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FXUS63 KILX 180630 AFDILX

Area Forecast Discussion National Weather Service Lincoln IL 130 AM CDT Sat May 18 2024

KEY MESSAGES

- Heat indices in the upper 80s to low 90s are forecast each day Sunday through Tuesday. This will pose at least a small risk for heat-related illness in vulnerable populations.

- There is a 20-30% chance for thunderstorms Sunday afternoon and evening. While the threat for severe weather is low, there is a nonzero chance for localized strong wind gusts and hail with any storm that develops.

- Thunderstorm chances increase early next work week, peaking at 70-80% Tuesday night and diminishing Wednesday. Some of these storms could be severe, especially northwest of I-57 Tuesday evening and night.

DISCUSSION
Issued at 130 AM CDT Sat May 18 2024

At 1:30am, long-wave IR satellite imagery shows the last wisp of convective debris clouds from thunderstorms that quickly dissolved near the I-72 corridor within the past couple hours. Nighttime microphysics suggests a lack of any other clouds, which will enhance radiational losses through daybreak, and, together with light to calm winds, may result in patchy fog. The CAMs are not as aggressive nor extensive in their depiction of diminished visibilities this morning, but both the HRRR and HiResARW have been highlighting an area along and south of a roughly Jacksonville to Danville line with transient sub- quarter mile visibility from 5-8am. We'll monitor surface observations to determine whether a DFA could become necessary before insolation "burns off" whatever fog develops, though at this juncture that's not looking particularly likely given the brevity and rather low (20-35%) probabilities for sub half mile visibilities.

A ridge of high pressure will continue to build into the region today to maintain dry conditions across central and southeast IL, although a decaying cold front accompanied by thunderstorms upstream (these will develop late afternoon) will attempt to intrude tonight, offering a 10-15% chance for a shower northwest of Peoria. Afternoon highs are forecast to reach the mid 80s as deep mixing transports drier mid-level air to the surface bringing dewpoints down into the low 50s. The second half of the weekend is tricker to forecast, however, as a piece of upper level energy lifts out of the Plains and attempts to flatten the ridge over IL. Given the remnant boundary from tonight will be somewhere in or near central Illinois, we might just have a trigger for afternoon storms which could significantly lower high temps Sunday. However, if storms either (1) don't develop or (2)
develop after peak heating, deep mixing during the afternoon could once again reduce low level moisture and aid temps in warming to around 90 degF, so we'd offer a reminder to those planning strenuous outdoor activities to stay hydrated and take breaks as needed in the shade or AC. Soundings would suggest a classic "inverted V" thermodynamic profile with 1500-3000 J/kg of SBCAPE (highest west of I-55) per the HRRR and NBM mean, suggesting a favorable atmosphere for at least isolated severe wind and hail with any pulse storms that fire tomorrow. However, it's distinctly possible the boundary from Saturday night is completely gone by tomorrow afternoon and/or the incoming wave is too far northwest of our area to initiate any storms and we consequently remain dry.
We'll have to wait and see.

Unfortunately, convective potential (and thus the dependent high temp forecast) becomes even less clear on Monday, as it will depend on any residual boundary (or lack thereof) from tomorrow's storms.
However, NBM suggests chances for measurable precip are highest west, at 60-75% beyond the IL River tapering to less than 25% east of the I-57 corridor. A low level temp inversion along and north of a warm front which will be lifting into our west Monday evening/night should limit convective potential during the day when shear is weak (0-6km generally less than 20 kt), but as the LLJ strengthens toward Tuesday morning we could have a couple storms drift into our area from the west, posing some risk for locally severe hail. Models' depiction of this system continue to slow and shift everything west, so most of the day Tuesday could very well wind up dry, but there's still quite a bit of model spread in the evolution of the surface cyclone. Nonetheless, deterministic models continue to advertise an unseasonably strong and quickly deepening low lifting out of the Plains and into the Upper Great Lakes late Tuesday into Wednesday, leading to increasing shear and moisture fostering a primed environment for severe thunderstorms, especially west of I-55 where the NBM suggests 20-40% chances for more than 3000 J/kg of SBCAPE and the deterministic GFS depicts 25-30+ kt of 0-1km bulk shear Tuesday evening. Storm mode is a bit difficult to pin down at this time range, but given the warm front will be well to our north and surface low to our west/northwest, we could wind up with some sort of broken line comprised of supercell-like structures capable of all hazards (again, highest risk west of I-55) Tuesday evening/night...although something more isolated could always fire along an outflow boundary during the afternoon further east.

Depending on cloud cover, Tuesday could be the hottest day in the period, but it will more likely be the most humid day as dewpoints climb into the 70s and heat indices consequently reach the low (to possibly mid) 90s. Fortunately, the same southerly winds advecting that rich low level moisture will also offset the oppressiveness of the early season warmth as gusts during the afternoon frequent the 20-25 mph range. Nevertheless, because Sunday through Tuesday will be the warmest conditions of the season (in other words, since early last autumn) so far and hence folks may not be quite acclimated to it just yet, it will pose some risk for heat-related illness in vulnerable populations.

Wednesday through Friday should be a bit cooler in the wake of the front's passage Tuesday night or early Wednesday, with Thursday and Friday highs forecast to hit the mid to upper 70s in a drier airmass (dewpoints in the 50s). NBM's 15-30% precip chances lingering through the end of the forecast period may be a bit overdone both temporally and spatially, but given a stationary boundary won't be particularly far to our southwest during this time period it's tough to justify lowering/removing PoPs; just be aware it's highly unlikely to be a total washout during the late Wednesday through Friday period.

Bumgardner

AVIATION
(For the 06z TAFs through 06z Saturday Night)
Issued at 1142 PM CDT Fri May 17 2024

Dissipating convection is keeping scattered mid clouds at BMI, DEC, and CMI through the night, but these clouds should dissipate later tonight. With the precip that was had, some areas could see fog development. DEC and CMI are the main sites that could see fog, so have kept a TEMPO group for a few hours in the morning at both sites with 3sm at DEC and 2sm at CMI. A little bit of CU should also develop in the morning with the moisture available, but only going with FEW to SCT at BMI, SPI, DEC, and CMI. This will dissipate in the late afternoon, but high cirrus will arrive for tomorrow evening. Winds will be light and variable overnight and then become southerly at 6-7kts during the daytime. Tomorrow evening wind speeds will decrease, but wind direction may be more southwest.

Auten

ILX WATCHES/WARNINGS/ADVISORIES
None.

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