Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 231135

Area Forecast Discussion
535 AM CST Tue Dec 23 2014

.SHORT TERM...(Today)

Initial shortwave associated with a closed low at 500 mb early this
morning was tracking across north central Illinois. Backedge of the
rain has shifted into our east and northeast counties...soon to exit
our area for this morning. Mid level dry slot seen on satellite data
tracking east into parts of west central Illinois. Most of the short
term models shift the lower clouds to our northeast this morning
allowing some breaks to develop. HRRR and RUC soundings showing
the low level moisture slowly lifting with time this morning, but
that shouldn`t last long as mid and high level clouds start to stream
northeast into our area ahead of the second shortwave that will bring
another round of rain, then snow to our area starting tonight. Air
mass in the wake of the weak frontal passage today not very cold at
all, in fact, 925-850 mb temperature forecasts suggests highs this
afternoon will range from the upper 40s north to the middle 50s


.LONG TERM...(Tonight through Monday)

Model guidance continues to throw us curve balls on how the current
system will depart the area on Christmas Eve. Their spread in the
details and lack of run-to-run consistency continues to make our
confidence in Christmas Eve`s snow potential much lower than
average. There has been more than a trivial trend back toward a
snowier day in the latest runs, although this trend is far from
unanimous. The upshot is that that travel difficulties could exist
for Christmas Eve and it is prudent to monitor later forecasts
closely. While confidence is too low to issue any winter weather
headlines at this time, have issued a Special Weather Statement
highlighting the potential as well as the uncertainty.

However, the main snow threat is still a couple days off. Until
then, we will have to contend with periods of rain, although
confidence is growing that sizable breaks in the rainy periods will
occur. A dry slot wrapping around the vertically stacked
upper/surface low should support most of Tuesday, and even into
Tuesday evening, ending up dry. This dry slot is showing up nicely
as a nearly cloud free area over the central High Plains, although
we do not expect skies to be quite so clear by the time the drier
airmass arrives here. The initial upper/surface lows will dissipate
with time as a more vigorous wave rounds the base of the mean Plains
trof later Tuesday into Wednesday.

Our main snow concern arrives as the above mentioned wave heads
northeast, developing a new surface cyclone as it does so. While all
models still keep the track of the surface low east of the forecast
area, the track has shifted far enough west in most model solutions
that the deformation zone on its back side tracks across at least
east-central and southeast Illinois. However, a wide range of model
solutions exist in the timing, track, and precipitation intensity
within this deformation zone. It is also unclear if all the
precipitation within the deformation zone will fall as snow, or it
will fall as rain or a rain/snow mix for a time. Obviously, this is
critical to snow accumulation potential. While most guidance has
surface temperatures above freezing through Wednesday afternoon, the
"warm" air depth is very shallow, generally no more than 1000-1500
feet above the ground. If snowfall rates are high enough, they will
quickly be able to erode this warm layer. There are signs that
convective banding within the deformation zone is possible, which
would certainly provide high enough rates to support a quicker
change over to snow. In any event, due to the persistent
uncertainty, have gone with no more than an inch or two of snowfall
on the back side of the system, mainly east of I-55. If the latest
model trends hold, these totals will need to be boosted.

The next couple waves/precipitation threat have slowed a bit from
previous days, centering more on Saturday/Sunday than
Friday/Saturday time frame. Temperatures just ahead of the lead wave
support rain or a rain/snow mix, with conditions eventually cooling
enough for all snow. Precipitation amounts during this time appear
fairly minor for the moment.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)

MVFR and local IFR cigs will become VFR this morning with PIA on
the edge of some wrap around moisture from the departing storm
system. Forecast soundings showing despite the loss of the low
level moisture, quite a bit of mid and high level cloud cover
will affect the TAF sites this morning into the afternoon hours
before more MVFR cigs approach the area for late this evening and
into the overnight hours ahead of the next storm system. As the
next storm system approaches from the south tonight, rain will
overspread the area from south to north after 04z with CMI and
DEC the first to see the rain followed by SPI and BMI aftr 06z
and then PIA during the early morning hours. Once the rain begins
it appears the cigs will quickly drop to MVFR and then eventually
IFR once again, but at this time will hold off bringing cigs and
or vsbys down too much in this forecast period as it appears the
IFR and LIFR cigs will be during the day Wed. Surface winds will
be out of the southwest today at 10 to 15 kts, and the gradually
turn into a light northwest to north direction tonight at 4 to
9 kts.




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