Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Lincoln Illinois
300 PM CST Sat Dec 20 2014

Short term...(tonight)
issued at 259 PM CST Sat Dec 20 2014

Cloudy skies persist across much of central Illinois this afternoon:
however, 20z/2pm visible satellite imagery shows a large tear in the
overcast developing from near Decatur southeastward to Flora.
Although the hrrr has been a bit overdone with the clearing, it
continues to suggest at least thinning clouds across the east/southeast kilx
County Warning Area late this afternoon into the evening. In fact, with 925mb winds
remaining light/variable and not really picking up a strong S/SW
component until Sunday, portions of the southeast may see partly cloudy
conditions persist through the night. Further north and west, low
clouds will be on the increase from the west. Forecast soundings
and model relative humidity profiles indicate that the lowest clouds/fog will
remain just west/northwest of central Illinois tonight. Due to cloud cover
and a light S/southeast wind, low temperatures will be a couple of degrees
warmer than last night. Coolest readings in the middle 20s will
occur south of I-70 where skies will likely remain partly cloudy,
while lows stay in the upper 20s/lower 30s elsewhere.


Long term...(sunday through saturday)
issued at 259 PM CST Sat Dec 20 2014

Still looking like only one major system to impact central and
southeast Illinois through the coming week. Unfortunately, the
potential exists for the biggest travel headaches with this system
to arrive later Tuesday night into Christmas eve. However, it should
be noted that no model consensus exists with respect to this
system's snow potential locally. In fact, at this point, more model
solutions keep the snowfall minimal than not. Nevertheless, it would
be prudent to keep an eye on the forecast heading into the Christmas
Holiday as the ultimate outcome becomes more clear.

High pressure at the surface and mostly neutral upper flow aloft
will keep its grip on the local weather for one more day on Sunday.
Southerly return flow on the back side of the surface ridge will
push temperatures a few degrees warmer than today. Meanwhile, a
vigorous upper wave, currently crashing ashore along the Pacific
northwest coast, will begin to dig a significant trof over the
plains, also inducing surface cyclogenesis.

Strong warm air advection/isentropic ascent ahead of this slow moving system will
begin to spread rain into the area on Monday. The rain chances will
continue into Monday night and Tuesday. This initial part of the
forecast is fairly well agreed upon by the models.

The initial surface low beneath the mean plains upper trof will not
move much across the northern plains into the upper Midwest between
Monday and Tuesday. Attention then turns to another vigorous wave
that will round the base of the mean trof Tuesday into Wednesday.
This wave should induce a second surface low, one that will become
dominant, as it lifts northeast into Canada by the end of the week.
The model agreement with the timing/track/strength of this secondary
low by Tuesday night into Wednesday is not very good. The track of
this low is critical to how quickly our precipitation will change
over to snow Tuesday night into Wednesday, and if we will be
impacted by the heavier snowfall within this system's deformation
zone. While the threat is certainly there, at this point only the
GFS has been impacting the forecast area very significantly.

Behind the midweek system, another wave will arrive by Friday.
However, the models have been trending weaker with this system
overall, as well as drier. So, have backed away from rain and/or
snow chances as this system arrives.


Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1140 am CST Sat Dec 20 2014

MVFR ceilings will gradually spread across central Illinois this
afternoon/evening. 17z/11am obs show MVFR conditions
along/southwest of a kgbg to kspi line, with VFR ceilings further
east across the remainder of the area. As light S/southeast flow
gradually becomes more S/SW, these lower clouds will return to all
terminals by evening. Based on the latest hrrr output, have
introduced MVFR at kpia by 21z, then further east to kcmi by 03z.


Ilx watches/warnings/advisories...


Short term...Barnes
long term...Bak

National Weather Service Glossary of Abbreviations