Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 231945

Area Forecast Discussion
National Weather Service Lincoln IL
245 PM CDT Wed Aug 23 2017

.SHORT TERM...(This evening through Thursday)
ISSUED AT 245 PM CDT Wed Aug 23 2017

High pressure at the surface will dominate the short term, despite
a weak upper shortwave in southern Manitoba expected to progress
into north-central Illinois late tonight and Thursday morning.
The RAP and NAM-nest show some convective elements developing in
NW Illinois after 10z, pushing the shortwave across Illinois
during the morning. The NAM/GFS/GEM/EC all point toward the wave
weakening and any radar returns that do develop will remain virga,
due to dry overall airmass. Have kept the forecast dry through
Thursday, with just an increase in cloud cover north of I-72
coincident with the passage of the wave.

The mid-afternoon minimum dewpoints point toward the potential for some
chilly low temps tonight. Lacon dewpoint of 48 and Peoria 49 give
credence that a few northern locations could even dip into the
upper 40s late tonight. Guidance numbers have not latched onto
that trend just yet, but we trimmed the northern temps a couple
degrees toward 50F. Highs on Thursday will be dependent on how
long cloud cover lingers in the NE half of the area, but in
general we expect highs to be a degree or two warmer than today,
in the upper 70s to around 80.


.LONG TERM...(Thursday night through Wednesday)
ISSUED AT 245 PM CDT Wed Aug 23 2017

The extended will be dominated by an upper trough in eastern
Canada/Great Lakes and an upper ridge in the Plains and Rockies,
setting up a persistent northwest flow into Illinois through this
weekend. The next weather disturbance of enough significance to
cause rain/storms looks to be as early as Sunday afternoon and
extend through as late as Tuesday afternoon due to a cut-off low
type situation. The confidence in the extended forecast past
Sunday is low, due to the unknown path of Tropical Depression Harvey,
and how close it will come to Illinois when it ejects from Texas
toward Tennessee/West Virginia. Harvey could linger for several
days in eastern Texas and the nearby waters of the Gulf of Mexico
from Friday through Tuesday, according to the ECMWF and GFS. The
Canadian drifts Harvey west into N Mexico and dissipates it there,
as the outlier solution.

In any event, the central Illinois area looks to be in the path of
a low pressure system arriving from the NW from Sunday afternoon
into Monday at least. However, an approaching Canadian upper
trough and the approach of Harvey could possibly cause that
precip potential to extend into Tuesday night and Wednesday as
the approaching systems work to amplify the eastern CONUS trough.
For now, we have limited PoPs to low chance and slight chance in
the Sunday through Wed time frame. The blended extended actually
has areas of dry conditions in portions of our forecast area
during that time, especially Monday night, due to forecast
uncertainties in strength of the low that arrives from the NW,
available moisture, and timing/path of Harvey. Moisture return
into Illinois from the south may be limited by Harvey to some
extent as low and mid level flows into IL remain relatively weak
during that time.

Temperatures look on track to remain below normal for late August,
with highs in the 70s and lows in the 50s, as the heat remains
bottled up in the western CONUS.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1244 PM CDT Wed Aug 23 2017

Minimal impacts expected in aviation weather over the next 24
hours. High pressure will remain firmly in control of our sensible
weather. Minimal fair weather cumulus this afternoon should not
reach BKN at any location, and will remain at VFR levels. Winds
will remain below 10kt in general, except for CMI during the last
hour. As the surface high axis drifts closer to Illinois later
today and tonight, the pressure gradient will weaken, keeping
winds below 10kt for the remainder of the 18z TAF period. A weak
shortwave in Manitoba is expected to progress into N Illinois late
tonight, but the very dry air in place across C and SE IL will
prevent any precip from reaching the ground. Tomorrow will be
similar to today, with FEW-SCT050 developing late




LONG TERM...Shimon
AVIATION...Shimon is the U.S. government's official web portal to all federal, state and local government web resources and services.