Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180729

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
129 AM CST Mon Dec 18 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.


129 AM CST

Tuesday through Sunday...

Big changes on tap during the long term period in the hemispheric
circulation over the north western hemisphere with a transition
from a more zonal split flow, to a more phased and highly
amplified meridional flow by the weekend. The result will be a
change from unseasonably mild conditions this week to a colder
pattern heading into the weekend, with a potentially wet, stormy
period as we make the transition. As is always the case, the
devil will be in the details, which get murkier and more uncertain
by Friday into the holiday weekend.

Tuesday, a cold front trailing southwest from a powerful cyclone
tracking across central Canada will move across the area during
the day. Moisture and forcing will be lacking with this front, so
not anticipating much more than a wind shift, with even the colder
air lagging behind the front. Should start the day Tuesday
unseasonably warm, so given the expected sunshine, it looks like
many areas could make a run at reaching/climbing above 50 degrees
Tuesday. Behind the weak cold front, look cooler temperatures
Tuesday night through Wednesday night, but even those cooler temps
still look to be a few degrees above average.

Pattern should get more interesting heading into the end of the
work week as strong shortwave digs into the southwest U.S. and
is progged to close off a rather deep tropospheric circulation.
Lee side cyclone will develop Thursday with surface warm
frontogenesis taking place east from that low into the mid MS
Valley Thursday. Initially, looks like most of our CWA could be
north of the warm front during the day Thursday.

By Thursday night, the sfc cyclone will move northeast to the
southern Iowa/northern Missouri with the associated warm front
progged to lift north across the CWA Thursday night. Expect rising
temperatures Thursday night behind the warm front with most areas
likely well into the 40s if not lower 50s by Friday morning. Cold
front should move across the area Friday afternoon, but not before
most (if not all) of the CWA warms into the 50s Friday. Could see
a few showers Thursday night, mainly northern CWA ahead of the
warm front and then again Friday into Friday evening ahead of the
cold front.

Forecast confidence decreases substantially at this point as
medium range models have been trending farther west with the axis
of the highly amplified long wave trough. The latest 00z
operational runs have continued this trend and that has
potentially big implications not only on temperatures over the
weekend but also precip chances. Unlike earlier runs which had
the heart of the arctic air mass spreading into the area in the
wake of the Thursday night/Friday morning`s shortwave, it now
holds the heart of the Arctic air farther west. While that would
keep temperatures more moderately cold this weekend, the bigger
implication is that it would keep us far closer to the baroclinic
zone and potential storm track. If we do end up close to the
baroclinic zone over the holiday weekend, that would bode well for
those dreaming of a white Christmas, as any shortwave energy
riding up the eastern side of the deep long wave trough could
easily lay down some accumulating snow. Important to keep in mind
that medium range models have yet to really lock into a solution
and until they do further refinements to the forecast could be
needed. If you have travel plans across the Midwest over the
holiday weekend, it would be advisable to pay attention to later
forecasts as there is potential for wintry precip making travel
dicey, but too soon to say to what extent and where.

- Izzi


For the 06Z TAFs...

CIGS will likely continue to lower through overnight hours to IFR
and potentially LIFR. Light fog with MVFR/IFR vsbys are likely,
but have recently seen some VLIFR dense fog develop out west
toward the MS river. Conditions will improve Monday mid-late
morning to MVFR and eventually clearing out, though confidence is
low on the timing of such clearing. As the low level westerly flow
increases late in the afternoon and early in the evening, expect
that to push the lower cloudiness east of the terminals either
late in the afternoon or early in the evening. Only other aviation
concern is the threat of LLWS Monday evening as winds just off the
deck ramp up to over 40kt.

- Izzi


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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