Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 172028

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
228 PM CST Fri Nov 17 2017

228 PM CST

Through Saturday...

A complicated forecast tonight through Saturday, which includes
potential for thunder area wide this evening and then south of
I-80 later tonight into early Saturday morning, an extended period
of rain/drizzle tonight through Saturday which will briefly
change over to a rain snow mix or even all snow very briefly late
Saturday morning, and then potential for Wind Advisory criteria
winds Saturday afternoon for locations south of I-80.

In the near term, narrow band of rain continues to progress
through portions of northeast Illinois and northwest Indiana
riding along WAA in place across the region. Had noted earlier
reports of UP with this precip, mainly across northwest Illinois.
Although initial saturation/cooling to wetbulb could support a
brief period of a wintry mix, trends and most guidance supportive
of lacking crystals this afternoon. This would support liquid,
along with a continued warming column. So, all rain still expected
in the near term. Later this afternoon into the evening, will see
this initial rain exit, and then already present rain over
eastern IA expand in coverage as it moves into northern IL and
northwest Indiana. This will occur as speed max and surface trough
approach, and while WAA increases. Instability is still trying to
increase across the region, with a ribbon of 100-250 J/kg in
place. With this in place and while forcing increases late this
afternoon into the evening, can`t rule out some embedded thunder.
Steadier rain should depart mid/late evening, but will see large
scale ascent remain overhead. This should support periodic light
rain or drizzle over much of the CWA. Continued warming through
tonight will support all liquid, with frozen precip not a concern
during this time.

Surface trough/low will continue to approach the southern CWA
late tonight into early Saturday morning, ahead of deeper trough
digging across the region. Although forcing will be in place over
much of the region, focus will likely be on sagging boundary which
by late tonight, will be along/south of I-80. Additional rain and
thunder chances are expected for this location especially into
early Saturday morning, as forcing really ramps up with the
arrival of this upper level trough and deepening low. Still not
anticipating any severe weather on Saturday with these thunder
chances, as most guidance fairly consistent with keeping any
surface based instability just south of the CWA Saturday. Thunder
chances should quickly depart by late morning and midday though.

Still monitoring an increase in precip by mid morning Saturday
across the entire CWA, including much of northern Illinois and
northwest Indiana. As this upper level trough and surface low
swing through, guidance still indicating the high likelihood of
more banded precip to push into northern Illinois through mid/late
morning. This deformation/fgen supported band of precip will
swing through, as much colder air pushes in behind this system.
Although lower confidence with the exact trends, there is a
definite possibility for at least a small window of a rain snow
mix. This low confidence is due to the variability to the extent
of the deeper moisture/crystals in place and timing of cooling of
the column. It`s possible that the pattern will only support a
rain snow mix briefly before the precip completely departs, but
it`s also very possible for even all snow to occur briefly. With
the highest chances of a mix of precip across northern Illinois,
have included mainly rain/snow but do have a couple of hours of
all snow beginning by mid morning and going into midday. If all
snow were to occur slightly longer, it`s possible to see some
minor snow accum mainly on grassy surfaces.

Warmest temperatures will occur in the morning Saturday, but with
increasing northerly winds and CAA behind the system, will see
temps fall into the afternoon. Strongest winds expected in the
afternoon with gusts around 40 MPH. It`s possible that sustained
speeds and gusts may be higher, and will need to monitor the
possibility for an Advisory Saturday afternoon.



211 PM CST

Saturday night through Friday...

Primarily dry conditions are expected throughout the extended time
frame, but temperatures will be quite variable as a series of lows
move across the northern Great Lakes region.

Saturday night, deep area of low pressure will be over the central
and eastern Great Lakes region with precipitation now east of the
local area but cold air arriving behind the cold front. 850mb
temperatures will bottom out around -10C Sunday morning but the
thermal trough will quickly shift off to the east with
temperatures moderating through the day Sunday. Strong subsidence
behind departing 500mb trough axis will overspread the area
through the day. Forecast soundings indicate stratus will be in
place early in the day Sunday, but subsidence helping to dry the
column and surface ridge axis building in from the west may allow
some sunshine later in the day.

Monday, another vort is progged to dig across the Canadian
Prairies and reach the Great Lakes region midday Tuesday.
Attendant surface low will stay over Canada, and precipitation
remains favored north of the local area so will maintain a dry
forecast through this time frame. Southwest flow out ahead of the
wave will help temperatures further moderate, reaching back into
the mid to upper 40s Monday and Tuesday. Expect a reinforcing
shot of cold air to push across the region Tuesday evening and
Tuesday night putting the area back into a deep freeze Tuesday
night with colder air remaining in place through early Friday.
Another clipper-like system will take a similar though slightly
more southerly track across the region Friday into Saturday with
temperatures undulating once more. This system may also provide
the next decent chance for precipitation in the area.



For the 18Z TAFs...

Complicated forecast, as approaching system will provide several
periods of rain that could include thunder late this
afternoon/early evening, as well as a possible rain/snow mix
Saturday morning. Varying winds and speeds are likely, as well as
IFR/LIFR ceilings and reduced vis under drizzle/fog tonight.

In the near term, overall dry conditions are in place. However,
beginning to see an increase in coverage of light rain over
portions of north central and northeast Illinois. Although
coverage will likely remain lower, this steady progression of the
current rain will likely move across all the terminals this
afternoon. As this rain moves overhead, would not be out of the
question to observe a very brief period of sleet, however, don`t
anticipate any impacts. As southerly winds remain elevated this
afternoon, VFR ceilings will lower to MVFR. This lowering will
continue this evening as ceilings fall to IFR, especially with the
arrival of more widespread rain. Late this afternoon and early
evening, some embedded thunder will be possible but with
confidence low, have not included it in the TAF. Low ceilings and
possible lower vis under rain/drizzle/fog are likely tonight. As
the system pushes through late tonight, winds will turn to the
northwest and then north. A lull in precip is then possible but
expect precip to move back overhead by mid morning. While colder
air is pushing overhead, its possible for a rain snow mix before
this precip departs during the day. At this time, highest chances
of this occurring are for RFD and have included a PROB30. Will
need to monitor for possible rain/snow for the other terminals.



211 PM CST

A strong and deepening low will lift across the Great Lakes region
resulting in a period of southerly gales today followed by stronger
high-end northerly gales Saturday into Saturday night. Winds look
to peak at around 45kt midday Saturday as the surface low tracks
east across the central Great Lakes. Gales should end across the
lake sometime early Sunday morning as the low pulls away across
Quebec and high pressure builds across the Great Plains. Surface
ridge will settle over the southeastern states Monday while
another low deepens over the Canadian Prairies. The low will track
across Ontario into Quebec on Tuesday with gales once again
possible across Lake Michigan Monday into Tuesday.



LM...Gale Warning...LMZ777-LMZ779 until 9 PM Friday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...9 AM Saturday to 6 AM Sunday.

     Small Craft Advisory...nearshore waters
     until 3 AM Saturday.




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