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FXUS63 KILX 211107 AFDILX

Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 607 AM CDT Tue May 21 2024

KEY MESSAGES

- There is a level 4 of 5 (moderate) risk for severe weather this evening west of the Illinois River, with gradually lower chances further east. While all severe hazards are in play, the greatest risks will be damaging wind gusts and brief tornadoes.
A level 1 of 5 (marginal) risk will linger into tomorrow evening south of I-70.

- There is a 70% chance heat indices surpass 90 this afternoon, posing a risk for heat-related illness in vulnerable populations.

- Strong south winds this afternoon are expected to frequently gust to 35 mph or higher, while there is a 15% chance gusts reach 45 mph. This may cause difficult driving conditions for those traveling on west-east oriented roads.

DISCUSSION
Issued at 138 AM CDT Tue May 21 2024

Current radar imagery shows a few decaying thunderstorms across northwest Indiana, with more sustained and, at times, severe convection rippling across Nebraska and western/central Iowa on the nose of a 45-55 kt LLJ. Ultimately, outflow boundaries from last night's storms will linger across portions of Iowa and northern Illinois this morning into early afternoon, offering instability gradients and locally enhanced low level environmental shear to govern storm evolution.

From a big picture perspective though, surface cyclogenesis will be fostered in the left exit region of a deepening upper trough taking on a negative tilt as it lifts across the central/northern Plains this afternoon. Strong warm advection ahead of that deepening low will thus gradually overspread the Prairie State today, a warm nose at 700mb leading to a rather strong capping inversion giving our forecast soundings the classic "loaded gun" look. This should offset mixing of the increasing winds aloft, but given (1) a lack of cloud cover (we'll likely have some altocumulus constellanus and/or cumulus, but not the typical WAA stratus shield if you believe HRRR), (2) the height of the inversion around 850mb, and (3) steep lapse rates in the mixed layer, I think we'll be quite breezy this afternoon. GFS and EC lamp guidance bring sustained speeds to around 25 kt (28 mph) at our ASOS sites this afternoon, while the EPS mean brings gusts to around 40 mph and HRRR suggests 40-70% chances for gusts surpassing 40 mph. Thinking is that if any site manages to hit advisory criteria (30 mph sustained or 45 mph gusts) it would be brief, and NBM probs are only around 10-15% anyway, but it'll be something to keep one eye on given the set up looks similar to (though perhaps slightly weaker than) March 3rd, April 16th, and April 28th when south winds flirted with criteria given overachieving afternoon mixing. At any rate, for many (if not most), these winds will pale in comparison to those brought by thunderstorms during the late evening/overnight hours. In addition, it'll be beneficial in one sense - it will make the summerlike warmth (high temps and dewpoints approaching 90 and 70, respectively) a bit less oppressive.

As the surface low draws nearer during the evening, temps aloft will cool leading to steepening 500-700mb lapse rates conducive to strong thunderstorm updrafts. In addition, the kinematic profile will become increasingly supportive of sustained convection, 0-3km shear values nearing 40 kt - highest west of I-55. While the greatest shear will be well to our north across Iowa, northern Illinois, and Wisconsin, our area should still have enough for sustained storms...though storm mode might be semidiscrete (vs a more solid line further north), particularly as the LLJ strengthens after 11pm or so. This means that all severe hazards will be possible, particularly with any isolated storm that develops as the line starts to break up, which most models suggest will happen between 10pm and midnight; however, the most widespread hazard still appears to be from straight-line winds, with a QLCS tornado or two possible given the aforementioned 0-3km bulk shear values are over 30 kt.
Timing of storms looks similar between CAMs, with the line entering our west around 7pm and storms becoming more scattered if and when (guidance is having this happen around 11pm..ish) it breaks up, after which time isolated wind and hail will pose a continued risk.
The straight-line wind (and QLCS tornado) risk will be highest across our north and west counties where shear is strongest due to closer proximity to the surface low. And some of these winds could be significant, as implied by HRRR's ensemble max which has small blotches of 80 mph or higher gusts, most of which are near or west of the IL River where SPC has a level 4 of 5 (moderate)
risk for severe weather driven by the wind hazard. This event could be classified as a derecho if enough significant (75+mph)
gusts are measured and its path is sufficiently long, which appears distinctly possible at this point.

Now, one thing we'll be keeping an eye on this afternoon is where outflow boundaries from the upstream early morning storms linger.
The stiff southerly winds in the boundary layer should help rid our area of such boundaries, but a couple CAMs have been hinting one stalls right on the edge of our northwest CWA, across/near Knox County, and that storms fire along it ahead of the main line during the 3-6pm timeframe. The 18z WRF-ARW had this, and of the hires models it handled convection the best yesterday; it's 00z iteration, however, which again matches radar the best of the HRRR ensemble, does not...So needless to say our confidence is low in this scenario, though if it did materialize storm mode would be supercellular and hence all severe hazards would be expected.

The upper low will lift well to the northwest as the cold front stalls near or just south of I-70 tomorrow, and as another mid level wave lifts northeast along/ahead of it additional storms are forecast to fire there. SPC maintains a level 2 of 5, slight, risk for severe weather in that area for wind and hail. Everywhere else, temps will fall to near/below normal as cold advection overspreads the area. Thereafter, guidance is murky on the location of a quasistationary boundary which will linger just to our southwest, lending to low confidence in the timing and placement of MCS activity riding along it amidst WNW flow. Anytime between Friday and Sunday looks conducive to episodes of severe weather across the Midwest, but whether we'll be close enough to the instability gradient to get in on any one/more of those MCSs remains yet to be seen. Confidence is also low in temps throughout the upcoming weekend, with NBM's 10th-90th percentile maxT range extending from 69 to 90 degF at the Decatur airport (arbitrarily-chosen point near the middle of the CWA) by Sunday. The lower and upper bounds of the 10-90th percentile range gives a reasonable low end and high end value, capturing the true high temp 80% of the time; in this case, this suggests there is a 90% chance high temps in Decatur surpass 69 degF on Sunday, and 10% chance they exceed 90 degF.

Bumgardner

AVIATION
(For the 12z TAFs through 12z Wednesday Morning)
Issued at 536 AM CDT Tue May 21 2024

South winds under 10 kt will increase sharply this morning, becoming sustained over 20 kt with frequent gusts near 30 kt by mid afternoon. A broken line of thunderstorms will bring brief torrential rain, frequent lightning, and widespread gusty winds which may locally exceed 50 kt (greatest chance at KPIA) between 7pm and 1am Tuesday night-Wednesday morning (00-06z Wednesday).
Behind that line of storms, clouds should become scattered to few and winds ease.

Bumgardner

ILX WATCHES/WARNINGS/ADVISORIES
None.

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