Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 280542

Area Forecast Discussion
1142 PM CST Sat Dec 27 2014


The cold air aloft driving into the north side of the system has
been changing the rain over to snow and sleet for a couple of
hours at the end of precipitation. There seems to be less evidence
of the snow/sleet band in the observations and reports over the last
hour or so, but the air temps are dropping toward freezing in the
back edge of the precip, so will continue to indicate a band of
snow/sleet at the western edge of precip until precip comes to an
end later tonight. Any light snow/sleet accumulation would create
slippery roads. Road temps have generally been reporting above
freezing, but some bridges, overpasses, and secondary roads would
be more susceptible to icing. The precip is increasing its forward
progress toward Indiana, and we have adjusted the weather grids to
end precip a few hours sooner tonight. All precip will likely be
out of our counties by 1 am and possibly midnight.

Gusty northwest winds will develop after precip ends, ushering the
colder air. Some clearing is forecast for our NW counties later
tonight, which would allow low temps to drop off a bit farther
than guidance. Will trim a couple degrees off lows toward
Galesburg/Lacon. Updated forecast grids and info will be available


.SHORT TERM...(Tonight)

Low clouds, drizzle, light rain showers still ongoing across the
forecast area this afternoon.  Cold front about out of ILX
area...just NW of Lawrenceville.  Some flakes mixing into the precip
on the extreme NW edge of the post frontal echos on radar.  Front
progression somewhat steady although running into more parallel flow
aloft.  Cold dense air from the NW really colder behind the post
frontal precip spread across the radar mosaic.  Timing of the cold
air a minor concern as time progresses.  Ground temps still close to
40 degrees, but with the arrival of the freezing temps, some
elevated surfaces could develop some slick spots. The arrival of the
coldest air will be after the precip has come to an
freezing should be confined to the  residual moisture/puddles from
today`s drizzle/rain. Another issue with the overnight forecast is
the cloud cover. GFS is quick to break it out...a little faster than
is currently represented on sat imagery.  NAM is far cloudier,
trapping the moisture in the boundary layer and preventing the mix
out entirely.  RUC looking more like the NAM with representing the
developing inversion behind the boundary.  Plenty of dry air up
there, but how quickly it will erode through the clouds is a huge
question mark.  Covered in cirrus all day, not convinced the diurnal
influence on the clouds is enough to erode them at sunset.
Considering how quickly sunset is approaching, going to split the
difference more or less with a slower clearing out...but not quite
as trapped as the NAM. Some breaking up could be seen in the far NW,
and forecast low temps are considerably cooler as a result.
However, going cloudier overall through the night.


.LONG TERM...(Sunday through Saturday)

High pressure behind the front will bring dry weather to the area
for tomorrow through Monday. However, this mild high pressure area
will be temporary as a stronger/colder high pressure area drops into
the region Tuesday. A weak boundary will be between these two high
pressure areas, but it will have very little moisture to work with.
This will be an Arctic front with much colder air behind it. Any
precip with this Arctic front will be very light and could occur
Monday night. With the colder temps and limited moisture, only pcpn
expected will be flurries. Also believe this will be across the
northern 2/3rds of the CWA, leaving the southeastern CWA dry as the
area flurries moves east.

Temps will be around normal tomorrow and Monday, but then drop to
below normal for Tue as the Arctic cold high pressure moves in.

Cold, dry Arctic high pressure will dominate the area for days 3-5,
but as the high pressure modifies and moves into southeastern US,
temps will begin to warm gradually and a weather system will develop
in the southwestern US and begin to move out into the plains. With
high pressure to the east and a surface low pressure area to the
west, warmer air will not be the only thing to return to the area.
Moisture will also return and this will bring pcpn to the area as
the moisture interacts with a warm front and lifts northward into
the area. P-type could be very interesting and be dependent on the
surface temperature and temps in the lower levels. Models
forecasting temps above zero at 850mb over the area, which would
indicate rain. However, surface temps look to be below freezing Fri
night; and this could mean frozen pcpn possible. Considering Fri
through Sat is Day6-7, will not get too detailed on pcpn type and
just go with snow or rain, or a mix of rain and snow. Thinking
mostly rain on Friday with a mix in the north, then a mix becoming
all snow on Fri night, and then all snow in the east on Sat. Am
expecting the p-type to change over the next several days, and when
the event becomes better defined by the models and closer.

As mentioned earlier, temps will start cold and then gradually warm
during the week.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)

Precip has shifted east of all the terminal sites, and MVFR clouds
will blanket the area the rest of the night and through much of
the morning tomorrow. CMI has a brief break in the MVFR clouds
until 07z/1am, but low clouds will fill in by 07z. The back edge
of the clouds is positioned to our west from south-central
Wisconsin through SE Iowa and down through the middle of Missouri.
The clearing progression has slowed later this evening, and that
trend should allow low clouds to linger over the terminal sites
into tomorrow morning. PIA should be the first to clear out,
possibly as early as 12-13z. Clearing trends should speed up with
turbulent mixing of dry air tomorrow, reaching BMI by 14z, SPI by
16z, CMI by 18z and DEC by 19z. The HRRR is quicker with clearing,
as is the RAP. Will stick with a more pessimistic clearing trend
until satellite images give more confidence in the models cloud
progressions. Wind speeds will decrease below 10kt by 08z-09z
tonight, and remain less than 10kt through the day tomorrow. Wind
direction will start out NW, then back to the west during the day
as high pressure builds farther into Illinois.




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