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FXUS63 KILX 191703 AFDILX

Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1203 PM CDT Sun May 19 2024

KEY MESSAGES

- Heat indices in the low 90s are forecast today and again Tuesday. This will pose at least a small risk for heat-related illness in vulnerable populations.

- There is a 20% chance for thunderstorms near and north of I-72 between 4 and 9 pm this afternoon and evening. While most of the area will remain dry, any of these thunderstorms could generate localized strong wind gusts and heavy rainfall surpassing 1.5 inches.

- Thunderstorm chances increase Monday into Tuesday, peaking around 80% Tuesday night and diminishing late Wednesday. Any of these storms could produce severe weather, though the greatest risk will be from damaging winds Tuesday night west of the IL River.

UPDATE
Issued at 1008 AM CDT Sun May 19 2024

At 920am/1420z, a weak, stalled front was positioned roughly near the I-72 corridor (NE winds at Quincy, SW winds at Springfield/Decatur/Champaign). This front may nudge northward during the day. The upper level support remain unfavorable for precip development, with a ridge axis draped across IL and modest height rises anticipated today. However, forecast soundings continue to show moderate instability (~2000 J/kg) developing by the afternoon, with little in the way of capping, and CAMs suggest that forcing near this weak front will be sufficient to kick off scattered storms this afternoon. Added a corridor of 30% PoPs roughly from Macomb to Champaign, rather than just a broad 20% chance north of I-72, because the CAMs have had a consistent signal for development this afternoon over the last several model cycles.

The environment near the front will feature rich sfc-based vorticity and steep low-level lapse rates, which are favorable ingredients for funnels or perhaps even an isolated, brief landspout tornado. The RAP shows a corridor where the non-supercell tornado parameter values are 0.5 to 1, and this corridor coincides with where we expect storm development this afternoon. Rotating features below the cloud base would be most likely to occur while cells are initially deepening. Once rain reaches the sfc and they become outflow dominant, the threat will subside.

Erwin

DISCUSSION
Issued at 150 AM CDT Sun May 19 2024

At 145am, nighttime microphysics satellite imagery shows a narrow band of broken stratocumulus around 5 to 8 kft - per sfc obs - is drifting slowly southeast near the IL River along a dissolving cold front, the remains of which will stall across central Illinois through this afternoon. The HRRR continues to suggest sfc temps, currently in the mid to upper 60s, will rapidly warm toward 90 degF this afternoon in the vicinity of that boundary as deep mixing (NBM mean for mixing heights is nearly 7 kft) dries the low levels a bit - resulting in a classic inverted V-type forecast sounding with 2000-2500 J/kg SBCAPE, per HRRR mean. If storms fire along that boundary, as virtually every CAM in the HRRR suggests, they'll be short-lived given the weakly sheared environment (HRRR's ensemble 90th percentile is generally less than 20 kt 0-6km shear south of I-74, while the mean is closer to 10 kt). However, since lapse rates in the mixed layer are dry adiabatic in forecast soundings, storms will pose a risk for gusty wind and torrential rain as they collapse, and new storms forming along their outflow boundaries could move over the same areas increasing the risk for localized hydrological issues. At this time, the greatest chance for thunderstorms this afternoon is between 4 and 9 pm along and just north of I-72, with high (80-90%) confidence in dry conditions area-wide prior to 2pm. Heat indices today should remain near maximum sfc temps since dewpoints will fall slightly with afternoon mixing, however because this afternoon will be the warmest we've seen since autumn it may catch some folks off guard and pose a risk for heat- related illness to vulnerable populations, including those participating in strenuous outdoor activities.

Tonight, the LLJ will set up to our west where scattered storms - some possibly generating severe hail - will percolate mainly across Iowa and northern Missouri, although a storm or two could approach the western fringe of our area; we'll keep one eye on this.
Tomorrow, whether and where storms form will be a function of where the outflow boundary from tonight's activity lands and whether it gets turned over by a combination of mixing and increasing low level southerly winds which, at the surface, will gust 20+ mph at times.
Anywhere from 15 to 50% - the greatest percentage along the I-74 corridor, nearer the warm front - of LREF membership produces measurable rain across our area during the afternoon and/or evening tomorrow. Since we'll have at least a little shear (20-30 kt, per HRRR mean) to work with given the proximity of the shortwave trough lifting into northern IL/WI during the afternoon, any of these storms could turn severe posing mainly a hail risk though possibly also an isolated damaging wind gust (given steep low level lapse rates once again). Highs tomorrow are tricky given potential for convection and convective debris clouds offsetting insolation, and I would not be surprised if we underachieve a bit in spots; we're still forecasting mid to upper 80s, though some locations could very well struggle to surpass 80 degF.

The warmth is expected to return, however, for Tuesday as moisture advection strengthens to bring dewpoints into the upper 60s to low 70s, though winds gusting to 25 to 30 mph during the afternoon should make it feel a bit less oppressive compared to today. While there's a small (10%) chance something fires in the warm sector, a strong capping inversion should keep us dry during the daylit hours.
Model guidance continues to trend slower and farther west with this system, and it makes one wonder how significant the severe threat will be for us after all. At this point, the greatest risk appears to be along and west of the IL River where the latest iteration of the ECMWF brings the leading edge of the line at around 10pm. Nonetheless, it maintains rigorous lightning activity with those storms all the way to I-57 (ETA 1am Wednesday), which suggests the low level jet could sustain storms well into the night with all hazards (though straight-line winds the most common) still in play given strong forcing along the cold front, 40-50 kt deep layer shear, and steep mid level lapse rates from the EML. And it's worth mentioning the GFS is several hours faster, bringing the line to I-55 around 7pm Tuesday, suggesting the entire area would see storms and many areas west of I-57 would see severe weather. The ECMWF has been more consistent in its depiction of this entire system for the past 5-6 days, so I'm inclined to lean in that direction but in any case severe weather is a concern area-wide with the greatest confidence west of the IL River.

Fortunately, the surface low will be rapidly occluding as it lifts well to our north on Wednesday, causing shear along and ahead of the cold front to dramatically weaken. Nonetheless, we don't want to quite rule out a couple feisty storms in our far (south)eastern counties (mainly southeast of I-70) late Wednesday morning-mid afternoon if the cold front should tarry long enough there. As strong cool advection overspreads the region, it'll feel chilly on Wednesday with northwest winds gusting 25-35 mph - highest along and northeast of the I-74 corridor - and temps only warming into the low to mid 70s for highs during the afternoon. We hang on to 20-35% PoPs Thursday into Friday, though the boundary Thursday will be far enough to our south that most, if not all, of the area should stay dry, and with a little sunshine we could make a run for 80 degF. Friday's a bit more uncertain as models and their respective ensembles struggle to handle the evolution of the next system which 50-70% of NBM membership suggests will bring measurable rain to the region by Saturday.

Bumgardner

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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