Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 162040

Area Forecast Discussion
240 PM CST Tue Dec 16 2014

.SHORT TERM...(Tonight)

Occluded low pressure system shifts into the eastern Great Lakes
this evening, allowing the gradient to gradually relax and decrease
wind gusts.  Latest NAM forecast soundings and HRRR ceiling guidance
suggest lower clouds will persist overnight, and given deep cyclonic
flow and in-building subsidence inversion at 900 MB, believe this
will hold true.  Therefore have increased cloud cover overnight.
Lows will be challenging with the full effect of CAA being offset by
insulating effects of cloud cover.  Have trended mins 2-3 degrees
warmer than MET/MAV blend, which gives lower 20s west of the IL
River, to mid 20s central/east.


.LONG TERM...(Wednesday through Tuesday)

High pressure to settle into the region on Wednesday bringing in dry
with more seasonal temperatures. Forecast soundings...although not
as aggressive as yesterday with the low level moisture...continues
to indicate moisture trapped beneath a subsidence inversion will
keep a bit more cloudiness around the area during the day. Soundings
do show some breaks developing by late morning into the afternoon
hours but just in time to see mid and high level moisture/clouds
stream in ahead of our next weather system for Thursday. That weather
system is expected to spread light snow into parts of extreme west
central through southeast Illinois Thursday morning. Model runs had
been suggesting as the precip spread northeast into a more confluent
flow pattern over the lower Great Lakes coverage would decrease
during the day Thursday. Most of the operational models have backed
off that idea for now and have brought some light precip further north
and east into our area on Thursday so have brought precip chances further
north into the forecast area. Snowfall could accumulate up to 1 inch
from Springfield south Thursday morning before the area begins to
decrease in coverage during the afternoon.

Thursday`s system should be off to our east by evening with our attention
turning to a more significant wave over eastern Texas that will push
a surface low east-northeast across the Gulf Coast states Friday. The
northern periphery of the precip associated with the southern wave will
track over southeast Illinois with once again light snowfall for later
Friday night into Saturday. Current indications suggest an inch or less
for southeast Illinois. High pressure will then shift east into our
area for Sunday into early Monday bringing quiet weather to the region.

Quite a bit of model spread with respect to the system for early next
week, which will be moving southeast out of the Northern Plains and
tracking a strong low pressure system at the surface to our north
Monday. The operational GFS is more progressive with the early week
system and sweeps the cold front thru our area Tuesday afternoon and
evening with little if any southern stream interaction showing up
on the latest model run. In contrast, the latest ECMWF was indicating
a more phased look between the northern and southern streams which
effectively holds up the front pushing across our area later Tuesday
until a surface wave shifts off to our northeast Tuesday night.
This would bring rain on Tuesday, which would eventually change to
snow later in the day and evening as the deepening surface wave shifts
away from our area by Wednesday.

Seems as if each model run has a different solution with respect to
the pattern over the Midwest the first half of next week so not making
any significant changes to the grids at this point for days 7 and 8.
After our brief warm up ahead of the system on Monday and Tuesday, it
turns colder again by midweek.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)

Remnant showers and drizzle with visibility reductions to 2-4 SM will
exit the eastern TAF sites by 20Z. Ceilings have already risen to
MVFR levels at PIA/SPI, and this trend will continue to the east
this afternoon, as low pressure shifts farther away from the
region. Main concern for this forecast is possibility/extent of
scattering out MVFR ceilings tonight into tomorrow morning.
Forecast soundings from higher resolution models point to a
familiar scene from the past few weeks, with a strong subsidence
inversion trapping moisture around 1500 FT. This looks likely to
keep ceilings in place at least through 15Z/Wed, even with
northwest winds bringing in drier air at the surface. Tight
pressure gradient will keep west winds gusting at 25 kts this
afternoon. Gusts will diminish after sunset with speeds tapering
to around 10 kts as the gradient relaxes, and under 10 kts towards
sunrise as surface ridge builds in from the NW.




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