Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 101642
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1042 AM CST Sat Dec 10 2016

.UPDATE...
Issued at 1042 AM CST Sat Dec 10 2016

16z/10am surface analysis shows ridge of high pressure extending
from the Upper Midwest to the Ohio River Valley. A band of light
snow has developed within the WAA regime behind the departing
high, with latest radar imagery/obs indicating snow reaching the
ground across much of Iowa west of Cedar Rapids/Iowa City. Models
have a good handle on the precip area thus far, with all solutions
spreading it slowly eastward into north-central Illinois this
afternoon. Based on current trajectories, think the snow will
remain along/north of a Canton to Danville line...with perhaps a
few flurries further south. Have sharpened the PoP gradient
accordingly, keeping the greatest chances for snow along/north of
I-74...and likely PoPs further north across Stark and Marshall
counties. Snowfall totals will be light, with only a couple tenths
of an inch accumulating across the far northern KILX CWA through
early evening.

&&

.SHORT TERM...(Today and tonight)
ISSUED AT 224 AM CST Sat Dec 10 2016

Surface data shows ridge of high pressure over western IL which will
be east today as a system strengthens in the lee of the Rockies.
Southerly flow will develop over MO and produce significant
baroclinic lift over IA by midday. Lift zone will shift to the east
through day, will resultant snow development IA into IL. Main area
of lift over northern 1/4 of IL, but the northern quarter of ILX cwa
will have snow fall this afternoon into tonight with this
overrunning pattern. By Sunday 12z, the main low pressure center
will be into the western plains.

.LONG TERM...(Sunday through Friday)
ISSUED AT 224 AM CST Sat Dec 10 2016

Precipitation type is the primary challenge at the start of the
week. Friday evening model runs largely track the low center roughly
along the Illinois/Iowa border Sunday afternoon, with the NAM even
farther west and the ECMWF a tad further east along the Illinois
River. This track is going to have significant ramifications on the
precip type through the day and into the evening. While precipitation
starts as snow, a transition from snow to rain will occur from
south to north through the morning, with the far north remaining
with a rain/sleet/snow mix into afternoon. With the warmer air
surging further into northern Illinois, have lowered total snowfall
amounts and largely stayed under an inch, except for 1-3 inches in
an area from Galesburg east to Lacon. The rain will mix with or
change to snow in most areas as the precipitation ends, but
accumulations will be minimal as the significant surge of cold air
arrives after the precip moves out.

Fairly sizable temperature spread forecast across the CWA early
next week, depending on the extent of any snow cover from the
weekend system. The somewhat zonal upper flow on Sunday will yield
to another lobe of Arctic air plunging into the northern Plains.
Both the GFS and the ECMWF keep our area on the periphery of the
cold air mass, although 850 mb temperatures will still be near -17C
mid week. Regarding the affiliated wave tracking across the central
U.S., the ECMWF has slowed a bit but still sends some light snow
zipping eastward on Wednesday between the I-70 and I-80 corridors.
Meanwhile, the GFS is about 24 hours slower. Have mainly gone with
30-40% PoP`s with this feature for now, due to the discrepancy.
Should we get any snow, the subsequent temperatures will probably
need to be lowered a few degrees, but many areas should at least
fall into the single digits by Thursday morning with wind chills
below zero.

Toward the end of the forecast period, both longer range models show
the next Arctic surge carving out a significant trough from central
Canada into the Southwest U.S. A fairly large area of overrunning
precipitation is shown by both models by late Friday over the mid-
Mississippi Valley. This is at the tail end of the current forecast
range, so only the western half of the forecast area will carry a
snow chance for now.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 546 AM CST Sat Dec 10 2016

VFR cigs over region ahead of the developing overrunning pcpn
system. Snow should begin developing and moving into region in the
afternoon. Northern TAF sites will be affected with the light snow
this afternoon and in the early evening. Conditions will be mainly
VFR with only a chance of lower VSB with the heavier snow showers.
Other TAF sites will continue with VFR cloudiness.


&&

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

$$

UPDATE...Barnes
SHORT TERM...Goetsch
LONG TERM...Geelhart
AVIATION...Goetsch



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