935
FXUS63 KILX 301738
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1238 PM CDT Mon Jun 30 2025

.KEY MESSAGES...

- Showers and storms are possible today (30-40% chance), with the
  best chance during the late afternoon into the evening. There is
  a 5% chance of severe storms, with damaging winds the main
  concern. Locally heavy rainfall is also possible.

- After a brief cooldown to seasonable temperatures for the middle
  of the week, hot and humid conditions will return late in the
  week. Afternoon heat indices of 95 to 105 degrees are expected
  each day Friday (July 4th) through Sunday.

&&

.DISCUSSION...
Issued at 236 AM CDT Mon Jun 30 2025

*** TODAY ***

A relatively diffuse sfc pattern is in place across the region,
along with a moist airmass with PWAT values of 1.5-2" (highest
across SE IL). Upstream, an upper trough axis was positioned
across the Dakotas, and this feature will swing through our area
today. In the short term (07z-12z Mon AM), scattered showers are
most likely south of I-70 where there is subtle convergence
through the low- levels. Given that rich moisture (PWATs around
2"), some locally heavy rainfall could occur. Elsewhere, mostly
dry conditions are favored through 12z. Some patchy fog could
develop (Macomb was reporting 1/4 mile as of 07z/2am), but high
clouds spreading in from convection across MO could slow
additional radiational cooling.

For today, the ambient environment remains as advertised in
previous discussions, with subpar deep layer shear (20-30 kts),
weak mid- level lapse rates, and moist profiles with well-mixed
PBLs. Collectively, this kind of environment favors similar
hazards to those seen with previous days convection: isolated
damaging winds due to precipitation loading in downdrafts, and
locally heavy rainfall. The one difference from previous days is
the approaching upper trough/cold front and marginally-improved
shear could lead to slightly greater storm coverage/organization.
Still, CAMs appear fairly sparse in terms of storm coverage across
the ILX CWA today, with most likely timeframe for precip being
22z-03z (5pm-10pm) as the front moves through. Continued the
downward trend in PoPs for today, now at 30-50% chance area-wide
as CAMs depict scattered or multi-cellular convection rather than
a coherent line. The front has trended a bit slower, so storms
could linger into late Mon eve (across E IL) before finally
departing to the SE.

*** MID-WEEK ***

There were no meaningful changes to the mid-week forecast. Weak
sfc high pressure (~1016 mb) will accompany the post-frontal
airmass into the region, resulting in ample sunshine on Tues/Wed
along with our long-advertised break from the heat/humidity. Highs
will be in the mid 80s Tues, then upper 80s on Wed, but dewpoints
in the 60s will keep heat indices below 90 degrees those days.

*** LATE WEEK INTO THE WEEKEND ***

Towards late week, guidance suggests the upper pattern will
feature an amplified ridge building over the central Plains, with
upper lows near the Hudson Bay as well as along the west coast of
Canada. Additionally, a weakening upper low over California should
eventually be swept northeast, and this shortwave could play a
role in late week precip chances for IL. As the upper ridge
approaches, it will allow the heat and humidity to build back into
the region. For Friday (the 4th) through Sunday, highs are
expected to be in the low 90s with dewpoints returning to the 70s,
which will result in several consecutive days of peak heat
indices between 95-105 degrees. Heat Risk is in the moderate
(impacts to those most susceptible to heat) to major (impacts to
anyone without adequate hydration/cooling) categories on those
days.

Guidance continues to depict the upper ridge axis shifting over
IL on the 4th of July, which should help limit precip coverage due
to large scale subsidence. Blended guidance currently has just a
15-20% chance of precip area-wide during the afternoon/evening of
the 4th, and the coarser resolution model blend has had a tendency
to overdue the PoPs during this summertime pattern, some I`m
optimistic that much of the area will stay dry. The aforementioned
shortwave is progged to be too far west to provide any meaningful
forcing for ascent across our area, and therefore any convective
development seems more likely to be driven by disorganized, pop-up
thunderstorms if we reach the convective temperature.

As the pattern evolves eastward in time, our local precip chances
increase into the weekend, with a 20-50% chance on Sat/Sun.
Despite the upper trough approaching during that time frame, 500mb
flow remains weak (less than 30 kts) and lapse rates uninspiring,
so the threat of organized severe storms from this system appears
fairly low. With another high moisture environment in place
(PWATs approaching 2"), sporadic strong wind gusts due to precip
loading will continue to be a concern with any convection.

Erwin

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1238 PM CDT Mon Jun 30 2025

The main aviation concern through 18z Tuesday will be scattered
thunderstorms late this afternoon through this evening. Primary
forcing will be a cold front now from southeastern IA into far
northwestern IL, combined with daytime heating. Isolated storms
ahead of the front this afternoon cannot be ruled out, though they
are fairly low probability. In addition to lightning and lowered
visibility from heavy rain, isolated wind gusts in excess of 40 kt
could occur with this evening`s storm activity.

Outside of storms, low VFR to brief MVFR ceilings are a
possibility this afternoon.

AAT

&&

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

$$