Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200853

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
253 AM CST Fri Jan 20 2017

235 AM CST

Through Today...

Another day of Pacific Northwest-like weather with concerns for
larger impact being dense fog this morning in parts of north
central Illinois.

The closed upper low across southwest Minnesota early this morning
is lifting nearly due north, with a negative tilt to its southern
extent. That vorticity lobe extension is lifting over the area
early this morning, supporting showers with even some convective
elements in east-central Illinois. A 2 a.m. surface hand analysis
indicates a broad 1002 mb low across south central Illinois, with
a gradually sloped warm frontal inversion inferred north of that.
As this low tracks into northern Indiana by mid-morning, the light
to moderate showers across the eastern half of the forecast area
will track northward out of the area. While fairly light low-
level flow, patchy drizzle seems possible areawide today.

Dense fog observations are noted across Iowa and into western
Illinois early this morning and there is potential for this to
expand into north central Illinois near or just after sunrise.
Will watch that closely. Otherwise, sort of a murky one half mile
to two mile visibility is expected across the rest of the area
this morning and for some into the afternoon.

Temperatures have struggled under the clouds in recent days, and
while winds will turn southeast especially south of I-80 today,
readings will fail to reach what the 925mb temperatures would
commonly support with no snow cover. Light warm advection,
especially in the south, should support mid 40s to even some upper
40s in the far south. Overall, only about a five degree climb is
expected today areawide, with light flow off the lake probably
keeping lakeside areas from climbing but two or three degrees.
The Chicago lakefront only had a three degree range yesterday,
further supporting this reasoning.



252 AM CST

Friday Night through Thursday...

An active weather pattern will persist through the long term
forecast period, with periodic chances for precipitation.

The general trend for the longer term forecast period will be for a
progressive high amplitude pattern.  Temperatures through the period
will generally be above seasonal normal levels. While a series of
systems will bring periodic chances for pcpn, p-type will likely be
mostly rain or a rain/snow mix durg the overnight hours.  Expect
little opportunity for any sgfnt snowfall through the period.

For Friday night into Saturday, a modest srn stream shortwave is
expected to track from the cntrl plains to the upper MS valley. The
associated sfc low should track should be well west of the area,
leaving the local area in a zone of warm/moist advection in the
lower levels.  The models are in good agreement on advecting drier
air aloft across the region, overspreading a saturated layer blw
5kft.  This pattern will suggest areas of fog and drizzle.  cloudy,
foggy, drizzly conditions will persist into Saturday.  The main
forecast concern for the weekend will be just how high temperatures
can climb. Much of the guidance is suggesting temps reaching well
into the 50s with uppers on Saturday.  Have some concern that the
models have been trending to warm.  While 925mb temps of arnd +8 C,
the extensive, persistent cloud cover may prevent temps from getting
that high.  So, confidence in the temperature forecast is only
moderate, and will be highly dependent on cloud cover.  If some
sunshine can break through, there will be the potential to mix down
the warmer air.  However, if skies remain overcast into the weekend,
then temps may need to be adjusted down a some.  Also, have
maintained the slight chance thunder for the srn portions of the
CWA.  But, that too is a low confidence, dependent on the track of a
weak srn stream shortwave.

Into next week, the active weather pattern will continue as the
pattern remains high amplitude and quickly progressive.  The trend
through at least mid week should be for above normal temperatures,
with highs in the 40s.  While the will be periodic chances for
pcpn, p-type should largely remain liquid, or perhaps a rain/snow
mix.  Late in the period, long wave upper troughing is expected to
develop over the cntrl plains and move east.  This would signal a
trend back to more seasonable temperatures with highs in the low to
middle 30s and lows in the 20s.


For the 06Z TAFs...

Main forecast concerns through the period are cigs/vis.

Rain continues to spread across the terminals late this evening
and the short term/hi res guidance has been in general agreement
keeping the rain across northeast IL and northwest Indiana for
much of the night...though with a slow diminishing trend. Extended
prevailing light rain/showers through 12z...and even at that time
drizzle is likely which may persist through much of Friday

While visibilities have dropped to 1-2sm with some of the heavier
rain...this rain has and likely will keep the vis from tanking
overnight. Eventually as the rain ends...the vis is expected to
lower as fog thickens but confidence regarding how low the vis
will drop remains only medium for ord/mdw so maintained 1sm with
this forecast. Further west at rfd/dpa...its possible that the vis
will drop to 1/2sm around and just after sunrise. Maintained tempo
but confidence also medium here. It is also possible that some
areas may develop dense trends will need to be monitored.

Ifr/lifr cigs have developed just about everywhere but are still
mainly sct/bkn in far northeast IL. Cigs will become prevailing
ifr and then lower to lifr overnight and likely quickly given the
trend at mdw, which dropped from mvfr to 400 ft between 05Z and
0530Z. There is still some uncertainty regarding how fast this
happens across all of northeast IL however. Cigs may lower to
vlifr Friday morning but this would most likely be associated with
vis under 1sm. Some minor improvement at least to ifr is expected
Friday afternoon and then cigs/vis will likely trend lower Friday
night into early Saturday morning but low confidence at the end of
the period. cms/mtf


237 AM CST

With increasing moisture over the cooler waters of the lake, will
need to watch early this morning for possible dense fog. It could
remain just a low stratus deck and more of a 1-2 SM visibility,
but confidence is low. Any visibility changes should be gradual as
opposed to very quick today. The fog threat will continue through
Sunday, especially across the south.

Otherwise, east winds today should turn southeast tonight and
persist into Saturday, before turning back east to northeast on
Sunday, as low pressure moves south of the lake. It is possible
for some Small Craft Advisory criteria waves into the Illinois and
Indiana nearshore on Sunday night and early Monday.






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