509
FXUS63 KILX 051952
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
252 PM CDT Sun May 5 2024

.KEY MESSAGES...

- There will be severe thunderstorm potential Tuesday, especially
  for locations along and southeast of a Champaign to Shelbyville
  line.

- Another round of potentially severe thunderstorms will impact
  parts of central Illinois Wednesday afternoon/evening.

- Much cooler conditions will return for the end of the week.

&&

.DISCUSSION...
Issued at 252 PM CDT Sun May 5 2024

Last nights`s cold front has stalled out near the Ohio River, while
an upper level shortwave/weak surface low is tracking along the
portion of the front trailing near the AR/MO state line. This system
will track along the front into the Ohio Valley tonight into Monday,
spreading showers into the I-70 corridor, with perhaps some stray
showers/light precipitation as far north as I-72. Instability
appears it will be meager to absent, especially north of I-70, so
only including a slight chance of thunder from around I-70 south.

A key player in the next significant forecast feature for central IL
is the upper level low currently spinning over the Great Basin. As
this feature emerges into the northern Plains Monday, it will lift a
warm front into the vicinity of central IL, which suggests a few
showers that afternoon/evening, but subsidence between the outgoing
wave and incoming low should dominate, and have trimmed out PoPs for
much of the area. However, a moist and unstable, strongly sheared,
warm sector will be developing in the southern-central Plains,
promoting significant severe thunderstorm potential that should
organize into a linear convective system, sweeping eastward toward
IL overnight Monday night. The degree of weakening of this system
overnight will be of paramount importance as far as the degree of
severity of thunderstorms arriving in central IL early Tuesday
morning. The 12Z NAM depicts 1500-2000 J/kg MLCAPE at 1AM Monday
night as the feature approaches IL, 1000-1500 at 4 AM moving into
western parts of central IL, and 500-1000 at 7 AM in eastern
central IL about to depart the area to the east. Some of the 12Z
models have sped up the timing of this feature, which could result
in more overnight/early morning threat of severe thunderstorms in
central IL, but overall decrease Tuesday`s threat due to
unfavorable timing for diurnal heating contributions to
instability. SPC has accordingly expanded Monday night`s Marginal
Risk area to Fulton-Jasper-Richland County southwestward. Re-
development of thunderstorms Tuesday afternoon could take place
along outflows from these early morning thunderstorms, as well as
from earlier thunderstorm activity to the southwest. However,
destabilization and lift following the early morning system is
questionable for much of central IL. A Marginal Risk for most of
central IL and a Slight Risk from I-57 eastward continues for a
conditional threat for severe thunderstorms given the strongly
sheared environment.

The closed upper low over the Northern Plains will linger into
Wednesday, with another embedded shortwave likely inducing another
convective system Wednesday. Current model suite has good
consistency with tracking this feature into central IL Wednesday
evening, which would be a diurnally favorable time for severe
thunderstorms. Current model suite depicts strong instability
1500+ J/kg for most of central IL and southward, as well as 50+
kts deep layer shear, so the environment looks ripe for severe
thunderstorm potential. SPC has a 15 percent severe thunderstorm
risk delineated for most of central IL southward.

Following Wednesday`s system, more of a northwesterly flow aloft
sets up for late week into the weekend, with the large scale trough
axis shifting east of IL. Although occasional precipitation chances
can be expected, amounts would be lighter, instability would be
weaker for a decrease in thunderstorm activity.

Temperatures should rise back to near summertime warmth through
Wednesday, as we see highs in the mid 70s Monday, and near 80
Tuesday and Wednesday. A downtrend will take place with the
northwesterly flow late in the week, as highs around 70-75 are
expected Thursday, around 65 Friday, and slowly trending into the
upper 60s-lower 70s next weekend. Lows could dip into the 40s Friday
into next weekend.

37

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1239 PM CDT Sun May 5 2024

SCT-BKN cloud cover over central IL this afternoon should
gradually lift and decrease in coverage this afternoon with
diurnal heating and increase in upper level cloud cover,
respectively. Have included a couple hours of at least tempo for
MVFR cigs due to this cloud cover for all sites SE of KPIA.
Overnight, lowering cigs should take place as a weather system
moves by to the south, and have included MVFR cigs after 14Z along
the I-72 corridor as a result, but will need to keep an eye on
trends for whether cigs this low develop. Winds NE 5-10 kts
initially, gradually veering to SE by around 14Z.

37

&&

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

$$