Area Forecast Discussion
Issued by NWS Central Illinois

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931
FXUS63 KILX 251800
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
100 PM CDT Tue Apr 25 2017

.UPDATE...
Issued at 1100 AM CDT Tue Apr 25 2017

A band of mid-level lift is producing some ACCAS clouds across
western Illinois, which are showing up on radar images. Based on
the very dry airmass below the cloud bases, no precipitation is
likely reaching the ground this morning. High resolution models
show the band of showers dissipating on the southern edges, with
better chances of some sprinkles remaining northwest of our
forecast area. Have increased cloud cover in the sky grids to
account for that area of forcing for precip, but have kept the
weather dry through the afternoon. The additional cloud cover
could slow down the hourly temperature rises, but we still expect
a majority of the area to reach the upper 70s to low 80s, with the
warmest readings mainly in our SW counties toward Jacksonville
and Springfield. HRRR and RAP models are generally holding off on
a line of showers/storms entering our western counties until after
midnight. Have therefore slowed down the start time of rain in our
far west counties by a couple hours to trend that way.

Updated forecast info will be available shortly.

&&

.SHORT TERM...(Today and tonight)
ISSUED AT 219 AM CDT Tue Apr 25 2017

Still quite a bit of dry air over the forecast area early this
morning, with 2 am dew points in the 30s along and west of I-55.
However, dew points in the 50s have pushed as far north as central
Indiana, and will spread westward with time through the day. With
temperatures a couple degrees warmer than yesterday -- generally
around 80 degrees -- the added moisture will become a bit more
noticeable.

Latest surface map showing low pressure over north central Iowa,
with a frontal boundary extending southwest to another low over
the Oklahoma panhandle. As this southern low rides northeast
today, the warm front will surge northward, and much of the rain
will be on the northern flank. However, showers and thunderstorms
will be developing along the trailing cold front this evening. ARW
model has a line of storms moving into the northwest CWA as early
as late evening, but other higher resolution models are more
focusing on Missouri with a brush-by over western Illinois after
midnight. Have included chance PoP`s as far east as Springfield
and Lincoln overnight.

.LONG TERM...(Wednesday through Monday)
ISSUED AT 219 AM CDT Tue Apr 25 2017

Lots to focus on during this part of the forecast, in terms of
severe weather potential and heavier rainfall.

Latest SPC Day1 outlook introduced a slight risk of severe weather
over most areas west of I-57 on Wednesday. Most models have the
cold front moving into the western CWA during the afternoon hours,
when 0-6km bulk shear is over 50 knots and MUCAPE`s reach the
1500-2000 J/kg range. High-res models are a bit variable with the
timing, with the ARW fastest (along I-57 by sunset) and the
NAM slowest (still west of the Mississippi River at the same
time). Highest PoP`s during the afternoon will be west of I-55 and
80-90% PoP`s arrive early evening. In terms of timing of the rain
end, the GFS has a fairly clean back edge, but most models linger
some showers through the night as the upper trough arrives, so
have only scaled back to around 50% over the far western CWA after
midnight and lingered some rain over the east into Thursday
morning. With precipitable water (PWAT) values around 1.5 to 1.7
inches Wednesday into Wednesday evening, some heavier rains are
likely mainly focused along the I-55 corridor.

Late week storm system remains a challenge. Evening model guidance
continues to dig a broad upper trough over the Rockies, closing
off a low near the 4-corners region by late Friday evening. Some
better agreement is being seen with this low lifting into the
central Plains, with the surface cyclone tracking across northwest
Illinois early Sunday. While a warm front sets up shop over
central Illinois, the speed of it lifting north is more uncertain,
with the GFS over the northern CWA by late Saturday afternoon and
the ECMWF about 12 hours slower with that position. However, the
heavy rain aspect is in better agreement, with an extended period
of 1.50+ inch PWAT`s from late Friday night into midday Sunday.
Widespread soaking rains are likely this weekend, which may start
to cause some impacts with time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 100 PM CDT Tue Apr 25 2017

VFR conditions should prevail the rest of today and this evening
as high clouds drift across Illinois ahead of approaching low
pressure. Winds will remain the primary concern this afternoon as
south winds gust to 25kt at times. Some gusts to near 20kt could
even linger several hours this evening, based on momentum transfer
plots in bufkit. High resolution guidance are generally agreeing
that storms will hold off until after midnight. PIA could see
storms as early as 07z, but trends have been to slow down the
arrival of precip in general. Have introduced a VCSH to start at
PIA at 09z, with VCTS around 12z. Most of thunder potential should
hold off at the other terminal sites until after 12z, as the
best forcing for precip remains west of I-55 until later morning.
Some brief MVFR conditions could develop as the storms reach near
a TAF site. However, prevailing cloud heights should remain low
VFR.

&&

.FIRE WEATHER...
Issued at 400 AM CDT Tue Apr 25 2017

Latest guidance showing 10-hour dead fuel values of 5-6% across
much of the northwest half of the forecast area. Dew points over
that area overnight have not recovered much and were still largely
in the 30s as of 3 am. While some improvement in dew point is
expected today, strong southerly winds will push temperatures to
around 80 degrees this afternoon with minimum relative humidity
values around 30%. Conditions not quite at red flag level, but
wind gusts to around 30 mph will raise some concern.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...Shimon
SHORT TERM...Geelhart
LONG TERM...Geelhart
AVIATION...Shimon
FIRE WEATHER...Geelhart



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