Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 170912

Area Forecast Discussion
312 AM CST Wed Dec 17 2014

.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 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.




LONG TERM...Geelhart
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