426
FXUS63 KILX 041741
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1241 PM 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.

&&

.UPDATE...
Issued at 1018 AM CDT Sat May 4 2024

Have opted to increase PoPs and speed-up the time of arrival of
afternoon convection based on latest 12z hi-res trends. Up until
this point, two limiting factors for afternoon convective
coverage has been the idea that deep-layer shear is somewhat
underwhelming (30 kts or less), and that synoptic forcing atop the
cold front is somewhat nebulous. If true, then updrafts may
struggle to get organized or stay organized ahead of the cold
front.

However, we have some concern about a decaying MCV currently
lifting across north central Missouri at this hour.  MCVs
notoriously augment forcing and vertical wind profiles, and it
bears watching over the next couple hours. If this feature can
hold together as it approaches the mid-Mississippi Valley, then
convective coverage could become greater than previously
anticipated.

A quick glance at the RAP-derived parameter space suggests the
mesoscale environment this afternoon will be sufficiently
unstable as steepening 0-3 km lapse rates and good solar
insolation work to erode convection inhibition. SBCAPE values are
now progged to be 2000-2500 J/kg ahead of the cold front; which is
a notable increase from yesterday`s modeled instability.

This all adds up to increased confidence in severe weather
coverage this afternoon and early evening in areas west of I-55.
The primary severe weather hazards are still hail and wind, and
recent HRRR guidance suggests that the latter could be the more
formidable feature as cold pools strengthen ahead of the front.

The expectation is still for severe convection to fade after
sunset, as the lack of an appreciable LLJ will make it difficult
for storms to maintain their punch.

MJA

&&

.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 18z TAFs through 18z Sunday Afternoon)
Issued at 1241 PM CDT Sat May 4 2024

There is a high chance (>80%) a line of storms will work across the
area later this afternoon, impacting TAF sites from west to east for
a few hours through early evening. South-southwest winds will swing
around to the northwest behind the storms as a cold front sweeps
through. Winds could be gusty with storms, but otherwise should
range from 10-15 kts through tonight. MVFR ceilings become common
late tonight and stick around at most airfields through at least
late Sunday morning.

NMA

&&

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

$$