387
FXUS63 KILX 250743
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
243 AM CDT Sat May 25 2024

.KEY MESSAGES...

- Today will be a great day for outdoor activities, with mostly
  sunny skies, light winds, reduced humidity, and high
  temperatures in the 70s.

- Multiple rounds of thunderstorms are expected (greater than 80%
  chance) on Sunday. The initial wave of storms on Sunday morning
  could pose a risk of large hail (level 1 out of 5 risk), while
  additional development later in the day will be capable of all
  severe hazards (level 3 of 5).

- Locally heavy rain from the thunderstorms on Sunday could lead
  to flash flooding, especially if it occurs in areas that
  received heavy rain on Friday.

- Conditions trend drier during the upcoming work week, with a few
  lingering showers/storms north of I-72 Monday/Tuesday (15-20%
  chance), followed by mostly dry weather Wednesday-Friday.

&&

.SHORT TERM...  (Through Late Tonight)
Issued at 238 AM CDT Sat May 25 2024

A cold front continues to progress east across the area, extending
roughly from Rantoul towards St. Louis at 215am/0715z. Nighttime
microphysics satellite imagery shows a band of clouds from
immediately behind the front back towards the IL River. Much of
the precip activity has come to an end within the ILX CWA, but
isolated pockets of light rain can`t be ruled out within that area
of cloud cover, which is expected to depart east into IN by mid-
morning.

A pleasant Saturday is in store as weak sfc high pressure,
currently positioned over Kansas City, shifts east following the
cold frontal passage. Skies will be mostly clear today, with light
sfc winds and temperatures peaking in the upper 70s to low 80s.
Lower dewpoints (in the upper 40s or 50s during the afternoon)
should feel quite refreshing.

Erwin

.LONG TERM... (Sunday through next Saturday)
Issued at 238 AM CDT Sat May 25 2024

Severe thunderstorms are possible on Sunday (level 3 of 5 risk).
Guidance depicts a concerning parameter space, but there is still
quite a bit of uncertainty as to how this event could unfold.

In response to a sfc low in the lee of the Rockies, t-storms are
expected to develop Sat eve across the Plains, and CAMs show the
remnants of this convection pushing towards W IL close to sunrise on
Sun, continuing across the area Sun AM. These storms would be
elevated. Often times, we see these convective complexes weaken
after daybreak as the LLJ weakens and cloud top heating occurs, but
with guidance showing around 2000 J/kg of MUCAPE present, it seems
these storms could pose a hail threat. The SPC included portions
of western IL in a level 1 (of 5) risk in the Day 1 outlook, but
that would be for these Sun AM storms occurring late in the Day 1
forecast period (towards 12z/7am on Sunday). These storms are a
key point of uncertainty in the subsequent forecast, as their
evolution will impact destabilization later in the
afternoon/evening. It`s also possible that if the timing of the
first round of storms starts to shift slightly later into the
morning/early afternoon, they could reintensify and pose a more
robust severe threat.

In the upper levels on Sun, two shortwaves are evident in guidance,
one accompanying the morning storms, and another approaching during
the late afternoon/evening. The presence of both of these waves does
increase confidence that the two waves of storms currently shown in
guidance will actually occur. For the afternoon/evening, current
forecast soundings suggest very strong sfc-based instability
(greater than 3000 J/kg) developing south of I-72 as dewpoints surge
into the low 70s. Even the oft over-mixed HRRR is showing dewpoints
in the 70s across southern IL Sun PM. The forecast soundings appear
quite favorable for all severe hazards, with minimal capping, strong
deep layer shear (50 knots) and strong low level helicity (over 200
J/kg 0-1km storm-relative helicity), but again, the impacts of the
wave of morning storms will go a long way towards determining if
this pristine environment becomes a reality or not. Mesoscale
boundaries associated with the morning storms will be key in
determining if/where storms occur Sun afternoon. For now, the 25.00z
HRRR and NAMNest are actually in decent agreement - depicting robust
t-storm development Sun afternoon, generally near and south of I-72.
Given the mesoscale uncertainties, would not be surprised at all if
the CAMs placement of storms shifts considerably in future runs, but
for now, this is where the severe storm chances appear highest Sun
PM.

Deep layer shear vectors are oriented roughly east-west Sun PM,
which is close to paralleling the anticipated mesoscale boundaries.
Accordingly, upscale growth appears likely and this is well-captured
by the CAMs. Such a storm mode favors damaging winds as the primary
hazard. 0-3km shear vectors are around 30 knots, which is sufficient
but not overly supportive of a QLCS tornado threat, and thus the 5%
tornado risk feels appropriate despite the otherwise favorable
hodographs that at first glance would seem to signal a greater
tornado threat. One concern due to this boundary-parallel flow is
locally heavy rainfall. HREF 24-hr LPMM shows rainfall amounts
generally under 1.5" thru 7pm Sun, with pockets of locally higher
amounts exceeding 2-3", which could cause some flash flooding
concerns, particularly if it occurs in areas that received heavy
rainfall on Fri (along/just south of I-72).

The presence of the passing shortwave could keep scattered post-
frontal showers around Sun night, but the severe threat should
diminish as the front pushes south of the area late Sun evening.
Into early next week, upper trough becomes a persistent feature over
the Great Lakes. Cyclonic flow persists across IL, with several
upper level shortwaves pivoting around the main trough, and thus
pivoting through IL. That could keep isolated showers/storms in the
storm chances in the forecast Mon-Tues afternoons when diurnal
heating is maximized, but with low instability (less than 1000 J/kg)
no severe weather is expected those days.

Finally, it looks like we could see a prolonged dry period for the
latter half of the work week as an amplified upper ridge builds over
the central Plains, resulting in relatively strong (~1026-1028mb)
sfc high pressure over the Midwest. Precip chances are less than 15%
on Wed-Thurs. Deterministic guidance looks dry for Fri and the
daytime hours next Sat, with any precip off to the west. Ensemble
guidance spread starts to increase, however, such that low end PoPs
do exist across western IL Fri/Sat. As guidance comes into better
agreement, I expect the 15-30% PoPs for Fri-Sat to trend lower, at
least across eastern IL (closer to the sfc high). Temps will be near
to slightly below normal for much of the upcoming work week, with
highs in the 70s gradually trending warmer as we move towards next
weekend.

Erwin

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1141 PM CDT Fri May 24 2024

A cold front will slowly work its way southeastward across the
central IL terminals overnight. Ahead of the front, scattered
showers and a remote possibility of a thunderstorm can be
expected. Winds will be S-SW around 10 kts. Behind the front,
winds will shift to NW around 10 kts, then high pressure will
settle into the area by mid morning bringing clear skies and light
and variable winds. Skies look to remain clear until high clouds
associated with the next storm system spread into the area
Saturday evening. Winds will return out of the SE 6-12 kts after
00Z.

37

&&

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

$$