Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 261712
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1212 PM CDT Wed Jul 26 2017

.UPDATE...
Issued at 1047 AM CDT Wed Jul 26 2017

Have lowered temps for today based on increasing high clouds
associated with MCS approaching from IA/MO.

Stationary front set up from south central Iowa through eastern MO
into southern IL will be focus for any convective development this
afternoon. With lower temps and dry atmosphere above the boundary
layer at ILX,DVN, and SGF, instability and rainfall efficiency may
be a bit less concerning than previous thinking.

At this point will keep FFA running as 12z NAM still indicating
some potential, but any flash flooding may be spotty or confined
to more vulnerable areas such as urban areas. Will re-evaluate
later this afternoon once entire 12z model suite is in and more
CAMS runs are available.

&&

.SHORT TERM...(Today and tonight)
ISSUED AT 251 AM CDT Wed Jul 26 2017

Lots to worry about in the short term, between the heat and the
threat of strong/severe storms and heavy rain.

Large MCS currently ongoing across central Nebraska early this
morning, along a warm front associated with low pressure over
southwest Nebraska. Ahead of expected redevelopment later this
afternoon and evening over Iowa, our area will primarily deal with
the cirrus blowoff during the day. This will cause some potential
headaches with temperatures and heat index values today. Southwest
CWA will be borderline on heat advisory criteria with values near
105 degrees this afternoon. The cirrus has the potential to keep
temperatures down a bit though, and for that reason will hold off
on any advisories at this point.

Isolated storms continue to be featured by the higher resolution
models late this afternoon across mainly west central into
southern Illinois, but the main threat remains with the MCS that
is expected to form over Iowa. The storms with this system should
start moving into the northwest CWA around sunset and push
southeast, although the areas south of I-70 will be fairly dry
until after midnight. Precipitable water values remain off the
charts for this area with values greater than 2.50" late evening
and overnight, with forecast soundings showing warm cloud depths
upwards of 14,000 feet. Rainfall over the last week in a large
part of the CWA has been less than an inch, and 1-hour flash flood
guidance is mostly 2-2.5 inches. However, given this atmosphere,
it`s certainly plausible that some of the heavier storms could
produce that much rain. The latest NAM and GFS guidance focus the
heavy rain axis in west central Illinois, potentially as far
southeast as Taylorville. Will go ahead and hoist a Flash Flood
Watch over the western half of the CWA (basically Lacon to
Taylorville westward) for tonight and Thursday morning.

Severe-wise, latest Day1 slight risk is focused northwest of the
Illinois River. While there is decent shear progged over the area,
it arrives during the evening with lesser instability, and
forecast soundings trend more toward skinny CAPE as the heavy rain
threat starts to become more of a concern.

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 251 AM CDT Wed Jul 26 2017

Some spread in the models on how exactly the surface low will
track southeast over the area. The NAM and GFS are slower and
staying southwest of our CWA, which would limit any severe
potential on Thursday, while the ECMWF and Canadian models are
much further north (near Bloomington Thursday morning) resulting
in a larger warm sector ahead of the front. Latest Day2 focuses
more on the former scenario and keeps the slight risk from about
I-70 southward, but the evolution of the nocturnal MCS and any
residual boundaries are obviously wild cards at this point. In any
event, have kept likely PoP`s into the afternoon south of I-70,
with a trend toward more drier conditions across the northwest CWA
during the afternoon.

The remainder of the forecast period looks to be pretty quiet,
with broad troughing aloft staying across the eastern U.S. into
the first part of next week. This will keep the western ridge from
building eastward, and temperatures around here will be near or
just below seasonal normals for the end of July.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1210 PM CDT Wed Jul 26 2017

Stationary front near the Mississippi River will begin to push
east ahead of a shortwave moving southeast into the Lower Missouri
Valley this afternoon. Scattered convection will likely develop
along this boundary but coverage limits due to relatively dry
atmosphere warrant leaving out of terminals at this time. Best
chances of anything impacting our area would be at KSPI.

Better chances of convection will move southeast with the area
overnight associated with the aforementioned shortwave. Periods of
Precipitation and possibly some MVFR ceilings will persist through
the morning Thursday as a cold front stalls over the area in wake
of the shortwave.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 7 PM CDT this evening through Thursday
morning for ILZ027>031-036-037-040>042-047>052.

&&

$$

UPDATE...Barker
SHORT TERM...Geelhart
LONG TERM...Geelhart
AVIATION...Barker



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