Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 292130

Area Forecast Discussion
330 PM CST Thu Jan 29 2015

.SHORT TERM...(Tonight)

Forecast challenge for tonight will be whether or not any partial
clearing will occur and the affect on low temperatures. The latest
satellite loop early this afternoon indicated an area of partly
cloudy skies in central/southern MN into central IA behind a
departing shortwave trough. This area was starting to fill in with a
few more clouds, though, the past hour. The latest guidance from the
RAP, HRRR and NAM all indicate that moisture in the lowest 100 mb
should stay high enough overnight for low clouds to persist until
Friday morning. Breaks in the clouds may occur at times late this
evening, but they shouldn`t low enough to impact low temperatures.
As a result will go with lows in the 20-25 range across all of
central and southeast IL.

Patches of drizzle/flurries may linger in parts of central Illinois
early this evening. However, this will have very small areal
coverage and will have little or no impact, so will not include in
the forecast for tonight.

As the low pressure center moves through the eastern Great Lakes
late this evening, the pressure gradient should loosen up, resulting
in gradually diminishing winds in central Illinois early tonight,
and in eastern Illinois by daybreak.


.LONG TERM...(Friday through Thursday)

Models have come into better agreement with the main focus for this
forecast, this weekend winter weather system. The GFS and ECMWF both
look very similar with location of the heaviest snowfall Sat night
through Sun. Only differences in the those two models is the timing
and track of the low press area as it moves northeast across the
southern Miss valley. Precip will begin to spread into the western
parts of the CWA Sat afternoon, but temps will be warm enough for
most of the pcpn to be either rain, or a mix of rain or snow. No
accumulations are expected during the afternoon due to the warm
temps. The major part of the snow will arrive late Sat night, after
midnight, through some light accumulations are possible Sat evening.
After midnight, the snow is expected to become moderate with rates
approaching 1/2 inch per hour. The snow will be during the
overrunning period of the system as it begins to lift out of the
southern plains. With the upper level part of the system being just
an open trough/wave, a heavy deformation area of snow will likely
not setup on the back side of the low pressure area. However, snow
will continue Sun afternoon through Sun evening, with accumulations
being on the lighter side. Preliminary indications are that snowfall
totals from Sat evening through Sun night will be in the 4.5 to 6
inch range. There is still some uncertainty with these amounts as
the models are just now coming into agreement and some shift south
or north with the track is still possible. Given the longer duration
of the snowfall, it does not meet criteria for a watch at this time,
so no headlines will be issued. A later forecast shift may decide to
issue one if any changes occur with the model forecasts.

Beyond this system this weekend, conditions will be relatively dry,
except for Tue night when another chance of snow is possible from a
system coming down from the northwest. Once this system pushes
through, temps will drop and become well below normal for Wed
through Thur.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)

The main aviation concerns will be with MVFR ceilings and the
strong northwest wind gusts across central and eastern Illinois.

The latest satellite loop and surface observations indicated a
large area of MVFR ceilings across all of Illinois and much of
Iowa. The main question is the advancement of an area of partial
clearing from southern MN and central IA, and when the MVFR
ceilings will dissipate. The vast majority of short range models
indicate that a shortwave trough axis will swing through central
and eastern IL late this evening. However, most of these same
models including the NAM, RAP and HRRR indicate that there will be
plenty of low level moisture trapped below a stout inversion. This
would keep at least broken MVFR ceilings in the area through the
night and into much of Friday morning. Will follow this line of
thinking as it can be very difficult to erode low level clouds
this time of year.

The tight pressure gradient will keep northwest winds gusting
around 25-30 knots through the afternoon into early evening at
most TAF sites - particularly in east central Illinois. The
lessening impact of the surface low moving through the eastern
Great Lakes will result in diminishing wind in central IL late
this evening, and in eastern Illinois before daybreak.




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