957
FXUS63 KILX 120208
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
908 PM CDT Fri Jul 11 2025

.KEY MESSAGES...

- Thunderstorms will sweep into the Illinois River Valley this
  evening. Areas of heavy rainfall are the main threat for
  isolated flooding, especially if it falls in urban and
  poor drainage areas.

- Daily chances for afternoon/evening thunderstorms will be seen
  through the weekend. SPC has a marginal risk for severe weather
  in place for Saturday and Sunday.

- Hot and humid conditions will be in place through the end of
  this week and again early next week. Temperatures look to trend
  cooler by the end of next week.

&&

.UPDATE...
Issued at 908 PM CDT Fri Jul 11 2025

A slow moving line of thunderstorms continues to progress
southeastward into the Illinois River Valley this evening. An
outflow evident on radar ahead of the storms signals a decrease in
severe weather potential, and high resolution model data corroborate
the decrease in intensity of storms late this evening with further
extent southeastward. Deep layer wind shear in the environment these
storms are moving into is under 30 kts, also on the weak side for
severe storms. Nevertheless, any storms able to catch up with the
outflow or develop an intense enough updraft could pose a threat for
an isolated storm with large hail or locally damaging wind gusts.
Main feature that will need to be watched for is flooding, as
portions of Knox, Stark, and Marshall counties already saw heavy
rainfall in the past day, and could start to see flooding develop
with a quick 1.5+ inch rainfall. For the most part, the line has
started to move fast enough that rainfall looks like it may avoid
reaching critical thresholds, but any segments that slow down could
be problematic. Otherwise, a warm night appears to be on track for
tonight, with lows in the lower to mid 70s, as the cold front
trailing this evenings storms remains northwest of the IL River
through the nighttime hours.

37

&&

.DISCUSSION...(through next Thursday night)
Issued at 208 PM CDT Fri Jul 11 2025

A seasonably hot and humid airmass will remain in place through
the weekend as a parade of shortwave troughs push across the Mid-
Mississippi Valley. The net effect will be daily chances for
thunderstorms, some of which may carry an attendant risk for
severe weather and/or flash flooding.

This afternoon, very buoyant boundary layer conditions have
evolved across the warm sector, with central and southeast
Illinois positioned south of a sharpening warm front. Sfc temps
have warmed into the upper 80s with dewpoints in the mid 70s. The
latest 18z RAP analysis suggests no residual capping exists amid a
backdrop of moderate MLCAPE (1500-3000 J/kg), with the greatest of
this instability focused across far west central Illinois. Within
the kinematic parameter space, an area of enhanced deep-layer
shear (30-40 kts) is being analyzed near and north of the I-80
corridor over eastern Iowa and northern Illinois, and is directly
tied to a narrow mid-level speed max lifting across that area.

A quick glance at water vapor imagery reveals a few subtle
shortwave impulses lifting northeastward over north-central
Missouri and south-central Iowa, ahead of both a stronger
convectively-induced shortwave and the main shortwave trough axis.
These more subtle shortwaves could provide the necessary forcing
to support scattered thunderstorm development as early as this
afternoon across west-central Illinois, mainly between 4-6pm local
time.

Short but cyclonically curved hodographs this afternoon support
skinny, cellular updrafts that may have a tendency to consolidate
and grow upscale fairly quickly. Mesoscale soundings from the HRRR
in areas north of the Illinois River valley reveal very steep low-
level lapse rates (> 9.5 C/km) and robust DCAPE values (> 1100
J/kg). This all adds up to a large hail (up to quarter size) and
damaging downburst (greater than 60 mph) risk with any storm that
may develop this afternoon.

The greater chance and better coverage of thunderstorms will come
this evening (between 7-10pm local time) with the arrival of the
stronger, convectively-induced shortwave. This disturbance will
once again augment deep-layer shear (30-40 kts) across portions of
southeast Iowa and west-central Illinois. In addition, a modest
low-level jet will begin to veer into west central Illinois,
helping to maintain a favorably buoyant parameter space.

The hodograph, while it does lengthen a bit due to the impinging
LLJ, is not supportive of isolated supercells, especially with the
mean wind and deep-layer shear vectors discouraging cross-boundary
flow. Instead, new updrafts will have a tendency to consolidate
and become linear. This again supports primarily a damaging wind
threat (gusts > 60 mph) across portions of west central and
central Illinois.

Modest low-level ambient shear and strong theta-e differentials
suggest the line of storms may quickly become outflow dominant as
they evolve eastward in time and space later this evening. This
lends a question as to how far east the damaging wind threat may
extend, with the greatest risk perhaps staying west of I-55.

Once the line of storms becomes outflow dominant later this
evening, the risk for heavy rainfall and flash flooding could
still remain overnight as the mature cold pool and modest LLJ work
together to help maintain updraft development beneath a sink of
healthy MUCAPE. Rainfall rates could eclipse 1/hr wherever
backbuilding occurs. But overall, the coverage of storms should
continue to fade overnight as the convectively-induced shortwave
departs our area.

Additional storm development is likely Saturday afternoon as the
synoptic cold front pushes in from the west. A favorably buoyant
environment will once again evolve by around midday, though deep-
layer shear will be notably less present tomorrow (< 20 kts). The
relatively weak deep-layer shear and poor mid-level lapse rates (<
6.5 C/km) suggests updrafts will struggle to hold together. Thus,
both coverage and intensity of thunderstorms tomorrow ought to be
less than today. Still, there will be enough sfc convergence along
the front to support at least scattered storms.

As we head into next week, some uncertainty remains with how far
south the cold front will drift before washing out on Sunday.
Right now, our southernmost counties remain clipped by a marginal
risk by SPC. Elsewhere across our area, conditions appear to
become drier and continued seasonably hot through at least Monday
as a ridge builds in. Rain chances then increase by the middle-to-
late stages of next week as shortwave activity increases across
the Plains and helps push a few sfc boundaries across central IL.

MJA

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 644 PM CDT Fri Jul 11 2025

A line of thunderstorms will work its way southeastward into the
central IL terminals late this evening, preceded by an earlier line
affecting only the KPIA terminal prior to 01Z. Conditions will be
primarily VFR through much of tonight aside from MVFR or locally
worse conditions and strong gusty wind potential in thunderstorms,
until MVFR cigs develop after 12Z-14Z. Thunderstorms look to become
more widespread after 02Z-04Z, lasting until 04Z-08Z, then only a
few lingering showers should continue through the night. Another
round of thunderstorms is possible mainly for KDEC-KCMI eastward
after 18Z Saturday. Winds S 10-12 with gusts to 20 kts this
evening, veering to W 8-12 kts by 14Z.

37

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 1 AM CDT Saturday for ILZ027-028-030.

&&

$$