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Area forecast discussion 
National Weather Service Lincoln Illinois
1221 PM CDT Sat Apr 18 2015

Update...
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 locations...it 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 18z tafs through 18z Sunday afternoon)
issued at 1215 PM CDT Sat Apr 18 2015

Main forecast concern will be timing of showers into the taf sites
late tonight/Sunday morning. Until that time period...VFR
conditions will continue. Satellite data and surface observations
continue to indicate some scattered-broken altocumulus with a higher
overcast ceiling around 18000 feet. We are seeing some more thin spots
developing in the high overcast early this afternoon but the
overall trend for the rest of this afternoon through most of this
evening will change little. Forecast soundings not showing much
in the way of support for any precipitation in or close to our taf sites
through this evening with most of the moisture/clouds expected to
be about 7000 feet. Next weather system expected to bring showers
into the forecast area from south to north Sunday morning but it
appears the bulk of this activity will hold off until around 12z
in spi and Dec and by 16z for points further north late Sunday
morning. Surface winds this afternoon will be out of the southeast
at 10 to 15 kts and east to southeast at around 10 kts tonight
through Sunday morning.

&&

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

$$

Update...Smith
short term...Barnes
long term...Barnes
aviation...Smith

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