Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 100825
AFDILX

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

.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 06Z TAFS through 06Z Saturday night)
Issued at 1105 PM CST Fri Dec 9 2016

The MVFR cigs have finally cleared most of our TAF sites late this
evening. A large area of mid and high level clouds will continue
to stream in from the west tonight and gradually lower during the
day on Saturday as a weather system approaches from the west.
Still expecting VFR cigs through the day with cigs lowering to
3000-4000 feet across the north after 20z with the possibility for
a brief period of snow into the evening hours at KPIA and KBMI.
Most model data keeps the bulk of the steadier snows north of the
TAF sites but our northern TAFs will be close to the precipitation
so will add PROB30s to both KBMI and KPIA after 20z. Further
south, still a question as to just when the snow will occur at
those sites, if at all. For now, will hold off mentioning in this
forecast issuance. Surface winds will remain light and variable
tonight and then gradually shift into the southeast at 10 to 15
kts by afternoon.


&&

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

$$

SHORT TERM...Goetsch
LONG TERM...Geelhart
AVIATION...Smith


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