Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 151149

Area Forecast Discussion
649 AM CDT Mon Sep 15 2014

.SHORT TERM...(Today)

Main concern for today is shower/storm timing/chances associated
with the passage of a weak cold front and driving upper level
energy. The upper level forcing is somewhat complicated as it will
be provided by two separate disturbances. The first/weaker wave is
tracking east across the central Plains, while the second/stronger
wave is diving across the northern Plains. The second wave should
end up being the dominant feature for us, although both waves should
tend to shear out to some degree as they push into the confluent
flow across the Midwest.

The best forcing/frontal arrival have slowed some from previous
forecast, with the front not expected to clear the entire forecast
area until this evening. This slower arrival will also delay the
better chances for precipitation until the midday/afternoon hours.
Have expanded thunder mention to the entire forecast area today
given the steep mid-level lapse rates on forecast soundings
area wide. This north/east shift of thunder chances is also supported
by storms currently tracking east across southern Iowa. These storms
were located on the nose of a 25kt low-level jet in advance of the
central Plains wave. The forcing of the LLJ and central Plains wave
should weaken before reaching the local area, however it is expected
to be replaced by upper-level divergence/mid-level frontogenesis as
a jet streak ahead of the northern Plains wave arrives. A fairly
wide N-S temperature range is likely across the forecast area today
considering the fairly early arrival of the front/precipitation in
our northwest counties, compared to the evening frontal arrival
across southeast Illinois.


.LONG TERM...(Tonight through Sunday)

Cold front should reach the I-70 corridor by sunset and be out of
the forecast area shortly afterward, taking the rain with it. Fairly
sharp clearing expected behind the front as high pressure builds in,
but clouds will linger much of the night in southeast Illinois.

Quiet midweek period expected as the high drifts across the Midwest.
Evening models have various interpretations of some convective
activity to our southwest on Wednesday, along the periphery of some
upper ridging across the Rockies. but all models keep it in Missouri
and will thus keep our forecast dry. Slow moderating trend in
temperatures with lower 70s widespread by Thursday.

Upper ridge breaks down late in the week before rebuilding along the
West Coast late in the weekend. Longer range models dig a sharpening
upper wave across the mid and upper Mississippi Valley by Saturday
night, with rain chances mainly focused in the Saturday afternoon
and evening time frame. Beyond that, the models show quite a bit of
divergence in the evolution of this wave. The latest ECMWF model
forms a closed low over Lake Superior Saturday night and drives a
large slug of cooler air across the Midwest later in the weekend,
while the steadier GFS is much more progressive and shows much less
of a temperature change. That sharper scenario is more of a recent
development and thus for now have only shown some small decrease in
temperatures for Sunday and Monday.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)

Spotty showers and thunderstorms have developed across northern
Illinois and upstream toward a cold front cutting northeast to
southwest across Iowa. The coverage is expected to increase over
the next few hours, with shower/storm chances lingering until the
cold front passes later today. Have included a VCTS at all central
Illinois terminals until FROPA, and have also included a 4 hour
tempo for showers for the four hours preceding FROPA when
rainfall coverage should be greatest. May need to amend to add a
TEMPO for thunder as well, but do not want to overdo the thunder
coverages until its ultimate extent is more clear. Carried VCSH
for a few hours post FROPA due to some guidance suggesting showers
may linger for a time.

VFR conditions to start the period may degrade to MVFR with the
steadier showers/storms. Then, expect MVFR conditions to develop
due to moisture/clouds trapped beneath the post frontal subsidence
inversion. Current thinking is that skies should clear before the
end of the period, but confidence it the timing of the clearing is
not high.




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