Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192026

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
226 PM CST Thu Jan 19 2017

156 PM CST

Through Friday...

Rain...fog...and drizzle, maybe a thunderstorm are all on the docket
for the short term. The deep/cutoff upper low across the plains
will slowly lift north tonight. The low will take on a negative
tilt which should keep forcing strong ahead of the low. In
addition to the strong height falls ahead of the low, an elevated
warm front will be lifting northward as well but will never
completely lift north through the area. Isentropic lift with the
moist southerly flow ahead of the low will lead to rain increasing
through the afternoon and continuing for at least a good portion
of the night. Guidance is not all that excited about thunder
chances, but have left a small thunder mention as there could be a
few storms with elevated lapse rates ahead of the low and modest
warm advection on the nose of the low level jet.

Once the lead wave moves through overnight, we will lose the deeper
moisture layer, but continued warm/moist advection ahead of the
low and over the somewhat stalled out warm front across Central
Illinois, in conjunction with continued waves rotating around and
ahead of the upper low would suggest continued clouds, fog, and
drizzle. The pattern would suggest fog/drizzle could easily lead
to at least localized dense fog, but certainly expect lingering
low cloudiness most if not all day. In spite of clouds being in
place to limit warming, dewpoints will be coming up through the
afternoon which should allow temperatures to get into the 40s in
most areas.



224 PM CST

Friday night through Thursday...

An active weather pattern is expected during the period, with
several chances for precipitation. The main concerns initially
during the period will be the threat of dense fog Friday night
into Saturday, and temperatures on Saturday. Attention will then
focus on what appears to be the beginnings towards a turn in the
direction to more winter-like conditions across the area by the
middle to end of next week.

At the beginning of the period (Friday night), the upper level
disturbance, currently shifting over southern California and
Nevada, is expected to shift northward into the Upper Midwest by
early Saturday morning. This system will drive a surface warm
frontal boundary northward over the area Friday night. This front
will not only mark to the leading edge to a very mild airmass, but
also much higher dewpoints. As a result, it appears likely that
fog will develop, either during the day Friday, or definitely by
Friday evening as surface dew points quickly surge well into the
40s. This certainly could result in areas of dense fog and
drizzle across the area for a period Friday night into Saturday

The main concern for Saturday will then be how warm we get during
the day. A very mild air mass (925 MB temperatures +7 to +9) will
be in place over the area during the day. This certainly could
support temperatures well into the 50s on Saturday. However, with
dew points also likely climbing into the mid to upper 40s and
areas of fog around in the morning, we might not loss the low
clouds at all during the day. With this in mind, it may be
difficult to warm much above the lower 50s. In spite of this
concern, however, I have not strayed much from the blended
guidance, which currently advertises low to mid 50s far north, to
the upper 50s south.

Mild, but a bit cooler conditions will continue into early next
week. However, model and ensemble forecast guidance is
suggesting the transition to a colder and active weather pattern
by the middle of the week. This active pattern may support the
development of a storm system that could impact the Lower Great
Lakes Region late Tuesday through Wednesday. This system could
bring us our next chance of precipitation, with some snow possible
by Wednesday. We will need to be watched in the coming days, as a
farther south track could support accumulating snow over the area
Tuesday night into Wednesday.

Following this system, it appears that colder weather will filter
in over the area under northwest flow. Some periods of light snow
will also continue to be possible at times later in the week as we
remain in a semi-active northwest flow aloft pattern.



For the 18Z TAFs...

Main TAF concerns are timing of IFR/LIFR cig/vis trends through
the period, primarily early this afternoon for KRFD, then tonight
through Friday morning for all sites. Showers/isolated embedded
thunder expected to spread across from the south area late this
afternoon, continuing past midnight CST before diminishing to
drizzle which will linger Friday morning.

Weak surface high pressure ridge across the forecast area this
morning has allowed extensive low stratus/fog to develop, though
obs trends over the past few hours continue to indicate a gradual
improvement in conditions with lower cigs scattering across
portions of the Chicago metro area. Low level moisture remains
trapped beneath shallow (but fairly strong) inversion north of an
area of weak low pressure and a warm front currently over the
lower mid-Mississippi and lower Ohio river valleys. A band of
showers from eastern Iowa to downstate IL/lower Ohio Valley will
spread northeast and increase in coverage across the terminals
later this afternoon, with a period of rain/showers expected to
persist through the evening hours in advance of a mid-level
disturbance approaching from the southwest. Forecast soundings and
RAP model analysis does depict an axis of weak elevated
instability coincident with this band of precipitation, which
would support the potential for isolated embedded thunderstorms
especially late this evening through midnight when strongest large
scale ascent is present. Though greatest instability and thunder
threat appears to remain just south of the terminals, did include
a VCTS mention late this evening to express the overall trend.

LIFR/VLIFR conditions are then expected after midnight through
Friday morning, as surface low pressure trough lifts across the
region. Forecast models are in good agreement in depicting
saturated boundary layer up through about 850 mb, which will
likely support an extended period of low cigs/fog and drizzle.
LIFR conditions could persist into Friday afternoon.



401 AM CST

Generally quiet conditions expected across the lake at this time.
High pressure will exit to the east today, and allow for a weak
low to lift up the Mississippi river today. However, it is
expected to dissipate as it moves north. Another low is forecast
to form over the Plains Friday night and reach Minnesota Saturday
night, with a stronger low expected to form over the southern
Plains Saturday night. However, neither systems expected to bring
any significant winds across the lake during this time.






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