Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 200535
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
1135 PM CST Fri Dec 19 2014

.UPDATE...
ISSUED AT 905 PM CST FRI DEC 19 2014

The surface ridge axis is finally making some progress to the east
of Illinois, allowing winds west of I-55 to start a trend out of
the southeast. A weak shortwave passing across southern IL could
trigger some light snow showers or flurries southeast of I-70.
Radar returns toward Lawrenceville are mostly virga, with no snow
reaching the ground yet. Cloud heights are lowering with time
under that wave, so some flurries were added to the forecast for
the far southeast counties toward midnight.

The next area of concern is NW of the IL river, where low clouds
and fog may develop into some freezing drizzle toward sunrise. The
precip could fall as flurries, but a lack of ice crystals to feed
into the low cloud layer will limit the ability for snowflakes to
develop. Thus, the concern for FZDZ to occur. Any icing would
cause slippery travel conditions. Have added a mention of FZDZ
between 12z-15z Sat morn, but may need a mention even earlier
depending on speed of saturation and progression of shortwave from
Nebraska across Iowa/N MO.

Low temps will not drop off appreciably from current temps due to
increasing clouds. Updates this evening were to the weather and
sky grids to match expected trends.

&&

.SHORT TERM...(Tonight)
ISSUED AT 328 PM CST FRI DEC 19 2014

20z/2pm visible satellite imagery continues to show a large break in
the overcast extending from southeast Iowa across much of central
Illinois into Kentucky. With high pressure directly overhead and
only very light low-level winds, partly to mostly clear conditions
will persist through the evening hours.  As the high begins to shift
eastward, a southerly return flow will gradually develop late
tonight into Saturday, allowing the low clouds currently across
Iowa/Missouri to spill back into the area.  End result will be a
mostly clear start to the night, with overcast conditions expected
across the board by dawn Saturday.  May also see some patchy fog
west of the I-55 corridor late tonight as low-level moisture
increases, but am not expecting any major restrictions to
visibility.  Due to the initially mostly clear skies and light
winds, another cold night is expected with low temperatures
bottoming out in the lower to middle 20s.

&&

.LONG TERM...(Saturday through Friday)
ISSUED AT 328 PM CST FRI DEC 19 2014

A rather quiet weekend is on tap across central and southeast
Illinois, but weather activity will be on the upswing heading into
the Holiday work week.

Model agreement is good through the weekend with mostly neutral
upper-level flow and high pressure expected to be the dominant
player at the surface. A week wave, currently pushing east across
the central Plains, will cross the area early Saturday. However, the
forcing appears too weak, and moisture depth too shallow, to warrant
a mention of any precipitation.

A strong, slow moving system will impact the area from Monday into
Christmas Eve. We will remain on the warm side of the system through
Tuesday, with all precipitation initially expected to fall in the
form of rain. Temperatures ahead of the system will also average
nearly 10 degrees above normal for late December.

The colder air on the back side of the deepening storm system will
arrive for Tuesday night into Christmas Eve. This would support any
remaining precipitation changing over to snow. At this point, the
threat for significant snowfall on the back side of the system seems
low, but some light snow accumulation is likely. The model
agreement/run-to-run consistency is still a little shaky heading
into midweek, so future model trends will need to be monitored very
closely.

Another system is progged to arrive by late in the week, generally
Friday into Saturday. However, model consistency with this system is
poorer than the first, so confidence in the ultimate outcome is not
very high.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
ISSUED AT 1118 PM CST FRI DEC 19 2014

VFR conditions will begin the TAF period for most of the TAF
sites...with the exception being CMI where MVFR VIS is already in
place. Some of the issues with the TAFs tonight are the timing of
MVFR VIS and lowering CIGs. The surface high that was over
Illinois earlier is slowly pushing east with light to calm winds
remaining over much of central Illinois. Winds are also slowly
shifting to the east southeast allowing the low level moisture to
filter in both from the east and the west. Locations over Missouri
and Indiana have MVFR VIS and CIGs with locations in Missouri
under IFR CIGs. Given the complexity involved with timing of lower
VIS and CIGs...confidence is fairly low in the TAF forecast past
the first 6 hours. Currently have MVFR conditions over PIA
starting at 09Z...at 08Z for BMI and DEC...and at 12Z for SPI.
Mainly went with the HRRR since it seemed to have a pretty good
handle on the previous forecast. Also included a TEMPO group for
CMI as all guidance had IFR VIS over the site between 09Z and 13Z.
Only included a TEMPO for now since confidence in the timing is
lower. VFR conditions could resume around 16Z for the sites after
the next upper level wave moves through Illinois...but this could
change with the next TAF issuance.

&&

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

$$

UPDATE...Shimon
SHORT TERM...Barnes
LONG TERM...Bak
AVIATION...ALW





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