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Area forecast discussion 
National Weather Service Lincoln Illinois
529 am CST Sat Jan 31 2015

Short term...(today)
issued at 329 am CST Sat Jan 31 2015

Quiet weather is expected to start the day across central and
southeast Illinois. A ridge of high pressure, currently paralleling
the Ohio River valley, will continue to pull away from the area.
This will allow a storm system, starting to get organized over the
southwest United States, to approach. Decent warm air advection/isentropic lift
ahead of the approaching storm will result in steadily
increasing/thickening cloud cover this morning. Rain will begin to
spread across the forecast area from southwest to northeast this
afternoon. A rain/snow mix can't be ruled out by late afternoon
across our far northwest counties, mainly Knox, Stark, and Marshall,
but no snow accumulation is anticipated before tonight. The
southerly winds will push temperatures above normal, despite the
cloud cover and precipitation breaking out, with highs near 40
degrees in most areas.


Long term...(tonight through friday)
issued at 329 am CST Sat Jan 31 2015

Surface wave moving out of the desert SW and phasing with an upper
level trof in the more northern portion of the 500mb flow over the
Continental U.S. To bring the next winter storm for the Midwest Saturday and
Sunday. Models continue to track a little further north with the
solutions...putting more and more warm air ahead of the low. This
warmer trend ahead of the system will keep the precipitation mainly
rain even into the evening hours. Cold air is not as deep and
abundant with this system, and the strong southerly flow in advance
of the sfc low keeping the low levels warm enough for rain. NAM
BUFKIT soundings not switching over to all snow for pia until
between 03-06z. However, have rain/snow mix for the evening, but
temps very at or above freezing through most of the evening up to
midnight will limit the accumulations and/or make for a very slushy
wet mix. Once the low draws closer and more cold air is pulled into
the system, the changeover to snow will become more widespread
across the area, mainly north of the I-70 corridor. Areas between
I-70 and roughly I-72 will see a mix a bit longer as well, but still
changing over to mostly snow by Sunday. Snow lingering into Sunday
evening before ending by Monday morning.

Significant accumulations are expected with this storm...over an
extended period of time. Winter Storm Warning is confined to a
Rushville to Bloomington line. Traditional Winter Storm Warning
criteria will be very close...though 6"-8" are expected in the
warned area in total...the snow will accumulate over an extended
period of time-18 to 24 hours-- with the best snowfall accumulations
expected between early Sunday morning and Sunday evening. The
Winter Weather Advisory south of the warning up to, but not
including, the I-70 corridor will also see the accumulations over a
longer duration event. Although the southeast likely to see mostly
rain, some snow accumulations will drop south of I-70 by Monday
morning as well.

Phasing of this sfc system with an open wave aloft resulting in
weaker dynamics in the bigger picture and a slower deepening of the
low. Lift at any given time good to moderate, but limited by lack
of deeper organization. Isentropic lift definitely there on 305-320k
sfcs, but never for long and angle of winds to gradient closer to 45
than to perpendicular. Models struggling with over deepening of the
system and potentially with the snowfall accumulations being
overdone somewhat with the warmer air moving into the system. That
being said...even reducing the snowfall totals from model qpf...over
such a long duration with a slow moving system, the totals are still
in warning/advisory criteria for much of the Midwest.

Winds increasing as the storm moves through, particularly on Sunday
afternoon could result in some blowing snow over the northern tier
of ilx, although the snow will initially be more wet and
the storm GOES on the slrs will increase and snow be more prone to
blowing and drifting.

Cold air moving in behind the system will result in a very chilly
Monday and another quick wave Monday will bring another chance for
some snow mainly across the northern third of the state.
Accumulations are expected to be low at this point with that
system. Beyond that, the forecast is relatively quiet through the
work week with moderate temperatures until deeper cold air moves
into the region dropping high temps at the end of the week into the
teens and 20s.


Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 520 am CST Sat Jan 31 2015

Condtions across the central Illinois terminals will start out
quiet/VFR as departing high pressure slowly loses its grip on the
area. This will allow a rapid thickening/lowering of the clouds as
a storm system approaches from the west. Rain will begin to break
out from these thicker clouds later this afternoon, with light
rain expected to develop as well. The persistent precipitation
will eventually see conditions degrade to IFR this evening. The
precipitation will also gradually mix with and change over to snow
as cooler air begins to filter into the area.


Ilx watches/warnings/advisories...
Winter Storm Warning from 9 PM this evening to 9 PM CST Sunday
for ilz027>031-036>038-040>043-047-048.

Winter Weather Advisory from 9 PM this evening to 9 PM CST
Sunday for ilz044>046-049>057-061.



Short term...Bak
long term...hjs

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