000
FXUS63 KILX 080309
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
909 PM CST Sat Dec 7 2013

.DISCUSSION...
ISSUED 900 PM CST Sat Dec 7 2013

Strong mid and upper level flows will continue to push increasing
moisture toward Illinois. Radar returns showing virga snows are
already reaching into SW Illinois. Saturation will develop from
10k-12k ft and downward toward the ground by mid morning on
Sunday. We could see a few flurries as early as 3 am across our
southwest counties, which was added to the grids. Chance PoPs
still look good for Sunday morning, with light snow the primary
precip type.

As warming aloft continues Sunday afternoon, and dry air aloft
removes a layer-source of ice crystals into the top of the lower
clouds. That could cause a transition to some freezing drizzle
instead of snow. Better chances of any freezing precip looks to be
southeast of I-70, but we will continue a mention in all areas,
especially during the afternoon and evening. If any icing does
occur, it should remain very light and mainly on elevated
surfaces and untreated bridges/overpasses.

Snow accumulations will be mainly a dusting east of I-55, with
around a half inch toward Galesburg. Accums of 1 inch or more
should remain confined to Iowa and far NW IL.

Adjusted the low temp grid down a degree or two over the snow-pack
in the southeast, otherwise lows looked on track. Increasing
clouds should help to keep lows from dropping off much below guid.
The remainder of the grids looked on track. Updated forecast info
will be available shortly.

Shimon
&&

.AVIATION...
ISSUED 553 PM CST Sat Dec 7 2013

Clear skies this evening will give way to increasing mid-high
clouds by sunrise tomorrow. Forecast soundings show MVFR ceilings
likely by mid morning as saturation occurs around PIA along the
-10C isotherm. Some ice may be present under those conditions, and
snowfall may be observed later Sunday morning at PIA and BMI as
the low pressure system lifts across eastern Iowa. Enough drying
aloft will shut down the supply of ice crystals as the lower
levels warm to -5C and remain relatively saturated. That scenario
could produce some light freezing drizzle if any lift remains
present in the low levels. We are not looking at a major icing
event, but we could get a light coating of ice on elevated
objects, and untreated bridges and overpasses.

The overall theme is that ice crystals will become less available
as we head through Sunday afternoon and evening, so snow chances
could evolve to -fzdz. Will have to watch this event closely for
the possible need of a freezing rain advisory.

The latest model runs show two distinct areas of precip, one to
the NW of our terminals and one to the southeast. Some expansion
of precip is indicated Sunday afternoon, but less than previous
models. Therefore the confidence is not high in a major ice event.

Winds will start out northerly this evening, then become northeast
as the surface high makes progress into the Great Lakes. Winds
will eventually shift to the SE by mid-day on Sunday and become
gusty to 20kt at times.

Shimon
&&

.PREV DISCUSSION...
ISSUED 225 PM CST Sat Dec 7 2013

Main concern this package has to do with the possible wintry mix
of freezing drizzle and snow tomorrow and tomorrow evening over
the area. Short term models seem to have a good handle on the
upper level pattern and the sfc features and their expected
trends. Differences, however, are seen with the actual
temperature profiles across the area. These differences will make
a big difference in precip type tomorrow through tomorrow evening.
Extended models seem in good agreement as well.

SHORT TERM...Tonight through Tuesday

Models have slowed the onset of pcpn with the next system moving
into the area tomorrow. Appears that cwa will be caught in the
middle of the best dynamics with the upcoming system. The sfc low
will move out of northwest MO and into southern WI, northwest of
the area, while another frontal system will be kept to the south.
Looks like enough dynamics over our area for some pcpn, though
main concern is precip type. Looks like once atmosphere becomes
saturated in the lower levels, the pcpn should start as snow.
However, just after that, we will lose our moisture at the level
where the ice crystals would be forming. So since where pcpn will
develop will be little warmer than needed for snow, the precip
will form as liquid and continue to fall through the cloud that
way. This means that the pcpn should reach the ground as liquid
water and with the sfc temperatures below freezing, then pcpn type
will be freezing; and given the slightly weaker dynamics than to
the northwest and south, the pcpn will be drizzle. So, expecting
freezing drizzle tomorrow afternoon through tomorrow night. Some
light snow could be mixed with this tomorrow afternoon, but
expecting all light freezing drizzle tomorrow evening. The pcpn
should end before midnight as the system pushes northeast and the
atmosphere dries out. Beyond this system, another arctic high
pressure area will build in for Mon and Tue.

Temps will warm a tad ahead of the system tomorrow evening, but
cold temps are expected again early Tue morning. Overall temps
will remain below freezing through the period, which is 10+
degrees below normal.

LONG TERM...Tuesday night through Saturday

High pressure will continue to dominate the weather across the
area through most of the week. There is a minor system that is
forecast to move across the northern part of the state Wed, but
expecting only clouds out of this system at the moment. Another
system approaches the area at the end of the week, but only a
slight chc of pcpn is expected at this time; which will only show
up in the grids and not in the worded forecast.

Temps will remain cold through the period with coldest temps being
Wed and Wed night. As the high pressure area shifts to the
southeast at end of the week, temps will warm to above freezing by
Friday. However, this will still be below normal for middle of
December.

Auten

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$