931
FXUS63 KILX 131759
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1259 PM CDT Sun Jul 13 2025

.KEY MESSAGES...

- This afternoon, areas near and south of Interstate 72 will
  experience scattered thunderstorms, posing a risk of localized
  damaging winds and flooding.

- Additional chances for strong thunderstorms and flooding return
  by the middle of the week (Wednesday - Thursday) and again by
  next weekend (Saturday).

&&

.DISCUSSION...(through next Saturday night)
Issued at 1250 PM CDT Sun Jul 13 2025

A weak front lies across central Illinois, near and south of the
I-72 corridor, as shown by early afternoon surface analysis.
Scattered showers have developed due to warm-air advection and
isentropic lift, driven by a very moist boundary layer (with PWATS
exceeding the 90th percentile for mid-July climatology) and weak
low-level southwest flow impinging on the surface front.

Scattered thunderstorms are expected to develop this afternoon as
instability increases south of the boundary. This will be
accompanied by an increase in forcing for ascent and deep-layer
shear, driven by a mid-level shortwave trough lifting into
southern Illinois.

Modest CAPE/Shear profiles are expected this afternoon south of
the front with the latest RAP guidance supporting MLCAPE values
between 1000-1850 J/kg and deep-layer shear between 20-30 kts.
Though poor mid-level lapse rates (<6.5 C/km) may hinder updraft
development, the presence of steep low-level lapse rates (>8 C/km)
near and south of I-70 suggests a risk of damaging thunderstorm
winds wherever the stronger updrafts do form. The risk of hail is
minimal due to limited instability and shear in the hail growth
zone (-10 to -30 C). Similarly, the tornado threat is low,
primarily attributed to a relatively straight hodograph and weak
streamwise vorticity within the effective inflow layer. This is
best illustrated by poor 0-1km SRH values (< 50 m2/s2). Still, any
squirrelly storm-scale interactions that could potentially boost
tornado potential will need to be monitored in addition to the
potential for landspouts that could anchor along the synoptic
boundary.

The more widespread impact today will be heavy rainfall and
localized urban flooding. The latest HREF LPMM QPF guidance
supports pockets of 1-3 of rainfall through about 9pm this
evening. This seems reasonable given our moisture-laden airmass
and the fact that the mean wind and shear vectors are parallel to
the slow-moving front.

Monday is looking like a dry day for most as the mid-level
shortwave departs east of our area and the sfc front washes out
somewhere near the HWY 50 corridor. The pattern becomes hard to
whip by the middle of the week, with the next chance for rain
returning as early as Tuesday. By then, another subtle shortwave
impulse appears to lift northward across the Missouri Ozarks,
helping to resharpen the washed-out sfc front and nudge it
northward back into central Illinois. Relatively weak deep-layer
shear but moderate buoyancy near and south of the front could
support scattered thunderstorms both Tuesday and Wednesday
afternoon.

Shower and thunderstorm coverage may then increase Wednesday and
Thursday as a pair of mid-level shortwaves lift across the Mid-
Mississippi Valley, which will again push a synoptic front through
our region. A quick assessment of deep-layer shear and PWATs
during this timeframe indicates a potential return of both strong
thunderstorms and localized flooding.

Chances for rain lower by Friday as the synoptic front exits our
area and is replaced by surface high pressure. Areas of dense fog
could become prevalent Friday night into Saturday morning across
portions of central and southeast Illinois. Rain chances then
increase Saturday afternoon and evening as ensemble guidance hints
at another shortwave trough moving through quasi-zonal, mid-level
flow, ultimately helping push a frontal system into central
Illinois.

MJA

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1222 PM CDT Sun Jul 13 2025

A slow-moving frontal system will gradually push south of the I-72
terminals this afternoon. Scattered thunderstorms in the vicinity
of this front are already underway and are anticipated to persist
through the afternoon. We have therefore introduced a TEMPO group
for TSRA at all terminals through about 21z-22z. Flight
conditions will largely remain VFR through 00z, though any
instance of TSRA could result in MVFR visibility.

By late tonight, areas of fog could develop across all terminals,
perhaps reducing visibility into the MVFR/IFR range across the
board before conditions improve after 12z-13z Saturday morning.

Otherwise, winds will be variable throughout this afternoon into
the evening hours, especially as nearby thunderstorm outflow work
to abruptly change wind directions. Winds then become nearly calm
overnight.

MJA

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$