Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Lincoln Illinois
1149 am CST sun Dec 21 2014
issued at 1035 am CST sun Dec 21 2014
16z/10am visible satellite imagery shows cloud cover blanketing
much of central illinois: however, skies remain mostly clear along
and south of the I-70 corridor into the Ohio River valley.
Short-wave ridging in advance of an upper-level disturbance over
the Central Plains will allow heights to slowly rise today.
This along with continued backed southeasterly low-level flow will
cause the leading edge of the cloud cover to erode
north/northwestward this afternoon. Am already beginning to see
evidence of this on satellite this morning, so have updated sky
grids accordingly. End result will be mostly sunny skies
along/south of I-70, with skies becoming partly sunny as far
northwest as a Shelbyville to Champaign line. Further west, mostly
cloudy conditions will prevail across the remainder of the area.
High temperatures will be a few degrees warmer than yesterday,
with readings reaching the upper 30s and lower 40s.
issued at 537 am CST sun Dec 21 2014
All but extreme southeast Illinois was cloudy early this morning and
the latest satellite trends suggest the southeast may see the clouds
move in for a brief time over the next couple of hours. With the
clear sky, a few locations in southeast Illinois have seen some
patchy fog develop, but with the clouds starting to move in we don't
expect the fog to become widespread. Further to the northwest, some
patchy drizzle has been occurring over parts of extreme eastern Iowa
and some of that, at least based on radar, was tracking into areas
mainly along and west of the Illinois River, so will carry some
patchy drizzle wording for early this morning before that shifts to
our north. Surface temperatures have risen into the middle 30s early
this morning along with dew points now into the lower 30s, so am not
expecting any problems from freezing drizzle at this point.
The slow moving high pressure system that had given parts of our
area some sunshine the past couple of days was finally drifting off
to our east allowing a southerly flow to develop over most of the
forecast area. Forecast soundings indicate our southeast counties
may see the lower cloud cover drift north-northeast out of their
area this morning allowing some sun for a time today. Further north
and west, short range models not as optimistic with a mostly cloudy
sky prevailing. We should see the temperatures continue to moderate
from the highs of yesterday with most areas close to or above the 40
degree mark this afternoon.
Long term...(tonight through saturday)
issued at 537 am CST sun Dec 21 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 Monday afternoon)
issued at 1149 am CST sun Dec 21 2014
MVFR ceilings prevail at the kilx terminals late this morning:
however, the cloud cover is beginning to erode across east-central
Illinois. 1730z visible satellite imagery shows clearing line from
kprg to just north of k1h2 and lifting slowly northward. Based on
timing tools and latest hrrr forecast, skies will partially clear
at kcmi between 20z and 21z. Further west, forecast soundings
suggest a continued MVFR overcast through the afternoon before
ceilings gradually improve to VFR this evening in advance of an
approaching storm system. IFR ceilings will develop/spread across
the area between 13z and 16z Mon as rain showers arrive from the
southwest. Winds will be from the S/southeast at 10-5kt through the