Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 160516
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1116 PM CST Thu Feb 15 2018

.SHORT TERM...
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
needed.

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.

KMD

&&

.LONG TERM...
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
night.

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.

JEE

&&

.AVIATION...
For the 06Z TAFs...

Relatively low confidence forecast for the overnight hours with
regards to CIGS. Overall, there has been a trend in guidance to
back off on the duration and to a lesser extent likelihood of
MVFR CIGS. Upstream observations and satellite data suggest MVFR
CIGS aren`t terribly widespread, but there is some moving south
behind a secondary cold front set to move across the terminals
overnight. In what appears to be an influence from Lake Michigan,
there is also an area of IFR stratus into SE WI immediately behind
the front. Biggest changes to previous TAF were to shorten
duration of MVFR Friday morning and to add a TEMPO for some IFR
CIGS for the terminals nearer to the lake.

- Izzi

&&

.MARINE...
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.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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