Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151557

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
957 AM CST Thu Feb 15 2018

957 AM CST

As feared earlier this morning, visibilities have tanked almost
area wide. A weak surface pressure pattern has allowed winds to
slacken off a bit more. Currently far north central Illinois and
Lake County Illinois have the highest visibilities, and would be
the first to clear. We still need to wait for a cold front to
arrive later today to allow for any real improvement in the north,
and the rain in the south may eventually help at a similar time
frame. Have kept the advisory simple and extended all areas in the
mid afternoon time frame for now.



309 AM CST

Through Friday...

This Thursday morning brings a very challenging fog forecast. In
summary, the Dense Fog Advisory continues through mid-late
morning with the most impactful low visibility being along the
general I-80 corridor. Sites in this area, including here at the
WFO in Romeoville, have had occasional visibility less than a few
hundred feet.

A somewhat sloppy surface pressure pattern over the region as of 3
am, with the primary warm front draped near the southern edge of
the snow pack across the far southern CWA, separating lower 50s to
the south from upper 30s to the north. Another system low
pressure across Lake Superior has stiffened the pressure gradient
across Wisconsin and the northern part of the CWA, allowing for
surface winds around 10 mph and occasional gusts. Regional VWP
data indicate 20-30 kt winds at 1000-2000 ft in this area, and
that`s allowing for some mixing and preventing the humidity from
staying at 100%. The result has been some marked improved
visibility in northern locations during the pre-dawn hours. Also
possibly helping is that dew points are not markedly
climbing...they are just staying in the mid 30s. While that was
enough of a moisture magnitude for dense fog development late
yesterday, it`s possible ground conditions, including the
temperature of the snowpack, have modified enough to keep dense
fog from being as widespread now in northern locations.

These winds will ease through early-morning, and guidance is
indicating some fog expansion back to the north. Confidence is
quite low though on how the sub quarter mile visibility will
unfold, especially I-88 and north. Felt it prudent to hold onto
the Advisory though, as a marked visibility drop is still forecast
by some guidance into the morning. To the south, along and just
north of the aforementioned warm front, conditions remain more
primed for dense fog. By later morning, rain showers will develop
along and south of I-88 in response to a short wave disturbance
overriding the boundary. This rain should help improve visibility
above dense status, though some at least lighter fog is likely to
linger through the entire day. Along lakeshore areas, it is
possible the fog becomes dense again in the afternoon with
possible light onshore cooler flow.

As for the rain today, the most widespread coverage is expected
late morning through the afternoon. Precipitable waters are
forecast to be over an inch...highly anomalous for this time of
year...and some isolated rainfall in excess of one half inch is
possible mainly east of I-57.

Temperatures should only climb slightly today with all the cloud
cover and then some showers. Far southern locations such as
Watseka and Fowler will be in the warm sector and with no snow
cover, and thus are likely to near 60 degrees. For the  north
around 40 is likely.

As the surface reflection moves along the boundary into Indiana
this evening, winds will start to increase from the north.
Temperatures will fall to below freezing in the pre-dawn hours of
Friday morning and continue to fall. Some slick conditions may
develop where puddles from melt/rain exist. As for any additional
precipitation, the upper trough itself could rattle out some brief
light snow/flurries in the pre-dawn hours, but no accumulation
forecast. Friday morning will be breezy and highs in the
afternoon will recover only slightly to mid to upper 20s despite



330 AM CST

Friday Night through Wednesday...

Attention on this period is first on a chance for a brief period
of snow Saturday that is likely to not accumulate. Second is on
the active first half of next week. Obviously being numerous days
out specifics remain uncertain, but the main possible impact for
most of the area could be from multiple rounds of rainfall on
ground that had and will continue to be experiencing snow melt.

The progressive upper level pattern will continue into the weekend
before becoming more amplified. In this pattern on Saturday, a
fairly strong northern stream short wave is forecast to accelerate
through the region. This has less moisture pull than some GFS runs
were indicating yesterday, and its progressive nature should limit
deep saturation and overall precipitation magnitude. Snow
currently is favored as the p-type for northern areas, although
the time of day right now favored is the afternoon with highs just
above freezing, so any accumulation looks light (under an inch),
and it is certainly possible this is a non-accumulating event.

