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
FXUS63 KLOT 051143

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
543 AM CST Mon Dec 5 2016

332 AM CST

Through Tuesday...

Main forecast concerns/challenges are with potential for dense
fog/freezing fog over mainly northern Illinois this morning,
brief/light wintry mix early Tuesday morning, and then with much
colder air later in the work week.

Surface ridge axis situated through much of the CWA, however,
gradient on the northern periphery allowing for a steady west wind
early this morning. These winds and the stratus in place helping
temps to remain nearly steady in the low 30s. Although temps may
fall a degree or two, don`t think temps will drop too far from
where they are currently, staying right around freezing. As the
high shifts to the east this morning backing winds will allow for
upstream WAA to shift east over the area. This setup over a snow
pack along with a strong inversion anticipated to be in place
today, will support continued cloudy skies today and likely into
tonight. Continue to monitor fog trends early this morning with
upstream trends still showing falling ceilings and visibility in
fog over eastern Iowa. Guidance varying significantly with the
extent of this easterly edge of more dense fog later this morning.
However, with the previous noted warm/moist advection occurring
over a snow pack, deteriorating conditions this morning is still a
very possibility over the CWA. Have maintained areas of fog over
the entire CWA through midday but have confined the dense
fog/freezing fog wording over the western half of the CWA. Think
these areas have the highest chance of getting into this dense fog
before winds continue to back to a south/southeast direction by
late morning. Although some patchy dense fog may work further into
other areas in northeast Illinois and northwest Indiana, it should
not be quite as dense. Areas along and west of I39 could get into
this dense fog around 12z, with it then possibly spreading east
soon there after. Given the temps hovering right around freezing,
will need to monitor for potentially slick conditions through mid

Trends in model guidance continue to take the next system lifting
northeast out of the southern Plains tonight into Tuesday more to
the east of the area. The bulk of the precip with this system will
likely stay away from the CWA, but do think there could be a small
window where the northwest extent of the precip shield moves
through the far southeast area of the CWA very late tonight into
early Tuesday morning. So have maintained low chance pops. Given
some drier and warmer air in the lower levels, precip type would
likely vary if it occurred. Do think a wintry mix that could even
include some light freezing rain may be possible, before this
precip departs by mid morning Tuesday. Large upper level system
will slowly move east through the northern Plains and upper
Midwest on Tuesday, with associated cold front expected to push
the region throughout the day. Have kept dry conditions over the
RFD area but some guidance is developing some light QPF along the
front in this area Tuesday morning. Upper level support along this
boundary is greatest in northwest Illinois, however, drier lower
levels once again complicating the situation. Once again, have
maintained dry conditions until a better signal arises.



337 AM CST

Tuesday night through Sunday...

Colder air returns at the start of the period getting well into
the 20s Tuesday night and for some locations, not getting out of
the 20s on Wednesday. Persistent CAA will usher in much colder
air, likely getting colder through the end of the work week.
Guidance has come into better agreement with the next chance of
snow, which would be Wednesday night into early Thursday morning.
Trends would be for lessening chances of snow, and so have reduced
pops. Snow may still occur mainly over the eastern CWA, but if it
does occur, it will likely be on the light side. Cold air in place
and setup favoring lake effect snow potential in northwest Indiana
by Thursday and possibly continuing into Friday before boundary
layer flow backs to more of a westerly direction. Details still to
fall out, but maintained chance wording in the forecast.



For the 12Z TAFs...

Concerns: Expansion of fog early this morning, how low visibility
will get, and how long it will last. This is coupled with lowering
of cigs to LIFR and even possibly VLIFR levels from their current
IFR and the question of whether the very low cigs even lift very
much at all today.

An extensive IFR stratus deck is locked in place across the region
this morning. There are no real indications that this deck will go
away, but short term guidance is suggesting a lowering of ceilings
and a corresponding drop in visibility as this occurs. There is a
subtle wind shift to west-southwest across western IL behind a
surface high in an area of warm advection atop the fresh snow
cover. Ceilings and visibilities continue to fall as the warm
advection wing spreads east. The trend should continue with cigs
likely falling to LIFR after 12z- 13z closer to the Chicagoland
terminals. There is some guidance bringing visibility in the tank
after daybreak and keeping it down for several hours, other
sources do show a lowering cig and vsby trend but not quite as low
on the vsby reduction.

At this point confidence is highest continued IFR stratus lowering
to LIFR and getting into the 300-400 foot range of LIFR. VLIFR
cigs of 100-200 ft is not out of the question if any drizzle gets
rung of the lower deck but have lower confidence on that.
Confidence is also high in a dip to IFR visibility, with some LIFR
getting into North Central IL and possibly across ORD/MDW. The low
level wind field turns more southwest through the morning such
that confidence on 1/2sm vsby or lower at ORD at MDW are still
very possible, but not a lock just yet. This is something that
will require mesoscale monitoring here in the near term as the
warm advective axis is going to shift right over our area too and
ORD/MDW do have a similar snowpack in place and the low levels are
very moist/wet ground. Forecast soundings from the RAP/HRRR
indicate very little lifting today after cigs retreat again later
today at all. Winds shift more southeasterly later today which
could bring some improvement but not likely at this point.

Precipitation chances look fairly low for most of the area. Low
pressure will sneak northeast into the Ohio valley Tuesday
morning, but most guidance keeps the light precip across northwest
Indiana and more so points farther south and east across central
and southern Indiana. There is also a weak trough axis that will
cross northern IL Tuesday morning but precip chances with this
also look low. Southeast winds ahead of this boundary will
eventually shift back to westerly.



256 AM CST

Modest west winds to 30 kt will transition to southwesterly and
remain elevated...though possible settling slightly to 15 to 25
kt as high pressure moves east across the Ohio Valley and low
pressure strengthens across the upper Midwest. This low will then
shift east across southern Ontario on Tuesday and then proceed
northeast to James Bay by Wednesday night. South-southeast winds
will strengthen to the south of this low later tonight. A strong
cold front associated with the low will pass across Lake Michigan
later Tuesday. The pressure gradient to the south of this low
coupled with a strengthening ridge of high pressure across the
northern plains and extending into the Ohio valley will create the
potential for low-end gales as early as Tuesday night,
particularlyacross the north half of the lake. Stiff west winds
shift northwesterly later in the week in the cold advection behind
this front, occasionally getting near gale levels again though
guidance is not super excited about persistent gales and holds
winds are 30 kt or so through an extended period. The gale
potential does still exist though.






WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.