942
FXUS63 KILX 040811
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
311 AM CDT Sat May 4 2024

.KEY MESSAGES...

- Level 1 (Marginal) Risk of severe storms from late Saturday
afternoon into evening for all of central and southeast IL.

- Periodic chances of showers and storms Monday through Friday,
  with severe storm potential elevated on Tuesday and Wednesday
  during the late afternoon into evening each day.

&&

.DISCUSSION...
Issued at 310 AM CDT Sat May 4 2024

The latest IR satellite images show Illinois in the coll zone
between two systems, with high pressure providing mostly clear
skies and light winds across the CWA. Ongoing storms across
eastern Nebraska and Kansas will continue to make slow progress
toward the mid-Mississippi River Valley as a cold front approaches.
Low level flow will increase ahead of that feature from the
southwest, boosting low level dewpoints into the low to mid 60s
this afternoon. That will support instability parameters climbing
into the 1000-1500 J/kg MLCAPE range ahead of the front, despite
thickening anvil cirrus and mid clouds this afternoon. Marginal
shear values around 30kts will be a limiting factor in severe
storm potential, as a broken line of storms progresses into
central Illinois between 3 pm and 5 pm. Chances for any rainfall
will peak at 60-70% west of I-55, and remain in the chance
category east of I-55. A marginal threat for large hail and
damaging wind gusts will be present 22z/5pm through around
01z-02z before diurnal stabilization limits updrafts. By the time
the line reaches I-57 the lightning threat will be
diminishing/ending.

The next period of concern will start on Tuesday as a stationary
front across southern Illinois lifts north as a warm front in
response to surface cyclogenesis in the Plains. Warm sector shear
and instability parameter space will be more favorable for severe
weather, as 0-6km shear climbs over 60kts along with MUCAPE
climbing toward 2500 J/kg under the support of 65+F deg SFC
dewpoints. All severe hazards will be in play as supercells
develop during the evening across central Illinois. The upper
trough will evolve eastward on Wednesday, dragging a cold front
across Illinois during the late afternoon and evening time frame.
There are some differences in the model guidance timing of the
trough evolution, but Grand Ensemble guidance is pointing toward
40-50% chance for bulk shear of at least 50kts and CAPE of at
least 2000 J/kg both Tuesday and Wednesday for areas south of
I-72. The best shear is along an axis northwest of the best
instability, but enough overlap exists for a wave of severe storms
each late afternoon and evening.

Additional storm chances will linger through Friday, but at a much
lower coverage and intensity per long range guidance. Total
rainfall over the next 7 days looks to climb into the 1 to 2 inch
range, especially south of I-72, where Grand Ensemble probs show
50-60% chance of over 2 inches. Some rivers may climb back toward
minor flood stage due to saturated soils allowing for higher
runoff into streams.

Shimon

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1216 AM CDT Sat May 4 2024

VFR conditions will prevail in the near term through mid afternoon
on Saturday, under surface high pressure. A non-zero chance of light
fog at our eastern TAF sites of DEC/CMI, but better chances of
MVFR/IFR fog should remain south of I-70. A weakening line of
thunderstorms is projected to impact (70% chance) the western TAF
sites (PIA/SPI/BMI/DEC) during the late afternoon through mid-
evening time frame, with primarily a 2-3 hour window of thunder.
More isolated thunder chances (20%) at CMI.

Shimon

&&

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

$$