Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170902

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

257 AM CST

Through tonight...

Early this morning, 500mb vort max is lifting across northern
Illinois with widespread showers and even a few thunderstorm out
ahead of this feature. Some of the heavier convective elements
have produced over an inch per hour rainfall rates which on top
of frozen conditions just beneath the surface has allowed for
rapid runoff causing localized flooding in portions of the
Chicago Metro. Fortunately, instability and the better forcing on
the nose of a 40-45 kt low level jet is starting to shift east of
the local forecast area along with the stronger convective
elements. More scattered light rain showers may persist through
shortly after sunrise, but then the vort max will also exit east
of the area with the threat for precip further diminishing.
Through the remainder of the day, there could be occasional light
sprinkles or drizzle as we remain locked under thick low stratus
and low level cyclonic flow persists, but any additional QPF will
be very light. Meanwhile, another vort max currently digging
across Minnesota will arrive across the local area this evening
and tonight and give a little bump in PoPs, though again expect
fairly low QPF given relatively shallow moisture and modest

The other concern for today and tonight is fog. Dense fog was in
place earlier across much of the forecast area, though the line of
showers and thunderstorm helped improve visibility some in the
near term. Visibility has been dropping again in far western
Illinois and eastern Iowa behind the precipitation, and anticipate
similar trends locally as the showers start to move off to our
east. Will maintain a dense fog advisory for all but the southern
tier of counties through mid morning. Anticipate improving
conditions from the west as northwest flow picks up later this
morning behind the departing low. Winds are expected to drop off
again late this evening and overnight as a surface ridge builds
across the Mississippi Valley. This could allow ceiling to build
back down with fog redeveloping overnight, especially late and in
the far west where the gradient is weakest.



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 06Z TAFs...

IFR/LIFR conditions in ra/dz will be the primary concern for the
rest of the night and into tomorrow morning.

Low pressure has moved over nern MO and will continue to track
northeast through the night with a warm front lifting north to
near a KPNT to KRZL line. In the cold sector north of the front,
cigs/vis has been a mixed bag ranging from lifr to mvfr, with
somewhat improving conditions in areas of more  convective pcpn
with shra and isold tsra stirring up sfc vis. Cigs have generally
remained lower ifr to lifr and will continue to remain in the
200-500ft range.

No sgfnt improvement is expected through the night. At 04Z, a
relative dry slot is moving over nern IL with one area of more
convective pcpn lifting across Lame Michigan. Under this dry slot,
expect cigs/vis to drop to lift for a short period, with pcpn more
dz than ra/shra. The upper low center will move over the terminal
areas btwn 07-12z. As the cold core of the upper low moves
overhear, there will be a very small chance for some isold ts, but
generally, expect a period of shra with slightly higher vis, but
still ifr.

Following the passage of the low, winds will gradually veer from
sely to swly durg the morning hours and then nwly by late
afternoon. The pressure gradient will be weak and wind speeds
should generally be under 10 kt through the period. As winds veer
from slwy to nwly, increasingly drier air will move into the area,
allowing prevailing vis to very gradually improve, but expect that
cigs will remain arnd 300ft through late afternoon.


235 PM CST

Fog will continue to be a problem into Tuesday morning across the
southern part of the lake, especially the far south. Webcam
visibility and reports indicate near 1/2SM visibility, and have a
marine fog advisory now issued. There will be times when rain
showers abate some of the fog, especially this evening. However
there is a good possibility of the visibility dropping again

Otherwise, this evening will see an increase in easterly winds
across the southern part of the lake. There could be a period
north of Chicago where winds and more so waves briefly reach Small
Craft Advisory criteria, but it should be short lasting. As the
low center moves over and past the lake, winds will turn northerly
across the entire lake, dissipating the fog. Winds turn southwest
on Wednesday and last into Thursday. Confidence in the wind
forecast Friday into the weekend is low, as trends are now
indicating more of an east to northwest wind given multiple slow-
moving low pressures to the southwest of the area.



IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ023-ILZ032 UNTIL 10 AM Tuesday.

IN...Dense Fog Advisory...INZ001-INZ002 UNTIL 10 AM Tuesday.

LM...Dense Fog Advisory...NEARSHORE WATERS
     UNTIL 10 AM Tuesday.




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