884
FXUS63 KILX 042314
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
614 PM CDT Sat May 4 2024

.KEY MESSAGES...

- Scattered thunderstorms will develop this afternoon west of
Interstate 57, mainly between 3pm and 9pm.  The potential severe
weather hazards include hail to the size of ping pong balls and
damaging wind gusts up to 60 mph.

- Confidence is medium that hazardous weather will occur both
Tuesday and Wednesday as a pair of disturbances track across the
region, bringing periods of thunderstorms.

&&

.DISCUSSION...
Issued at 141 PM CDT Sat May 4 2024

*********************************************************************
 Today`s Severe Weather Potential:
*********************************************************************

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 struggle to stay organized -- thus
limiting the severe risk.

However, we have recalibrate our expectations due to a decaying
MCV currently lifting across northeast Missouri. 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 crosses the mid-Mississippi Valley, then convective coverage
should 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.

Any attendant flash flood risk with storm activity looks low.  The 3-
hr flash flood guidance in areas west of I-55 is currently between
1.5" - 2.0".  When overlaying the HREF 3-hr PMM QPF, this same area
only achieves 0.5" - 1.0".  It`s at least worth noting that recent
runs of the HRRR offer a couple localized QPF pockets of 1.75", but
any flash flooding that occurs would be the exception and not the
rule.

Our 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.

*********************************************************************
 Tuesday-Wednesday Severe Weather Potential:
*********************************************************************

A seasonable and otherwise low-drama forecast has been drawn up for
Sunday and Monday, but concern quickly focuses on the convective
potential unfolding during the Tuesday-Wednesday timeframe.  Global
deterministic guidance continues to resolve a synoptic pattern
favorable for a multi-day severe weather outbreak, and this signal
is reflected in other analogs (CIPS) and machine learning tools (CSU
MLP Severe).

In a sentence or two, we`re monitoring the evolution of what looks
to be a deepening upper-level low positioned over the Northern
Plains by Monday night, with a strong jetcore nosing into the Mid-
Mississippi Valley above multiple shortwaves traversing meridional
flow.  This combination of strong forcing, strong kinematics, and
favorable thermodynamics evolving in deep southwest flow could lead
to widespread severe weather within a broadening warm sector.

Our confidence at this time is only medium, as we cannot possibly
imagine how certain mesoscale processes (convective feedback,
debris, outflow boundaries, etc) evolve in time and space. But,
the synoptic setup currently being modeled for Tuesday-Wednesday
is concerning.

When leveraging our 100-member ensemble comprised of the GEFS, EPS,
and CMCE to assess a favorable parameter space for supercell
development (SBCAPE > 1000 J/kg, SBCINH > -75 J/kg, and bulk shear >
40 kts), we see a 50-70% chance of achieving these conditions on
Tuesday in areas along and south of I-72.  When looking toward
Wednesday, we see probabilities of 50-60% in areas south of I-72.
And so while the emphasis appears to currently favor the southern
half of our CWA on both days, our confidence is only medium.  The
severe weather outlook will undoubtedly change in coming days, so do
check back.

MJA

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 613 PM CDT Sat May 4 2024

A cold front is pushing across central Illinois this evening with
a line of showers and thunderstorms along the front. Precip
should only last around an hour before moving out. Winds will turn
NW behind the front and continue to veer to N then NE overnight
into Sunday. A period of MVFR ceilings is expected along and east
of I-55 overnight through around midday Sunday. Clouds should
scatter back to VFR midday with VFR conditions prevailing the
remainder of the period.

Deubelbeiss

&&

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

$$