Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 241105

Area Forecast Discussion
505 AM CST Wed Dec 24 2014

.SHORT TERM...(Today)

Latest surface analysis indicating the storm system of interest for
us was located over northwest Alabama and is now forecast to track
further east than earlier thought and should be in extreme eastern
Indiana or western Ohio by this afternoon. This will push the
band of heavier precip east as well which would suggest the
accumulating snow will set up east of I-55, more like along and east
of I-57 for any significant accumulations. Forecast soundings off just
about any model you look at was indicating boundary layer temperatures
still too warm to support any long duration of accumulating snow
until this afternoon or early this evening over extreme east central
Illinois. Even in these locations, wet bulb zero heights were above
1200 feet, so we will need to see some significant forcing associated
with the shortwave trof/upper low to produce some dynamic cooling that
may be able to overcome the warm boundary layer temperatures. Not
seeing any strong trends back to that solution with the past couple
of model runs, as a result, have started to trim back on the advisory
across our west for now. Have added some counties across extreme east
central Illinois due the eastward shift to the storm track and where
the better and longer duration lift will set up this afternoon and
early this evening. As far as snowfall is concerned, very low end
advisory snowfall totals across the area with generally 1 to 3 inches
east of I-55, and if model trends continue to shift the surface low
track further east, the better setup for accumulating snow will be
east of I-57.


.LONG TERM...(Tonight through Tuesday)

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 12Z TAFS through 12Z Thursday morning)

Cigs expected to deteriorate to IFR conditions by later this
morning as precipitation spreads north across the forecast
area. The storm system that was to bring snow to most of the
area is now shifting further east than originally thought,
as a result the heavier snowfall is now expected mainly east
of the TAF sites this afternoon. We will see rain, occasionally
mixed with snow at PIA this morning, and over the entire area this
afternoon, change over to all snow from west to east later this
afternoon as the storm shifts well off to our east. Latest model
runs suggest the precip will begin to shift off to our east by
00z with MVFR cigs holding thru most of tonight. Surface winds
will gradually increase out of the northwest today with speeds
of 10 to 15 kts by late this morning into the afternoon hours.
We will see some occasional gusts around 20 kts at times as
well into the early evening hours. Winds will be from a westerly
direction tonight at 7 to 12 kts.


WINTER WEATHER ADVISORY from 9 AM this morning to 9 PM CST this
evening FOR ILZ038-043>046-052>057-061.



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