Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 172350

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
550 PM CST Sun Dec 17 2017

211 PM CST

Through Monday...

Primary forecast challenges in the short term period revolve around
impacts from fog and drizzle potential overnight, along with a
chance of showers this evening.

A diffuse warm front withing a weak sfc trough extends across nrn
IL/srn Lake Michigan this afternoon.  As the warm front oozes north,
winds are gradually veering to sly-swly, with temperatures
inching into the mid 40s, with decreasing cloud cover. As of 2PM
CST, the northern tier counties remain in the cooler air, under
east flow and a low stratus deck. By this evening what`s left of
the weak warm front will lift north of the Wisconsin border with
slightly higher low level moisture advecting northeast into the
area. There is still good consensus in the short range guidance
for the development of drizzle likely, along with at least some
threat for fog development. A weak shortwave moving through the
swly flow along, along the invof the sfc frontal boundary, may
bring some sct showers this evening. Warm advection and widespread
cloud cover should result in nearly steady temps all night. It`s
unclear how dense the fog will be tonight, but given swly winds
arnd 5 kt and thick cloud cover, it is not a particularly
favorable set-up for widespread dense fog, though visibility down
to 1 mile is quite likely.

Monday`s forecast is still a bit tricky as there is some concern
about the timing of lingering stratus and fog burning off. Most
guidance suggests that this stratus will quickly clear out Monday,
however, given the time of year and the expectation that it will be
a thick stratus deck, it seems likely that most of the area should
stay remain socked in much of the day. So, temperatures Monday may
not be much different than today.


101 PM CST

Monday night through Sunday...

The extended features generally a milder pattern through mid to
late week, a rain to snow type system Thursday night into Friday,
then a return to colder temperatures for the weekend and even
colder air into next week. Some snow chances to appear near the
tail end of the forecast cycle near the Christmas holiday.

Mild southwest flow will continue Monday night into Tuesday ahead
of a cold front that will also largely remain to our north, but it
will graze the area Tuesday. The main question will be how much of
the stratus will erode Monday night. The mesoscale NAM suggests
some lingering stratus overnight into Tuesday under an inversion,
while global guidance generally suggests clouds do fade. If the
global models are right and the NAM is exhibiting its typical high
bias of low level RH, it could be a fairly warm day like this
past Saturday. Still high spread in ensemble guidance to say one
way or the other, but either way we will remain above normal.
High pressure will build in Tuesday night into Wednesday, and
will bring a brief cooler shot on Wednesday behind the cold front.
There will be a weak wave to our north Wednesday, but just expect
an increase in mid to higher clouds.

The reservoir of cold air locked up well north in Canada will get
dislodged later this week as low pressure in the Gulf of Alaska
shifts into the western United States. This low will eventually
dig and close off (if only briefly) near the four corners region
of the southwest. Deep southwest flow ahead of this low will
bring the last gasp in the the mild pattern across our region
Thursday into Friday.

The next precipitation maker for the region will be driven by this
system, as a surface low in lee of the central Rockies will eject
toward the midwest. As outlined fairly well the overnight shift
discussion, the general trend with this system has been for the surface
to track either over the area or farther westward. This would act
to slow the cold air arrival and leave northeast IL and northwest
IN on the mild and windy side of things for most of the event.
There is still much up in the air with this system with timing and
p-type concerns, but it looks like we will remain on the liquid
side of this one for most of it until maybe the very end. A with
such a dynamic and mild system this time of year, there is a non
zero thunder chance.

There is a decent signal from the long range guidance on return to
a colder pattern, and possibly a fairly potent one for early next
week. A deep upper level longwave trough will shift into the
continental United States, while the southeastern ridge will also
strengthen which will initially slow the arrival of the coldest
air. Confidence on colder temperatures is medium-high, but the
magnitude of the cold may be mitigated by the lack of snow cover
across the region also.

For those dreaming of a white Christmas, all hope is not lost, as
several mid to long range models do advertise energy building
around the base of the trough and then move this forcing northeast
sometime in the Christmas Eve/early Christmas Day time frame. At
this point there have still been plenty of model run changes to
say much more than that.



For the 00Z TAFs...

550 pm...Multiple forecast concerns this period including a
chance of showers this evening...ifr cigs/vis with drizzle
overnight into Monday morning...wind speeds Monday and possible
low level wind shear Monday evening.

A weak upper wave will move across the area this evening with
scattered showers across northwest IL which will move into
southern Wisconsin. How far southeast these extend into the
Chicago terminals remains uncertain so maintained vicinity mention
for now but may need a tempo or short prevailing for light showers
as trends emerge in the next few hours. Cigs likely to remain
vfr until after these showers move northeast of the area late this
evening...when cigs will drop to low mvfr. Cigs are expected to
continue to drop to ifr overnight but confidence on timing and how
low is only medium. As these lower cigs develop...some drizzle is
possible which may lower vis to 1-2sm but trends will need to be
monitored for drizzle/vis as well. Cigs will lift through mvfr
mid/late Monday morning and then scatter out early Monday

Light and variable winds to start will turn light southwesterly
this evening and remain light southwesterly through sunrise Monday
morning. Speeds should slowly increase into the 10-15kt range but
forecast soundings suggest gusts toward 20kts may develop perhaps
as lower cigs begin to lift...but confidence for gusts is low. Low
level winds begin to increase Monday evening with the potential
for low level wind shear after sunset. cms


101 PM CST

Marine concerns center on several gale force wind
episodes. The first is associated with low pressure tracking
across central Canada Monday night ahead of a cold front.
Forecast guidance still depicts the the best shot of gales is
over the northern portions of the lake with the southwest winds
Monday night. Behind the front Tuesday afternoon into Tuesday
evening, it is looking more like 30 kt winds south with the best
chance for prevailing lower end gales over the central and
northern portion of the lake. There is a little better agreement
on the system for late in the week, and the associated
strengthening surface low will likely pass over Lake Michigan.
The strength of the low is still a bit in question, but there will
be gusty south winds ahead of it, followed by a shot of west-
northwest winds. Either time could pose at least a brief gale



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...6 PM Monday
     to noon Tuesday.

     Small Craft Advisory...LMZ744-LMZ745...6 PM Monday to 3 PM




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