Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171802

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1202 PM 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 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.



LM...Dense Fog Advisory...NEARSHORE WATERS
     UNTIL NOON Tuesday.




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