Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151720

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1120 AM CST Fri Dec 15 2017


There is a little lake enhancement to the shortwave across
northwest Indiana. Expecting visibility reductions to be the main
effect, but could be a dusting to a tenth or two briefly.
Otherwise, periods of flurries can be expected elsewhere, and even
though there will be cloud breaks, we do expect more clouds than
sun today, especially in the afternoon as the next weak wave



208 AM CST

Through tonight...

Yet another shortwave pivoting around the longwave trough that has
been established over the region will clip the CWA today as the
parent trough retreats. At the surface, a weak cold front
trailing south from low pressure over the Upper Great Lakes will
move across the area this morning. Before cooler air has much
chance to filter in, winds will likely begin to back in advance
of the next system with warm air advection beginning this evening.

While there could be a few breaks, overall it appears that post
frontal stratus should be extensive today. Stratus could begin to
clear out tonight as low level southwesterly flow advects in drier
low level air mass, however mid-high level cloudiness should have
already moved in resulting in mainly cloudy skies tonight. Cannot
rule out some flurries from the stratus deck today into this
evening, particularly early this evening as mid level deck arrives
potentially seeding the stratus. Not expecting accumulations, but
could see some flakes flying at times today into this evening.
Cloud cover tonight is expected to keep temps from falling off and
have raised forecast lows generally above guidance.

- Izzi


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

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.



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.



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


LM...Small Craft Advisory...nearshore waters
     until 9 PM Friday.




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