Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 101737
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1137 AM CST Tue Dec 10 2019

.UPDATE...
1000 AM CST

Main forecast update this morning was to increase cloud cover and
add scattered flurries across far northeast IL. Lingering low
level convergence on the backside of departing surface trough is
allowing for cloud and flurry development across parts of southern
WI and northeast IL. Don`t expect anymore than flurries this
morning with this cloud cover and flurries likely diminishing
through midday, as the convergence weakens and drier air arrives.

Rodriguez

&&

.SHORT TERM...
138 AM CST

Through Tonight...

The cold air has taken up residence for the short term with teens
areawide and wind chills of 5 below to 5 above to start today.
This cold, near 15 degrees below normal for early December, is the
main story today.

Early morning water vapor imagery depicts the source of the cold
air in the upper levels -- a well-defined long wave trough --
carving into the Great Lakes region. A 165 kt upper jet is square
atop the area as of 130 am and leading this trough in.
Underneath, cold advection profiles continue with stratocumulus
that has been slow to progress eastward, and some of this has
brought flurries. Local aircraft soundings indicate the depth of
these clouds are about 2,000 ft and locked between the subsidence
inversion aloft and the boundary layer. The cold advection
profiles at 925-850 mb will weaken from daybreak through mid-
morning, and expect the clouds to show considerable scattering
from west to east during that time.

Temperatures should continue to drop through 9 a.m. or so with
westerly winds gusting at times to 20 mph. Readings will only
rebound slightly with lower to mid 20s for highs from north to
south with high confidence.

There should be a period of sun after early morning clouds depart
that persists through most of the afternoon. There is, however, a
large area of clouds to the west with a leading edge in central
Iowa early this morning, and this is tied to a sheared wave
rounding the trough into southern Wisconsin this afternoon. The
RAP does bring some of this 5-8,000 ft moisture over north central
Illinois this afternoon, but if will be fighting suppression and
drier air in advance. If conditions were to cloud back up in
north central Illinois this afternoon, there could be flurries.

Depending on the duration of clearing tonight, lows will have
potential to reach single digits for outlying areas north of I-80,
but most favored north of I-88. Clouds should start to thicken
late in advance of a quick-moving system noted below.

MTF

&&

.LONG TERM...
330 AM CST

Wednesday through Monday...

There are a couple periods of interest for precipitation potential,
first off with light snow chances for some on Wednesday morning.
Then over the weekend, a more noteworthy system could bring
rain/snow and accumulating snow somewhere in the region. Early
next work week offers some chance for wintry precip as well,
though confidence is very low in that portion of the forecast.

Wednesday morning`s potential brief light snow producer is a
subtle mid-level short-wave that will relatively maximize in
intensity with a modest warm advection response from central and
eastern Iowa to near the MS River. The wave will then likely get
sheared/weaken with eastward extent. Also adding large scale lift
will be a strong 125+ kt 300 mb jet streak nosing in. Uncertainty
is rather high even this close in due to the subtle/low-amplitude
nature of the short-wave, as well as the very dry low level Arctic
air mass that it will be encountering. On the other hand, with
such a cold air mass in place, it doesn`t take much saturation to
wring out dendrites. The model guidance is unsurprisingly pretty
variable with the placement of light QPF, with a sizable spread
in the CAMs as well. Overall, the last few runs of the 3km NAMNest
are most bullish and farther north in holding the wave together a
bit farther east.

Despite the mixed signals in the guidance, the slightly better
chances for a period of morning light snow look to line up for
roughly the southwest 1/3 of the CWA. In collaboration with WFO
DVN and ILX, bumped up PoPs to the chance range along/south of a
Sterling to Kankakee to Iroquois line, with slight chances up to
near or just north of I-90 corridor. As would be expected, the
DGZ will be quite deep in the Arctic air mass, so areas that do
get into light snow could get a quick couple tenths of
accumulation. With cold temps in the teens, even a coating of snow
could cause some travel impacts during the morning commute.

The rest of Wednesday will clear out as strong 1035 mb high
pressure quickly overspreads from the northwest. After highs in
the 20s, light to calm winds and clear skies for a few to several
hours after sunset should support a quick temperature fall to the
teens in many areas, with low teens in favored northern IL cold
spots. Can`t rule out some isolated upper single digits evening
lows if skies stay clear long enough. Warm advection will then
quickly ensue with increasing clouds and gradually increasing east
to southeast winds overnight, so temps will level out and then
rise into Thursday morning. Thursday will be a breezy and much
milder day on the back side of the high pressure as a weak surface
low lifts to our northwest. Current signs suggest that cloud cover
will be mid and high level, so southerly winds gusting to 25 to
perhaps 30 mph, mild 925 mb temps and no snow cover should
translate into highs into the low and even mid 40s. Rain is
likely to stay north of the Wisconsin state line.

The close of the work week into the first half of the weekend
offers a challenging, complex surface and mid-upper level pattern.
A split flow pattern and active Pacific jet will likely have a few
stronger embedded short-waves passing nearby, with questions on
any possible phasing with an incoming northern stream trough
digging southward. The daytime hours Friday should be mostly
quiet, but cloudy with a better chance for return southerly flow
stratus, so low 40s highs might be generous.

A weak surface low will be taking shape over the mid Missouri
valley by Friday evening, with potential for light warm advection
precip ahead of it (possibly a rain/snow mix). The exact track of
the surface low, as well as evolution of mid-upper level features
is certainly far enough out for changes. However, broad agreement
in general idea of surface low tracking over and then east of the
area on Saturday supports mid range chance PoPs. Saturday is a
period to watch, as we could see a scenario of rain to snow or
even all snow for parts of the area depending on overall evolution.

The 00z ECMWF and ensemble suite was most concerning on this
note, though even operational GFS and GEM not too far off. We`ll
be monitoring for potential accumulating snow and travel impacts
in at least portions of the region, which may be coupled with
steadily falling temps through the evening after earlier temps
above freezing. Sunday and especially Monday are lower confidence,
with models waffling on whether next possible system impacts our
region or aims farther east. The cold shot behind Saturday`s
system could also be more robust than indicated in official
forecast, especially if snow cover is laid down in portions of the
area.

Castro

&&

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

Any lingering MVFR stratus and flurries just to the north of ORD
will continue to diminish this afternoon, with VFR and dry
conditions then expected into tonight. A fast moving system will
move across the region Wednesday morning, with VFR ceilings
returning across the terminals. Light snow and lower ceilings
associated with this system are expected to remain just to the
south of the terminals at this time. However, it`s still possible
that the northern extent of this light snow could briefly clip the
terminals. If this were to occur, the window for this light snow
would be small with any snow accumulation still not likely. This
system quickly departs midday Wednesday, with clearing skies the
trend into the afternoon. A steady westerly wind, occasionally
gusting this afternoon, will persist this period. Winds will
likely quickly weaken Wednesday afternoon, as high pressure builds
across the region.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Gary to Michigan City IN until 3 PM Tuesday.

&&

$$

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