Area Forecast Discussion
Issued by NWS Central Illinois

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296
FXUS63 KILX 180930
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
330 AM CST Wed Jan 18 2017

.SHORT TERM...(Today and tonight)
ISSUED AT 330 AM CST Wed Jan 18 2017

Main short-term forecast challenge will be determining if any
substantial clearing will occur today.  08z/2am IR satellite imagery
shows low clouds blanketing Illinois, with the closest clearing
across far western Iowa/Missouri.  Cloud-bearing winds are currently
quite light from the west, but will gradually become southwest by
this afternoon as a weak surface ridge axis passes to the east.
Given the light flow and an increasingly strong subsidence inversion
noted on forecast soundings, think clearing will be slow to occur.
While the GFS remains bullish with clearing as early as this
morning, the latest HRRR is on the other end of the spectrum and
suggests clouds may persist through the entire day.  Based on
current satellite loops, think clouds will hold firm through midday,
then begin to break up as boundary layer winds back to the southwest
this afternoon.  As a result, prefer the middle-of-the-road NAM
solution which shows clearing taking place across the western KILX
CWA early this afternoon then further east to the Indiana border by
mid to late afternoon.  Given the cloudy start and only weak WAA
later in the day, have opted to follow the coolest numeric guidance
for highs, with afternoon temps only reaching the lower to middle
40s.

Closed upper low currently evident on water vapor imagery over New
Mexico will begin to lift northeastward tonight.  As it does, a
plume of deep-layer moisture will approach from the south overnight.
Models have been consistently speeding up the progress of this
system and its associated precip, so have included low chance PoPs
for rain showers after midnight along/south of a Jacksonville to
Effingham line.  Before the clouds return, think patchy fog will
develop across the entire area tonight...especially given limited
mixing expected this afternoon.

&&

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 330 AM CST Wed Jan 18 2017

Copious amounts of Pacific and Gulf of Mexico moisture will get
drawn northward ahead of the upper low as it tracks into the Plains
on Thursday, with NAM showing near record precipitable water values
for this time of year of 1 to 1.30 by afternoon/evening.  Given
ample deep-layer moisture and strong forcing associated with the
approaching wave, have gone with likely to categorical PoPs Thursday
afternoon and evening.  System is moving through a bit faster than
previously thought, so it now appears precip will exit into the
Great Lakes by Friday morning.  Total rainfall will generally be
around one half inch...with as much as three quarters of an inch in
the Wabash River Valley.

Once the initial wave tracks into the Great Lakes, a period of
warm/dry weather will be on tap for Friday and Saturday.  With 850mb
temps progged to reach 8-10C, high temperatures will climb well into
the 50s and perhaps even into the lower 60s by Saturday.  In fact,
both the GFS and ECMWF numeric guidance suggest potential record
highs in the middle 60s.

After that, unsettled weather will return for early next week as yet
another southern stream system approaches the region.  00z Jan 18
models are in disagreement with how far north the wave will track,
with the ECMWF now taking a more easterly track across the Deep
South.  If the ECMWF verifies, rainfall amounts Sunday through
Monday may be lighter than previously thought.  For now, will
maintain high chance to likely PoPs for rain showers Sunday into
Monday, followed by dry weather by Tuesday.  Warm conditions will
continue through the entire period, with any substantial cooling
holding off until the middle and end of next week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1131 PM CST Tue Jan 17 2017

Main challenge for this TAF update is the timing of when the
IFR/LIFR ceilings will depart central and eastern Illinois
Wednesday. In the meantime, am expecting the clouds to persist
through the remainder of the night, with the lowest ceilings
ranging from 300-700 ft. Visibility is expected to be lowest in
the slightly cooler air along the I-74 corridor, with 2-3SM common
toward daybreak.

Many of the short range models seem to be overly optimistic about
pushing the low level moisture and associated cloudiness out of
the area shortly after daybreak Wednesday. The typical rule of
thumb this time of the year is for the low clouds to depart slower
than indicated by the models. As a result, will hold off on
scattering out the cloud cover about 3 hours or so slower than the
models. KSPI should be the first to clear just before 17z or so,
with KCMI scattering out around 19-20z.


&&

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

$$

SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...Miller



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