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fxus63 klot 111716 

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
issued by National Weather Service Lincoln Illinois
1116 am CST Tue Dec 11 2018

1025 am CST

Clear skies southwest winds will continue for a majority of the
day, as a cold front slowly drifts southward toward northern
Illinois. Cirrus clouds are poised to progress into northern
Illinois this afternoon, with cold frontal passage projected for the 22z to
02z time frame. A dry frontal passage is expected initially, with
a wind shift to the east-southeast the initial result. However, a
mixed precip event is still projected for later tonight into
Wednesday morning, with some light icing from freezing rain or
freezing drizzle, along with some light snow accum possible. Still
in the analysis phase of any forecast changes to tonight and
Wednesday, but some travel impacts for the Wed morning commute
could develop.

No major changes were needed for this morning's update. The
forecast database appears on track.



Short term...
330 am CST

Through Wednesday...

The main concern is the potential for a quick hitting shot of
wintry precip for parts of the area on Wednesday morning. Today
will be a fairly pleasant day for December. It appears we won't be
dealing with low clouds during the daylight hours, so just some
patchy mid/high clouds overhead. Low mixing depth may be a
limiting factor on temps being even milder than forecast. However,
am suspicious of the colder raw guidance from NCEP and CMC vs
European model (ecmwf) and MOS guidance which has been consistently warmer for
today's highs. Bumped up highs to mid 30s far north to upper 30s
to lower 40s elsewhere. If mixing is deep enough, climo of progged
925 mb temps would support some mid 40s readings south of I-80.
Temperatures will likely drop fairly quickly this evening as we
have a brief period of a weak pressure gradient ahead of our
clipper. Given relatively low temp/dew point spreads, could see
some haze or light fog develop during the early evening.

Parent short-wave for the clipper can be seen on west/v imagery over
the northern rockies early this morning. Trend for the clipper
wave has been much quicker and a less favorable track for
anything more than a few hours of precip, with the surface low
forecast to track into southern WI by mid day Wednesday. There
will be a warm nose of +1 to +3c aloft in place in advance of the
system, but with a very dry column. The potent nature of the mid
and upper level system, favoring it closing off as it tracks
across Iowa early Wednesday will result in rapid height falls.
Evaporative cooling and height falls should both support to cool
the column as brief window of top down saturation occurs. Cams
including the 3km namnest and WRF-nmm appear reasonable
conceptually for this system, with a fairly narrow fast moving
band/warm advection Wing of precip pivoting across from the pre-
dawn hours through the mid to late morning. Track of surface low
favors north of I-80 for highest pops in high chance to likely
category and light quantitative precipitation forecast of up to 0.05, highest near WI border.

While forecast soundings are a bit concerning for lacking ice
aloft at times, suspect that these soundings are a product of the
model struggling with how brief the period of precip will be after
a very dry column to start. Think that the saturation will occur
and at most 1-3 hours of precip in any given location, likely
followed by it mostly ending. Can't completely rule out some
spotty sprinkles/drizzle/flurries behind the main band of precip,
though suspect most areas will be dry after whatever occurs. Given
expectation of column to cool quickly to snow supportive, window
for any freezing rain (maybe brief ice pellets) looks to be short
right at onset, so do not have any ice accums in the grids.

Temperatures will likely be near to a few degrees above freezing
while the brief window of precip occurs, so a slushy coating to
maybe up to a half inch far north is possible, highest far north,
and mostly on grassy/colder surfaces. Steep lapse rates and strong
lift with the potent wave could support precip coming down at a
decent clip, but the very short duration will be a major limiting
factor. The afternoon will be cloudy and breezy with west-southwest
winds gusting up to 25 mph in the wake of the low. Temperatures
will top out in the mid to upper 30s during early afternoon after
any precip ends.



Long term...
330 am CST

Thursday through Monday...

A decent Pacific system will move onshore across the Pacific
northwest this morning and will be shifting east of The Rockies to
start the period. The energy associated with this system will be
split across a broad trough axis spanning the country, with a more
open wave across the northern Great Lakes, while strong upper jet
on the back side of the trough will allow the southern stream to
develop a closed low. Models still dig this energy from either the
Ohio Valley then northeast to the eastern Great Lakes, or dig it
farther south into the arklatex region and then back into the Ohio
Valley. Regardless, warm/moist southwest flow advection will feed
1"+ precipitable water values into the approaching system. While
those values may not get this far north, a decent inverted surface
trough will setup. Therefore, expect a blossoming shield of
precipitation Thursday afternoon and especially Thursday night.
Model thermal progs suggests the column will be warm enough for
all liquid precipitation.

Where the model discrepancies of low placement become more
noticeable is in determining how long precipitation will linger
into Friday and even Saturday. The ec has the farthest south track
and would keep the latter portion of the storm's precip south and
east across the Ohio Valley. And the Gem is the model that phases
the northern and southern streams and is able to manufacture/bring
in colder northern stream air that is largely cut off on the
other models.

As noted earlier, the upper low by Saturday afternoon on the
various medium range models is anywhere from the eastern Great
Lakes to south of the Tennessee Valley. Therefore confidence on
precipitation and associated p-type beyond the early Friday period
is certainly lower, and we could just be back under high pressure
by early Saturday based on the European model (ecmwf). The model blend is weighted
a bit colder, and thus there is some snow mention Friday night. But
if the lack of phasing pans out, the very little precip will be
left in the colder sector of this system.

Beyond this, we will fluctuate between ridging aloft Saturday
night to another weaker wave on Sunday with a slightly better
chance of precip. Another wave in northwest flow may be on the
horizon for early next week.



for the 18z tafs...

Main concern will be after about 09z, as an area of low pressure
moves into far southern Wisconsin. Forecast soundings continue to
suggest a short period of -fzra potential before a quick switch to
light snow. Have maintained the prob30 mention with some slight
timing refinement. Ceilings will steadily deteriorate and should
be around IFR levels as early as 12-13z near krfd and 14-15z near
kord/kmdw. Slight improvement is expected by midday as the low
lifts further northeast, but any significant lifting of cloud
heights should be late afternoon.



Lot watches/warnings/advisories...



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