Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 300048

Area Forecast Discussion
648 PM CST Thu Jan 29 2015


Clouds continue to blanket central Illinois early this evening:
however, quite a bit of clearing is developing upstream across
Minnesota into far northern Iowa. This clearing is occurring within
the subsident region behind a short-wave trough noted on water
vapor imagery over Lake Superior/Wisconsin. As this wave tracks
into Michigan later tonight, the clearing will develop further
southeastward into portions of central Illinois after midnight.
HRRR continues to suggest a gradual dissipation of the cloud cover
overnight, while the NAM holds on to the clouds right through
Friday. Based on latest satellite loop, think the NAM is too
aggressive with its low-level moisture so will trend toward the
HRRR for the short-term forecast. Satellite timing tools continue
to indicate at least partial clearing arriving northwest of the
Illinois River by 06-07z, then further east and south across most
of the area by dawn. Some locations near the Indiana border and
along/south of I-70 may stay overcast until Friday morning. Have
updated sky grids to better reflect current and expected trends.
Have also updated hourly temps to slow the initial fall due to the
lingering cloud cover this evening. Once skies begin to clear,
temps will drop off steadily after midnight, with lows ranging
from the teens northwest of the Illinois River to the middle 20s
along/south of I-70 where clouds will persist longest.


.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 00Z TAFS through 00Z Friday evening)

Main aviation forecast challenges will be the current MVFR
ceilings and when to clear them out. As is typically the case
with these wintertime low stratus events, the models are all over
the place concerning how quickly it will clear. The GFS is very
aggressive with bringing clearing into the area as early as 06z,
while the NAM keeps low-level moisture in place through much of
Friday. Meanwhile, the latest HRRR is suggesting a gradual
dissipation of the cloud cover late tonight, with mostly clear
skies across the board by early to mid-morning. 23z/5pm satellite
imagery shows clouds blanketing much of the Midwest: however, a
large hole has developed across Minnesota and is steadily expanding
S/SE. This is in association with subsidence on the back side of a
short-wave trough evident in water vapor imagery over Lake
Superior/Wisconsin. As this feature continues to drop
southeastward, so will the subsidence and clearing. Based on
satellite timing tools, clouds should begin to scatter at KPIA by
06z, then further E/SE to KCMI by around 10z. Winds will initially
be strong/gusty from the NW this evening, but will decrease to
around 10kt late tonight. After that, high pressure will build
into the region on Friday, providing mostly clear skies and light
winds through 00z.




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