Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS63 KLOT 102346
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
546 PM CST Sun Dec 10 2017

.SHORT TERM...
119 PM CST

Through Monday...

Main concern for the short term forecast period will be another
chance of snow tomorrow.

For the remainder of this afternoon, a stratus deck is quickly
pushing south out of Wisconsin with the northern tier counties
already under overcast skies and the stratus deck will continue to
spread south and east, bringing an end to the brief period of
sunshine today.  Temperatures will top out in the upper 20s to low
30s over the northern tier counties near the Wisconsin border and
middle to upper 30s elsewhere.  Winds are also shifting to
northwestward, which will usher in cooler air along with the
increasing cloud cover.  Under persistent cold advection,
temperatures overnight tonight will drop into the upper teens to
lower 20s.

Forecast concern will quickly shift to snow chances.  latest
guidance is a little slower in bringing the warm advection wing of
pcpn into the area, ahead of a weakening low dropping out of Canada.
The sfc low track is expected through central Wisconsin and central
Lake Michigan.  There is relatively decent agreement among the
models in this system being on a weakening trend as it tracks just
north of the CWA.  However, timing is still a concern with the
possibility that the snow will still impact the morning rush hours.
So, while some snow is expected with the warm advection zone and
amounts/rates/accumulations should be light, there may still be some
travel impacts for the morning rush hours.  As the sfc low continues
to track east through the day, another shortwave taking a more
southerly track will induce development of another weak low over the
middle Mississippi Valley by mid day.  As this system tracks east
through the afternoon, there is some indication in the guidance that
an f-gen band may set up across nrn IL/srn WI durg the afternoon
hours.  However, there is not a strong model consensus with this
scenario, but feel that there is enough confidence to at least
increase PoPs and QPF over the nrn tier counties tomorrow afternoon
with a chance for around an inch of new snowfall with the f-gen
band.

&&

.LONG TERM...
135 PM CST

Monday night through Sunday...

Meridional upper level flow will begin the period with
the extensive upper ridge across the west extending up into
Alaska. What goes up must come down, and on the downstream side of
the trough the flow aloft will be headed our way from northern
Canada. A surface low pressure will deepen just to the east of our
area Monday night as additional shortwave energy will dig into
amplifying upper trough over the eastern Great Lakes, and will
bring the leading edge of a cold front to southern Lake Michigan.
Model guidance paints some scattered light QPF Monday evening
with the front and maybe some lingering snow bands from the
afternoon, but soundings do not suggest much precip will be left
at this point. By the time the low levels get into the better snow
production region, low level moisture will be scant Tuesday
morning. This pattern will support a reinforcing shot of cold air
for Tuesday. Highs on Tuesday will only reach the low to mid 20s,
and it will be a blustery day with northwest winds of 10-20 mph,
making for daytime wind chills struggling to get into the teens.
Low level flow will be northwest which again favors LaPorte county
eastward for lake effect snow, but there is enough of a component
into Porter county on the GFS for a mention of snow showers as
low level forcing/thermodynamics are fairly good with an unstable
layer right in a prime snow production zone. It is not as deep of
a mixed layer as this weekend, but sufficient.

Warm air advection will only be modest Tuesday night, and with
weakening winds Tuesday night will be a cold one with teens area
wide, even some single digits.

The northwest flow wave train will continue as the ridge in the
west, while weakening a little, still holds for a bit longer. Warm
advection ahead of these waves and a lingering baroclinic zone in
the region will bring occasional chances for light snow through
Thursday night, but no big snow storms. Our next chance for light
snow would be Wednesday, but this appears to just clip the area.
There could also be some snow shower potential behind another
weaker cold front Thursday.

The upper ridge in the west shows signs of finally breaking down
late in the week and the upper level flow will flatten. This will
bring a bit of a milder pattern to the midwest this weekend. With
a ridge expected to be positioned across the south, any
moisture/forcing will be confined to additional northern stream
waves that, at least through the weekend remain to our north.

KMD

&&

.AVIATION...
For the 00Z TAFs...

Complicated forecast with lower than average confidence regarding
snow timing/potential Monday. Looks like a decent shot of a band
of some light snow moving across the terminal near sunrise, with
brief vsby reductions and possibly a coating of snow. Should be a
break in the snow much of the morning, with potential for another
brief shot of some snow midday/early afternoon, though only a
couple models are showing this. Main band of more impressive snow
is likely to set up over southern WI or far northern IL (north of
the Chicago terminals) tomorrow afternoon, then this band should
shift southeast across the terminals tomorrow evening, but likely
in a weakening phase as it does so raising doubts about the
duration and magnitude of impact it will have.

Monday evening, as the system pulls out looks for strong and gusty
northwest winds to lock in with MVFR cigs and probably wind
whipped flurries with perhaps a couple brief shots of light snow
showers.

- Izzi

&&

.MARINE...
135 PM CST

Next concern is for a period of northwest gale force
winds Monday evening into early Tuesday, and possibly some
freezing spray.

Another upper level disturbance will allow surface low pressure to
take aim at southern Lake Michigan by Monday afternoon and
evening. The low will undergo more significant deepening Monday
night as it moves to the eastern Great Lakes. Meanwhile, cold
arctic air will move over the lake as high pressure builds south
across the Plains. Winds will shift to the northwest by Monday
evening as the low and associated cold front move through, with
winds quickly increasing to 40 kt gales by late Monday evening.
Cold air, with temperatures in the teens to lower 20`s, will be
drawn across the lake by these strong winds, with the potential
for freezing spray increasing by Tuesday morning, and persisting
into Tuesday night before the winds gradually ease. A gale watch
has been hoisted for all of the open waters of Lake Michigan for
the Monday night into Tuesday afternoon. It is possible that a
gale may be needed for the IL nearshore waters also, but the low
track favors the open waters and the Indiana nearshore.

Winds will then diminish fairly quickly late Tuesday night into
Wednesday, as weak high pressure ridging spreads across the lake.
Northwest flow aloft may bring another low or two across the area
Wednesday night and Thursday.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Watch...LMZ777-LMZ779...10 PM Monday to noon Tuesday.

     Gale Watch...LMZ743-LMZ744-LMZ745...midnight Tuesday to noon
     Tuesday.

&&

$$

VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSCHICAGO
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.