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FXUS63 KLSX 260429 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1129 PM CDT Thu Apr 25 2024

KEY MESSAGES

- The central United States is on the verge of a multi-day severe weather event beginning tonight. The bi-state region will be on the eastern edge of this threat initially, with glancing threats tonight and late Saturday night, before the threat shifts east on Sunday.

- Sunday evening and night stand out as our highest threat for severe thunderstorms ahead of a cold front. Large hail, damaging wind, and tornadoes will all be a concern. Heavy rain may also result in localized flash flooding.



SHORT TERM
(Through Late Friday Night)
Issued at 247 PM CDT Thu Apr 25 2024

High clouds continue to stream into the region from ongoing convection along and ahead of a warm front, currently stalled from the central Great Plains down through the southern Ozark Mountains. The majority of the forecast area is dry thanks to a stalled surface high centered over the Great Lakes promoting cool and dry east winds. Overhead, a ridge is building over the central CONUS with a trough digging in the Intermountain West that will be the focus for our active weather through the weekend. The warm front is forecast to lift north tonight as a low-level jet intensifies, which will continue to promote elevated convection overnight into Friday morning. With meager forcing and instability in place, these thunderstorms will very likely (80%+ chance)
possess an isolated threat for marginally-severe hail. As the LLJ weakens after sunrise, the intensity for these thunderstorms will drop even further as the convection spreads north and east.
Depending on the speed of the warm front and these thunderstorms, some areas have a low (20% chance) threat of heavy rain and localized flooding. I'm not confident that any one area is under a greater threat given the widespread nature of the thunderstorms, however with highly- anomalous (above the 99th climatological percentile) precipitable water values in place and relatively-slow motion, heavy rain will be closely monitored.

The greater impacts from this convection will be its influence on available instability for the afternoon and evening, when a shortwave glances the Mid-Mississippi Valley to its northwest. While forcing is marginally more impressive, and deep-layer shear increases a bit, available instability will be in question if morning thunderstorms linger into the early/mid-afternoon. There's also no discernible surface or low-level lifting mechanism outside of a possible remnant outflow boundary from the earlier convection. All of this is to say the threat for any thunderstorms, much less severe thunderstorms, is highly conditional on favorable instability and forcing existing at the same time. The best threat for this now seems to be further south, closer to the I-70 corridor and south, but that area is also more divorced from the best mid/upper-level ascent. With better kinematics to our north and west, this round of severe weather will largely stay confined to western Missouri and the central Great Plains. Regardless, if a severe thunderstorm can develop in our area, it will threaten primarily large hail and damaging wind.
If the boundary on which convection develops is still somewhat defined, I can't rule out a brief tornado as well.

MRB

LONG TERM
(Saturday through Next Thursday)
Issued at 247 PM CDT Thu Apr 25 2024

Any lingering convection gradually diminishes overnight, though with the entire region within an open warm sector and the warm front to our north, instability will be more appreciable by the hour as we enter Saturday. Instability increases markedly during the day under strong warm air advection within a moisture-rich airmass, particularly across the eastern Plains and western Missouri where that 925/850mb WAA is strongest. However, some degree of ridging overhead will act to inhibit deep convection in a widespread sense during the day. It's worth noting that the amplitude of the ridge isn't as strong in the guidance as it was 24 hours ago, which gives me pause in saying Saturday will be largely dry. However, with very little forcing or lift to focus the instability, any convection that develops will be more nebulous and not very intense as a result.

The second of two amplified shortwaves approach the region Saturday evening, helping to advance the synoptic system further east. There is potential for a round of nocturnal convection to develop overnight Saturday into Sunday morning in western in northern Missouri on the nose of the low-level jet, but most indication are that the threat area will once again be confined to central/northern Missouri and west-central Illinois at the worst. This round would likely grow into some convective complex and threaten damaging wind and a few tornadoes. The threat for severe weather across the bi- state region becomes more widespread Sunday afternoon and night. Our airmass will once more destabilize, but now amidst much stronger ascent and forcing. The second mid-level wave will cross over much of the region, and while the speeds themselves aren't very strong, there is some upper-level jet streak support for synoptic ascent.
Instability is still somewhat appreciable, but not as impressive as in days past. That said, the shear and kinematic fields along with subtle pre-frontal surface convergence may be able to compensate and initiate strong to severe thunderstorms in the afternoon across western Missouri. Thunderstorms look to start discrete with strong deep-layer shear, but storm vectors will favor upscale growth at some point in the late afternoon. Exactly when is unclear, which affects the evolution of the convection and overall threats. That said, all severe hazards (and heavy rain) are realistic on Sunday.

There continues to be a gradual slowing of the cold front that would essentially end the severe weather threat compared to previous forecasts. It now looks like the front will still be either west of the region or just entering central Missouri Monday morning. While this would lead to a buildup of instability ahead of the boundary, leading to a threat for more showers and thunderstorms, the shear parameter space becomes less favorable for severe weather in the wake of the strong shortwave on Sunday. While the threat for any strong convection is low on Monday, subtle changes in the pattern may result in large differences in the potential. The front and any ascent aloft finally exits late Monday, but the boundary itself may waver north and south amidst near-zonal upper-level flow. As such, persistent 15-40% PoPs exist through the rest of the forecast period. I don't expect it to be raining that entire time, but there is very little certainty this far out regarding exactly when rain chances peak in a given location.

MRB

AVIATION
(For the 06z TAFs through 06z Friday Night)
Issued at 1115 PM CDT Thu Apr 25 2024

A broad area of MVFR ceilings will overspread the area through the overnight hours ahead of a warm front. Some IFR is possible, however ensemble guidance is showing IFR probabilities mostly below 50 percent, so confidence in IFR is low and will therefore not go lower than 1000ft in the 06Z TAFs. Winds will increase from the southeast and become gusty through the early morning.
Widely scattered showers and likely a few thunderstorms are expected to develop near the Mississippi River between 09-12Z and move northeast through the morning. Another, more widespread area of showers and thunderstorms is expected to develop over central or west central Missouri between 12-15Z, and this area will move east to affect much of the area through 18-20Z. Ceilings behind this second area of convection should rise to between 2500-4000ft.

SPECIFICS FOR KSTL:

Latest indications are that widely scattered showers will develop in the vicinity of the terminal between 10-13Z. I do not expect much thunder in this first batch of convection, although it's not a zero chance at Lambert. The second batch of convection looks likely to affect the terminal between 18Z-21Z. This looks more organized and should have more coverage as it moves in from the west.

Carney

LSX WATCHES/WARNINGS/ADVISORIES
MO...None.
IL...None.

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