Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230857

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
257 AM CST Mon Jan 23 2017

241 AM CST

Through Today...

It`s the third straight morning with a Dense Fog Advisory in
effect for a portion of the CWA, though this time the areal
coverage threat of dense fog is quite minimal. The advisory
remains in effect for just the Lake Michigan shore counties of
northwest Indiana until mid-morning. Webcams along the I-80/90/94
stretch in far northwest Indiana indicate some dense fog. This
marine advected fog should gradually ease in intensity later this
morning into the afternoon. The rest of the CWA has mainly light
fog early this morning. An area of marine fog had moved into part
of northeast Illinois earlier (took ORD down to 1/2 mile
visibility), but since the winds have turned back north-northwest
and offshore. Cannot 100 percent rule out some dense fog along
mainly the Illinois lakefront early this morning, but it should be
temporary if it were to occur again.

With little in the way of a scouring weather system, the stratus
shield remains massive across the Upper Midwest and Great Lakes.
Any sun today would be just peeks it would seem. There has been
patchy drizzle reports in southern WI and northwest IL associated
with even lower stratus, and may see some of this in the northern
CWA this morning. Otherwise, synoptic shower activity should
remain just east of the CWA.

Not expecting much of a temperature climb again today given the
clouds and some cold advection aloft brought in by the departing
Mid-Atlantic system. Have forecast afternoon temperatures inching
into the lower 40s, which would be the 5th straight 40+ degree
day. The last time that happened in Chicago and Rockford during
the month of January was January 7-12, 2013.



256 AM CST

Monday Night through Sunday...

Unseasonably mild conditions will continue for a few more days
before a major pattern shift brings back winter with more seasonable
conditions expected by Thursday.

In the mean time, conditions will be mild and dry Monday night and
into Tuesday.  By Tuesday afternoon, an upper low and associated sfc
reflection will drop out of the rockies and track across the Central
Plains.  The longer term guidance remains relatively consistent on
this feature, though timing and intensity differences show up in the
models almost as soon as it drops out of the Rockies.  In spite of
these, relatively minor, differences, the main signal remains strong
that the next wave of pcpn should begin to move into the CWA as
early as Tuesday afternoon.  With temperatures remaining
unseasonably mild, predominant p-type will start out as rain. Strong
warm, moist advection will set up in the strong swly flow in advance
of the system. There may be a few hour period where a little snow
could mix in with the rain, especially close to the Wisconsin
border, but the temperature trend through the night will likely not
be typically diurnal and temps will likely reach the overnight lows
before midnight and then rise through the night as the should rise
through the late night hours as warm, moist air continues to stream
into the area.  The models are also in relatively good agreement on
tracking the upper low through the middle Mississippi Valley and
over srn Wisconsin before lifting to the northeast by Wednesday
evening.  While there are also minor timing and intensity
differences with the associated sfc low, there is good overall
agreement that the low track will go through eastern Iowa and
southern Wisconsin.  With conditions still relatively warm, this
track is not at all conducive to bringing any significant snow to
the CWA. As the low lifts through srn Wisconsin through the day on
Wednesday, some colder air may be able to filter in behind the
system, and some snow could mix in with the expected rain.
Temperatures should continue to trend downward Wednesday night and
pcpn should gradually transition from predominantly rain to a rain
snow mix overnight, with a little light snow possibly lingering over
the area into Thursday morning.  The major pattern shift will begin
Wednesday night and Thursday as the main upper low lifts into sern
Canada while upper ridging builds over the west coast. A series of
northern stream shortwaves are expected to drop out of central
Canada, carving out a broad upper trough over the ern 2/3 of the
conus while the ridge builds over the west.  These weak waves may be
sufficient to generate a little light snow late Wednesday night into
Thursday, but no accumulation is expected.

By Thursday night and through the remainder of the period, the upper
level pattern will shift to deep layer cold advection under nwly
flow.  This will bringing temperatures back to more seasonable
levels with lows in the upper 20s to around 30 F Thursday morning.
Thursday should be the transitional day with highs in the middle
30s, but by Friday and through the weekend, highs will be in the
upper 20s to around 30 F with lows in the upper teens to low 20s.


For the 06Z TAFs...

Expansive area of stratus will continue to blanket the terminals
through the TAF cycle. Biggest forecast challenge is determining
when/where CIGS will drop to IFR vs staying/improving to MVFR.
Would seem a gradual trend toward lowering CIGS will take place
tonight with many locations probably dropping to IFR, though
based on trends this evening there could be variability in flight
categories. Some fog could also develop, though not expecting
conditions as low or as widespread as last night. While not
reflected in the GYY TAF, its possible some more dense fog could
roll into GYY off the lake at any point in time, but hard to say
where any fog over the lake is let alone where it will move to.
CIGS are expected to begin the typical diurnal improvement Monday
by mid to late morning and it seems probable most TAF sites will
improve to MVFR at some point Monday late morning or afternoon.
Stratus will likely linger Monday night and it is possible there
could be another lowering of CIGS, but confidence at this distance
is low in those details.



241 AM CST

Fog is evident on numerous lake-oriented webcams early this
morning, and given still minimal air mass change, it can be
inferred it continues over much of the open waters. Have extended
the Marine Fog Advisory some today, especially for the north,
where the wind flow is lighter. Uncertainty is higher on the
south in terms of ending the fog, and it is likely western
portions of the south half will see the fog threat end early this

North winds today will become light and possibly variable by
midday Tuesday as a ridge quickly moves over the lake. A strong
low will then approach, increasing southeast winds sharply on
Tuesday night. These winds will turn southwest by Wednesday
morning and then northwest behind the low center on Wednesday
evening, with multiple days of northwest flow to follow. Gusts
look like they could top 30 kt over the open waters at times from
late Tuesday night through the end of the week.



IN...Dense Fog Advisory...INZ001-INZ002 UNTIL 9 AM Monday.


     Dense Fog Advisory...LMZ743-LMZ744-LMZ745-LMZ777-LMZ779 UNTIL
     NOON Monday.




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