Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 232024

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

223 PM CST

Through Tuesday...

Despite expansive cloud shield in place today, above normal temps
in the low 40s were still able to be realized. It does appear that
the stretch of above normal temps will continue on Tuesday,
however, first with a cooler night tonight. Anticipate some cooler
air to swing across the region tonight, while surface high to the
west build across the CWA. Persistent cloud cover should limit the
extent of the cooling tonight, but with temps expected to fall to
the low 30s. With high in place, not expecting any precip but will
continue to monitor fog potential later tonight into early Tuesday
morning. At this time, widespread dense fog is not appearing
likely but locations across the CWA could see fog with some patchy
dense fog also possible. Most of Tuesday appearing to stay dry
ahead of an approaching system from the Plains. Some light precip
could spread north into northwest Illinois late Tuesday afternoon,
but once again, expect the focus to be just to the west of the CWA
during the day Tuesday.



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 18Z TAFs...

Present MVFR clouds situated across the region will remain in
place through the forecast period. Some slight improvement has
occurred this morning with this trend likely continuing into the
afternoon. However, do expect a downward trend in these ceilings
to occur once again later tonight into Tuesday morning. During
this time, IFR ceilings will likely return as well as reduced vis
in fog. At this time, it does appear that vis should remain in the
3-5SM range but will monitor for possible lower vis especially
late tonight. Winds have varied from the 340 to 020 directions
today and expect this to remain possible this afternoon, but with
winds to then be more of a westerly direction tonight. Stratus
slowly improves on Tuesday, while winds turn to the east ahead of
the next approaching system.



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.






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