Sunrise 5:56 AM   Sunset 7:56 PM Moonrise 3:59 AM   Moonset 4:02 PM

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FXUS63 KLSX 041125 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 625 AM CDT Sat May 4 2024

KEY MESSAGES

- Isolated strong to severe thunderstorms will be capable of quarter- size hail and damaging wind gusts today from roughly 2-8pm.

- An active weather pattern will bring multiple rounds of severe thunderstorms to portions of the Midwest early to mid next week. Specifics on exact timing, location, and magnitude are still unclear, but confidence continues to increase that the area will experience at least a round of severe weather.



SHORT TERM
(Through Sunday Night)
Issued at 319 AM CDT Sat May4 2024

A shortwave is digging into the Central Plains per recent water vapor imagery, and is aiding in driving ongoing convection along a cold front stretched across that portion of the CONUS. Ahead of this front across the Midwest, surface high pressure is moving eastward, with low-level winds becoming increasingly southerly. However, winds have been calm enough for patchy fog, some locally dense to develop where skies have been mostly clear. This fog will dissipate with the rising sun this morning.

This morning, these southerly winds will begin advecting deeper moisture into the CWA ahead of the decaying Plains convection as it enters the area. This decaying convection is not expected to pose a severe thunderstorm threat when it enters portions of central Missouri during the late morning. This afternoon, the increasing boundary layer moisture will yield roughly 1,500-2,000 J/kg of SBCAPE per 00z deterministic guidance and the HRRR mean. If convective debris from the morning storms is denser and more widespread, then surface heating and thusly instability will be limited to some degree. Given 0-6 km bulk shear is expected to only be around 20 kts at best, lower instability would greatly reduce the already isolated severe thunderstorm threat. If the previously mentioned levels of instability are realized, then an isolated threat of severe thunderstorms can be expected along the cold front and any lingering outflow boundaries this afternoon as a shortwave passes overhead. Hail up to around the size of quarters and damaging wind gusts are the main threats from roughly 2-8pm.

Another shortwave within the southwesterly flow aloft will move out of the Southern Plains and toward the Middle Mississippi Valley tomorrow. Guidance consensus has slowed this shortwave and its surface reflection, leading to much of the area being dry through the day tomorrow. During the afternoon and into the evening, scattered showers and thunderstorms will begin moving into the CWA from the southwest, with the best rain chances (50-70%) remaining along and south of the I-70 corridor through Sunday night.

Elmore

LONG TERM
(Monday through Friday)
Issued at 319 AM CDT Sat May 4 2024

Monday will be the start of a multi-day stretch of severe weather potential across the Great Plains and Midwest. Aloft, a trough will broaden over the western CONUS as a shortwave pivots through the Great Plains and into the Upper-Midwest. At the surface, a low will occlude across the Northern Plains as it sweeps a cold front through the Central Plains and lifts a warm front through the Midwest.
Guidance consensus is that the cold front will stay west of the CWA during the day Monday, but the warm front will lift northward through the area. Scattered showers and thunderstorms are possible (40-70% chance) with the warm front, but 0-6 km bulk shear of 20 kts or less leads to low confidence in severe weather occurring with the warm front. If severe weather does impact the CWA Monday, it would likely do so late Monday night as decaying convection associated with the Plains cold front moves into the area.

The cold front moves into the CWA Tuesday as another shortwave passes through the Midwest along the southeastern periphery of the trough. This will increase 0-6 km shear across the warm sector to roughly 50 kts among SBCAPE peaking around 1,500 J/kg, setting the stage for a potent environment for severe thunderstorms. Given the current lead time, there's still plenty of time for details to chance, such as the phasing of the shortwave, timing of the cold front, and ability of the atmosphere to sufficiently destabilize if there's residual cloud cover from overnight/early morning convection.

For Wednesday, the trough reloads as another shortwave moves through the Central Plains and into the Midwest, spawning another surface low that will follow a similar trajectory. In turn, the previous day's cold front will lift northward as a warm front, placing portions of the Middle Mississippi Valley in the warm sector similarly characterized as the one the day prior and leading to another potential round of severe weather. The placement of this threat will be dependent on how far northward the warm front can travel and the timing of the cold front.

In the wake of the front to end the week, the upper-level trough will move eastward and place the CWA beneath deep northwesterly flow per ensemble clusters and deterministic guidance. This flow will advect cooler, more seasonable to slightly cooler than normal air into the region. The 25th percentile of ensemble guidance continues to hover right around climatology, building confidence in this outcome.

Elmore

AVIATION
(For the 12z TAFs through 12z Sunday Morning)
Issued at 616 AM CDT Sat May 4 2024

A decaying band of showers and thunderstorms is moving toward central Missouri and the KJEF and KCOU terminals. I can't rule out that this convection impacts the terminals earlier this morning, but confidence is low. Additional scattered showers and thunderstorms are expected this afternoon and evening along any remaining outflow boundaries and/or a cold front moving into the area. Other than KUIN where coverage is expected to be greater, the scattered nature of this afternoon/evening convection yields low confidence in direct impacts to local terminals. If showers or thunderstorms are to directly impact terminals, low visibilities, frequent lightning, and sporadic, gusty wind changes are possible.

Elmore

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

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