL this afternoon. Differential
heating between the lake and land should result in a cooler/denser
air mass over the lake, and while a true lake breeze may not form,
the lake may add a bit of an extra push to the front this
afternoon.

Cloudiness is likely to increase from the south overnight as
weakening southern stream shortwave lifts northeast toward the
region. No precip is expected tonight, however as cloudiness
increases look for temps to level off and likely begin to rebound
prior to dawn, especially southern CWA.

- Izzi

&&

.LONG TERM...
324 AM CST

Sunday through Friday...

The area will temporarily be in an above normal temperature regime
as the mid and upper pattern across North America reshuffles,
resulting in mainly positive mid-level height anomalies. An
exception to this will be a cold front/trough passage Monday night
into Tuesday. Later in the work week, significant amplification
of poleward ridging up to and north of Alaska will set the stage
for the likelihood of the return of a colder pattern to close the
week and heading toward Christmas. A storm system of uncertain
magnitude, track and evolution from the Plains toward the Midwest
and Great Lakes will likely affect the region Thursday and Friday
and herald the return of the colder pattern. Accumulating snow
will be likely with this system in the greater region, but it`s
still far too early to tell if these accumulations will fall
within the CWA and how much.

A weakening/shearing positively tilted short-wave trough will
affect lift into the area on Sunday, with a shot of light rain
probable south of I-80. The cutoff in rain chances will likely be
sharper than in current grids, so expect refining of this, with
parts of the area likely to stay dry. The warm frontal boundary
expected to sag south into northern IL today will likely lift and
stall just north of the CWA on Sunday. Given skies quickly
becoming overcast and also precip for parts of area, undercut
guidance high temperatures on Sunday to generally upper 30s to
around 40.

Sunday night will feature a weak gradient/light flow regime,
likely without much if any dry advection. Anticipating skies to
remain overcast, with any breaks favored over far northern
Illinois. The setup seems to be one favored for patchy fog
potential, so have added it to the grids into Monday morning, with
the better chance for any dense fog south of I-80 where higher
dew points will be. Furthermore, with broad ascent over the area,
but the column devoid of ice crystals due to dry air aloft, patchy
drizzle is possible. A consideration for this period is that the
NAM/WRF guidance is exhibiting typical low level moist bias, but
nonetheless, synoptically, patchy fog/drizzle mention seems
reasonable.

On Monday, it`s uncertain how long expected overcast will take to
scatter, and have leaned more pessimistic with the sky cover in
the grids. This could have some impact on high temperatures, but
with a mild start to the day well into the 30s most areas, highs
should still reach into the 40s for most locations except perhaps
far north. Southwest winds will increase during the afternoon
ahead of a cold front trailing from strong low pressure over
northern Manitoba. The cold front will move across the area Monday
night into early Tuesday, with breezy west winds in its wake. The
models are still varying fairly significantly on the magnitude of
the colder air behind the front. With this being said, even the
colder solutions are not all that cold for December, so
temperatures have a good chance of reaching the 40s in many areas
and if the warmer models verify, more areas could approach or
exceed 50.

Tuesday night through Wednesday will be quiet and cooler under
surface high pressure ridging. Attention then turns to the low
pressure area over the Plains. The mid/upper pattern will be
complex with this setup, with none of the major players being
sampled by the RAOB network for the next few days. Thus expect
continued model/ensemble variability until everything gets sorted
out. It`s likely that some energy will dive into the southwest
CONUS and could get cut off. Enough looks to escape to form a
positively tilted trough over the region, as 250-300 mb jet
strengthens to nearly 150 kt by Friday. The surface low over the
High Plains with an inverted trough extending toward Iowa will be
followed on its heels by strong Arctic high pressure spurred from
the west coast/Alaskan ridge amplification mentioned earlier.

This will serve to ooze a cold front toward the area on Thursday
while the surface low (more of an elongated trough on some
models) weakens and tracks over or south of the CWA Thursday night
into Friday. How this all evolves will determine if and how much
accumulating wintry precipitation occurs within the CWA into
Friday. The pattern does look to support a swath of accumulating
snow somewhere in the region. Thus, this could offer an
opportunity for at least parts of the CWA to enjoy a white
Christmas, as colder than normal conditions return for Christmas
weekend, maintaining any snow cover (if any) that is put down. For
now, have broad rain/snow mention in forecast for Thursday into
Friday. Stay tuned.

Castro

&&

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

VFR conditions through the day Sunday. Southwest winds will
gradually ease the next few hours, but likely to maintain a
southwest to south-southwest flow over the area Sunday. Later in
the afternoon, look for winds to flip to light easterly and likely
remain easterly Sunday night. Some potential exists for some
patchy fog to develop after midnight, mainly at the more rural
airfields.

- Izzi

&&

.MARINE...
116 AM CST

Primary marine concern is with low pressure tracking across
central Canada early-mid week. Initially, it will result in a
period of marginal southwest gales Monday night into early
Tuesday, especially northern portions of the lake. In the wake of
the front another period of gale or near gale force west-northwest
winds looks likely later Tuesday into Tuesday night in the wake of
the trailing cold front that will move across the lake early
Tuesday. Another storm system is likely to move across the region
late in the week, but still a lot of spread in the models
regarding the strength and track of this system resulting in lower
than average confidence in wind speed/direction late in the week.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Gary to Michigan City IN until 9 AM Saturday.

&&

$$

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