Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 170524

Area Forecast Discussion
1124 PM CST Tue Dec 16 2014


Cloudy skies will continue overnight. Even with cooler and drier
air advecting into the area overnight, clouds should still keep
temps from dropping much overnight. Models still trying to bring
in some clearing during the early morning in western Illinois, but
with cyclonic flow, a deep layer of moisture, and an inversion,
believe clouds will hang around the area through late tomorrow.
Gradient will continue to loosen overnight, so gusts should
diminish later tonight, though wind speeds will still be 10-15kts
through early morning. so, current forecast looks good, so no
update needed at this time.


.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 06Z TAFS through 06Z Wednesday night)

Cloudy skies will continue at all sites through most of the day.
Some clearing is being forecast by some of the models for PIA and
SPI during the afternoon, but rather be cautious and keep some
clouds around into the early evening hours. Even with drier and
cooler temps coming in for tomorrow, the inversion seems to hang
around til evening, trapping some of the moisture in. All clouds
will be MVFR levels, with BMI at IFR now and lasting until around
13z. So will have PIA and SPI scattering out first, but keeping a
4hr TEMPO group during the late morning and early evening hours
for broken MVFR cigs. For the other three sites, BMI, DEC, and
CMI, will maintain cloud cover at all sites into the evening
hours. DEC could scatter out sometime in the evening, but enough
uncertainty at the moment. BMI and CMI might begin to scatter out
around midnight. Northwest winds will continue next 24hrs with
wind speeds decreasing through the period as high pressure ridge
builds into the area. Wind gusts have diminish, but some sites
still maintaining some gusts. These gusts should drop off overnight.




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