Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 280406

Area Forecast Discussion
National Weather Service Lincoln IL
1106 PM CDT SAT AUG 27 2016

Issued at 832 PM CDT Sat Aug 27 2016

Convection finally developed late this afternoon from near
Springfield east-northeast to just north of Decatur to
just west of Champaign. The storms developed in a weakly
sheared but rather unstable airmass with mixed layer capes
around 2500 to 3000 j/kg. The multicell storms began to train
across the northern half of Macon county to extreme western
Piatt county with radar estimates in the 3 to 4 inch range
just north of Warrensburg in north central Macon county, with
a broad swath of 2 to 3 inch estimates running from just west
of Warrensburg to near Cisco in extreme northeast Piatt county.
The storm tops reached to 50,000 feet and took full advantage
of the tremendous amount of moisture available in the atmosphere
with precipitable water values at or above 2 inches late this
afternoon. Those storms were slowly winding down, but occasionally
we get a stronger updraft develop with the cluster over north of
Forsyth in north central Macon county.

Will continue with the scattered wording for the next few hours
for the area running from near Springfield to Champaign for the
next few hours before reducing POPs to just slight chance for the
remainder of the night. We should have the updated zones out by
845 pm.


.SHORT TERM...(This evening through Sunday)
ISSUED AT 236 PM CDT Sat Aug 27 2016

18z/1pm water vapor imagery shows a distinct fetch of deep-layer
moisture flowing northward from the western Gulf of Mexico into
Missouri and west-central Illinois.  While the synoptic warm front
has lifted well to the north into southern Wisconsin, outflow
boundaries left behind by early morning convection will likely serve
as the focusing mechanisms for additional thunderstorms within this
moisture axis later this afternoon and evening. HRRR/WRF-NMM/WRF-ARW
have all been consistently showing scattered convection developing
mainly across the western half of the KILX CWA after 21/22z...with
the storms then gradually dissipating by mid-evening.  As a result,
will continue to focus low chance PoPs west of I-57 this afternoon
and evening, with just slight chance further southeast.  Once
daytime instability wanes, only isolated showers/thunder will linger
overnight.  With skies partially clearing, winds becoming very
light, and ample low-level moisture in place, think fog will develop
overnight.  HRRR is showing fog becoming prevalent after midnight,
while both NAM and GFS forecast soundings point to fog development
as well.  It is difficult to pinpoint exactly where the fog will
become most widespread/dense, but think this may be focused over the
Illinois River Valley where heavy rainfall occurred earlier this
morning.  At this point, will include patchy fog mention everywhere,
but this may need to be boosted by later shifts as trends become
more apparent.  Once any early morning fog dissipates, another
partly sunny, warm, and humid day will be on tap for Sunday.  With
very little focus for convection present, will once again only carry
20-30 PoPs.  High temperatures will top out in the middle to upper

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 236 PM CDT Sat Aug 27 2016

A broad subtropical high pressure ridge will be in the process of
building by Sunday evening promoting a storm track to the north of
central IL as 500 mb height gradients and stronger westerly flow
become confined to the northern tier of states and southern Canada.
Central IL looks to remain in an air mass characterized by 1.5 to
2.0 inches precipitable water and afternoon MLCAPE instability
values around 1500-2000 J/KG through Tuesday. With the flow aloft
weak, organized or severe storms are not expected although isolated
to scattered afternoon/early evening showers and thunderstorms will
be possible. Expect highs in the mid 80s to around 90 with the
cooler temperatures along I-74 and the warmer temperatures south of
I-70. Dewpoints in the lower 70s will bring heat index values around
90 to 95 each afternoon. Lows upper 60s to around 70.

As an upper level trough develops over the northeast U.S. and
northwest flow develops over the upper Midwest...a cold front
continues to be advertised by the 12Z model runs...pushing southward
into the Midwest. For Wednesday afternoon the front could trigger
scattered thunderstorms as it moves into central IL. Highs will
depend on exact timing, but have estimated low to mid 80s for now.
Wednesday night and beyond dry conditions can be expected as cooler,
stable, and dry air push into the area with high pressure and
subsidence predominating. Expect highs in the upper 70s to lower 80s
with a slow warming trend through the period. Dewpoints in the low
to mid 60s will make conditions considerably less humid. Lows
expected to range from the upper 50s to lower 60s.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1100 PM CDT Sat Aug 27 2016

Main forecast concern will be with the threat for fog and low
clouds in the 08z-13z time frame Sunday morning. Most of the
convection that occurred over parts of the area Saturday
evening has diminished, however there was still some widely
scattered showers and storms to our north associated with a
weak frontal boundary that is expected to drop into our
area Sunday morning. Only negative factor for widespread
fog will be with the extensive mid level cloud shield that
is across the area. The is what the HRRR model has been showing
most of the evening with the mid level clouds extending from
southwest of SPI northeast thru CMI and BMI with possibly some
MVFR vsbs developing late tonight as the cloud shield shifts to
our north and east by morning.

What fog and low clouds that are able to develop late tonight
will lift rather quickly by 15 or 16z Sunday with a return to VFR
conditions by afternoon. As was the case today, and especially
with the boundary slipping south into our area Sunday, diurnal
cumululs will develop by early afternoon with isolated to widely
scattered thunderstorms possible. Any storms the do develop during
the afternoon should dissipate Sunday evening.


.ILX Watches/Warnings/Advisories...


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