Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 172001

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
201 PM CST Tue Jan 17 2017

158 PM CST

Through Wednesday...

Damp and cloudy conditions continue overnight.  An upper level wave
currently over Iowa will pass overhead through this evening. Light
rain showers and drizzle will form ahead of the wave late this
afternoon. Rainfall amounts will be very minor at around or less
0.05 inches, and nothing like the rainfall we received Monday night.
The majority of rain will fall north and east of a Rochelle, IL to
Rensselaer, IN line.  Rain and drizzle come to an end around
midnight. Fog may redevelop especially across north central IL, but
have low confidence in how dense fog will be.  Due to my lack of
confidence, kept did not include dense fog in the forecast.

High pressure spreads east Wednesday and we should be on an
improving trend. Winds become southwest during the day and usher in
warmer air. Models differ on how quickly clouds will clear out and
there is a chance we may not see the sun tomorrow.  Decided to lean
on clearing in the afternoon and raised high temps into the low to
mid 40s. Local climatology would suggest we could see slightly
higher temps if clouds clear quicker, but on the other hand, temps
could be cooler if clouds linger.



257 AM CST

Wednesday through Monday...

Increasingly mild and primarily dry conditions will be in place
Wednesday and Thursday. Upper ridge is progged to amplify across
the mid section of the country while surface high builds across
portions of the Ohio Valley allowing warmer return flow to spread
across the Mississippi Valley. Temperatures are expected to warm
into the low 40s Wednesday and mid to upper 40s Thursday. 925mb
flow becomes anticyclonic during the day Wednesday which should
help to remove any lingering stratus and anticipate partly sunny
to mostly sunny conditions to continue at least through the first
half of the day Thursday. Cloud cover increases again later
Thursday and Thursday night and PoPs gradually return from the
south as closed upper low lifts from the Southern Plains across
the mid Mississippi Valley. The column is sufficiently warm for
any precip to fall as rain Thursday night into Friday. Models are
in fairly good agreement on another closed upper low moving east
across the Mississippi Valley to the Ohio Valley Sunday into
Monday which will bring additional chances for precipitation to
the area.



For the 18Z TAFs...

Visibilities are finally coming up across the board, but cigs
will remain IFR to LIFR through tonight and into Tuesday morning.
An upper level wave currently over Iowa will pass overhead this
afternoon and evening. Guidance is hinting that drizzle and a few
light rain showers will form ahead of the disturbance. Have medium
confidence in how many showers will form, so kept drizzle as the
prevailing precip type and a tempo for showers. Showers may lower
vsby slightly, and no frozen precip is expected. Some guidance
members suggest dense fog may form again tonight especially at RFD
and DPA. Have very low confidence in how low vsbys will go so did
not go as low as some guidance members would suggest.

West winds continue Wednesday. Drier air moves in helping any
lingering fog dissipate. Clouds should scatter out, but only have
medium timing in when clouds will scatter. We should see VFR
conditions by Wed afternoon at the latest.



419 AM CST

Low pressure moving across the southern end of the lake early
this morning, and allowing for varying winds over the entire lake.
With this in place, highly variable winds will be in place over
the southern half of the lake and nearshore through mid morning.
Then expect more of a northerly direction to be in place through
the remainder of the day. Expect northeast winds to be in place
over the north half today, increasing this morning but diminishing
once again later today. This low departs later this evening while
high pressure builds across the mid Mississippi valley. This will
allow winds to turn more west tonight and then southwest on
Wednesday. Gradient over the lake will tighten during this time,
with highest speeds to 30 KT returning across the north half into
Wednesday night.

Patchy dense fog still being observed across the nearshore waters
early this morning and expect this to still be in place through
mid/late morning. This patchy dense fog may be affecting portions
of the open waters across the south half, but confidence is low
with extent of this fog. So have not expanded the advisory to the
open waters but maintained fog wording with the possibility for
more dense fog.






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