Area Forecast Discussion
Issued by NWS Chicago, IL

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764
FXUS63 KLOT 202049
AFDLOT

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

.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...
252 AM CST

Friday Night through Thursday...

An active weather pattern will persist through the long term
forecast period, with periodic chances for precipitation.

The general trend for the longer term forecast period will be for a
progressive high amplitude pattern.  Temperatures through the period
will generally be above seasonal normal levels. While a series of
systems will bring periodic chances for pcpn, p-type will likely be
mostly rain or a rain/snow mix durg the overnight hours.  Expect
little opportunity for any sgfnt snowfall through the period.

For Friday night into Saturday, a modest srn stream shortwave is
expected to track from the cntrl plains to the upper MS valley. The
associated sfc low should track should be well west of the area,
leaving the local area in a zone of warm/moist advection in the
lower levels.  The models are in good agreement on advecting drier
air aloft across the region, overspreading a saturated layer blw
5kft.  This pattern will suggest areas of fog and drizzle.  cloudy,
foggy, drizzly conditions will persist into Saturday.  The main
forecast concern for the weekend will be just how high temperatures
can climb. Much of the guidance is suggesting temps reaching well
into the 50s with uppers on Saturday.  Have some concern that the
models have been trending to warm.  While 925mb temps of arnd +8 C,
the extensive, persistent cloud cover may prevent temps from getting
that high.  So, confidence in the temperature forecast is only
moderate, and will be highly dependent on cloud cover.  If some
sunshine can break through, there will be the potential to mix down
the warmer air.  However, if skies remain overcast into the weekend,
then temps may need to be adjusted down a some.  Also, have
maintained the slight chance thunder for the srn portions of the
CWA.  But, that too is a low confidence, dependent on the track of a
weak srn stream shortwave.

Into next week, the active weather pattern will continue as the
pattern remains high amplitude and quickly progressive.  The trend
through at least mid week should be for above normal temperatures,
with highs in the 40s.  While the will be periodic chances for
pcpn, p-type should largely remain liquid, or perhaps a rain/snow
mix.  Late in the period, long wave upper troughing is expected to
develop over the cntrl plains and move east.  This would signal a
trend back to more seasonable temperatures with highs in the low to
middle 30s and lows in the 20s.

&&

.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...
237 AM CST

With increasing moisture over the cooler waters of the lake, will
need to watch early this morning for possible dense fog. It could
remain just a low stratus deck and more of a 1-2 SM visibility,
but confidence is low. Any visibility changes should be gradual as
opposed to very quick today. The fog threat will continue through
Sunday, especially across the south.

Otherwise, east winds today should turn southeast tonight and
persist into Saturday, before turning back east to northeast on
Sunday, as low pressure moves south of the lake. It is possible
for some Small Craft Advisory criteria waves into the Illinois and
Indiana nearshore on Sunday night and early Monday.

MTF

&&

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

$$

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