917
FXUS63 KILX 260430
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1130 PM CDT Sat May 25 2024

.KEY MESSAGES...

- Severe weather returns tomorrow morning. The line of
  thunderstorms is expected to move through from 5am to noon.
  Main hazards with this line is large hail and damaging wind.

- There is a second round of severe weather tomorrow afternoon. It
  will be a mix of discrete supercells that eventually merge into
  a line. This will be an all hazards possible round: large hail,
  damaging winds, tornado or two, and flooding.

- Flash flooding is most likely to occur where the outflow
  boundary will set up over the are that was affected yesterday
  and is very saturated already. A Flood Watch is in effect from
  7pm to midnight tomorrow.

- After tomorrow the weather will be dry and pretty through most
  of next week.

&&

.UPDATE...
Issued at 900 PM CDT Sat May 25 2024

Low pressure has migrated into western KS, while the high pressure
center that was over IL today as moved northeast of the area. The
resulting pressure gradient has allowed a light southeast wind to
initiate across much of central IL after a mostly calm wind day,
and this wind will increase to a breezy 15 mph or so by Sunday
morning as the low continues toward IL overnight. Supercell
thunderstorms ahead of the low this evening will likely develop
into a more organized line or cluster of storms as they head
toward central IL by morning. High resolution models bring the
first storms into the central IL forecast area around 5 to 6 AM
and continue them eastward through the I-57 corridor by 10 AM.
These storms continue to have a risk of large hail and severe wind
gusts. Timing is a bit earlier than previously forecast, so have
updated forecast with storm timing/location an hour or two/county
or two earlier/farther east than the afternoon package. Otherwise,
lows mainly in the lower 60s look on track for tonight.

37

&&

.DISCUSSION...
Issued at 252 PM CDT Sat May 25 2024

With a high pressure in place today, weather has been calm and
beautiful. It has provided a nice breather before the next round of
active, potentially severe, weather tomorrow morning. Skies have
been clear and sunny. Highs today have been around the upper 70s.
Lows tonight are expected to drop down to the low 60s.

Tomorrow is a different story. POPs increase again late tonight into
early tomorrow morning. The models struggled with the previous event
that moved through yesterday and once again today, they are not
agreeing on timing and evolution of the event. Both the NAM 3km and
HRRR show an initial line moving across the CWA tomorrow morning,
but the way the afternoon unfolds is uncertain. However, that
uncertainty could mostly play into if the environment can recover
sufficiently from the morning convection, which could also be
playing a factor in how the models are depicting the afternoon
"mess".

Timing on the HRRR is showing the initial line of storms moving
across central and southeastern Illinois from 10z to 18z tomorrow
morning. Whereas, the NAM 3km is showing 13z to 18z. So the
consensus is that the line of storms should be out of the area by
early afternoon, but the start time is still variable by a couple
hours. The ARW model shows it entering around 12z. To generally
summarize when we think the line will begin to affect our area, it
should be around the time everyone is waking up in the morning.

Mesoscale Factors....... The morning time has MUCAPE of 2000-2500
J/kg, lapse rates aren`t super impressive (~7.5-8.5 C/km), and bulk
shear 30-40 knots. This round of convection appears to be elevated,
bringing the primary risk of large hail and damaging winds and
limiting the tornado threat. In the afternoon, the risk shifts some.
The CAPE becomes more surface based (SBCAPE of 1500-2000 J/kg). The
lapse rates are impressive just as the CI begins (LR of 9 C/km) but
quickly weaken as we go into the evening hours. Bulk shear increases
to 40-60 knots. The initial storm mode in the afternoon could be
supercells with a tornado and large hail risk but will transition to
a more linear mode as we go into the evening, shifting the hazards
to more a flash flooding, large hail, and damaging wind set up.

This initial line will arrive well before the front and will be
associated with a strong shortwave passage, with the afternoon
convection potentially firing along the outflow boundary of the
morning line. If the afternoon round sets up along and south of I-72
again, we could have some flooding concerns. Forecast soundings from
the GFS and NAM show PWAT values in that area of 1.5 to 2 inches and
somewhat long, skinny CAPE. Our storm motion is too quick so the
threat will come from training storms over the same areas that have
already been saturated to the max over the past week. The cold front
should move through tomorrow evening.

After tomorrow the next real chance for any rain comes on Friday.
Temperatures will slowly rebound this week. The first part of the
week the highs will be in the low to mid 70s and by Thursday high
will be getting to the low 80s. The skies should be blue and clear
for the most part this coming week after Sunday.


Copple

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1130 PM CDT Sat May 25 2024

A morning line of strong to severe thunderstorms and a less
certain afternoon to early evening round of scattered storms are
in the forecast for the central IL terminals over the upcoming
day. The first round looks to sweep across the area from around
12Z-16Z, then there is much higher uncertainty in timing and
location with subsequent thunderstorms. MVFR to possible brief
IFR conditions are possible with these storms, especially from
12Z-14Z until 17Z-20Z when MVFR cigs will likely be predominant.
Winds SE 5-10 kts overnight, shifting to S and increasing to
12-16 kts with gusts 20-28 kts by around 16Z. Winds shifting to W
around 10 kts by 02Z-04Z after a cold front sweeps through the
area.

37

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 7 AM CDT Sunday through late Sunday night for
ILZ044>046-049>057-061>063-066>068-071>073.

&&

$$