Area Forecast Discussion
Issued by NWS Central Illinois
FXUS63 KILX 080553
Area Forecast Discussion
National Weather Service Lincoln IL
1153 PM CST Sat Dec 7 2013
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
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.
ISSUED 1153 PM CST Sat Dec 7 2013
An initial wave of isentropic lift will work to saturate the low
to mid levels, but little in the way of snow accumulation is
expected from now through around mid-day Sunday. Flurries may
begin as early as 12z for SPI/DEC with mid morning flurries for
the remainder of the TAF sites. Some light accumulation of a few
tenths to near one half inch will be possible across a majority of
central and eastern IL. As much as one inch could accumulate
northwest of Peoria, but the higher totals of an inch or more
should remain mainly in Iowa and far NW IL.
The 00z models continue to show a split in the focus zones for
precipitation generation, with central IL in a coll zone between
jet dynamics to the southeast and the surface low to the
northwest. Overall, precip type could become mixed phase, with
freezing drizzle possible later Sunday into Sunday evening as we
lose ice crystals aloft but remain below freezing through the
lowest 5k ft of the boundary layer. Lift will be diminishing
during that time, as dry air progressively erodes the low level
moisture from aloft. Precip chances should come to an end before
midnight/06z Sunday night as the low pulls away to the northeast.
Winds will start out northeast the rest of tonight, then become
southeast as low level moisture increases. Some gusts to 20kt will
be seen as mixing depth increases. Weaker winds will return Sunday
evening as winds become due south by 06z Sunday night.
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