Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 201607
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1107 AM CDT SAT AUG 20 2016

.UPDATE...
Issued at 1106 AM CDT Sat Aug 20 2016

Cold front currently approaching the Illinois River with a series
of relatively weak pre-frontal lines of showers and thunderstorms
ahead of the front. Marginal instability up to around 1000 J/KG
and sfc-6km bulk shear to 30 kts ahead of the front indicates a
marginal risk for severe thunderstorms possible as daytime heating
takes place. This will be primarily east of I-55 from noon to 4
p.m. due to the timing of the front...which should reach the
Indiana border around 6 p.m. and end the threat of any severe
weather behind it. Will be making some adjustments to PoPs and
coverage to account for trends in showers and thunderstorms.
Otherwise highs ranging from near 77 at Galesburg to the low 80s
south of I-70 look good. Winds becoming breezy at SW 10-15 mph
with gusts over 20 mph...switching to WNW behind the front.

&&

.SHORT TERM...(Today and tonight)
ISSUED AT 349 AM CDT Sat Aug 20 2016

Low pressure area and associated frontal boundary is just to the
west of the state this morning and will continue to move east into
the CWA and through the CWA today. Convective models are now faster
with the timing of the front moving through the area. Convection
that developed ahead of the front early this evening has diminished
quite a bit. Satellite trends show cold cloud tops diminishing in
size and limited to where the most intense convection is located at,
which is very small areas in the line of showers and storms. This
area will move across the northern parts of the CWA this morning
with the front trailing behind, forecasted to reach west of a PIA to
SPI line around 12z. So, have diminished pops in the southwest to
reflect current radar trends. Expecting additional convection to
redevelop along and just ahead of the front later this morning and
afternoon with increased heating and instability ahead of the front,
but mainly in the east and southeast parts of the CWA. So will keep
higher pops in the east and southeast for this afternoon, with lower
pops back to the west. With HiRes models showing faster movement of
precip and the front through the area today, it is possible that all
precip could be out of the CWA by this evening. However, a weak sfc
trough will still be moving through the area, so will keep a chance
of precip in the east and southeast for early this evening. Then dry
weather is expected after midnight.

Temps will be warmest ahead of the front in southeast CWA, with much
cooler temps elsewhere...at around 80 degrees and below. Temps will
also be cooler during the night with mostly clear skies and light
winds.

.LONG TERM...(Sunday through Friday)
ISSUED AT 349 AM CDT Sat Aug 20 2016

Models in relatively good agreement in the Sunday through Wednesday
period as the deep ern NA trough pushes east of the area. Model
solutions are a bit more progressive with the passage then earlier
runs suggesting that the axis of the cold pool associated with the
trough axis may be east of the forecast area by the time of maximum
heating Sunday. Still looks like lapse rates may be steep enough for
some diurnal cumulus, particularly over eastern Illinois, but maybe
not as prevalent as first thought.

With the faster movement of the trough heights begin to build and
850 mb temps begin to climb already Sunday evening. However with the
surface high overhead boundary layer decoupling is likely and temps
should fall into the low-mid 50s. The 00z NAM and GEM are
significantly colder than GFS and particularly 12z ECMWF and would
will generally follow GEFS mean as a compromise with the much warmer
EC. Still should be some of the coolest temps we have seen since
early June in many locations.

Heights build rather quickly as a short-wave ridge develops behind
the exiting system and ahead of the next developing trough in this
rather active and progressive pattern for August. Temperatures are
expected to be several degrees warmer each day as southerly surface
flow develops. Dew points will also climb reaching into the upper
60s ahead of the next system.

Model spread develops quickly with this mid-late week system,
particularly with the cold air advection behind the wave. The 12z
ECMWF is deeper with the overall system then the GFS and breaks the
energy into two separate surges - each able to tap much cooler
Canadian air then the GFS. By Friday afternoon, 850 mb temps are
some 3-6C colder in the EC then GFS. The GEFS also suggests
uncertianty given a large solution envelope with Fridays high
somewhere between the mid-70s and the mid-80s. Will go with a blend
on Friday with a lean toward the cooler side of the spread.

With differences in strength/timing of the main wave, the model
solutions regarding the surface front and the intensity of any
convection associated with it vary substantially as well. Given the
suggested speed of the system and high dew points ahead of it, shear
and instability may be sufficient to warrant watching for a
hazardous convective threat Wednesday afternoon with a heavy rain
threat also possible given the 1.8-2.0 inch PWs forecasted by the
majority of GEFS members. The climatological 90 percentile is near
1.7 inches for late August.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 754 AM CDT Sat Aug 20 2016

Broken line of showers and isolated storms is moving through the
area this morning and based on HiRes models, this could be it for
the some of the TAFs today. However, the main front that these
storms formed on last night has not moved through the state yet.
So, will keep some showers going at all sites with VCTS. Will also
have a 2hr tempo group at all sites except PIA for storms. Looks
like could be an end to this round of precip, but with cyclonic
flow continuing and lots of low level moisture, and front still
west, will keep vcsh remainder of the morning at all sites. Once
front moves into area, believe storms will redevelop, so will
maintain a VCTS at all sites for the afternoon only. Once this
moves through, skis should clear or become mostly clear. Winds
will be south to southwest this morning ahead of the front, then
become west to northwest behind the front. Gradient tighter with
this system, so have gust of 22-24kts at the sites during the day
and afternoon, mainly just ahead of the front and when the front
passes.


&&

.ILX Watches/Warnings/Advisories...
NONE.
&&

$$

UPDATE...Onton
SHORT TERM...Auten
LONG TERM...Barker
AVIATION...Auten



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