A western U.S. trough remains forecast to take shape into Monday
with strong southwest flow aloft and at least some at the surface
to steer in higher moisture. The initial precipitation looks to
be as early as Sunday night as a very anomalous low-level jet and
moisture flux are forecast by the 00Z GFS and EC. Surface
temperatures should be climbing and are favored to be above
freezing during later Sunday night, which continues to be a trend,
lowering the threat of icing in that initial precipitation time.
Almost all ensemble members of the GFS as well as its solution
and the deterministic EC support a low to the west of or over the
area on Monday, allowing for mild temperatures (possibly very
mild) and rain as the precipitation type. Also the forecast
elevated instability is enough to warrant occasional heavier
rainfall rates as well as some thunder. Obviously quite a bit of
uncertainty on coverage and it would likely be occasional waves of
showers as it looks now. This may be enough to cause some minor
flooding issues including on rivers which will be elevated and at
risk of some hastened ice break-up.

The period Tuesday-Wednesday remains very low confidence. The
elevated baroclinic zone will likely be over the area in southwest
flow aloft, which still raises some concern for mixed
precipitation, though that threat has diminished. The EC is
slower progressing with the cooler surface air, and keeps Tuesday
very mild with waves of showers and possibly storms continuing.
With the upper long wave trough remaining more positively tilted
from central Canada into the U.S. Rockies, do not see an overall
quick progression with the zone of rain. It also does not look
like any of the surface lows propagation along the boundary are
able to tap into a cold conveyor belt with that setup, so snow, at
least any appreciable snow, does not look particularly favored
for our area at this time.

Accumulated rainfall amounts on guidance really pick up just south
of the CWA (2"+ on Monday-Wednesday), and with that close
proximity it will definitely be a time period to watch for those
with hydrologic concerns.



For the 12Z TAFs...

554 am...Forecast concerns remain fog/vis/cigs through late this

Dense fog is widespread over much of the area...just south of a
rfd/ord/mdw/gyy line. This dense fog is likely to remain in place
for much of this morning with slow improvement. Additional
improvement is possible as light rain develops this afternoon...
generally along and south of the I-80 corridor. But there remains
uncertainty regarding how much of this dense fog spreads/
redevelops across the rest of terminals. Confidence for 1/4sm is
only high enough to include at dpa/gyy with this forecast. If
light rain develops further north across ord/mdw/dpa by early
afternoon...this will likely improve vis more than currently
advertised. The light rain or showers are expected to remain
southeast of rfd. Patchy drizzle is also possible this morning.

A cold front will move south across the terminals late this
afternoon. Further improvement is expected with this front but
trends/timing will likely need refinement with later forecasts.
Cigs/vis should lift through ifr and eventually into mvfr by early
this evening. Mvfr cigs may persist through the overnight hours
with scattering by Friday morning expected.

Southwesterly winds generally under 10kts to start will slowly
diminish to 5kts by late morning and turn westerly by early
afternoon. Winds may actually become light and variable for a time
early this afternoon and there is some potential for winds to turn
light northeasterly off the lake but confidence is low and trends
will need to be monitored...though even if winds did turn
northeast...speeds should still be light. Winds will shift to the
north/northwest behind the cold front mid/late afternoon but
speeds will be slow to increase behind the front...eventually
increasing to 10-15kts later this evening with gusts to 20kts
early Friday morning. cms



200 am...Low pressure over Lake Superior early this morning will
move into Quebec by this evening. A trailing cold front from this
low will move south across Lake Michigan today...shifting winds
northerly. High pressure will build across the Plains tonight and
the gradient will tighten over the western lakes overnight into
Friday morning with wind speeds to 30kts. There may be occasional
gale gusts early Friday morning...mainly across southern portions
of the lake. The high will quickly move across the southern lakes
Friday evening...reaching New England Saturday morning. Winds will
shift westerly Friday and diminish but will become southwesterly
Saturday with speeds back to 30kts. Its possible a period of gales
may develop over northern portions of the lake...but confidence is

Low pressure will move across northern Ontario Saturday with a
trailing cold front crossing Lake Michigan Saturday night. A large
trough of low pressure will develop over the Plains Sunday which
will gradually shift east across the lakes region early next week.
Southerly winds Sunday/Sunday night will slowly shift north/
northeast Monday across the north...then shift northerly across
the rest of the lake by Tuesday. However...wind directions will be
dependent on the timing of this trough and any low pressures that
develop along changes are possible with later forecasts.


IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ023-ILZ032-ILZ033-ILZ039 until 4 PM Thursday.

IN...Dense Fog Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 until 4
     PM Thursday.

LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 until 6 PM Thursday.




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