Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 152024

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

1217 PM CST

Through Friday...

Concerns center on the improvement of dense fog conditions across
the area through the afternoon and evening.

A weak pressure gradient is in place across the area today ahead
of a cold front which is nosing into north central Illinois. It
is the drier airmass with this front that will provide the impetus
to help improve visibility. There is also a sheared out shortwave
along and south of I-80 which is not only driving some rain and
drizzle, but is leading to some local improvement in visibility,
but not widespread yet. Expect the bulk of the rain/drizzle will
largely remain along and southeast of I-55. There is also one
additional pocket of locally improved conditions just along and
north of Ohare in northern Cook and most of Lake County IL. With
the front still upstream, expect soupy conditions through at least
mid to late afternoon.

The front will get more of a shove southeastward through the area
this afternoon, but it will likely take some time to get to areas
southeast of I-55/I-57 such that lower conditions may linger
beyond the late afternoon end time of the dense fog advisory. Will
have some time to assess how widespread the fog will be as we
head into the evening commute and if any local extensions are

While there will not be any real precip on the leading edge of the
front, the upper trough axis will shift into the area later
tonight, with some weaker shortwave energy riding along the
secondary cold push overnight. Forecast soundings, do not show
much deep moisture with this, but there are some hints from the
short term hi-resolution guidance that there could be some light
snow/flurries late tonight with this. This is not depicted in the
global guidance or even the NAM.

Friday will just be seasonably cold with breezy northwest winds in
the morning. Otherwise with high pressure coming in, expect
clearing skies and weakening winds. Highs will top out below the
freezing mark.



221 PM CST

Friday night through Thursday...

Overview...A brief period of snow is expected Saturday afternoon
north of I-80.  The highest forecast snow totals will be up to an
inch near Rockford, IL. Mainly rain is expected south of I-80.  The
main forecast challenge then turns to Sunday night through Tuesday.
An extended period of rain showers is expected and a few embedded
thunderstorms are also possible. Rainfall amounts around 1-2 inches
are possible early next week.

High pressure passes over northern IL Friday night, and an upper
level trough follows the high Saturday afternoon. Precip is expected
across the region, and snow is most likely north of I-80, especially
along the IL/WI state line. High temperatures in the upper 30s
should keep precip as all rain south of I-80.  Precip will end
Saturday evening, and snow amounts will be highest near Rockford
with up to an inch expected.  Model spreads near Rockford indicate
anywhere from a half inch to just shy of 2 inches, so 1 inch seems
reasonable. Model spreads support less snow further south and east.

A high pressure ridge passes over the region Saturday night, and the
next series of low pressure systems sets up over the Plains.  The
first low reaches the upper Mississippi Valley Sunday afternoon.
Winds become southerly and usher in warmer air. Moisture accompanies
the warmer air, and rain should start Sunday evening or night. The
ECMWF is quicker with the rain than the GFS.

Monday morning will bring widespread rain showers. A few embedded
thunderstorms are possible as the second low pressure center moves
over northern IL Monday afternoon through Mon night.  Severe storms
are not expected.

High temperatures early next week will be well above average. Monday
looks like the warmest day with highs near 60 possible south of I-
80.  Lows Monday night will range from near freezing at RFD to upper
40s in NW IN. Very little snow is expected though thanks to a stout
upper level warm layer that will melt snowflakes as they fall.  At
best, areas near Rockford may see a mix of rain and snow Monday

The low pressure trough remains over the region through Tuesday
afternoon bringing more rain and thunderstorms.  WPC rainfall
estimates for Sunday night through Tuesday night are 1-2", but
locally higher rainfall amounts will be possible with thunderstorms.
 This much rain could lead to river flooding, especially with
relatively frozen ground.

The low`s cold front comes through Tuesday shifting winds to the
northwest.  I decided to keep a transition from rain to snow as
colder air works southeast, but we could see a period of mixed
precip.  Forecast soundings hint that the saturated layer could be
shallow and below the DGZ suggesting ice crystals, but it`s a bit
too far out to get that specific just yet. A 20 degree temperature
drop from Tuesday afternoon to lows Tuesday night/Wednesday morning
will likely be the most noticeable difference.

High pressure brings drier conditions mid week.



For the 18Z TAFs...

Fog, low ceilings and visibility improvement times are the main
forecast concern.

A recipe for VLIFR is indeed in place with a weak pressure
gradient in place coupled with continued snowmelt increasing
dewpoints not allowing any improvement. The batch of rain to the
south of the terminals is not providing much improvement if any.
Therefore we need to wait for a cold front to drift in from the
northwest today to provide the dry advection needed to improve
conditions. Therefore, will likely carry 1/4 mile vsby (or lower)
out the gate with the 18z TAFs with the exception of RFD which
will hold MVFR or occasional lower IFR. ORD is a wild card with
the clear pocket just to its north that may bring occasional
improvements. The most optimistic guidance does allow the clear
pocket across Lake County Illinois, and currently as close as
PWK, to ease into ORD after 1830-19z, a tad later at MDW. RAP,
HRRR are a bit more delayed with the improvement, more like 21z at
ORD and 22-23z at MDW. Will just need to watch trends as they
unfold this afternoon. Current TAFs are likely the most
optimistic of conditions. The remainder of the terminals can also
expect improvement late this afternoon, lingering the longest near
GYY and southeast of the terminals into the evening.

Only modest concern after this will be breezy NNW winds overnight
into early Friday, though at this point speeds look to remain
below 22L concerns at ORD, with gusts holding under 20 kt and
sustained under 15 kt. Winds will back to more NW during the day
and ease.



221 PM CST

A pair of low pressure troughs oriented west to east across far
northern Lake Michigan and far southern Lake Michigan will shift to
the Lower Great Lakes through this evening. Ongoing dense fog across
primarily extreme southern Lake Michigan will diminish with the
trough passage late this afternoon into early evening. N to NNW
winds to 30 knots are then expected across much of Lake Michigan
late this evening into Friday morning. Southwesterly winds to 30
knots are then expected Saturday between high pressure across the
Ohio River Valley and a low pressure trough across the Upper
Mississippi River Valley. A brief period of gales to 35 knots will
be possible across northern Lake Michigan Saturday morning.

After a brief passage of a high pressure ridge Sunday morning,
southerly winds to 30 knots are expected late Sunday afternoon
through Sunday night as low pressure approaches from the northern
Plains. An associated trough will settle across Lake Michigan Sunday
night into Tuesday, resulting in an extended period of southerly
winds to 30 knots across southern Lake Michigan, and northerly winds
to 25 knots across northern Lake Michigan. Winds under 25 knots are
then expected into Thursday as high pressure drifts across the Great
Lakes region.



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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