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FXUS63 KILX 191938 AFDILX

Area Forecast Discussion National Weather Service Lincoln IL 238 PM CDT Sun May 19 2024

KEY MESSAGES

- Temperatures remain above normal through Tuesday, with highs in the upper 80s or low 90s. Peak heat indices will be in the low 90s each day.

- Scattered thunderstorms, mainly north of I-72 (20-30% chance), through this evening. Severe weather is unlikely but funnel clouds or a brief landspout can't be ruled out. Slow storm motions could lead to locally heavy rainfall exceeding 1.5".

- Severe thunderstorms are possible Monday through Wednesday, although the areas at risk vary each day. The greatest risk is the potential for damaging winds northwest of the IL River late Tuesday evening, as well as south of I-70 on Wednesday afternoon.



SHORT TERM
(Through Late Monday Afternoon)
Issued at 234 PM CDT Sun May 19 2024

As of 2pm/19z, the diffuse frontal boundary that's been draped across the area today does seem to have lifted back to the north, with NE winds at Pontiac and Peoria, but a southerly component to the wind at Lincoln/Bloomington/Champaign. To this point, Cu development had been fairly limited, but day-cloud phase satellite imagery shows sporadic signs of glaciation occurring in Vermilion and Menard County. We'll likely see increasing vertical development eventually leading to deep convective initiation over the next few hours, which is consistent with what the CAMs have shown. The latest RAP mesoanalysis is still showing a corridor near that frontal boundary where non-supercell tornado parameter values range from 0.5 to 1, so a funnel or isolated landspout could occur as cells deepen this afternoon. With weak shear, storms should be relatively short- lived, but they also won't be moving fast and if an updraft manages to persist longer than expected locally heavy rainfall could occur (isolated totals over 1.5" possible). These storms should wane after sunset.

A shortwave is progged to lift NE from the Plains towards the upper Midwest early Mon AM, which could produce a few showers/storms NW of the IL River before midday. The forecast gets a little more complicated into the afternoon. Zooming out and stepping back for a moment, explosive thunderstorm development is expected over KS this evening. This matters for us because CAMs depict a remnant MCV lifting NE thru IA toward N IL, which could provide additional forcing for storms Mon PM, even though the primary upper wave will be positioned well north of the area by that time. The environment does look marginally supportive of severe weather, with moderate instability and weak shear (less than 30 knots). Forecast soundings show an inverted-V profile below the cloud deck, with over 1000 J/kg of DCAPE. The weak shear means storms will likely lack organization, but that thermo profile could support sporadic hail or strong wind gusts. As is often the case with MCVs, there is still a good deal of uncertainty as to where it ultimately tracks, how expansive the associated cirrus shield will be, and what impacts that subsequently has on the destabilization. For now, most of the CWA north of I-70 is in a marginal risk (level 1 of 5) for severe weather, but far northern portions of the CWA (Marshall and northern Woodford/McLean Counties) are in a slight risk (level 2 of 5).

Erwin

LONG TERM
(Monday Night through Next Sunday)
Issued at 234 PM CDT Sun May 19 2024

Severe t-storm potential exists across portions of the ILX CWA both Tues evening (level 3 of 5 risk west of IL River) and Wed (best chance south of I-70), driven by a volatile environment ahead of an advancing cold front. Unfortunately, we're at a time of year where the mesoscale details become quite murky more than 24 hours in advance, owing to multiple rounds of convection progressing across the Plains and Midwest between now and Wednesday.

The current expectation is that an emerging upper level trough will result in a deepening sfc low across the west-central Plains by late Mon, which eventually lifts northeast to the upper Midwest on Tues. WAA could result in a few elevated (not surface- based) storms Mon night into early Tues AM. Forecast soundings don't look particularly impressive, but there could be a low-end hail threat from these storms. For Tues afternoon, forecast soundings show strong capping across the entire ILX CWA, owing to a +18 to +20 degC warm nose. Storms are expected to form along a cold front across IA Tues afternoon and eventually progress SE into the CWA, but they likely won't move in until after 7pm Tues/00z Wed. Given the expected storm mode (linear or quasi- linear), damaging winds remains the top concern, but all hazards are possible. Forecast hodographs do become elongated in the low- level as the LLJ kicks in, with nearly 50 knot winds just 1km AGL Tues eve, resulting in roughly 30 knots of 0-1km shear and around 200 J/kg of 0-1 SRH. 0-3 km shear values are around 40 knots, which gives some wiggle room for embedded surges within the line to hit 30 knots of line-normal 0-3 km shear. All of those aforementioned low-level shear/helicity values suggest a tornado threat will exist in addition to the damaging wind potential.
Exactly how long, and how far east, these threats persist into the night remains uncertain, and this is well reflected in the Day 3 convective outlook which gradually lowers the risk category with eastward extent.

Wednesday's severe threat will still ultimately depend on where the front/outflow from Tuesday night's storms set up. Some guidance has been trending slower/further north with the frontal placement, so while the most likely area for severe weather is still south of I- 70, the I-72 corridor could wind up being at risk as well. Between the mesoscale uncertainty and the northward trends, was not as comfortable reducing the northwestward extent of PoPs compared to previous forecast cycles. Ahead of the front, the environment is expected to remain quite volatile with ample instability (approaching 2500 J/kg). Despite the primary upper wave associated with Tuesday's storms lifting well off to the north, a secondary short wave will place a new strong mid-level jet streak over the region (60+ knot flow at 500mb), resulting in 40-50 knots of deep layer shear across the warm sector. Shear profiles continue to look rather unidirectional, and deep layer shear vectors parallel to the front suggest a linear storm mode with damaging winds as the primary threats, while hail is also a concern.

Temperatures cool off a bit behind the front, with highs in the upper 70s/low 80s for the end of the work week. The upper wave pattern remains quite progressive into next weekend, so the extended forecast continues to be littered with precip chances - but model spread in the timing and placement of any specific disturbance is quite large. The CPC 6-10 day precip outlook (May 25-29) shows a 40- 50% chance of above normal precip.

Erwin

AVIATION
(For the 18z TAFs through 18z Monday Afternoon)
Issued at 1202 PM CDT Sun May 19 2024

A weak, stalled front is draped east-west across the region, and continues to make the wind direction forecast tricky through the afternoon. There is also a 20-30% chance for showers/storms this afternoon. The addition of a PROB30 for -TSRA was considered, mainly for the 20-23z time frame, but with storms expected to be widely scattered, opted to stick with just VCSH. Winds will shift to SE overnight, before shifting back to SW after sunrise Mon. VFR conditions should prevail through the period.

Erwin

ILX WATCHES/WARNINGS/ADVISORIES
None.

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