Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 102051

Area Forecast Discussion
National Weather Service Lincoln IL
251 PM CST Sat Dec 10 2016

.SHORT TERM...(This evening through Sunday)
ISSUED AT 250 PM CST Sat Dec 10 2016

Light snow has developed across north-central Illinois this
afternoon...primarily along and northwest of a Macomb to Peoria
line.  The snow is occurring within a pronounced band of 800-700mb
layer frontogenetic forcing caused by warm advection on the back
side of departing high pressure.  NAM shows the strongest forcing
lifting northeastward out of the KILX CWA by this evening, so have
hit snow chances hardest during the late afternoon/early evening
when likely PoPs are warranted.  By early to mid-evening, the
primary snow band will be near I-80, while lighter snow lingers
further south to near I-74.  Accumulations with this initial push of
snow will be minor, with amounts of 1 to 1.5 expected across Stark
and Marshall counties.

A stronger storm system currently taking shape in the lee of the
Rockies across eastern Colorado will take aim on the region on
Sunday, spreading a mixed bag of precipitation into central
Illinois.  12z Dec 10 models are all in relatively good agreement
with the storm track from eastern Colorado this
southern Wisconsin by Sunday evening.  Southerly flow ahead of the
system will bring warmer air into Illinois, which will present
precip type issues.  Forecast soundings are initially cold enough to
support all snow Sunday morning, but as the boundary layer warms,
the snow will mix with sleet and freezing rain as the morning
progresses...before switching to all rain across much of the area by
afternoon.  It appears rain will be the primary precip type across
the majority of central/southeast Illinois Sunday afternoon...with a
mix of snow, rain, and sleet persisting further north from the
Peoria area northward.  Any ice accumulations Sunday morning are
expected to be minimal.  Snowfall amounts will remain very light as
well, but will increase steadily further north.  The latest forecast
indicates storm total snowfall of around 1 inch along a Danville line.  Higher totals of 3 to 4 inches will
be concentrated further north across Stark and Marshall counties,
where a Winter Weather Advisory is in effect through Sunday evening.


.LONG TERM...(Sunday night through Saturday)
ISSUED AT 250 PM CST Sat Dec 10 2016

The cold front associated with the system currently taking shape
over the Plains into the Midwest will traverse central IL Sunday
evening with precipitation coming to an end behind the front. Around
6 p.m. the front looks to be near the Mississippi River...then
reaching the Indiana border near to just after Midnight. As central
IL will be in the warm sector at this point...precipitation type
will be switched over to rain...other than perhaps north of I-74. A
brief switch back to snow will be possible behind the front before
precipitation completely ends. Accumulations up to a tenth or so are
possible after this point from I-74 northward...but not much
significant accumulation.

Cooler zonal flow and dry conditions with surface high are on track
for Monday. Highs should range from the upper 20s along I-74 to the
upper 30s south of I-70. Monday evening a disturbance looks to pass
through northern IL/southern WI however precipitation is likely to
remain just to the north of this forecast area. Arctic air settling
southward into the region will follow this wave bringing
temperatures decreasing through Midweek. Highs Tuesday should
already be near 20 degrees for Galesburg, Peoria, and Bloomington,
but still remaining as high as the mid 30s south of I-70. Wednesday
and Thursday highs should be in the mid teens near I-74, to the low
20s south of I-70. Models have been trending away from a fast moving
shortwave feature producing a quick period of light snow
Wednesday...but have kept 20-30 percent PoPs for about the northern
half of the forecast area with under 1/2 inch snow accumulation as
ECMWF continues to advertise. Cold temperatures and wind chills as
low as 0 to -10 degrees Thursday morning will be the main impact
through the midweek period.

The next system in longer range models continues to be timed for
Friday into Saturday. Track of the low center and trailing high
continue to vary run to run so forecast uncertainty remains.
Nevertheless...there is some potential for significant snowfall or
periods of freezing rain depending on the track. For now kept
precipitation type as snow given the uncertainty in freezing rain


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1156 AM CST Sat Dec 10 2016

A weak disturbance will spread light snow into north-central
Illinois this afternoon and evening. Based on upstream radar loops
and the latest model data, it appears KPIA will be the only
central Illinois terminal that is impacted. Will bring a period of
light snow and low VFR ceilings into KPIA between 21z and 02z
accordingly. There is a potential for MVFR ceilings at KPIA during
this time frame, but confidence is not high enough to include in
the forecast. After that, VFR conditions will prevail at all TAF
sites through the evening before a stronger storm system
approaches late tonight. 12z NAM/GFS are in good agreement with
light snow and MVFR ceilings spreading across the area by Sunday
morning. Based on forecast soundings, have introduced MVFR
ceilings at KPIA by 11z, then further southeast to the I-72
terminals by 16z. Precip will gradually mix with and change to
rain by afternoon.


Winter Weather Advisory until midnight CST Sunday night for



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