Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192332

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
532 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 00Z TAFs...

532 pm...Several forecast concerns this period including cigs/vis
later this evening through the remainder of the period.

Periods of rain will move across the terminals this evening and
slowly begin to shift east after midnight with mainly drizzle
continuing overnight into Friday morning. Best instability remains
south of the area tonight and threat of any thunderstorms should
remain well south of the terminals as removed vicinity
thunder mention with this forecast.

Cigs have become vfr and guidance would suggest this should
continue for a few more hours but with light easterly winds off
the lake...expect low mvfr or ifr cigs to develop during the
evening hours and while confidence is high this will occur...only
medium confidence on timing and cigs may end up lower sooner than
currently advertised. Cigs will lower to lifr late this evening
and possibly to vlifr by Friday morning...slowly lifting back to
ifr Friday afternoon.

Visibilities will likely hold in the 2-3sm this evening while the
rain is occurring but eventually will fall to 1sm overnight.
There remains some uncertainty regarding how low visibilities will
fall in the immediate Chicago metro area. Vis may drop to 1/2sm at
ord/mdw by sunrise and continue into mid morning Friday but
confidence only medium at this time and opted to hold vis at 1sm
with this forecast. May need to introduce a tempo for lower
conditions with later forecasts. Confidence a bit higher outside
the metro area and tempo 1/2sm at rfd/dpa with this forecast where
dense fog may develop Friday morning.

Winds remain generally easterly under 10kts through Friday morning
and then may turn a bit more southeasterly Friday afternoon...
speeds still under 10kts. cms


235 PM CST

The main concerns over the lake will be the potential for periods
of fog through much of the weekend. The greatest threat for
widespread dense fog looks to be Friday night and Saturday. During
this period, a warm front is expected to lift northward over the
lake. A very moist, high dew point, air mass expected to accompany
this front, and this may result in periods of dense fog, possibly
lingering into Sunday.

In the extended period, it appears that a storm system could take
aim on the Lower Great Lakes by the middle of next week. This
system may result in a period of enhanced northerly winds over
the lake.






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