Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 162036

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

205 PM CST

Through Sunday...

Concerns center around fog and low cloud development tonight,
associated sky cover and temperature trends, and precipitation
chances Sunday.

Southwest winds and plentiful sunshine have created a fairly nice
day in the weather department as area temperatures range from the
low 40s in far northeast IL to near 50 across central IL. Onshore
flow has setup near the lake front in Lake County IL where readings
hold near 40. Surface analysis paints a warm front just south of
Milwaukee. Stratus is found in the northeast wind regime just north
of the front, with fairly clear skies to the south of the front.
Initial clear skies and lighter winds this evening should promote a
fairly quick drop off in temperatures. In response to low pressure
that is organizing to our southwest, the warm front will likely
shift south into northern IL tonight. Expect stratus and at least
patchy fog to accompany the front under cool northeast flow.
Moisture return is not great yet, so we don`t expect fog to get too
dense, but at least some patchy ground fog or freezing fog is
possible. Confidence on how far south the front gets is not super

Water vapor imagery depicts the moisture return associated with an
upper low across Texas. This energy will attempt to overrun our
shallow cold low level airmass into Sunday. Model guidance generally
agrees in delaying precipitation onset Sunday morning until low
level temperatures get above or a few degrees above freezing, and
the profile aloft is supportive of liquid precipitation. If
anything gets out ahead of the main precip shield, there could be
some cool low lying areas where a small window of very light
freezing rain could exist, but feel this is not going to be the
norm as forecast soundings have a ton of dry air ahead of the
front. The upper low will weaken as it enters northeast IL and
northwest IN such that the main precip shield will remain south of
I-80, but clouds and drizzle very light rain will likely shift
north through the afternoon as the better moisture return begins.



235 PM CST

Sunday night through Saturday...

Main concern for the long term forecast period will be a pattern
shift, bringing an end to the above normal temperatures back to more
seasonable conditions.

By Sunday night, the region will be firmly under the influence of
broad riding over ern 2/3 CONUS, a deep closed low over the Desert
Southwest and a building ridge poking into the Pacific Northwest.
The resultant swly flow aloft will keep the region under a
persistent pattern of deep layer warm advection and the resultant
above normal temperatures.  For Sunday night through Monday, a weak
sfc trough and diffuse warm front will set up across nrn IL/srn WI,
keeping the region under a weak pressure gradient and a shallow
saturated layer extended up to 850 mb with much drier air aloft.
This should set up a scenario for fog and drizzle for Sunday night,
and likely through much of the day on Monday.  The latest guidance
suggests that a weak cold front will move through the area late
Monday night or early Tuesday, with high pressure building across
the nrn plains and Upper Mississippi Valley.  This should usher in
some drier air, but since the airmass will be more of a Pacific-
modified continental polar nature, rather than an arctic air mass,
there should be little impact on temperatures trends and just a
scouring out of the low level moisture and associated fog and

The long range guidance remains in good temporal agreement on
lifting the closed upper low out of the Desert Southwest early next
week.  The system will evolve into an open wave, tracking to the
Lower Ohio Valley by Wednesday morning.  While there are some
intensity differences among the long range models, there is
relatively good agreement on timing and path.  So, confidence is
relatively high in the general trend in the longer range forecast
period.  The associated sfc system with the southern stream wave
will likely be shunted south by the building high pressure, keeping
any associated pcpn well south of the CWA.  The next feature of
concern to nrn IL/nwrn IN will be the pattern shift expected for
late next week, as has been discussed the past few discussion.  The
models are still in good agreement on the idea of northern stream
shortwave energy breaking down the upper ridge and allowing a
Pacific-origin system to drop into the nrn plains.  However, there
is some concern over the evolution of another closed upper low
moving through the Four Corners region Thursday.  The ECMWF is very
progressive with this system, lifting it out as an open wave and
then phasing with a nrn stream shortwave and tracks a sfc low across
nrn IL on Friday.  The GFS is significantly different in cutting off
the upper low, indicating it digging in south into northern Mexico.
Given this kind of uncertainty, specific details of a sfc low track
late next week will be questionable.  However, in either case, still
feel that the going temperature trend is going to be for a return to
more seasonable conditions and that chances for pcpn will be high. P-
type and amounts will remain questionable for late next week.


For the 18Z TAFs...

Concern is for stratus or fog late tonight into early Sunday.
Stratus formed just north of the warm front in Wisconsin this
morning. Model guidance shifts the front south into northern
Illinois late tonight into early Sunday. Confidence is highest on
low clouds getting into RFD, a bit higher than medium confidence.
Confidence is lower at ORD and MDW but still possible, so we
included at SCT IFR deck in for now. This will bear monitoring
with guidance mixed as to how for south clouds will make it, but
the pattern would suggest a period of low clouds or fog if the
RAP/HRRR are correct. If clouds do come in they will be IFR,
especially right at the front location. Precipitation will shift
into the area from the southwest Sunday morning, but remain south
of the terminals in the morning, possibly not even making it north
of I-80 through the day, but expect lowering clouds again late in
the day.

Winds will remain southeast today, shifting to east or
variable around 0z. Winds will hold east/southeast until the front
lifts back north mid day Sunday.



235 PM CST

A relatively quiet period of weather will continue for Lake
Michigan for the next several days as a series of weak clipper-
type lows track across the upper Great lakes region. The next
chance for impactful weather and Gale Force winds will not be
until early next week when the passage of a cold front. There is
still some uncertainty as to the timing of the cold front passage,
but the latest model guidance continues to suggest Monday night
or early Tuesday for the frontal passage, with a short period of
southwest gales for the northern portions of the lake in advance
of the front on Monday and then a period of west to northwest
gales for Tuesday into Tuesday night. The active pattern looks to
persist late next week with another system bringing another round
of strong winds and colder air to the lake.





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