Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200837

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
237 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.



224 PM CST

Friday night through Thursday...

An active weather pattern is expected during the period, with
several chances for precipitation. The main concerns initially
during the period will be the threat of dense fog Friday night
into Saturday, and temperatures on Saturday. Attention will then
focus on what appears to be the beginnings towards a turn in the
direction to more winter-like conditions across the area by the
middle to end of next week.

At the beginning of the period (Friday night), the upper level
disturbance, currently shifting over southern California and
Nevada, is expected to shift northward into the Upper Midwest by
early Saturday morning. This system will drive a surface warm
frontal boundary northward over the area Friday night. This front
will not only mark to the leading edge to a very mild airmass, but
also much higher dewpoints. As a result, it appears likely that
fog will develop, either during the day Friday, or definitely by
Friday evening as surface dew points quickly surge well into the
40s. This certainly could result in areas of dense fog and
drizzle across the area for a period Friday night into Saturday

The main concern for Saturday will then be how warm we get during
the day. A very mild air mass (925 MB temperatures +7 to +9) will
be in place over the area during the day. This certainly could
support temperatures well into the 50s on Saturday. However, with
dew points also likely climbing into the mid to upper 40s and
areas of fog around in the morning, we might not loss the low
clouds at all during the day. With this in mind, it may be
difficult to warm much above the lower 50s. In spite of this
concern, however, I have not strayed much from the blended
guidance, which currently advertises low to mid 50s far north, to
the upper 50s south.

Mild, but a bit cooler conditions will continue into early next
week. However, model and ensemble forecast guidance is
suggesting the transition to a colder and active weather pattern
by the middle of the week. This active pattern may support the
development of a storm system that could impact the Lower Great
Lakes Region late Tuesday through Wednesday. This system could
bring us our next chance of precipitation, with some snow possible
by Wednesday. We will need to be watched in the coming days, as a
farther south track could support accumulating snow over the area
Tuesday night into Wednesday.

Following this system, it appears that colder weather will filter
in over the area under northwest flow. Some periods of light snow
will also continue to be possible at times later in the week as we
remain in a semi-active northwest flow aloft pattern.



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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