Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 151803
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1203 PM CST Fri Dec 15 2017

.SHORT TERM...
1202 PM CST

Through Saturday...

There are several short lived periods for snow chances
today, first with a weak disturbance moving through early this
afternoon. Flurries have been reported fora few hours or less at
any one location upstream in northeast Iowa. This evening a bit
stronger NW flow disturbance will interact with a a northward
moving warm frontal feature. Isentropic lift over this front will
lead to some snow or more likely flurries, mainly northeast of a
RFD-MDW-VPZ line during the 0z-5z time frame, or mid to late
evening. This would also be brief, only a few hours or so. After
this, the final push with this clipper will come overnight, but by
this point the forcing will have shifted north of the Wisconsin
border. Some guidance tries to break out snow farther south, but
at this point the main frontal boundary will have shifted into
Wisconsin and the air temperatures in the cloud layer will be less
than optimal for snow production.

The story for Saturday will be the milder air mass that will advect
into the area from the southwest as we at least temporarily break
the cycle of northwest flow aloft. There is a closed positively
tilted trough well south into the Baja peninsula.  Surface high
pressure to our south will also shift into the southeast on
Saturday, which will promote deep layer southwesterly flow. With the
front to the north, we will be in the dry and largely cloud free
warm sector. The surface front may not actually get north of the
Wisconsin border, so cooler southeast winds will keep readings
closer to 40 in these areas, where across central Illinois highs
will approach 50. The gulf moisture is still far enough away to
make for a pleasant and not too breezy day than typically occurs
when we get into southerly winds in the winter time.

KMD

&&

.LONG TERM...
329 AM CST

Saturday through Thursday...

In a change from the past week, generally mild conditions will be
in store as primarily positive mid-level height anomalies yield
warmer temperatures aloft, with the colder air mass temporarily
being locked north of the area. The noteworthy cold front/trough
passage will be on Monday night, though this will still be
followed by above normal temperatures on Tuesday. Later in the
work week, significant amplification of poleward ridging up to and
north of Alaska will set the stage for the likelihood of the
return of a colder pattern to close the week and heading toward
Christmas. A storm system of uncertain magnitude and track from
the Plains toward the Midwest and Great Lakes will likely affect
the region Thursday and Friday and herald the return of the colder
pattern.

On Saturday, clouds to start the day should lift north as a warm
front lifts north and strong warm advection aloft ensues. This
will likely send temperatures into the 40s area wide except
perhaps far north near or just north of the warm front. Upper 40s
are likely for some areas, especially south of I-80. The front
will stay stalled north on Saturday night with a weak pressure
gradient and light/calm winds in the vicinity of the boundary.
Mid-high clouds may stream overhead overnight, but not enough to
prohibit radiational cooling of temperatures into the mid 20s to
lower 30s. Patchy fog is possible along and north of I-80 outside
of Chicago into early Sunday.

A shearing/weakening positively tilted mid-level wave will move
across the region on Sunday. Forecast soundings indicate a stout
lower level dry wedge will need to be eroded before precipitation
can reach the surface, so the earlier morning hours should start
dry. Light rain should be the primary p-type by the time
precipitation reaches the surface, though thermal profiles suggest
some wet snow could mix in far north in the late afternoon if it
is precipitating. The 00z Global Canadian model appears for now to
be an outlier in its handling of the wave on Sunday, keeping it
stronger with a much more cohesive precip shield implied, while
the other guidance suggests spotty precip at best. Lingering low
clouds Sunday night should prevent much of a temperature drop, so
went above blended guidance with mainly above freezing lows. With
weak flow, no dry advection and broad ascent, Sunday night appears
to conceptually be a candidate for stratus building down to fog
and patchy drizzle but will defer fog/drizzle mention until trends
can be ascertained.

Monday will be another mild day in the 40s, though likely cloudy
and possibly with spotty light rain/drizzle. The aforementioned
cold front passage will occur Monday night and guidance varies on
the magnitude of the cold shot behind it, with the operational GFS
currently on the colder side (highs only in 30s with other models
supportive of 40s). It will be breezy/windy in the wake of strong
low pressure north of the Great Lakes. Tuesday night through
Wednesday will be quiet under surface high pressure ridging. This
will be followed by the likely storm system affecting the region
Thursday and Friday, though it is far too soon to tell how things
will evolve regarding temperatures, precipitation type etc.

Castro

&&

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

After a brief clearing period in wake of this morning`s flurries,
there are several additional short lived periods for snow chances today
and tonight. With each wave expect at least some MVFR clouds.
The first is with a weak disturbance moving through early this
afternoon. This evening a bit stronger NW flow disturbance will
interact with a a northward moving warm frontal feature.
Isentropic lift over this front will lead to some light snow or
more likely flurries, mainly northeast of a RFD-ORD line during
the 0z-5z time frame, or mid to late evening. This would also be
brief, no more than a few hours. After this, the final push with
this clipper will come overnight, but by this point the forcing
will have shifted north of the Wisconsin border.

Gusty west/northwest winds will shift to southwest tonight. Later
Saturday expect a wind shift to southeast, but confidence on
timing is not all that high just yet.

KMD

&&

.MARINE...
208 AM CST

Fresh west winds today will gradually subside tonight as low
pressure over Lake Superior moves east tonight. Next bout of
significant winds looks to arrive Monday night with increasing
southwest winds initially, possibly nearing gale force, then west
to northwest gales appear to be fairly likely behind the front
Tuesday through early Wednesday. Active pattern looks to persist
late next week with another storm system bringing next bout of
high winds to the lake late in the week.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters
     until 9 PM Friday.

&&

$$

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