Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 261946

Area Forecast Discussion
National Weather Service Lincoln IL
246 PM CDT FRI AUG 26 2016

.SHORT TERM...(This evening through Saturday)
ISSUED AT 246 PM CDT Fri Aug 26 2016

19z/2pm surface analysis shows stationary frontal boundary draped
along the I-70 corridor.  To the north of the front, widespread
cloud cover has kept temperatures in the upper 70s and lower 80s
across much of the KILX CWA.  Meanwhile south of the boundary,
sunshine earlier in the day boosted temps into the upper 80s and
lower 90s. Due to these very warm temperatures and dewpoints in the
middle 70s, SBCAPE values have climbed to around 4000 J/kg
along/south of I-70.  Deep layer shear is quite weak in this area:
however, the combination of strong instability and convergence along
the synoptic boundary has led to numerous thunderstorms blossoming
this afternoon.  Some of the storms will be capable of producing
gusty 40-50mph winds, frequent lightning, and torrential rainfall.
Have raised PoPs considerably across the SE counties through the
remainder of the afternoon to account for latest radar trends.

Think the storms across the southeast will continue into early
evening before gradually shifting eastward into Indiana and
dissipating after sunset.  The focus for additional convection will
then shift further west overnight, as WAA over the frontal boundary
produces a complex of storms across Missouri/Iowa/western Illinois.
Many of the synoptic models keep most of the precip west of the
Mississippi River until after midnight, while the Rapid Refresh
tends to develop it further east sooner.  Given poor performance of
Rapid Refresh this afternoon, have trended toward the NAM/GFS
consensus for this forecast package.  As a result, have maintained
low chance PoPs through the evening, followed by an increase to high
chance or likely after midnight.  Think the main push of
showers/storms will lift into northern Illinois and the Great Lakes
Saturday morning, but trailing boundary will provide focus for
additional development through the day.  Have gone with high chance
to likely PoPs through Saturday accordingly.


.LONG TERM...(Saturday night through Friday)
ISSUED AT 246 PM CDT Fri Aug 26 2016

The axis of an upper level shortwave ejecting northeastward
through the upper Midwest should be into northeast Illinois mid-
evening Saturday with the right-rear jet entrance in the area. A
moist and confluent low level flow will contribute to instability of
around 1500 J/KG CAPE for continued thunderstorms. These features
will continue to produce high chance thunderstorms along/northeast
of I-74 and have 50% PoPs for Saturday evening, 30-40% to the
southwest. Overnight, as the upper level feature continues to shift
northeast chances for precipitation will decrease.

Broad upper level ridging will build into the southern U.S. Sunday
through midweek, pushing the stronger upper level westerly flow
northward through the northern tier of states and southern Canada.
Central IL will become firmly within a warm moist air mass with
precipitable water around 1.75 inches, and broad surface high
pressure centered in the region. This should produce primarily
diurnal, scattered thunderstorms with lower chances at night. Have
included generally chance PoPs of 30-40% Sunday through Tuesday,
with slight chance (20%) at night. Highs expected to range from the
mid to upper 80s Sunday through Wednesday with the warmer
temperatures generally south of I-70. Lows upper 60s to around 70.

12Z models advertise a large trough deepening into the northeast
U.S. for the latter half of the week. This looks to drive a cold
front southward into central IL around Wednesday-Thursday. This
looks to bring a minor cooling/drying after it crosses over the area
Wednesday night. Have only included slight chances for thunderstorms
Wednesday night and beyond as it looks like thunderstorm activity
will be pushed off to the south and west. For temperatures have
trended highs down to the low to mid 80s for Thursday and Friday and
lows down to the low to mid 60s.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1249 PM CDT Fri Aug 26 2016

Widespread mid-level cloud cover will continue to blanket the area
through the afternoon. Latest satellite/radar trends suggest most
of the convection will develop closer to the stationary frontal
boundary across southeast Illinois...with only isolated showers
further north at the central Illinois terminals. Will only carry
VCSH accordingly.

Main aviation question will be timing/extent of convection later
tonight into Saturday morning, as front begins to return
northward. Model solutions vary, with the Rapid Refresh showing
widespread showers/thunder developing along/west of I-55 as early
as this evening, while the NAM/GFS tend to keep the majority of
the precip further west across Iowa/Missouri until after midnight.
Based on current radar trends, have sided with the NAM/GFS
consensus here. As a result, have introduced predominant rain
showers with VCTS at KPIA by 07z...then further east to KCMI by
09z. At this time, will maintain low VFR ceilings during the
precip, but have reduced visbys down to around 4 miles.


.ILX Watches/Warnings/Advisories...


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