481
FXUS63 KILX 182330
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
630 PM CDT Fri Jul 18 2025

.KEY MESSAGES...

- Thunderstorms are likely on Saturday...with a 15-30% chance of
  severe weather along and north of a Jacksonville...to
  Taylorville...to Marshall line.

- Additional rounds of showers and thunderstorms will occur late
  Saturday night through Monday...with an attendant severe weather
  and excessive rainfall risk.

- Heat and humidity will build next week...with heat index values
  soaring well above 100 degrees Tuesday through Thursday.

&&

.DISCUSSION...
Issued at 125 PM CDT Fri Jul 18 2025

...Severe Weather Risk on Saturday...

The latest Day 2 Convective Outlook from SPC highlights locations
along/north of a Jacksonville...to Taylorville...to Marshall line
for a Slight Risk (15-30% chance) for severe weather on Saturday.
A cluster of thunderstorms is expected to develop upstream in
advance of a short-wave trough skirting along the U.S./Canada
border across southern Minnesota and northern Iowa later this
evening. While CAMs exhibit a significant spread in timing
solutions, the general consensus suggests storms will push into
the Illinois River Valley in a weakening state after daybreak
Saturday morning. They will then push eastward...generally
along/north of I-72 during the morning into the early afternoon
before exiting into Indiana. Gusty winds will be possible with the
strongest storms across the Illinois River Valley initially, with
a further weakening trend toward midday as they cross I-55. The
storms may then re-intensify as they encounter diurnal destabilization
across east-central Illinois, but this process may occur further
east into Indiana.

While this initial push of convection will generally be sub-severe,
a higher probability of severe weather will materialize along a
trailing outflow boundary by late afternoon and evening. The exact
position of the boundary will be determined by the extent and
track of the morning convection and therefore remains in question.
Based on 12z and 18z model runs, the most likely location will be
along/north of the I-72 corridor. Deterministic models such as the
GFS and NAM indicate strong destabilization after passage of the
morning storms...with SBCAPEs exceeding 4000J/kg. The 12z HREF is
not quite so bullish, but still shows ensemble mean SBCAPEs
increasing to 2500-3000J/kg. In addition, deep layer shear will be
stronger than it has been recently, with 0-6km values of 30-35kt.
Given the presence of a boundary, a highly unstable environment,
and adequate wind shear...think scattered strong to severe cells
will form along the outflow from 3pm to 8pm. Most CAMs are sparse
with their late day convective development, but think they are
under done. Any storms that develop will be capable of damaging
wind gusts in excess of 60mph, hail larger than quarters, and
isolated tornadoes.


...Active Weather Late Saturday Night through Monday...

Additional nocturnal convection will develop both Saturday night
and Sunday night. Early projections suggest the Saturday night
into Sunday morning storms may focus along/north of I-74...while
the Sunday night into Monday morning convection may be across the
heart of central Illinois. This will be a largely mesoscale-driven
event, so the details regarding the individual waves of convection
will likely shift slightly with subsequent forecast packages. For
now, suffice it to say the weekend will be unsettled at times...with
the potential for severe weather and locally heavy rainfall. The
SPC Day 3 outlook places all of central and southeast Illinois
under a Marginal Risk (5-15% chance) for severe weather on
Sunday...while WPC highlights nearly all of central and southeast
Illinois with a Slight Risk (15% or greater chance) for excessive
rainfall both Sunday and Monday.  Initial projections suggest
widespread rainfall of 1-2 inches...with perhaps as much as 2-3
inches north of the I-74 corridor. Given the recent dry conditions,
the rainfall will be quite beneficial:however, excessive rainfall
rates within the stronger storms may lead to scattered flash
flooding of urban areas and low-lying spots prone to flooding.


...Heatwave Next Week...

The upper ridge is progged to steadily strengthen this weekend and
by early next week will shift the "ring of fire" northward into
the Upper Midwest and Great Lakes. As a result, rain chances will
decrease on Monday...then will disappear by Tuesday and Wednesday.
Thanks to rising upper heights, lesser rain chances, and more
sunshine...temperatures will climb into the lower 90s by Tuesday
and into the middle 90s by Wednesday. Given the expected rainfall
this weekend, the moist soils and actively growing crops will
contribute to dewpoints in the upper 70s and perhaps lower 80s in
a few spots. This will push heat index values well above 100
degrees and into Heat Advisory/Warning territory by next Tuesday
through Thursday.

Barnes

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 550 PM CDT Fri Jul 18 2025

Isolated showers and thunderstorms may develop overnight in the
vicinity of a slowly lifting warm front. The confidence and
coverage remain too low to include a PROB30 at this time, but the
possibility is not zero. Otherwise, VFR conditions are anticipated
to persist through much of the overnight period, with ceilings
lowering near MVFR levels beyond 12z. By then, a decaying complex
of thunderstorms is forecast to enter the central Illinois
terminals, and have maintained the TEMPO/Prob30 groups between
14z-18z. Additional thunderstorms may develop late in this TAF
cycle. Confidence and coverage are both low beyond 18z, but wanted
to introduce a PROB30 group at all terminals from 18z-23z as a
hedge for impactful severe weather.

MJA

&&

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

$$