Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 182109

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 PM CST Mon Dec 18 2017

159 PM CST

Through Tuesday...

Dry advection from the west has resulted in at least partial
clearing skies, with more clouds hanging on along and southeast of
Interstate 55. Models appear overdone with the moisture this
afternoon, and so while there will be a few pockets of stratus,
expect a continued clearing trend into the evening hours. Deep low
pressure will continue east across Hudson Bay tonight. Breezy
southwest winds tonight ahead of this low will allow temperatures
to hold above freezing, and even up near 40 in downtown Chicago.

Tuesday, a cold front trailing southwest from the powerful
cyclone in Canada will graze the area during the day. With the
moisture and forcing focused farther north, we do not
expect much more than a wind shift. Tuesday will start off
unseasonably warm, so given the expected sunshine, it looks like
many areas could make a run at reaching/climbing above 50 degrees
Tuesday. The colder air lags the front a bit, but expect a quick
fall off in temperatures Tuesday evening and a colder night
Tuesday night.



309 PM CST

Tuesday night through Monday...

Overview...Quiet conditions continue as high pressure builds over
the region. The next low brings precip to the region Thursday night
and Friday, and much colder air follows the low. Snow is possible
Friday night. Temperatures drop back into the 20s and 30s Saturday,
and even colder temperatures are possible to begin the next work

High pressure moves over the region Wednesday under a zonal upper
level pattern. High temps Wednesday will be in the mid 30s to around
40. The next low forms over the Plains Wednesday.  The low shifts
east to northeast, and warmer air returns to the region Thursday.
Highs will range from the upper 30s to mid 40s. It looks like the
low`s warm front will be over southern WI Thursday morning, and I
think the majority of precip will stayed tied to the warm front. If
any precip occurs, I think it will be north of I-88. The NAM is the
most aggressive model with precip Thursday morning, but I have low
confidence in precip occurring. If it does, forecast soundings
indicate precip will be all liquid, but surface temps below freezing
could result in a brief period of freezing rain. Temps will warm so
if any freezing rain does occur, the ice should quickly melt.

The majority of precip will arrive with the surface low and it`s
upper level vorticity streamer Friday. Mainly rain is expected
Friday. The GFS has a much faster arrival time for cold air behind
the low later this week compared to the ECMWF. As such, leaned on
the blend for temperatures. This gave me a slow transition from rain
to snow Friday night. Given the differences in how quickly the
atmosphere cools, I have low confidence in exactly when rain will
turn over to snow so the main message is rain perhaps mixing with
and then changing over to snow Friday night/Saturday morning. The
GFS holds on to a little snow Saturday morning while the ECMWF is
dry so I only have a slight chance to chance of snow.

The bigger story Saturday will be the much cooler air mass, with
high temps in the 20s to 30s. If the GFS is right, lows could only
be in the low 20s. Since the 12Z ECMWF is still slower with the cold
air, I did not adjust temperatures cooler. The same is true for
temperatures through Monday aka they could be cooler than forecast,
but due to model differences, I left Superblend temps as is.

We could see a little precip Sunday as an upper level trough passes
overhead, but my confidence is low.



For the 18Z TAFs...1136 PM CST...

IFR cigs transitioning to MVFR and specific clearing times, along
with LLWS overnight.

Areas of drizzle/light rain will drift southeast of the terminals
this afternoon. Ceilings lift to low end MVFR behind the drizzle.
There is a small area of clearing across western Illinois that
will likely drift into the terminals this afternoon, but there are
indications that the MVFR cloud shield behind the clearing line
will become at least scattered to broken. A more significant push
of drier air from the west should eventually allow for a for
clearing of the lower clouds after 3z or so, but there could still
be some pockets of stratus lingering through the overnight hours.

The low level jet will increase tonight ahead of a cold front.
Marginal LLWS conditions will be in place and so have left the
conditions as is in the TAF but shortened the window. There will
no significant impacts with the front other than a subtle wind
shift from SW to W.



309 PM CST

The Gale Warning and Small Craft Advisory remain on track, so no
changes were made to them. Winds will likely only be to 30 kt
Tuesday morning over the north half, but winds will increase to
gales in the afternoon.

Deep low pressure will continue across Hudson Bay tonight, and
southwest winds increase to gales over the northern end of the lake.
The low`s cold front passes over the lake Tuesday morning, and west
to northwest gales are expected over the northern end of the lake
again. A brief period of gales is possible early Tuesday evening
over the southeast end of the lake.

Winds diminish and veer to the east as high pressure builds overhead
Wednesday.  East to southeast winds increase ahead of the next low
Thursday afternoon and evening. The low is forecast to pass just
south of the lake Friday morning and then continue to Quebec
Saturday. Winds become northwest behind the low. A weak low may form
over Lake Superior Saturday night, shift east across southern
Ontario, and then merge with a low pressure trough that stretches
across the Appalachians Sunday night.



LM...Small Craft Advisory...Winthrop Harbor to Gary IN until noon

     Small Craft Advisory...Gary to Michigan City IN until 3 PM Tuesday.




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