Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180930

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
330 AM CST Mon Dec 18 2017

330 AM CST

Through tonight...

The main concerns are with areas of fog and drizzle this morning
and then cloud cover trends this afternoon along with the impact
of these trends on temperatures. A warm front is lifting north
across far northern Illinois this morning, with light to moderate
southwest winds. Patchy to areas of fog and drizzle (with even
pockets of steadier drizzle) are in place early this morning, with
RFD reporting 1/4 SM visibility at this hour. Based off observations
just to the west and southwest of the RFD area, it appears that
the dense fog is likely to be short-lived, so not planning any
headlines for it. Any lingering fog should erode by the mid to
late morning hours, with patchy drizzle gradually sliding east,
and a completely dry afternoon favored.

Returning to cloud cover trends, GOES-16 advanced nighttime
microphysics RGB indicates a hole in the low stratus poking into
northeast IA, but otherwise solid coverage of it extending north
of the hole and south of it into northern MO. Cloud cover trends
are as usual tricky. Have overall leaned a bit more pessimistic
than much of the guidance. Boundary layer flow driving the
movement of the stratus is not expected to significantly pick up
until after dark, remaining out of the west-southwest 10-20 kt
range through that time. Thus, anticipate a slow movement of the
cloud deck, keeping much of the daytime hours OVC. Temperatures
have gradually risen through the night, so the mild starting point
will still enable low 40s highs along the WI border counties and
mid to upper 40s everywhere else. Should the thinking on clouds be
incorrect, a quicker clearing could result in going highs being a
bit too cool.

Southwest winds will pick up tonight ahead of a cold front and
become gusty at times, along with some cloud cover. Therefore,
temperatures are unlikely to drop much. Leaned closer to raw model
guidance for lows with this in mind, and a bit above previous
forecast, with mid 30s to around 40 indicated. Would not be
surprised if some locations remain above 40 through the night.



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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