Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 301734
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1134 AM CST Wed Nov 30 2016

.UPDATE...
1134 AM CST

Expansive low cloud deck under the vertically stacked upper low
has quickly enveloped the area, with just eastern Illinois and
northwest Indiana still observing sun as of this writing. Going
forecast today in pretty good shape, with tweaks to hourly sky,
temp and dew point grids to account for trends. Initial shortwave
expected to bring isolated to scattered shower activity can be
seen on W/V across central IA. This wave will give a glancing blow
to north central Illinois, where the highest chance range PoPs
will be focused late this afternoon into this evening. Farther
south and east, activity will be isolated at best. Latest guidance
suggests that the wave more likely to bring higher coverage of
scattered showers a bit farther south will pivot across the area
overnight, so also trended PoPs this way. Temperatures are likely
to remain primarily above freezing, with rain-snow showers favored
earlier on and then transitioning to all snow showers with time
this evening. Still not anticipating any impacts other than
perhaps brief occasional reduced visibility, as air temps above
freezing and mild road temps will prevent any accumulations, with
a dusting at most on grassy/colder surfaces.

Castro

&&

.SHORT TERM...
217 AM CST

Through tonight...

Primary concern in the short term is with precip chances very late
this afternoon and more so this evening. Cold front has just
about cleared our eastern CWA with post frontal stratus deck
nearing the MS River early this morning. The stratus will rotate
east and overspread the area today. Temps will be a bit tricky
with some rise expected through the morning, mainly where the sun
is out, followed by temps leveling out and likely slowly falling
area wide this afternoon.

A couple of shortwaves will pinwheel around the large deep
tropospheric cyclone, the first of which shows up nicely on
satellite imagery this morning over SD. This feature will lead to
increased ascent/deepening moisture, which combine with the steep
low level lapse rates/instability should support at least some
flurries but probably some scattered snow showers as well. Precip
could briefly start as some graupel or liquid given the marginal
temp profiles and anticipated convective nature of the precip.
Difficult to find a model that spits out much measurable precip so
opted to keep pops in the chance category and at worst, if there
is a heavier snow shower, it may be able to briefly whiten grassy
surface. Temps should largely stay a bit above freezing so any
accum would be brief and melt, so impacts would be minimal
tonight.

Izzi

&&

.LONG TERM...
217 AM CST

Thursday through Tuesday...

One final shortwave pivoting around the upper low could allow a
few flurries to be squeezed out Thursday morning, before precip
chances with this system wane. While precip should wrap up, it`s
likely we will be entering a prolonged period of overcast skies as
stratus becomes lodged beneath pronounced frontal inversion, with
no strong signal in the models to suggest impressive drying. In
fact, it is plausible that we may not see any sunshine again until
sometime next week mid-high level cloudiness this weekend could
offset any potential clearing of the lower clouds.

Forecast confidence by late in the weekend and particularly next
week is pretty low. ECMWF/GFS agree in digging shortwave trough
into northern old Mexico this weekend closing off/cutting off mid
level circulation. Northern stream shortwave should track across
the northern half of the CONUS this weekend, but GFS/ECMWF vary on
the degree of phasing between this shortwave and the Mexican
system. Even without significant phasing, the northern stream wave
could be enough to produce light precipitation Sunday. Thermal
profiles at the time look borderline for rain/snow and given so
many uncertainties in how the system/pattern evolve, too soon to
say if there could be an light snow accumulation risk with this
system.

The 00z operational runs of the GFS and ECMWF both agree in
digging a deep long wave trough into the western CONUS early next
week with an accompanying dump of arctic air. Detail on when and
how that cold air spreads east are major points of contention, so
again this morning made no changes to blended model guidance for
next week.

Izzi

&&

.AVIATION...
For the 12Z TAFs...

MVFR stratus spreading east will spread over the terminals through
the morning. The stratus has slowed significantly over the past
hour, so stratus may arrive slightly later than forecast if the
trend continues. Southwest winds gust up to 25 kt through the
period and become west tonight.

An upper level low will rotate over the region tonight.
Disturbances associated with the low will help produce areas of
flurries from late this afternoon through Thursday morning. Cigs
will remain MVFR with the flurries, although some guidance members
suggest IFR is psbl especially for RFD. Snow will not accumulate,
lower vsby beyond 6SM, nor cause any significant impacts.

JEE

&&

.MARINE...
253 AM CST

Headlines...Issued a small craft for the IN nearshore waters as
winds will be gusting up to 30 kt tonight. The SCY extends into
Thursday evening for points east of Gary, IN due to hazardous
waves.  Have lower confidence in seeing hazardous wind gusts over
the IL waters. Therefore, did not issue a small craft advisory
for the IL NSH waters.

The low over Minnesota finally crosses over the Great Lakes today on
its way to Quebec.  Winds become southwest behind the low and
stronger gusts to 30 kt are expected over the southern half of the
lake through Thurs morning.  West to northwest winds 15-25 kt are
then expected through Saturday. A high pressure ridge passes over
the lake Saturday and winds become south to southwest behind it.

The models show significant differences in how the pattern will
evolve early next week. However, both models feature a low forming
over the plains and shifting northeast at varying intensities and
speeds.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...LMZ743-LMZ744-LMZ745...3 PM Wednesday TO
     9 PM Thursday.

&&

$$

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