Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 200309
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
909 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 00Z TAFS through 00Z Saturday evening)
ISSUED AT 544 PM CST FRI DEC 19 2014

Latest satellite imagery indicated a thin layer of high clouds has
streamed across much of central Illinois early this evening, with
no cloud ceiling showing up in the observations at any TAF site at
23z. Further west, MVFR/IFR ceilings blanket Iowa/Missouri, as
well as just to the NE across NE IL and Indiana. The HRRR is
indicating that mid level clouds 5-6k FT could advance over our
terminal sites later tonight. Eventually cloud heights are
projected to drop to MVFR Saturday morning, with light fog at 4SM
BR restrictions. Can not rule out some flurries or possibly even
patchy freezing drizzle around PIA and BMI tomorrow morning. For
now, did not include any mention of precip, but any FZDZ could
cause slippery travel surfaces. Will monitor upstream and
forecast soundings for possible FZDZ occurring toward sunrise and
through the morning.

With high pressure directly overhead this evening, low-level winds
continue to remain light. Expect no change in that area tonight. Southeast
winds will develop tomorrow morning as the high shifts off to the
east, which will drag MVFR clouds back into central IL the
remainder of the day, with spotty precip possible.


&&

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

$$

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






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