021
FXUS63 KILX 161048
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
548 AM CDT Mon Jun 16 2025

.KEY MESSAGES...

- Seasonably hot temperatures return this week, becoming very hot
  by the weekend. The risk for excessive heat is highest Saturday
  through Monday when afternoon temperatures warm into the lower
  90s with heat index values approaching 100 F.

- Daily chances for thunderstorms exist through Wednesday. Any
  potential for severe weather and flooding appears to be confined
  to the Tuesday - Wednesday period.

&&

.DISCUSSION...
Issued at 357 AM CDT Mon Jun 16 2025

A highly complex pattern will begin to take shape across the
Plains today as a barrage of shortwave impulses crest the top of
the ridge. Each shortwave has the potential to spawn its own area
of organized convection this afternoon before congealing into an
MCS overnight. Further upstream across the Midwest, a decaying
complex of storms is currently lifting toward the confluence of
the Mississippi and Lower Ohio River valleys. This feature appears
to be riding along a weak baroclinic zone, and is forecast to
bring scattered showers and isolated thunderstorms to areas near
and south of our I-70 counties through this afternoon. Weak flow
aloft and poor lapse rates will preclude a severe weather risk for
today, but we could see a few pockets of heavy rain in this
airmass.

The main thesis for Tuesday is for thunderstorm coverage to
expand eastward into central Illinois, but it can be defended a
couple different ways.

The first way is via a decaying MCV, which pushes across Iowa
Tuesday morning and slumps into west central Illinois in time for
peak heating. Its a curious solution, one modeled primarily by
the HRRR. Its curious because the organized convective activity
being modeled precedes the arrival of the more favorable
kinematics (low- and mid-level jets) by several hours. We cannot
completely discount this solution. There have been many occasions
where the coarser-resolution global models exhibit unfavorable
kinematics, only for a decaying MCV to unexpectedly move in and
augment the shear profile. A lot would have to fall perfectly into
place for the recent HRRR guidance to verify, and it just seems a
little far-fetched at this point.

The second way fits the conceptual model much better, which is for a
nocturnal MCS to develop Tuesday evening somewhere over the Corn
Belt, ahead of a well-defined shortwave trough and on the nose of a
strengthening LLJ.  A quick glance at CAPE, shear, lapse rates and
the mean wind support healthy cold pool maintenance deep into the
overnight period as the purported MCS pushes toward the Mississippi
Valley. This narrative is generally supported by the 3km NAM and
FV3, though both sets of guidance suggest the MCS running into dry
air and losing steam over Illinois by Wednesday morning. While the
increasing kinematic fields could support an attendant thunderstorm
wind risk late Tuesday night, the risk for flooding could be more
germane. The NBM Mean QPF continues to average around 0.5" across
the ILX CWA during the Tuesday night period. The spread (10th to
90th percentile QPF) remains large; generally 0"-1.2. And then,
there are a few outliers that support 3-4 of QPF with MCS activity
Tuesday night.

To be clear, the possible outcomes for Tuesday range anywhere
between getting missed completely to getting hit squarely by an MCS.
No clear trend exists among current hi-res guidance, though it might
be a matter of when the storms move in rather than if.

Wednesdays risk for flooding and severe storms will be highly
conditional on whatever happens Tuesday night. Either the boundary
layer will recover/destabilize Wednesday afternoon ahead of the
impending cold front, or it wont. If it does, the parameter space
is very favorable for deep/organized convection and somewhat
favorable for flooding.

The oppressive heat and humidity is likely to return Friday. By
then, a broad 588-mb ridge axis is modeled to push across the Corn
Belt, becoming amplified (594mb) by Saturday and Sunday as a deep
western trough emerges. Daily temperatures are forecast to warm into
upper 80s by Friday, then lower 90s Saturday through Monday amid low
70s dewpoints. This all adds up to triple-digit heat index values by
the weekend.


MJA

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 528 AM CDT Mon Jun 16 2025

Surface high pressure will shift eastward and weaken as a cold
front approaches from the west. Winds will remain light under
influence of high pressure, but cloud cover will be prevalent
much of the TAF period as a disturbance moves into the Ohio
Valley and the front approaches from the west. Regional surface
obs early this morning reveal MVFR vsby near KSPI and KDEC where
shallow fog developed overnight. The reduced visibility may
continue a couple hours past sunrise. Beyond then, initial cumulus
development should result in a short period of MVFR cigs from
around 15Z-19Z until diurnal heating lifts cigs above 3000 ft AGL.
Have added a prob30 group at KDEC, KCMI and KBMI for isolated
thunderstorms this afternoon, primarily between 20Z-01Z.
Otherwise, winds E-NE 3-6 kts to start the TAF period, becoming S
5-10kts by 19Z.

MJA/37

&&

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

$$