089
FXUS63 KILX 151059
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
559 AM CDT Mon Jul 15 2024

.KEY MESSAGES...

- Oppressive heat and humid is expected today, with highs in the
  mid 90s and peak heat indices ranging from 105 to as much as
  115 degrees in some places. Cooler air arrives by mid-week, but
  areas south of I- 70 could see heat indices remain above 100
  degrees on Tuesday.

- Another round of severe thunderstorms is likely this evening
  after 7 or 8 PM (level 3 of 5 risk). The primary hazard is
  damaging winds, but a few tornadoes are also possible.

- The thunderstorm chances shift south of I-72 Tuesday evening
  (50- 70% chance), with some potential for severe thunderstorms
  (level 1 of 5) or excessive rainfall (level 2 of 4).

&&

.DISCUSSION...
Issued at 313 AM CDT Mon Jul 15 2024

The "ring of fire" remains active along the northern periphery of
an upper ridge that remains centered over the SW US, with several
MCS developing over the last few days - including one ongoing
across central IL as of 2am/07z - and another anticipated Monday
evening.

Regarding the ongoing MCS, it has largely become outflow dominant
with cloud temps trending warmer, with the exception of the
western flank. New cell generation continues to occur along the
surging outflow. Forward-propagation Corfidi vectors from the RAP
favor a continued southeastward motion of this MCS, but as it does
so it will steadily be moving into a less sheared environment -
and thus the expectation is that the severe threat is and will
continue to trend lower. Will have to keep an eye out for locally
heavy rainfall on the western flank of this MCS over the next few
hours, as recent radar imagery shows backbuilding storms tracking
across the same parts of Fulton/Tazewell counties. 3-hour flash
flood guidance in these areas is a little over 2", and radar
estimates are starting to approach 2" is western Tazewell. The
latest forecast has PoPs progressing all the way into
southeastern IL for this MCS, gradually diminishing with
southeastward extent as confidence decreases in its longevity.

The MCS will likely result in a cloudier start to the day across
the southeastern half of the forecast area. While there`s still
some uncertainty, the expectation is that skies transition to
mostly clear/partly cloudy by the afternoon. Assuming the cloud
cover does in fact clear out, forecast highs should push into the
mid 90s, with dewpoints in the mid/upper 70s resulting in
oppressive heat. Peak heat index values could climb over 110
degrees, particularly in locations west of I-55, which is
approaching excessive heat warning criteria. However, given the
uncertainties in the cloud cover forecast opted to stick with the
existing heat advisory.

Regarding the severe potential this evening (level 3 of 5 risk),
the HREF depicts a strongly unstable environment, with mean
MUCAPE values approaching 4000 J/kg coincident with 30-35 knots
of deep layer shear. Forecast soundings show very high DCAPE
values approaching 1400 J/kg. These instability parameters favor
strong updrafts/downdrafts capable of producing severe wind gusts.
CAMs like the HRRR support this notion, with some runs of the
HRRR depicting thunderstorms producing scattered wind gusts over
70 mph. The latest CAMs are actually in fairly good agreement
regarding the storm evolution on Monday, with storms forming first
over IA during the late afternoon/early evening, then quickly
growing upscale and reaching the northwest part of the ILX CWA
after 8-9 PM. Once these storms move into the ILX CWA, they`re
expected to pose a severe threat for at least several hours, but
the exact evolution and potential end time of this expected MCS
remains uncertain. Damaging winds are the main hazard, but the
potential for line-embedded tornadoes also appears concerning.
Guidance has a strong 925mb LLJ developing around the time storms
move into the ILX CWA, with 925 mb winds approaching 40-45 knots,
which results in 0-1 km shear vectors of almost 30 knots.

The pattern shift beings on Tuesday, as a cold front begins to
sink into the area. Additional storm development is possible
along/ahead of the front, mainly south of I-72 Tues PM (50-70%
chance). The environment ahead of the front continues to exhibit
ample instability, but weak shear that makes an organized severe
threat look less likely compared to Mon night. There is a level 1
of 5 risk of severe weather on Tuesday. Areas ahead of the front
will remain hot and humid, and the heat advisory may need to be
extended for areas south of I-70 on Tuesday for heat indices over
100 degrees. The other concern with the storms Tues is the
potential for locally heavy rainfall/isolated flash flooding, as
forecast PWAT values approach 2.3". While we are near the
climatological peak of PWAT values in central IL, those values are
still quite extreme, well above the 90th percentile and
approaching the maximum of the ILX sounding climatology,
regardless of time of year. While isolated, some CAMs do depict
isolated rainfall totals in excess of 2-3".

No significant changes occurred in the extended forecast, as the
pattern shifts to below normal temps and dry, save for some
lingering precip chances in SE IL near the frontal boundary
Wednesday.

Erwin

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 558 AM CDT Mon Jul 15 2024

An area of weakening showers moving away from the terminals has
resulted in a disorganized wind field across east-central IL as of
10z Monday, but southwesterly flow is expected to become
predominant into the morning. VFR conditions should prevail
through most of the period. A line of strong thunderstorms are
expected to progress across the area from northwest to southeast
this evening/tonight. Some shifts in t-storm timing are possible,
but the current PROB30 groups represent the most likely timing
based on current guidance (04-11z). If these storms do impact the
terminals, expect a sharp shift to breezy northwest winds and
reductions in visibility.

Erwin

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT this evening for ILZ027>031-036>038-
040>057-061>063-066>068-071>073.

&&

$$