Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201758
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1158 AM CST Sat Jan 20 2018

.SHORT TERM...
315 AM CST

Through Tonight...

The main concern with the short term forecast is the potential for
dense fog tonight.

Southwest winds have remained elevated through early this morning
and have allowed temperatures to remain at or above normal highs
for mid-January. While wind speeds will be far less than
yesterday, the southwest direction will continue warm advection
and snow melt. With the edge of the snow pack already into the
southwest CWA, see no reason most locations will not climb above
40 with even mid 40s expected in the snow-free areas including in
Chicago.

Stratus is expanding quickly upstream over the Ozarks early this
morning and low-level isentropic surfaces indicate some of this
will expand in during the afternoon. The deeper stratus, and
forecast low-level omega for drizzle, will creep northward into
the area this evening and expand more so overnight. Some high-
resolution guidance, including consistently the RAP, are
indicating splotches of a few hundreds of an inch. Expect there
to be areas of drizzle with possibly pockets of light rain,
mainly overnight. Temperatures in the far northern CWA could be
around freezing at the start of drizzle, but any freezing drizzle
is low confidence, and some guidance looks biased too cool from
now erroneous deeper snow cover.

Dew points will increase in the southwest flow though gradually
compared to the system early last week. Upper 30s to near 40 are
expected to be expanding into and over the CWA after dark. With
that likely hastening snow melt, and given some high-resolution
guidance indications, fog is likely and continues in the forecast.
A few factors that may limit dense fog of 1/4 mile or less
visibility is 1.) the stratus will have moved in first, 2.) the
dew point increase again is not rapid, and 3.) snow
melt/sublimation has already been occurring and even less snow
pack will remain by tonight. That said, it`s still a synoptic
pattern to be recognized for dense fog potential, including
through the day and evening Sunday, and will include the possible
mention in messaging.

MTF

&&

.LONG TERM...
315 AM CST

Sunday through Friday...

This time period`s focus is primarily through Tuesday morning, as
a strong mid-latitude cyclone crosses northeast over the Midwest
and Great Lakes. This system is still an open wave over southern
CA but closing off. Model guidance has trended slightly slower and
south with this, which is not a big surprise given the closed
upper center. Despite that, the system remains mature and even
deepening through Tuesday, thanks in large part to the upper jet
structure. Impact weather concerns are focused on fog Sunday and
Sunday night, rain with the heaviest precipitation time Sunday
night into early Monday morning (with possibly a few elevated
thunderstorms), and then the likelihood of a mainly brief period
of wet snow late Monday night into early Tuesday.

With the slight southward trend in guidance mass fields over the
past 24 hours, the strengthening surface warm front looks like it
will remain along/south of I-80 through Sunday evening and
possibly even to early Monday morning before pushing north.
Synoptically this continues the likelihood of fog, possibly
dense, especially over the northern half of the CWA. Drizzle with
pockets of light rain should continue, with temperatures forecast
to peak in the upper 30s north to mid 40s south.

Temperatures will hold steady to slowly rise Sunday night with the
inching north of the warm front. The surface low is expected to be
near the IA/MO border overnight Sunday night, with the moist
conveyor belt advancing over the CWA. Precipitable waters are
forecast to peak near or slightly over one inch (99th percentile
for time of year) juxtaposed with strong forcing ahead of the dry
slot nose. Widespread showers by overnight Sunday into early
Monday are expected, with some elevated thunder likely in the
region. Have forecast rainfall amounts on average around 0.75" for
that time period, but isolated amounts over an inch are certainly
possible. See Hydrology discussion below for more.

With the warm front lifting north Monday morning, the fog threat
should end and temperatures will continue to climb. Depending on
the precise path of the surface low and northward advance of the
warm sector, we should at least peak around 50 degrees. The
surface cold front will push eastward over the area Monday night.
With the system closed, the better push of cold advection actually
is more south than east and takes longer to spread over the area
than previous run. This pushes the start time of any wraparound
snow later into Monday night. The system is still maturing, and
there looks to be decent enough lift and even some deformation-
induced f-gen near the WI/IL border on global solutions to support
some snow into the early part of the Tuesday morning rush hour.
However, the forecast warmer surface temperatures and lower
ratios still indicated at that time, are favored to limit
noteworthy accumulations. Right now have some locations
along/north of I-88 receiving around one inch but confidence is
somewhat low in those values. Temperatures will drop into the
upper 20s to lower 30s during early-mid Tuesday morning.

Looking beyond that system, a very brief midweek period of
northwest flow aloft is forecast, with a clipper crossing the
Great Lakes region Wednesday. The pattern then shifts
southwesterly and gradually amplifies through early next weekend.
Blended guidance provided temperatures for late week may end up
being too cool, especially if indeed none to little snow cover.

MTF

&&

.HYDROLOGY...
315 AM CST

Snow melt has been ongoing the past 24 hours and will continue
today and tonight, with little snow cover likely remaining by the
time of main rainfall (Sunday night/early Monday). The rainfall
during that time period though could exceed one inch with an areal
basin average of 0.75" presently forecast. With many frozen
waterways this could create some issues. The rise in temperatures
and dew points could also cause some ice break ups. It should be
noted that this warm up is not as significant of a climb as seen
early last week that did cause some limited ice break up jams, but
will need to be monitored. Ice jams tend to be favored at bends
in area rivers and at choke-points and obstructions to free water
flow such as bridges.

&&

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

Primary aviation concern is arrival of low CIGS/VSBY and possible
drizzle/fog. Satellite imagery shows low MVFR/high end IFR
stratus rapidly expanding north and east through central IL/IN
with the leading edge roughly from KPNT to KDVN.  As low level
flow weakens through the afternoon and into this evening ,
expect that the progression of the leading edge of the stratus
deck will slow its progression. Latest guidance continues to trend
a little slower and won`t arrive at the terminals until around or
slightly after 00z. Still looks like an extended period of fog and
drizzle will develop late tonight with conditions continuing to
deteriorate. Lower end IFR or LIFR conditions should eventually
develop, but with snow cover rapidly melting, set up for dense
advection fog is not ideal. Greater chances of lower VSBY and
dense fog still may not occur after sunset Sunday evening.

&&

.MARINE...
119 AM CST

A low pressure system will develop over the Southern Highs Plains
tonight then track gradually northeast to northern IL/southern WI
by Monday. To the north of the low, set-up appears favorable for a
period of east gales Monday over northern portions of Lake
Michigan. As the low moves across the lake winds will briefly ease
Monday night before northerly gales develop on the backside of the
low Tuesday. Northerly winds will ease later Tues night into Wed
as high pressure approaches the western Great Lakes. A couple
periods of strong southerly winds with at least some potential for
gales exists late in the week into next weekend.

- Izzi

&&

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

$$

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