173
FXUS63 KILX 011046
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
546 AM CDT Tue Jul 1 2025

.KEY MESSAGES...

- Seasonable temperatures and less humid conditions are expected
  today and Wednesday. Hot and humid conditions return Friday
  (July 4th) through Sunday, with peak heat indices of 95 to 100
  degrees.

- Lingering showers across eastern Illinois will exit the area
  this morning, then mostly dry conditions should prevail through
  Friday (July 4). Daily chances for showers and storms return
  Saturday into next week.

&&

.DISCUSSION...
Issued at 239 AM CDT Tue Jul 1 2025

*** THROUGH TUESDAY AM ***

An MCS that produced heavy rainfall across SE IL (MRMS estimates
up to 4" in some spots) has shifted east of the area, and radar
coverage/intensity has reduced considerably in the last hour (as
of 07z/2am). However, we`re not done with the precipitation just
yet, as a new slow-moving axis of showers has formed and extends
from near Taylorville to SE Vermilion Co. RAP-based mesoanalysis
suggests this precip is being driven by a corridor of 850-mb FGEN.
This has not been well modeled by the CAMs, though the HRRR does
have some hints of this feature (albeit too far to the SE). Those
model runs suggest this axis of rain will steadily shift SE,
eventually exiting by 12-13z (7-8am). Current storm motion is
quite slow, and extrapolating the current motion with timing
tools suggests 12-13z is too fast of a precip departure, so did
extend the PoPs into the morning across SE IL. PWAT values are
still elevated, ranging from 1.6" along this new axis of rainfall
to 2" in the far SE portions of the CWA. The southeastward motion
of the new rainfall should help limit flooding, although if it
holds together into areas south of I-70 where higher rainfall
occurred it could lead to issues.

*** SEASONABLE AND LESS HUMID MID-WEEK ***

Zooming out, the start of the period feature a closed upper low
off the coast of California, with upper ridging over the Four
Corners region and troughing over the upper Midwest. A sfc cold
front had pushed south of the ILX CWA, extending across the Ohio
River Valley to Lake Erie. Behind this front, cooler and drier air
is advecting into the region. As of 07z/2am, temps and dewpoints
across the ILX CWA were in the upper 60s/low 70s, highest east of
I-55 where widespread mid-level cloud cover was still in place,
but further northwest across IA temps/dewpoints had fallen into
the low 60s.

Weak sfc high pressure (~1016-1018 mb) will be in place across the
cntrl CONUS today, shifting slightly east tomorrow. Seasonable and
less humid conditions are on track for today, with highs in the
mid 80s and dewpoints only in the low 60s. Temps warm a few
degrees for Wed, but should stay below 90.

*** LATE THIS WEEK INTO EARLY NEXT WEEK ***

Upper ridging is expected to amplify over the Plains, with the
ridge axis eventually shifting over the ILX CWA by Fri eve (4th
of July). This will result in a warming trend, with continued dry
weather. Previous iterations of the blended guidance had a low
chance (15- 20%) of precip on the 4th of July, which seemed
unlikely given the ridge axis overhead. The latest blend is now in
line with that assessment, with precip chances below 15% through
the evening of the 4th. A trough ejects and lifts across the
northern Plains into the weekend, resulting in the development of
a sfc low that is expected to track well north of the ILX CWA.
This sfc pattern will keep a southerly component to the sfc flow
across IL, resulting in increasing heat/humidity. The peak heat
indices are expected to be 95-100 degrees each day from Fri (July
4) through Sun.

As the cold frontal zone approaches the ILX CWA over the weekend,
precip chances increase (20-40% Sat, 30-50% Sun/Mon). Fcst
soundings out ahead of this front continue to show weak shear (20
knots of 0-6 km shear) and moderate instability (MLCAPE values
1500-2000 J/kg), but very high moisture content (PWAT values in
excess of 2"). Locally heavy rain, as well as isolated gusty
winds, remain the top concerns with any precip over the weekend.

Into early next week, guidance suggests a new upper ridge becomes
anchored over the Four Corners region. This results in an upper
flow pattern that places IL in the northwest flow regime of the
`Ring of Fire`, keeping occasional storm chances in the forecast
beyond Mon.

Erwin

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 546 AM CDT Tue Jul 1 2025

A mid-level cloud shield is shifting southeast of the terminals
presently. Some scattered diurnal Cu will develop around 3-4 kft
today, otherwise mostly clear skies are expected through the
period. VFR conditions should prevail. Winds will be out of the
northwest, with a few gusts around 18 kts mainly during the
afternoon.

Erwin

&&

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

$$