579
FXUS63 KILX 291040
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
540 AM CDT Sun Jun 29 2025

.KEY MESSAGES...

- Hot and humid through Monday, with heat indices in the mid 90s.
  Conditions cool slightly for Tuesday and Wednesday, with highs
  in the mid 80s and heat indices below 90 degrees.

- Isolated to scattered storms are forecast through Monday.
  Locally heavy rainfall is possible, and there is a marginal risk
  (level 1 of 5) of severe storms, mainly on Monday. The primary
  hazard with any severe storms will be strong winds.

&&

.DISCUSSION...
Issued at 241 AM CDT Sun Jun 29 2025

*** THROUGH MONDAY ***

Early Sun AM, convection was ongoing near a cold front draped
across parts of WI/MN. Elsewhere, convergence along the leading
edge of the LLJ has led to showers/storms over cntrl MO. A moist
airmass remains in place across the region, with sfc Td in the low
70s and PWAT values around 1.5" sampled by the 29.00z KILX
sounding (with models suggesting that values approach 2" across
southern IL). With highs near 90 today, afternoon heat indicies
will once again climb to 95- 100 degF.

Over the next 24-36 hours, confidence is low in the convective
evolution as the environment remains weakly sheared and several
areas of convection are already ongoing. In this type of regime,
it`s difficult to put much stock in specific CAM runs. Overall,
CAM storm coverage is fairly low, so tried to keep PoPs at 30% or
lower through Sun night. Think the most likely time for scattered
precip today will be in the afternoon as a weak impulse aloft
shifts towards central IL in conjunction with peak diurnal
heating. Forecast soundings show mean cloud layer winds of less
than 10 kts, and given the high PWATs could see localized heavy
rain this afternoon. Similar to previous days in this pattern, an
isolated strong wind gust can`t be ruled out.

A more focused opportunity for storms is still expected on Monday
as the approaching cold front moves across the region, but even
for this front, CAM precip coverage appears scattered at best.
Felt the NBM PoPs (80-90%) were far too aggressive, and instead
went with 60- 70% (and wouldn`t be surprised if these are reduced
in later forecast updates). The main upper level trough will be
displaced to the north, resulting in unimpressive mid-level lapse
rates. There is modest mid-level flow, however, which results in
marginal deep layer shear of around 30 kts, perhaps sufficient to
offer some convective organization. Moderate instability is
expected to develop ahead of the front, particularly into Mon
afternoon (2000-3000 J/kg of SBCAPE). PWATs also remain very high,
around 2" which is above the 90th percentile for this time of
year. The main hazards will be damaging winds (particularly if a
more organized convective line forms) and locally heavy rainfall.
The front and any associated storms should shift completely SE of
the ILX CWA by late Mon eve.

*** TUESDAY - SATURDAY ***

No significant changes to the forecast for the middle of the week,
with a brief cooldown to seasonable temps on track as the post-
frontal airmass and sfc high pressure settle into the region. Tues
and Wed will feature highs in the mid 80s, lows in the mid 60s,
and afternoon heat indices in the mid/upper 80s. Confidence is
high that temps will trend warmer late week as an upper ridge
builds/shifts to the Midwest. Daily high temps push back to the
upper 80s/low 90s Thurs-Sun, with increasing humidity as well. The
upper ridge strength doesn`t appear as strong as our heat wave
last week, at least, but we`re still on track to see afternoon
heat indices in the upper 90s each day.

Precip chances have trended a bit lower for the 4th of July, now
at 15-20%, rather than 20-30%. This reduction is due to the fact
that the upper ridge axis is now progged to be directly over IL on
the 4th, which would promote synoptic-scale subsidence and help
limit storm chances/coverage. Guidance does have disturbances
tracking along the northern periphery of the ridge on the 4th,
resulting in better precip chances across the northern
Plains/upper Midwest, so we will still need to monitor this
pattern for shifts, as it would not take much to shift the precip
chances into our area. Even without an obvious lifting mechanism,
fcst soundings for the afternoon of the 4th show a well-mixed PBL
with uncapped profiles nearing the convective temp, which could
support isolated storms (hence the 15-20%). Precip chances
increase slightly into the holiday weekend (30% Sat, 40% Sun).

Erwin

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 540 AM CDT Sun Jun 29 2025

The period begins with light winds and patchy BR resulting in MVFR
conditions. Any such visbys reductions will be relatively short-
lived, with VFR conditions expected to prevail after 13-14z. There
is a 10-20% chance of thunderstorms this afternoon, but potential
was too low to include a mention in the TAF. Southerly winds
develop by mid-morning and persist through the rest of the period.

Erwin

&&

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

$$