Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161142

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
542 AM CST Thu Nov 16 2017

319 AM CST

Through Friday...

As feared last night, stratus is indeed a forecast issue early
this morning. First batch of stratus is about to clear our far
southern/eastern counties, but another deck is pivoting in from
the northwest. Tricky cloud cover forecast today with majority of
guidance seemingly unaware of this stratus deck`s presence!
Generally leaned on the NAM/MET guidance for sky cover today, with
the expectation that the secondary stratus deck moving in will
blanket the area this morning before low level flow collapses and
stratus deck`s advance halts. Subsidence and some mixing should
allow stratus to erode from the edges, but as it looks now, NE IL
may be one of the last areas to clear out later this afternoon.
Areas that remain socked in under the stratus the longest will
obviously see the greatest impacts on temps today and attempted to
account for this in the high temp grids, but given the low
confidence in the exact cloud trends, it isn`t easy to know how
much to adjust temps down and where!

Tonight, assuming all of the stratus is gone (not a sure thing),
then skies should start out clear and with nearby (though
departing) high pressure, could see a quick drop in temps this
evening. As the high continues off to the east and mid-high
cloudiness increases, would expect temps to level off and probably
even begin to rebound prior to sunrise.

Strong low pressure tracking east along the U.S./Canadian border
Friday will result in strong and quite southerly winds, which will
be advecting in warmer air, though the strength of the winds and
resulting wind chills will largely make the warm up feel like a
wash! A lowering overcast is expected with widespread virga and
perhaps some sprinkles or light showers penetrating the dry low
level layer in the afternoon. Advection should be enough to drag
temps into the 40s Friday, despite the cloud cover.

- Izzi


319 AM CST

Friday night through Wednesday...

Friday night, the northern stream low lifts north into Canada,
while a secondary low pressure deepens develops over the Central
Plains in association with the southern stream shortwave trough.
As this low approaches, brisk southerly winds will continue and
plume of fairly moist low level air will surge north into IL in
advance of this secondary low. As surface dewpoints in the 50s
surge north, expect to see rising temperatures Friday night, with
many areas probably climbing into the 50s at night (especially
south of I-80). Along with the increase in low level moisture,
will come some meager instability, which given the strong WAA
regime and approaching strong trough will probably be enough for
embedded t-storms along with the increasing coverage of showers
Friday night.

This secondary sfc low will track across the CWA early Saturday
morning with strong northerly winds locking in behind the system
and sending temps falling again during the day Saturday. Will
probably be a dry intrusion and break in the rain (besides
drizzle) Saturday morning over parts of the area. Guidance has
trended a bit sharper/deeper with this secondary southern stream
shortwave and now is developing a respectable deformation zone on
the backside of the system Saturday over northern IL. The NAM is
most bullish with this feature and even suggests the combo of
strong CAA and strong dynamic cooling could flip precip over to
wet snow Sat afternoon, mainly western CWA. Have introduced some
rain/snow in the grids during this time period to account for this
potential, but worth noting the GFS/ECMWF are less bullish and
would suggest just a windy, cold, rain in the defo band.

The cold air moving in behind this system will have about 36 hours
to call our area home, before strengthening WAA commences Sunday
night and gets into full swing Monday. While Monday morning will
likely start on the chilly side, the strong southerly winds in
association with a strong clipper moving toward the Northern
Plains will quickly advect warmer air north and with very little
moisture at first. The dry air means stratus is not likely, so
with strong southerly winds and sunshine Monday, have opted to
raise temps over the blended model guidance in coordination with
neighboring offices. The strong clipper will pass north of our CWA
Tuesday with continued windy and mild conditions until the
associated cold front moves through Tuesday afternoon (with little
precip threat). The cold shot behind this system looks short lived
too, but medium range models have been seriously struggling to
come to a consistent solution for next week`s large scale pattern,
so forecast confidence next week is lower than average at this

- Izzi


For the 12Z TAFs...

The main concern for the period will be duration of mvfr cigs and
wind trends.

A hole in the expansive, thin, mvfr stratus deck moved across the
region early this morning. However, latest satellite imagery
indicates the the hold in the cloud deck has pushed south and east
of the terminals. RH profiles and satellite trends suggest that
the mvfr deck should hang in until early afternoon before
scattering out. Some mid and high cloud is expected to move across
the region this afternoon and overnight tonight, but any cloud
cover will be operationally insignificant.

Following the passage of the cold front, winds are shifting to
nwly this morning. Some residual gustiness has been observed early
this morning, but expect that this should diminish through the
morning hours as high pressure begins to build in from the west.
The ridge axis should move across the region this afternoon and
evening, bringing a period of light and variable winds. As the
ridge quickly moves off to the east tonight, winds will increase
gradually while veering from nely to sely. By late tomorrow
morning, another area of low pressure lifting through the central
plains will strengthen the pressure gradient over the midwest. The
combination of the strengthening gradient and pressure falls in
advance of the approaching low will set up strengthening, gusty
south winds for much of the day tomorrow, with gusts increasing to
25 kt, or slightly higher, persisting through the rest of the day.


330 AM CST

A fairly active pattern will produce multiple periods of gales
through the weekend. some lingering northwest gales will continue
into this morning as a cold front continues to move east of the
area. Winds will relax today and veer to southeasterly and then
southerly this evening and overnight tonight. The approach of
another cold front will tighten the gradient over the lake and
support another period of southerly gales from Friday morning into
Friday evening. Following this front the winds relax and veer to
northwesterly overnight Friday into early Saturday, eventually
increasing again to gales beginning by late morning on Saturday
and persisting into early Sunday.


LM...Gale Warning...LMZ777-LMZ779 until 6 AM Thursday.

     Gale Watch...LMZ777-LMZ779...9 AM Friday to 9 PM Friday.

     Small Craft Advisory...IN nearshore waters until 11 PM Thursday.

     Small Craft Advisory...IL nearshore waters until 4 PM Thursday.




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