Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 040529

Area Forecast Discussion
National Weather Service Lincoln IL
1129 PM CST Sat Dec 3 2016

Issued at 939 PM CST Sat Dec 3 2016

The upper level shortwave over the central Plains continues to
approach this evening with observations/radar echoes of snow ahead
of the wave axis in SE Nebraska and western Iowa, rain just to
the south. first spotty echoes reaching eastern Iowa, but with
plenty of dry air in mid levels ahead of the wave...little of this
seems to be reaching the ground. Another precipitation feature
bringing rain through the lower Mississippi Valley region as far
north as SE Missouri/far southern IL at this time also moving
east. The latter feature should largely pass by to the south.
Still looking to see first snow spreading into areas west of the
Illinois River after midnight and to the Indiana border around 6-8
a.m. based on current timing. Although a few temperatures slipped
just below freezing earlier this evening with partial
clearing...extensive mid and high clouds should bring temperatures
back to near or just above freezing later this evening through the
onset of precipitation overnight. No significant updates needed
at this time as progress is largely as planned.


.SHORT TERM...(This evening through Sunday)
ISSUED AT 304 PM CST Sat Dec 3 2016

Precipitation shield ahead of the storm system currently over the
lower Mississippi Valley and Texas has lifted as far north as the
far southern edges of Missouri as of early afternoon, with snow also
edging eastward toward central Iowa ahead of a sharp shortwave
pushing through the central Plains. Residual effects of the high
pressure currently over our area will slow the arrival of these
features, with forecast soundings showing a substantial dry wedge
between 850-700 mb through the evening. As the column moistens from
the top down, the snow will spread in from the west/southwest after
midnight. While starting out as snow before sunrise, top-down method
and surface temperatures above freezing suggest a quick change over
to a rain/snow mix from south to north. Northern CWA will likely
stay all snow much of the morning before mixing, and thus highest
accumulations will be in that area. Most areas from about Macomb-
Bloomington northward will see 1-2 inches of slushy snow
accumulations, potentially approaching 3 inches around Galesburg.
Given that road temperatures are currently in the mid 30s and air
temperatures will be near freezing, the pavement should largely
remain wet unless the snow falls in a particularly heavy fashion.
Therefore, will hold off on any winter weather headlines at this

Precipitation will end from west to east during the afternoon, as
the Plains shortwave quickly sweeps east across the state.

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 304 PM CST Sat Dec 3 2016

Sunday night will feature the departure of our first measurable snow
event of the season. Clouds may thin out west of I-55 after
midnight, helping low temps to dip into the mid 20s toward
Galesburg, with low 30s toward Lawrenceville. Additional clearing is
expected Monday morning. Partly cloudy skies should prevail during
the first half of the afternoon, helping high temps climb into the
low to mid 40s. Clouds are expected to thicken from the south later
in the afternoon as the next system develops south of Illinois. A
few light rain showers could reach as far north as Springfield to
Effingham by 00z/6pm Monday. However, based on the track of the low
pressure system, it appears the best window for precipitation looks
to be from midnight Monday night to mid morning on Tuesday. Rain
looks to be the primary precip type, with some snow possibly mixing
in at times north of Lincoln to Champaign from 3 am to 9 am Tuesday.
Little to no snow accumulation is expected, due to warm ground and
mix with rain. The ECMWF is the farthest northwest with the track of
the low along the Ohio River Valley, and likewise has the
precipitation shield expanding farther into central Illinois. The
GFS and Canadian limit precip with this event to areas southeast of
I-72, where the precip should remain all rain the entire time. The
ECMWF pulls colder air toward I-74, which could trigger a few snow
showers late Mon night into Tues. Have kept the highest PoPs east of
I-55 where better agreement remains, with only slight chance and
chance PoPs west of I-55.

The fast moving low should be far enough east of Illinois for precip
to end by Noon on Tuesday. Cold air should lag enough that highs
reach near seasonal normals in the upper 30s to mid 40s.

Only a brief break in the precip is expected, as another system from
approaches IL from the central Plains on Wednesday. The system is
mainly driven by a 500mb shortwave, with little to no surface
reflection in the pressure pattern. The ECMWF and Canadian are more
aggressive than the GFS with the development of snow showers in
central IL. The track of the central portion of the wave seems to be
focused south of our forecast area, passing across southern
Illinois, so any snow accumulations should be on the light side.
Snow amount trends are down from yesterday`s 12z models, and PoPs
were lowered due to uncertainty on the amount of moisture available
and the strength of the forcing for precip north of the shortwave.
We could see a dusting of snow farther north of Peoria, with up to
an inch along and south of I-70, closer to the wave center. Plenty
of uncertainty remains with any snow amounts, but the airmass at
least appears cold enough to support mainly snow in all of central

Much colder air will arrive after that wave passes, due to the upper
trough shifting from the Plains to the Heartland. By 12z Friday,
850mb temp forecasts range from -13 to -15C in the GFS to the
Canadians -19 to -21C, with the ECMWF in the middle at -16 to -17F.
Thursday`s high temps will only reach the around 20 toward Galesburg
and around 25 toward Lawrenceville. Any clearing that develops
Thursday night could allow for lows in the single digits, otherwise
most areas will bottom out between 11 and 15F. Highs on Friday may
creep a couple degrees warmer than Thursday, but temps will still
only top out in the low to mid 20s. A blanket of high clouds into
Saturday will limit highs some, but increasing southerly winds will
help temperatures top out in the upper 20s north of I-74 with lower
30s south of I-70.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1129 PM CST Sat Dec 3 2016

Snow and ceilings lowering into MVFR category to spread eastward
into central IL after 8-12Z...likely becoming IFR/LIFR by 12-15Z.
Rain mixing into snow by late morning to early afternoon
Sunday...followed by precipitation ending west to east from
20-24Z. IFR or low MVFR ceilings likely to persist through the
end of the period. Winds SE around 6 kts initially...veering to
S and SW through the period.





SHORT TERM...Geelhart
LONG TERM...Shimon
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