Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 301955

Area Forecast Discussion
255 PM CDT Tue Sep 30 2014

.SHORT TERM...(Tonight)

A cold front had pushed into southern IL and northeast MO this
afternoon bringing cooler temperatures today. Meanwhile 1022 mb high
pressure north of the Great Lakes was nosing southward into IL and
combined with upper level ridge over IL had decrease the low clouds
by early afternoon leaving patches of stratocumulus clouds along
with a veil of cirrus clouds. Temps at mid afternoon ranged from the
mid 60s over Knox, Stark and Marshall counties where low clouds
still lingered, to upper 70s and lower 80s along and south of
highway 50 in southeast IL.

Upper level ridge axis near the MS river will shift east across IL
and into Indiana by Wed morning while surface high pressure ridge
extends into the Ohio river valley from the Great Lakes region. As
the ridge axis shifts east of IL, ene winds to veer se on Wed as
frontal boundary retreats back north as a warm front over central IL
Wed. Dry conditions expected to prevail through Wed morning with
fair skies. Patchy shallow/ground fog could develop during overnight
due to low level moisture trapped in a strong inversion, but drier
air advecting in today on ne winds with dewpoints slipping into the
upper 40s and lower 50s from I-74 ne to limit fog formation.
Southern and sw areas may have best chance of seeing patchy fog
where higher dewpoints in the upper 50s to around 60F closer to the
frontal boundary. Similar MAV/MET guidance lows tonight have trended
cooler and leaned in that direction for tonights lows in the upper
40s and lower 50s, with mid 50s from Jacksonville sw.


.LONG TERM...(Wednesday through Tuesday)

Big changes on the horizon for Central and Southeast Illinois by the
end of the week as much colder air is expected to push southeast
into the Midwest. Energy in the form of a strong 500mb trough over
Alaska is expected to dive down the west coast of North America and
reinforce a developing longwave trough over the Central U.S. by late

In the meantime, the cold front that pushed through the area last
night is expected to retreat back toward the northeast ahead of the
initial trough developing currently over the Plains.  Some questions
remain as to how fast and far the front will be able to push across
the area on Wednesday with the answer having major implications on the
high temperature forecast.  The GFS suggests that progress should be
as far as I-74 while the NAM keeps temps below normal with the
boundary remaining sw of the area.  The 00z ECMWF is somewhere in
between.  SREF from 09z favors the warmer solution and will lean
toward a compromise of the GFS/MAV and the ECMWF.

As the mid-level trough deepens, with 130+ kt jet plunging into it,
on Wednesday Night and Thursday, scattered precip is expect to
develop across the Midwest. BUFkit model soundings suggest some
capping possibly through middle afternoon in the warm sector with
the NAM being the most unstable and loosing the cap by late
afternoon in SW forecast area.  In addition the cold front
associated with the main trough is expected to enter western
Illinois during the evening. The speed of the front is fairly
consistent through the operational model suite and has been slowing
with each run set. There should be some reduction in instability as
the evening wears on but dynamics will likely be enough to keep
strong storms going well into the evening along and ahead of the
boundary.  At this time it appears there may be some threat of hail
with the storms ahead of the line with straight-line winds being the
primary threat with the actual QLCS.

Much colder air is expected Friday with lingering clouds and showers
as 500mb temps approaching -30C are progged. Would not be surprised
to see near steady temperatures and brisk conditions.   Models may
be underplaying winds as is often the case in these type of
situatuions.  By Saturday morning 850 mb temps drop blo 0C in all
the operational suite. Clouds and winds should keep reduce the
threat of frost, but would expect at least a few wind protected
locations to have lows into the 30s.

Reinforcing shots of energy move through the trough over the weekend
and into next week.  Timing of individual features are difficult
this far out and the models show generally cool and partly to mostly
cloudy conditions but varying timing and chances for precipitation.
Overall heights slowly build early next week, but will generally
reduce precip and introduce only gradual warming given the


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

Shallow layer of stratocumulus clouds 1-2k ft thick dissipating
rapidly over central IL by midday due to strong subsidence from
surface and upper level ridge over IL. Cloud bases lifted to 1500
ft and have recently scattered out over the central IL airports
past hour or two, most recently at western airports of PIA and
SPI. RAP and NAM curule does not show much redevelopment of
cumulus clouds this afternoon so will keep just scattered cumulus
clouds 2-4k ft. Shallow moisture layer will be trapped by strong
inversion overnight and lead to some fog development. For now have
introduced MVFR vsbys from 08-14Z, but could see locally lower
vsbys late tonight/early Wed morning especially at SPI and DEC
where dewpoints a few degrees higher. HRRR model shows stratus
clouds and some fog advecting westward from Lake MI across
northern IL with light east flow and these clouds to stay north of
central IL tonight. Scattered cumulus clouds 4-5k ft to appear
after 15Z/10 am Wed across central IL. NE winds near 10 kts this
afternoon to become light east after sunset and SE near 10 kts
after 15Z Tue as front in southern IL retreats back north across
central IL Wed.




LONG TERM...Barker
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