Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 050530
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1130 PM CST Sun Dec 4 2016

.UPDATE...
Issued at 847 PM CST Sun Dec 4 2016

Upper level trough axis is now well east of the central IL
forecast area per water vapor satellite imagery with low level
moisture and cloud cover trapped below the subsidence above. Light
westerly winds in place across the region are slowly advancing a
back edge to the low clouds...approaching the Mississippi River at
this time. Patchy fog is noted in observations across central IL
with visibilities around 2 miles, however fog may become dense
behind the low cloud shield as radiational cooling increases and
winds decrease. Have made updates for fog development overnight.
Otherwise...expect temperatures to fall to the upper 20s and low
30s allowing some wet areas to freeze and become slick
overnight...especially in areas with fresh snow
accumulations...mainly I-72 northward.

&&

.SHORT TERM...(This evening through Monday)
ISSUED AT 323 PM CST Sun Dec 4 2016

Back edge of the precipitation is steadily advancing eastward at
mid afternoon, about to end from Peoria-Springfield and should
largely be done west of I-57 by sunset. Heavier snowfall amounts at
or above 6 inches were observed from Knox-Marshall Counties with a
couple inches closer to Springfield. Precipitation will likely end
as a period of very light rain based on upstream reports. Will go
ahead and cancel the Winter Weather Advisory, as the last of the
heavier bands will soon be exiting McLean County and any additional
snow through sunset will be minimal.

Main concern overnight is the cloud trends and potential for any
fog. Clear skies pushing into western Iowa and northwest Missouri
this afternoon, and wild card is how far east it advances. A high
pressure ridge will be drifting east over the state after midnight,
and any clearing will likely quickly fill back in with stratus and
fog. High-resolution models spread this in over most of central
Illinois after midnight. Will add some fog mention overnight and
early Monday over the area, but with the uncertainty, will not go
indicate dense fog at this point.

The ridge will drift east of the area on Monday, with southerly
winds trying to bring some warmer air into the area. Have tempered
the highs over the north where the heavier snow was today, but areas
along and south of I-72 probably should make it into the 40s.

.LONG TERM...(Monday night through Sunday)
ISSUED AT 323 PM CST Sun Dec 4 2016

The extended forecast will start with a low pressure system ejecting
from Louisiana to central Tennessee by 12z/6am Tuesday, then to West
Virginia by 6pm Tuesday. There has been a colder shift in the 12z
models, with respect to precipitation type for this event. The snow
cover from Sunday`s snow may play a roll in colder night time temps,
and possibly even daytime temps. The end result could be a mix of
snow and/or freezing rain developing later Monday night and Tuesday
morning north of I-72, with mainly rain south of I-72. Still
uncertainties with how much warm air will wrap around to the north
side of that system into Illinois, but have introduced freezing rain
and expanded snow in the forecast grids Mon night-Tues Morn. Even
light FZRA could cause travel problems, so will monitor thermal
profiles closely for future updates.

Tuesday afternoon could see enough dry air get pulled into Illinois
from the west to create some breaks of sunshine west of I-55.
However, the concurrent arrival of colder air with the clearing will
limit high temps to the mid to upper 30s N of I-72 with some low 40s
south of I-72. That clearing will not continue Tues night and Wed,
as mid and high clouds stream into the area ahead of an upper level
wave approaching from the Plains.

There has been some better agreement between the ECMWF, GFS and
Canadian that some snowfall will occur Wed night, when previously
the GFS and Canadian had been primarily dry. The EC trended toward
weaker forcing for precip and lower moisture content, to align
better with the light snowfall in the GFS/Can. Due to recent model
shifts, have kept the chance PoPs instead of increasing to Likely at
this point. Snowfall amounts at this point look to range from 0.5 to
1.5", with the higher totals oriented between I-72 and I-70.

An influx of very cold Canadian air will occur after that system
departs to the east on Thursday. The coldest night looks to be
Thursday night when lows bottom out in the 10-12F range near
Galesburg, with 16-18F south of I-70. Highs on Thursday and Friday
will remain below normal in the 20s.

Some warming is expected for the weekend as southerly winds develop
ahead of an approaching cold front. The 12z EC and GFS have
downplayed the snow potential from that system, so have limited the
PoPs to 30 pct for Sat night and Sunday. Moisture appears relatively
limited with the fast moving system.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1130 PM CST Sun Dec 4 2016

Primarily IFR cigs and few vsby restrictions this evening across
central IL as a low cloud shield lingers over the area following
the recent storm system. Overnight...fog should develop as low
cloud thins and eventually passes east of the area. This should
result in visibibility reducing to IFR/LIFR. With low sun angle
and light winds early in the day Monday...fog and low ceilings
should take until 18-19Z to dissipate to VFR conditions. Winds
W-SW under 10 kts backing to SE Monday morning through Monday
evening.

&&

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

$$

UPDATE...37
SHORT TERM...Geelhart
LONG TERM...Shimon
AVIATION...37



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