Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 222323

Area Forecast Discussion
523 PM CST Mon Dec 22 2014

.SHORT TERM...(Tonight)

Short-wave trough evident on 19z/1pm water vapor imagery over
Kansas/Oklahoma will lift northeastward tonight, reaching central
Illinois by 12z Tue.  A large area of light rain associated with
this feature is currently ongoing across the KILX CWA: however,
radar imagery is showing the precip becoming more scattered upstream
across Missouri into southern Illinois.  With a continued moist flow
and strong lift ahead of the upper wave, think numerous showers will
re-develop behind the main rain area later this afternoon into the
evening.  As a result, will continue to carry categorical PoPs
across the board through the evening.  After that, models agree that
a mid-level dry slot will approach from the southwest later tonight
into Tuesday morning.  Both the 12z NAM and the 17z HRRR suggest the
showers will taper off and come to an end from southwest to
northeast across the area by dawn Tuesday.  Will continue likely to
categorical PoPs along/north of I-74 through the entire night, but
will drop to just chance PoPs further south across the rest of the
CWA after midnight.  Southerly winds of 10 to 15 mph will continue
tonight, preventing temps from dropping much.  Overnight lows will
be several degrees above numeric guidance, ranging from around 40
across the north to the middle 40s south of I-70.


.LONG TERM...(Tuesday 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 00Z TAFS through 00Z Tuesday evening)

Cloudy skies and pcpn will continue at all TAF sites tonight.
Based on model data and radar trends, looks like pcpn might end
between after midnight and just before morning. When the pcpn
ends, expecting cigs to rise to above 1kft at all sites. However,
vis will remain around 4-5sm with fog. These conditions are
expected to continue into the morning hours. The HRRR model is
showing clearing in the morning moving from southwest to
northeast. Not confident with this, so will just have cigs
increasing to higher MVFR levels...about 2kft and lasting until
end of TAF period. Winds will be southeast to south through the
period at around 10-14kts.




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