Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 082040

Area Forecast Discussion
National Weather Service Lincoln IL
240 PM CST Thu Dec 8 2016

.SHORT TERM...(This evening through Friday)
ISSUED AT 240 PM CST Thu Dec 8 2016

1045mb high extending from Alberta/Saskatchewan southeastward to
Oklahoma will shift slowly eastward over the next 24 hours...with
the ridge axis eventually reaching the Mississippi River by Friday
evening.  In the meantime...central Illinois will continue to be
influenced by cyclonic flow around a deep upper low over Quebec.
Widespread cloud cover currently blankets central/northern
Illinois...with the southern edge of the cloud shield roughly along
a Des Moines, Flora line.  Main short-
term challenge will be predicting how quickly the clouds shift
eastward.  With deep-layer northwesterly flow in place, any
appreciable clearing will be slow to take place.  While model
solutions vary, think slow is the way to go in this particular
pattern.  As a result, will continue with a mostly cloudy forecast
across much of the CWA tonight and Friday.  Skies will at least
partially clear along the S/SW periphery of the CWA...especially
along/southwest of a Robinson line.
A few snow flurries have been reported from time to time beneath the
overcast, so have included mention of flurries across the northern
CWA through tonight.  Due to persistent cloud cover and northwest
winds, much below normal temperatures will continue...with lows
tonight in the teens and highs on Friday remaining in the 20s.


.LONG TERM...(Friday night through Thursday)
ISSUED AT 240 PM CST Thu Dec 8 2016

Temperatures will begin to moderate to start the period. These
milder, but still below normal, temperatures will last through the
beginning of next week before the next shot of Arctic air arrives.

The precipitation scenario for the weekend has changed a bit since
yesterday, and model agreement is better. A clipper type wave will
streak across the area Saturday afternoon and night bringing a quick
shot of snow to northern portions of the forecast area. This wave
has trended further north the last few model runs, and would expect
most of the snowfall to occur along/north of the I-74 corridor.

A more substantial wave will dig into the Plains on Sunday,
developing a surface low that will track northeast across the
forecast area Sunday night. This system looks increasingly like it
will tap into enough warm air that the precipitation on the warm
side of the low will mostly fall as rain, while mostly snow is
expected on the cold side. This is resulting in a complicated
weather type forecast across the forecast area from Sunday afternoon
through Sunday night, as there are still important differences in
the forecast storm track. A blended solution remains the way to go
given the uncertainty, and expect several forecast adjustments to be
needed to the details between now and Sunday. At this point, the
window for all snow, with sufficient intensity, looks too small for
significant snow accumulation.

Quiet weather returns for Monday. The passage of an Arctic cold
front Tuesday/Tuesday night will be accompanied by snow showers, but
they should not be significant. However, the cold air behind the
front will be (significant).


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1241 PM CST Thu Dec 8 2016

MVFR ceilings will persist at the central Illinois terminals
through the afternoon and likely beyond. Latest satellite imagery
shows widespread cloud cover across the region...with the
southwest edge of the cloud area extending from central Iowa into
southwest Illinois. With deep layer northwesterly flow expected to
persist...think clouds will be very slow to clear later tonight
into Friday. While Rapid Refresh forecast soundings suggest
clearing skies at KPIA as early as 11/12z...the NAM is much more
pessimistic and keeps clouds in place through the day Friday.
Given current and expected flow pattern, have opted to keep
overcast conditions in place at the I-74 terminals through the
entire 18z TAF period. Further southwest, trajectories suggest
clearing conditions at KSPI and KDEC. Exact timing of clearing
remains in question: however, model consensus is between 14 and




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