Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 102339
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
539 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 evening...to
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 Peoria...to
Bloomington...to 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
potential.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 525 PM CST Sat Dec 10 2016

Initial band of snow has shifted mainly north of the I-74 corridor
early this evening with another round expected late tonight into
Sunday as the main storm system moves across Illinois Sunday
afternoon and evening. MVFR cigs reported with some of the heavier
snow bursts with the lower cigs moving out of the area for a time
this evening with most of our sites seeing VFR cigs into the early
morning hours. Snow will increase in coverage again as the main
area of low pressure approaches our area on Sunday with cigs
lowering to MVFR, with the potential for some IFR cigs as warmer
air moves north in the late morning and afternoon hours. The snow
will mix with some sleet at times as temperatures aloft warm with
a low probability for a period of freezing rain mixing in with
the snow and sleet from around 15z through 19z before surface
temperatures warm to just above freezing. Will not include any
freezing rain in this set of TAFs but if new model data this
evening continues to suggest the threat, will include in KPIA and
KBMI for a couple of hours late Sunday morning.

Surface winds will be southeast at 10 to 15 kts tonight and then
veer into a southerly direction on Sunday as the storm approaches
with speeds of 12 to 17 kts with a few gusts up to 22 kts at
times.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight CST Sunday night for
ILZ028-030.

&&

$$

SHORT TERM...Barnes
LONG TERM...37
AVIATION...Smith


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