Area Forecast Discussion
Issued by NWS Chicago, IL

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923
FXUS63 KLOT 202255
AFDLOT

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

.UPDATE...
455 PM CST

Dense fog has been rolling in off the lake with UGN recently down
to 1/16SM. Webcams show dense fog farther south along the IL
shore as well. Clearly a dense fog advisory was needed to cover
this fog. Elsewhere, guidance is generally fairly bullish on dense
fog developing this evening, primarily north of I-80. Recent
trends have already shown vsbys dropping in most locations, given
the favorable synoptic set-up, wet grounds, and strong signal from
guidance, opted to not wait and just hoist the dense fog advisory
for all areas north of I-80 until 15z Sat morning. Climo for this
time of year would favor going longer than 15z, however models
indicate warm front down state will steadily move north tonight
into Saturday morning with visibilities likely to quickly improve
south of the warm front. In fact, many areas could see
visibilities improve prior to 15z expiration time and perhaps even
prior to sunrise as the warm front lifts north.

Updated gridded and text products are all out.

Izzi

&&

.SHORT TERM...
249 PM CST

Tonight and Saturday...

Subtle pseudo-warm front was draped west-east across roughly the
I-80 corridor this afternoon, which had lifted north with the
mid-level short wave which brought rain to the region last night
and early this morning. Another pair of mid-level short waves were
noted in GOES water vapor imagery within the complex larger-scale
upper trough across the central CONUS, and the second and sharper
of these (over KS-OK) is progged to lift northeast across the
forecast area through Saturday morning. In the process, warm/moist
advection will increase across the true warm frontal boundary
over central MO/downstate IL, resulting in redevelopment of
drizzle and light rain this evening. Fog will likely thicken as
well, particularly after midnight as the more organized precip
lifts north as the warm front approaches from the south. May well
end up needing a dense fog advisory over portions of the area
later tonight, though how quickly it develops depends on how much
rain (as opposed to drizzle) develops. Rain, even though progged
at only a few hundredths of an inch, would likely keep visibility
up a bit through the evening hours. As the surface low lifts from
the mid- Missouri Valley to the upper Mississippi Valley later
tonight and Saturday, the warm front will eventually lift north of
the area. Winds shifting to the south and slightly deeper mixing
in the warm sector should help clear out at least the lower clouds
and fog by Saturday afternoon, allowing temperatures to warm into
the 50`s in most areas, and to near 60 closer to central IL/IN.
Overnight temps tonight should hold steady or rise very slowly,
maintaining very mild conditions for mid-January.

Ratzer

&&

.LONG TERM...
259 PM CST

Saturday night through Friday...

An active weather pattern is expected at times during the period,
with several chances for precipitation. The main concerns
will be the threat for additional periods of fog Saturday night
into Sunday, as well as a small chance of thunderstorms over my
southeast Saturday night. Attention will then focus on what
appears to be the beginnings towards a turn in the direction to
colder conditions, along with periods of snow showers across the
area by the middle to end of next week.

At the start of the period (Saturday night), an upper level
disturbance is expected to lift northeastward over the Upper Great
Lakes, while a stronger Pacific system begins to shift eastward
across the Ozarks. The northern system will likely push a weak
cold front over the area during the night, and this could act as
the focus for a few showers and thunderstorms, mainly for areas
east of the I-55 corridor. Otherwise, it appears that surface
winds will weaken over the area overnight as a weak surface
gradient sets up over the area. This could end up setting the
stage for yet another night of fog across the region.

Late Sunday through Monday, the surface flow is expected to
become northerly across the region in response to the stout
southern CONUS storm system, which should be approaching the
Central Atlantic area by late Monday. Precipitation chances over
the area look low during this period, with the main precipitation
with this system likely to remain south and east of the area.
Temperatures will cool off into the lower 40s for Monday, but
this is still a good 10 degrees above average for this time of
year.

Model and ensemble forecast guidance continues to advertise the
transition to a colder and active weather pattern by the middle of
the week. As this transition occurs, it appears that another
storm system will develop over over the Plains, then shift
eastwards towards the Lower Great Lakes Region by midweek. The
current forecast track of this system (which could still change),
may take it across southern Wisconsin, which would favor northern
Illinois residing in the warm sector of the storm system into
Wednesday morning. As a result, rain would be the primary P-type
into Wednesday. However, thereafter colder air will be shifting
over the area, and this should result to a change over to snow at
some point by Wednesday night. Periods of snow showers and colder
weather will be the main story for the later half of the week.

KJB

&&

.AVIATION...
For the 18Z TAFs...

Primary concerns continue to be LIFR cigs/IFR vis across terminals
through the period.

VLIFR conditions have persisted away from terminals this morning,
with slightly better LIFR/IFR cigs across TAF sites. Have seen
some slow deterioration of vsbys to the south as weak moist
advection works against diurnal improvement, and while some modest
deterioration in vis is possible would expect TAF sites to hold
fairly steady through the afternoon.

Secondary disturbance approaches from the southwest this evening,
which is expected to result in redevelopment of drizzle/light rain
by late evening. Renewed precip development and loss of diurnal
effects are expected to result in a deterioration of cigs back
into LIFR range, with IFR vsbys. VLIFR cigs not out of the
question, though most guidance generally 300 foot or so and
confidence of anything lower than that is small. This second
disturbance allows warm front (which is currently far downstate
IL/IN) to lift across the area Saturday, shifting winds to the
south and increasing speeds, and improving cig/vis conditions
during the late morning and afternoon hours. Forecast soundings
support some gusts in the 15-20 kt range by afternoon.

Ratzer

&&

.MARINE...
308 PM CST

The main concerns over the lake will be the potential
for periods of fog through the weekend. The greatest threat for
widespread dense fog will be tonight and Saturday. A warm front is
expected to lift northward over the lake during this period. A
very moist, high dew point, air mass expected to accompany this
front, and this may result in periods of dense fog, possibly
lingering into Sunday. At the present time, we have issued a dense
fog advisory for the near shore waters. The open waters may also
need to be added to the advisory at a later time.

Otherwise, expect east-southeast tonight into Saturday, before
turning back east to northeast on Sunday, as potent area of low
pressure tracks well south of Lake Michigan. Northerly winds could
become strong enough to result in waves of 3-6 feet over southern
Lake Michigan Sunday night and Monday. This could require a Small
Craft Advisory for the Illinois and Indiana nearshores during this
period.

In the extended period, another storm system looks to take aim on
the Lower Great Lakes by the middle of next week. This system may
result in a period of enhanced northerly winds over the lake by
midweek.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ020-ILZ022 UNTIL 9 AM
     Saturday.

IN...Dense Fog Advisory...INZ001-INZ002 UNTIL 9 AM Saturday.

LM...Dense Fog Advisory...NEARSHORE WATERS
     UNTIL 10 AM Saturday.

     Dense Fog Advisory...LMZ777-LMZ779 UNTIL 9 AM Saturday.

&&

$$

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