Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 240306
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
906 PM CST Tue Dec 23 2014

.UPDATE...
ISSUED AT 900 PM CST TUE DEC 23 2014

The 00z NAM is indicating an eastward shift in the track of the
low and eventual placement of the heavier band of precip farther
east of I-55. Also, thermal profiles are generally still showing
rain lingering through the morning during some of the heavier
precipitation times, which could diminish snow totals that the
model algorithms are trying to pump out already by noon east of
I-55 and even I-57. Will not make quick headline changes with the
evening update, but will discuss with HPC and surrounding offices
after additional models come in on what adjustments seem prudent.
For now, may trim a little on the snowfall amounts, especially
west of I-55. Am not as convinced on bumping snow totals east of
I-57 where warm air will linger the longest.

In the immediate short term tonight, overrunning rains will
continue to affect east-southeast counties, with a gradual
expansion of the rain farther west toward I-55. By sunrise, light
rain should be affecting areas along the IL river, with a few
flakes of snow even possible NW of the river.

The change-over to snow will progress from west to east during the
day tomorrow, and a farther east track of the low could allow
colder air to advance farther east toward I-57 a little sooner. So
will not completely discount some snowfall as far east as I-57
before noon.

Temps tonight will only drop another 5 degrees or so, which is
handled well in the grids. Updated weather to match expected
trends.

&&

.SHORT TERM...(Tonight)
ISSUED AT 247 PM CST TUE DEC 23 2014

Weak low pressure moving off to the NE out of Wisconsin this
afternoon as more cirrus spread over the region ahead of the next
system.  Deep troffing over the center of the CONUS part of the
development of the next sfc low to spread weather up into the
region, beginning as rain showers this evening and overnight.  Rain
will increase in coverage as well, spreading northward as the
surface low deepens.  Overnight lows will be moderate as cloud cover
will prevent a lot of radiational cooling, except in the NW that may
see a few holes in the clouds before the bulk of the cloud cover and
precip move northward. Forecast tonight rather calm by comparison to
the forecast for Wednesday with regards to track of the low and the
advancing storm.

&&

.LONG TERM...(Wednesday through Tuesday)
ISSUED AT 247 PM CST TUE DEC 23 2014

Plan to issue a Winter Weather Advisory for the central portion of
the forecast area, mainly between the Illinois River and I-57. This
advisory will cover the accumulating snow expected to develop
tomorrow, and persist into the evening hours. Confidence remains
lower than usual with respect to the exact details where the
heaviest snow will fall, and how high the accumulation may be.
However, considering the impacts when combining snowfall with a
busy travel day, decided to issue the headline to bring attention to
the snow potential. Later adjustments to the advisory remain a
distinct possibility.

The vertically stacked and decaying upper/surface low to our north
has done about all it is going to do to us precipitation wise, with
our attention now turning to the vigorous wave rounding the base of
the mean trof over the southern Plains. The surface low associated
with this wave will reach the vicinity of central Indiana or Ohio by
this time tomorrow, but there is still some appreciable spread in
the exact location within the models. The ultimate track will have a
significant impact on our forecast. Currently favor a solution
toward the western end of the model envelope which would place the
heavier snow within the deformation zone of the low across central
Illinois. The rain that initially accompanies the approaching
surface low should change over to snow fairly rapidly by midday
Wednesday. While some guidance suggests near surface temperatures
will remain warm enough for rain for much of the day, prefer the
more rapid cooling exhibited by the NAM given the potentially high
snowfall rates. Regarding the potentially high snowfall rates, the
models continue to support convective banding within the deformation
zone of this system. So, while we are currently forecasting 2-4
inches of snow with our currently favored track of the deformation
zone, the potential for locally higher amounts is certainly there.
The snow should rapidly taper off Wednesday evening as the system
pulls away.

In the wake of the Christmas Eve storm, Christmas Day and Friday
should be quiet and unseasonably mild as southerly low-level return
flow quickly develops. These warmer temperatures should help to
quickly melt the snow that falls tomorrow.

Further out, at least a couple waves may impact the area in the
Saturday-Monday time-frame. The waves will eject toward the region
from the southwest, and should mainly impact our southeast counties.
Thermal profiles support mainly rain ahead of the waves, with a
gradual trend toward snow as they depart. At this point, none of the
waves look like significant snow makers.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
ISSUED AT 552 PM CST TUE DEC 23 2014

Model differences remain on the location of the heavier snow band
tomorrow. The snow will have the biggest impact over the next 24
hours. GFS/ECMWF have the heavier snow mainly affecting I-57 and
east, while the NAM/SREF show the band of 2-4" mainly bordering
the I-55 corridor and east to Champaign, with localized bands of
higher amounts from Champaign and N to Chicago. The timing of
change-over from rain to snow will also be critical to snow
amounts. If rainfall takes up more the precip total than snow,
snow amounts will be lower. For now, will continue with a trend of
rain-snow mix after 15-16z, and a change to all snow between
18-19z. Some heavier periods of snow will be possible during the
afternoon, with LIFR clouds and vis possible. Even some VLIFR
could affect BMI.

Have trended from VFR at the start of the TAF period down to MVFR
clouds between 07z-08z, then dropping to IFR clouds and MVFR vis as
the rain-snow transition begins tomorrow morning. The LIFR
conditions should hold off until after the rain completely changes
to snow tomorrow afternoon.


&&

.ILX WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY from 9 AM to 9 PM CST Wednesday FOR
ILZ031-037-038-042>045-051>054-061.

&&

$$

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






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