Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 171803

Area Forecast Discussion
1203 PM CST Wed Dec 17 2014


Clouds continue to blanket central and southeast Illinois this
morning, thanks to low-level moisture trapped beneath a subsidence
inversion at around 900mb. Enough synoptic lift is being generated
by a short-wave trough sinking southward through Wisconsin to trigger
a few snow flurries across the area. Radar imagery has shown some
weak echoes from near Peoria southeastward along the I-74
corridor, with both KBMI and KCMI reporting flurries at some
point this morning. Have updated the forecast to include scattered
flurries across the northern KILX CWA accordingly. Main question
of the day will be whether or not any clearing will occur. Latest
visible satellite imagery shows clearing across much of Iowa, as
well as central/southern Missouri. Based on timing tools, skies
will clear across the Illinois River Valley this afternoon, then
toward the I-55 corridor by late afternoon/early evening. Have
adjusted sky grids to better reflect current trends, featuring
partly sunny skies across the west this afternoon and continued
overcast conditions further east toward the Indiana border.


.SHORT TERM...(Today)

A north-south oriented elongated high pressure ridge will edge
eastward toward Illinois today bringing with it a much lighter
surface pressure gradient and subsidence aloft. The result will be
wind speeds and gusts tapering off throughout the day as well as
thinning low cloud cover. Seasonably cold temperatures that arrived
after yesterday`s cold front will continue, with highs today only
around the freezing mark, just a few degrees below normal.
Although cloud cover will thin out through the day, current
indications are that the GFS and HRRR models are too aggressive in
clearing out low level moisture trapped below an inversion based
just below 900 mb. Forecast cloud cover is based more in line with
the NAM which keeps a saturated layer through the early part of
the day, slowly thinning through the afternoon. Meanwhile, periods
of high cloud cover will be arriving in the afternoon as well.

Also watching a band of snow flurries moving southward through Iowa
this morning. This looks as if it will remain west of the central IL
forecast area before the disturbance responsible for it ejects
eastward. This could move through portions of central IL later this
morning, but would be of very little consequence and is a fairly low
probability at this point.


.LONG TERM...(Tonight through Tuesday)

Upper wave currently pushing through Arizona on track to reach the
southern Plains tonight, before dampening out somewhat as it moves
across Missouri. Forecast soundings off most of the models are
showing the column moistening from the top down overnight, but very
dry air lingers just past sunrise below 800 mb. A short period of
isentropic lift on the 305K plane will move through central Illinois
Thursday morning, coinciding with the moistening of the lower
layers, resulting in a period of light snow across the southwestern
CWA during the morning. Current thinking is only a few tenths from
Rushville through Springfield to the Effingham/Flora areas. The
soundings further northeast toward Peoria and Bloomington generally
never really moisten the lower levels (except for the NAM), and will
keep those areas dry on Thursday.

Model trends for the Friday night/Saturday system continue to shift
more to the south, as the next deepening wave tracks into the lower
Mississippi Valley. Have limited the PoP`s for Friday night and
Saturday morning to areas south of I-70, where accumulations look to
be less than an inch. The ECMWF tries to draw some of the moisture
northward Saturday morning as a separate wave in the northern stream
swings into the Great Lakes region, so have kept some slight chances
during the morning across east central Illinois.

Active period setting up next week, as a broad long wave trough digs
southward through the central U.S. Some timing differences as to the
depth of this wave, with the ECMWF and parallel GFS cutting off a
low over southeast South Dakota and drifting it east, while the
operational GFS waits until mid week to cut off a broad low over the
Midwest. Both solutions yield precipitation as a storm system
organizes just to our northwest. Have continued to concentrate the
precip chances Monday night into Tuesday, mostly rain but perhaps
some snow mixed in across the far north. Will need to watch the
evolution of this closely, with the pre-holiday travel rush
beginning around this time.


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

MVFR ceilings will prevail at the central Illinois terminals for
the next few hours before clearing gradually develops from west to
east across the area. 1730z visible satellite imagery shows
clearing beginning to work into the northwest KILX CWA around
Galesburg, and based on timing tools, this should reach KPIA by
around 21/22z. Will continue to monitor satellite to see if clouds
can erode faster, but for now will remain pessimistic. Clouds will
hold firm further east at KCMI until early evening when forecast
soundings finally show low-level drying. After a brief period of
mostly clear skies tonight, the next approaching system will
spread clouds back into the area on Thursday. Forecast soundings
suggest VFR ceilings developing across central Illinois between 11z
and 14z. May see enough moistening of the profile to produce light
snow at KSPI after 15z, but dry conditions should prevail




LONG TERM...Geelhart
AVIATION...Barnes is the U.S. government's official web portal to all federal, state and local government web resources and services.