484
FXUS63 KILX 161832
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
132 PM CDT Wed Jul 16 2025

.KEY MESSAGES...

- A broken line of thunderstorms will push across parts of central
  Illinois this afternoon...bringing a risk of strong winds and
  locally heavy rainfall.

- Heat and humidity will build next week...with early projections
  suggesting heat index values climbing well above 100 degrees by
  next Tuesday and Wednesday.

&&

.DISCUSSION...
Issued at 132 PM CDT Wed Jul 16 2025

...Severe Weather Risk This Afternoon...

18z/1pm water vapor imagery shows a vigorous short-wave trough
over northeast Iowa that will be the primary forcing mechanism for
severe convection across Wisconsin and far northern Illinois as
the afternoon progresses. Further south across central Illinois,
it appears a broken line of thunderstorms will form along a
trailing surface trough currently just west of the Illinois River.
The latest SPC mesoanalysis shows a moderately unstable, but
weakly sheared environment with MLCAPE values of 2000-2500J/kg and
0-6km bulk shear of just 20-25kt. As temperatures rise into the
lower 90s, MLCAPEs will increase into the 2500-3000J/kg range over
the next couple of hours...while shear increases to 25-30kt north
of the I-72 corridor. As is typically the case, the CAMs are a bit
too slow with their convective development...as radar is already
showing thunderstorms approaching the Illinois River. The WRF-ARW
and NSSL WRF were the fastest models and seem to have the best
handle on the situation albeit 2-3 hours too slow. Based on the
latest radar trends, think the broken line of storms will cross
the Illinois River and reach the I-55 corridor by 3pm...then
further east to I-57 by around 5pm. Given the current instability/shear
parameters, think a few of the storms will be capable of producing
damaging wind gusts in excess of 60mph and brief heavy downpours.
Most of the activity will be confined to areas north of
I-70...with hot and dry weather persisting further south.

The line of storms will settle southward toward I-70 early this
evening: however, models suggest it will be in a weakening state
as it moves further away from the marginally favorable wind shear
and daytime instability gradually wanes. While previous solutions
suggested showers/storms would potentially re-develop along the
resulting outflow boundary overnight, the latest guidance has
backed off considerably. NAM shows only a weak 25-35kt 850mb
nocturnal jet developing from Texas oriented northward into
Kansas, which is where most of the overnight convection will
focus. Further east, think only scattered showers/thunder will
develop mainly along/south of I-72.


...Additional Rain Chances Over the Weekend...

A cold front will sink southward through central Illinois on
Thursday, then become stationary near the I-64 corridor Thursday
night into Friday. The boundary will remain close enough to
warrant low chance PoPs mainly across the S/SW KILX CWA during
that time: however, think many areas will stay dry. As upper
ridging gradually builds over the Lower Mississippi River Valley,
the front will get nudged back northward over the weekend. Given
the presence of the boundary, a series of short-waves riding along
the northern periphery of the upper ridge, and ample deep-layer
moisture...the stage will be set for a few rounds of showers and
thunderstorms across Illinois from Friday night through Monday.
While timing of individual waves will be difficult, confidence is
growing that upper support will lead to a significant uptick in
PoPs during the day Saturday, then again by Sunday afternoon and
evening. While the exact track of the waves and their corresponding
swaths of enhanced rainfall are still somewhat in question, the
latest NBM guidance suggests widespread rainfall of 1-2 inches
across all of central Illinois this weekend...with the potential
for even higher totals north of the I-74 corridor. WPC has
highlighted much of central/northern Illinois with a Slight Risk
of excessive rainfall both Saturday and Sunday accordingly.


...Building Heat 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 likely push heat index values well above
100 degrees and into Heat Advisory/Warning territory by next
Tuesday and Wednesday.

Barnes

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1244 PM CDT Wed Jul 16 2025

1730z/1230pm radar/satellite imagery shows a broken line of
convection developing just west of the Illinois River. CAMs are
about 2-3 hours too slow with convective evolution, so am relying
heavily upon observational data for the immediate short-term
aviation forecast. With that said, have opted to include a TEMPO
group for thunder at KPIA between 18z and 19z...then further east
to the I-55 terminals between 19z and 21z...and to KCMI between
20z and 22z. Once the line passes, any lingering storms will
settle south of the TAF sites by early evening. Earlier solutions
suggesting the potential for additional widespread showers/storms
overnight have backed off considerably and now focus much of the
activity further west across Kansas/Missouri. As a result, have
dropped thunder mention everywhere tonight and have only carried a
PROB30 for showers at KSPI/KDEC/KCMI between 08z and 13z. Winds
will initially be S/SW at 10-15kt, then will veer to W/SW after
passage of the convective line. Winds will then gradually veer to
NW late tonight into Thursday morning as a cold front settles
southward through the region. As winds become NW, the HRRR/RAP
show low clouds developing and spilling into the area Thursday
morning. Have therefore lowered ceilings to MVFR after the 11z-13z
time frame.

Barnes

&&

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

$$