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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.

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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.

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ILX WATCHES/WARNINGS/ADVISORIES
None.

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