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Area forecast discussion 
National Weather Service Lincoln Illinois
1042 am CDT Sat Apr 18 2015

issued at 1035 am CDT Sat Apr 18 2015

Main forecast concern will be with shower chances over our far
western counties this afternoon. Latest surface map indicates
dew points have risen into the middle and upper 50s over our forecast
area late this morning. The better return flow of moisture was
noted over eastern Missouri northward into southeast Iowa...but
even in these appears the NAM forecasts were a
bit aggressive with the dew point projections for late this
morning. Current satellite and surface observations across the
area indicating quite a bit of middle and high level cloud cover
over central Illinois with some thinning noted in the cloud shield
over southeast Illinois. Latest rapid refresh model soundings indicating
the better instability over far western Illinois later this afternoon
into this evening...however...the better lift and deeper moisture
is forecast to remain west of our area. Further north and east...a
weak frontal boundary is forecast to slip south into our northern
and eastern counties this afternoon with models indicating some
weak moisture convergence along this feature. However...forecast
soundings not showing much in the way of any support for precipitation
this afternoon.

Have adjusted the temperature trends into early this afternoon
based on the thicker cloud cover and have tweaked the precipitation
timing out west as well. Zone update should be out by 11 am.


Short term...(today and tonight)
issued at 324 am CDT Sat Apr 18 2015

High pressure will remain in control of the weather across central
Illinois today, resulting in another warm and dry day. Clouds and
showers associated with a slow-moving upper low over Colorado will
remain west of the kilx County Warning Area. While the GFS tries to bring a few
light showers into the area during the afternoon, most high-res
model solutions remain dry. Given initially dry airmass below 750mb
as evidenced by the 00z kilx upper air sounding and the fact that
the system in question remains closed at 500mb and thus slow-moving,
prefer the slower/drier solution. As a result, will only feature
slight chance probability of precipitation across the far southwest around Jacksonville
during the afternoon, with dry weather elsewhere. Afternoon high
temperatures will once again rise well into the 70s.

Long term...(sunday through friday)
issued at 324 am CDT Sat Apr 18 2015

Northern-stream short-wave noted on latest water vapor imagery over
the Pacific northwest will provide the necessary kick to finally
dislodge the persistent closed low over Colorado. As a result,
models are in good agreement that the low will eject eastward and
open up over the next 48 hours as it gradually becomes absorbed into
the northern stream. As this feature nears, deeper moisture being
drawn northward from the Gulf of Mexico will arrive, resulting in
increasing rain chances. 00z Apr 18 NAM continues to slow the
eastward advance of the precipitation tonight, keeping all but the southeast County Warning Area
dry through the entire night. European model (ecmwf)/Gem seem to be following suit,
while the GFS is the most aggressive in spreading precipitation into the
area. Given slow movement of system and a continued dry easterly
low-level fetch, think the GFS is too fast. Have therefore trimmed
probability of precipitation tonight, keeping rain chances south of the I-74 corridor. Have
also restricted thunder mention to the far southeast in closer
proximity to deepest moisture and strongest upper dynamics
associated with lead wave.

Forecast soundings saturate on Sunday as low-level flow veers to the
south ahead of the approaching system. Given much more favorable
forcing/moisture, will carry categorical probability of precipitation across the board.
Showers and a few thunderstorms will occur throughout the day,
keeping high temperatures in the 60s. Scattered showers/thunder
will continue through Sunday night, with a few showers even
lingering into Monday morning across east-central Illinois before
the 500mb vorticity maximum passes to the east of the region.

Forecast confidence decreases beyond Tuesday, as model solutions
begin to diverge substantially. Latest run of the European model (ecmwf) shows a
weak cold front passing through Illinois Tuesday night into early
Wednesday, followed by cool/dry weather through the end of the week.
By the end of the period, it shows a wave passing generally south of
the area into the Ohio/Tennessee River valleys by Friday night into
Saturday. Meanwhile, the GFS features a very different solution,
showing a cold front dropping through Illinois late Wednesday, then
the southern stream system at the end of the week coming further
north and producing rain across central Illinois Friday and
Saturday. Given the discrepancies between the models, have opted to
keep precipitation mention in the extended to a minimum until better
consistency is achieved. Bottom line for the forecast next week is
a trend toward cooler conditions, with highs mainly in the lower to
middle 60s and lows in the 40s.


Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 627 am CDT Sat Apr 18 2015

Cirrus shield continues to spread over the taf sites this morning,
and should continue through the afternoon. Then middle clouds will
begin to advect into the area as the next system slowly lifts out
of the southwest. Middle clouds will continue into the evening hours
as well as some lower cumulus/SC advecting into the area in advance of
the next system. Models have slowed the next system even more, so
have decided to keep mention of any precipitation out of the tafs at this
time, even tonight. Winds will be out of the southeast through
most of the period, with winds becoming easterly this evening.


Ilx watches/warnings/advisories...


short term...Barnes
long term...Barnes

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