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Area forecast discussion 
National Weather Service Lincoln Illinois
1146 PM CST sun Dec 28 2014

Update...
issued at 900 PM CST sun Dec 28 2014

Clear skies early this evening allowed temps to drop off quickly
toward overnight lows. Some cirrus streaming into the northwest half of
our forecast area will help to slow down the temp falls the rest
of the night. Still expect a few more degrees of temp drop under
those clouds, and another 3-5 deg where skies remain clear. Temps
are approaching the dewpoints already, which will also work to
slow additional temp falls. However, still decided to trim a few
degrees from lows in most areas, with higher changes toward
Danville.

Winds will remain light and variable under the surface ridge axis,
extending across Illinois from northeast Oklahoma to Northwest
Indiana. The remainder of the forecast looks on track with no
weather makers until possibly Monday night when flurries develop
with the next shortwave. Updated info will be available shortly.

&&

Short term...(tonight)
issued at 310 PM CST sun Dec 28 2014

High pressure briefly building into the region this evening as
clouds slowly erode and move off to the southeast. Colder air
moving in with clearing skies tonight will drop overnight lows a few
more degrees from last nights lows. With the ridge axis moving into
the region, the winds become light and variable in the overnight,
allowing for more efficient radiational cooling in addition to the
colder airmass. Other than the cold start to the morning, weather
tonight rather quiet for the Midwest.

&&

Long term...(monday through sunday)
issued at 310 PM CST sun Dec 28 2014

As the high pressure shifts to the southeast, a larger, colder high
pressure area will slide in with Arctic air. There will be a thin
boundary between the two air masses and with this, there will be a
small chance of flurries as it passes. Timing of this feature looks
to be Monday night and Tuesday morning. This colder Arctic high will
bring well below normal temps to the region for Tue night through
Wed night. The coldest lows are expected to be Tue night and the
coldest highs on Wed. Breezy northwest winds during the period will
also create wind chills below zero across the northern part of the
state, especially Tue night and Wed morning.

This high pressure will dominate the weather through thur night and
will slide into the eastern US by thur evening. With this high to
the east and an upper level low in the southwest US, moisture and
warmer temps will begin to advect back northward into the region.
Model differ on the timing of this return flow and this results in
differences in timing of the return of pcpn to the area. The European model (ecmwf)
is quicker and further east with the track of a low pressure area,
along with the associated pcpn. The GFS is slower and further west
to northwest with the track, and thereby brings more warm air into
the region ahead of the low pressure area, and more pcpn. Looks like
the regional blend leans toward the GFS with a western track, more
warm air ahead of the system and more of a spread of pcpn over the
area. The next question then, is what will p-type be during the
period. Due to the spread of the models, confidence is low, so will
keep p-type simple and just go with snow becoming rain or snow, and
then just rain, and then back to rain or snow. 850mb temps look to
be couple of degrees above freezing, so would expect rain well north
into the cwa. However, the question is the sfc temps. Anywhere with
temps below freezing has the potential of receiving freezing rain.
But if the rain is heavy enough, then temps could remain at or just
above freezing overnight Friday. To repeat, forecast confidence is
low due to model spread, so will not be adding any freezing pcpn at
this time. As the system gets closer and the models reach some
agreement and consistency, forecast changes are likely. Beyond this
system, dry and cooler temps are again expected for Sat night and
Sunday.

&&

Aviation...(for the 06z tafs through 06z Monday night)
issued at 1146 PM CST sun Dec 28 2014

High pressure centered across our counties will keep VFR
conditions in general. Some fog is indicated in the hrrr and rap
models along the ridge axis for cmi and Dec. The rap GOES all the
way to vlifr with the fog across cmi/Dec/spi, while the hrrr keeps
the LIFR fog east of cmi. Will add MVFR fog at cmi and Dec as
light fog is developing along that axis already near-by.

A weak cold front extending from southeast Nebraska to southern Michigan
will slowly drift to the southeast over the next day. As the cold
front approaches Monday evening, MVFR clouds are expected to
arrive from northwest to southeast.

The increasing clouds will be triggered by a shortwave moving
east along the front as it arrives in the area. A few flurries
could develop for the northern terminals of pia and bmi. Models
have trended drier with the last 2 runs, so do not expect any snow
accumulation as the NAM was suggesting previously. Will add -sn at
pia and bmi Monday evening, and keep the other taf sites dry.

Winds will remain calm under the surface ridge axis tonight. A
northeast wind will develop by mid-morning on Monday and continue
through the rest of the taf period. Wind speeds will remain less
than 10kt.

&&

Ilx watches/warnings/advisories...
none.
&&

$$

Update...shimon
short term...hjs
long term...auten
aviation...shimon

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