Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 050933

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
333 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.




Monday through Sunday...

231 pm...Forecast concerns included potential for mixed precip
Monday night/Tuesday morning...chance of snow Wednesday night/
Thursday morning and much colder for the middle/end of the week.

High pressure over the Ohio Valley will be moving east Monday
afternoon into Monday night. With the new snow pack and low sun
angle...expect high temps Monday to be below guidance levels even
if some sunshine develops though low level moisture fields would
suggest that some low cloud cover could remain with increasing
mid/high clouds later Monday.

Models remain fairly similar with one low moving northeast
through the Ohio Valley/Mid Atlantic Tuesday and a second low
moving across the upper midwest Monday night and into Ontario
Tuesday...with a trailing cold front moving across the area on
Tuesday. There has been some consistency with the low moving
across the Ohio Valley remaining far enough to the southeast that
precip would only reach the far southeast cwa late Monday night
into Tuesday morning. Only the NAM remains further northwest
across the cwa and confidence with this solution is low. Mainly
chance pops with this system but confidence regarding where the
precip actually occurs is low. Thermal profiles continue to
suggest that precip would be a mix and possibly rain or freezing
rain depending on surface temps. With the new snow pack...its
likely that temps will drop to then hover near freezing Monday
night into Tuesday morning...before slowly warming. Too close to
call from this distance but some mention of light freezing rain
seems prudent at this time. If temps end up a few degrees warmer
or the precip shield continues to shift to the east...then changes
to precip type/timing can be expected.

Cold air begins to spread into the area Tuesday and continues
through the end of the week. Models continue to try to develop
light snow across the area Wednesday afternoon into Wednesday
night as the coldest air arrives. The Ecmwf is most developed...
though still a weak surface low through the Ohio Valley.
Confidence regarding system development and associated snow
remains low with just chance pops for this time period.

Another wave rotates across the area Thursday/Thursday evening
that could bring some snow showers or light snow to the area.
Wind direction with the colder air should keep any significant
lake effect snow east of Porter County but still chance mention
for far northeast Porter County later in the week. A clipper like
system is possible next weekend but too early for any specifics
with this system. cms


9z TAF AMDs.

Concerns: Expansion of fog early this morning, how low visibility
will get, and how long it will last. This is coupled with a
potential lowering of cigs to LIFR levels from their current IFR
and the question of whether the cigs even lift very much at all
today. All of these features are only of medium confidence, and
low confidence on ORD/MDW on the extent of vsby reduction. Higher
confidence in lowering of cigs.

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. In this area there has been some clearing of the stratus
leading to fog, but in the areas without clearing the stratus is
lowering to LIFR. The trend should continue farther more so after
12z- 13z closer to the Chicagoland termianals. 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. At this point
confidence is highest continued IFR stratus with a dip to LIFR
this morning with a downward trend in vsby as well. 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 overnight as the warm advective
axis is going to shift right over our area too and ORD/MDW do
have a similar snowpack in place. Forecast soundings from the
RAP/HRRR indicate very little lifting today after cigs retreat
again later today until possibly later tonight as winds shift more

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 Tuesdsay 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 strenghten 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, particuarly
across 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.






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