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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1156 am CST Tue Dec 12 2017

Short term...
243 am CST

Through tonight...

Blustery and colder weather is in store for the area today, in
the wake of monday's clipper. Clouds and a few flurries early will
give way to partly cloudy skies, though with chilly temps/wind
chills.

Surface low pressure associated with monday's system continues to
track away to the east early this morning, with it's trailing
cold front well south of the Ohio River. Gusty north-northwest
winds 25 to 30+ mph were pulling colder, Arctic air into the
region early this morning, with lingering cold-air stratocu deck
producing a few flurries and light snow showers across the eastern
2/3 or so of the cwa. Cloud layer is thinning per GOES 11-3.9
micron imagery and model forecast soundings indicate this trend
should continue this morning, thus would anticipate any flurries
to end fairly early. The exception will be in Northwest Indiana,
where lake effect snow showers will develop, though the best low
level convergence focus should remain just east of the
Porter/LaPorte County line. Still, snow showers may produce some
minor accumulations through midday mainly across eastern Porter
County. Skies become partly cloudy over the remainder of the area,
with model thermal fields supporting daytime highs in the low-mid
20's, and coolest temps north. While winds will very slowly ease
today, wind chills are already in the single digits over far
northern Illinois as of 2 am, and these will likely dip to around zero
to 5 above across the area this morning before rebounding slightly
later today.

Weak surface high pressure ridge drifts across the forecast area
this evening, allowing winds to diminish further. Decreasing
winds, new snow cover especially across far northern IL, and dry
Arctic air may allow for a quick drop-off in temps after sunset
this evening. Have leaned toward coldest guidance temps for areas
toward the Illinois/WI border where 2-3 inches of snow fell Monday
afternoon in a few spots, and the typically colder Fox Valley,
where temps may fall quickly into the single digits early. Another
quick-moving clipper system will be approaching from the
northwest overnight however, with increasing mid-level cloud cover
which may inhibit further temp drop. Surface winds will also back
to the south overnight as the ridge drifts east and surface low
pressure moves into the upper Midwest. This combination of
increasing/thickening cloud cover and developing south wind will
likely allow for steady or slowly rising temps after midnight.
Precip looks to hold off until Wednesday morning.

Ratzer

&&

Long term...
314 am CST

Wednesday through Monday...

Overview...another clipper system will bring mainly snow to the
region Wednesday. Accumulating snow is most likely in the Lake
Michigan collar counties. Precipitation type could be an issue
along and south of I-80. Lake effect snow will impact Southern Lake
Michigan Wednesday night into Thursday morning. While there are a
couple additional chances for precip through the weekend, I have low
confidence in seeing measurable precip. I have higher confidence in
a warming trend Friday into early next week.

Another clipper system will travel over the forecast area Wednesday,
and similar to the past few clippers, the placement of the low will
dictate precip type. Most models have settled on a path through
northern Illinois and northwest in, while the NAM has a more northerly
path over southern WI and the lake. I leaned toward the more
southern solutions. Areas north of the low should see light snow
Wednesday assuming we are able to saturate. Like the system Monday,
the majority of snow may fall behind the low as a secondary upper
level vorticity streamer moves through. If an f-gen band forms, we
could see another narrow band of moderate to heavy snow like we saw
Monday evening.

Areas along and south of I-80 may see mixed precip or periods of
rain as temperatures should climb into the mid to upper 30s. If the
low is further north, mixed precip and warmer temperatures will also
shift north.

A very favorable lake effect snow regime sets up behind the low
Wednesday night. Delta t's look to be around 17 degrees and flow
will be down The Heart of the lake. Unlike the last Few Lake effect
snow events, this one should impact northeast Illinois Wednesday evening.
Winds back to northwest pushing the band over lake and Porter
counties overnight. It's a bit far out to pinpoint snow amounts, but
i'm thinking snow totals will probably be higher than currently
forecast wherever the band sets up. The band will then push east
Thursday morning.

An upper level trough passes overhead Friday, and it could force a
little snow. The GFS has much more quantitative precipitation forecast than other models so I capped
precip chances in the chance range. A surface ridge and warm air
advection follow the trough leading to high temps in the upper 30s
to low 40s Saturday. Another low moves over the upper Great Lakes
over the weekend, but it looks like the majority of precip will fall
north of the forecast area.

Jee

&&

Aviation...
for the 18z tafs...

Northwest winds over the area this afternoon will continue to ease
through the afternoon, but the main focus during the period is
with another clipper system set to shift over the area on
Wednesday. The current track of this low would put if over
northern Illinois by midday Wednesday. Given this track, the wind
forecast will be tricky at the terminals as any minor fluctuation
in the expected track of this low could result in changes in the
favored wind directions and speeds. Currently we expect the winds
to become south-southeasterly by early Wednesday morning, then
shift west-southwesterly later in the morning as the low
approaches, and finally back to the northwest again during the
afternoon.

In addition to the winds, it appears that they may be a period or
two of snow over the eastern terminals on Wednesday. While the
better chances for snow is likely to remain north of the area in
Wisconsin, it does appear that there could be a short 1 to 3 hour
period of snow early Wednesday morning over northeastern Illinois in
association with increasing warm air advection. There is still
uncertainties in the extent of this band of snow, but if it
materializes could produce a short period of low visibilities in
moderate to heavier snow before it quickly shifts out of the area
by mid to late morning. While the snow is likely to end during the
morning, it appears that another quick moving band of snow could
shift over the terminals during the afternoon, with another period
of reduced visibilities possible. Given uncertainties in timing
and the extent of the snow, I have only mentioned the lowest
visibilities in tempos at this time.

Kjb

&&

Marine...
314 am CST

Current gale headlines look on track. Small craft advisories will be
needed for the nearshore waters after the gale warnings as winds and
waves slowly subside.

The low over the eastern Great Lakes will continue to New England
today allowing a high pressure ridge to pass overhead tonight. Gales
will diminish late this afternoon into the early evening as the
pressure gradient relaxes between the departing low and approaching
ridge.

The next low pressure system will pass just over or just south of
the southern tip of Lake Michigan Wednesday afternoon. Gales are not
expected at this time, but gusts up to 25 kt are expected over the
southern half of the lake mid Wednesday morning through Wednesday
night. West winds are then forecast through Friday. Another weak low
is forecast to develop over Lake Superior Thursday evening and then
deepen as it moves over the eastern Great Lakes Friday evening.
Winds may increase to 30 kt over the southern end of the lake. I
have low confidence in the forecast Saturday and beyond due to
significant differences in how guidance handles the pattern
progression.

Jee

&&

Lot watches/warnings/advisories...
Illinois...none.
In...Lakeshore flood advisory...inz002 until 3 PM Tuesday.

Lm...Small Craft Advisory...Illinois nearshore waters until 10 PM Tuesday.

Gale Warning...in nearshore waters until 3 PM Tuesday.

Gale Warning...lmz777-lmz779 until 8 PM Tuesday.

&&

$$

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