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FXUS63 KILX 050328 AFDILX

Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1028 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.

UPDATE
Issued at 807 PM CDT Sat May 4 2024

A cold front is working across central Illinois this evening with a line of showers and thunderstorms preceding the front. In addition, a gust front has surged ahead of the leading edge of storms and is kicking up some dust resulting in lowered visibility in some areas. Overall, the severe threat has diminished from earlier and will continue to trend down, but will need to continue to monitor for a stronger storm or two over the next couple hours. Gusts of 40 to 50 mph will be possible in spots with the strongest storms. Leading edge of the storms should clear the forecast area by around 1000/1030 pm with precip chances ending altogether in the following hour or two. Behind the front, breezy northwest winds are overspreading the Illinois River Valley with upstream dewpoints in the 40s expected to work across at least the Illinois River Valley through the night.

Deubelbeiss

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 HRRR 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 06z TAFs through 06z Sunday Night)
Issued at 1028 PM CDT Sat May 4 2024

A cold front has pushed across central Illinois leaving NW winds in its wake. NW winds in place overnight will veer to the NE Sunday morning and east Sunday evening. MVFR ceilings are expected to develop across portions of the area, mainly along and east of I-55 overnight into Sunday morning, but any ceilings should scatter back to VFR by around midday Sunday.

Deubelbeiss

ILX WATCHES/WARNINGS/ADVISORIES
None.

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