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FXUS63 KLSX 051724 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1224 PM CDT Sun May 5 2024

KEY MESSAGES

- Scattered strong to severe thunderstorms are expected late Monday night into early Tuesday morning mainly along and west of the Mississippi River.

- Additional rounds of severe thunderstorms are possible on Tuesday and Wednesday across portions of the area. However, severe weather on each day will depend on exactly what happens the day prior, which is uncertain at this time.



SHORT TERM
(Through Monday Night)
Issued at 319 AM CDT Sun May 5 2024

A shortwave can be seen in water vapor imagery drifting slowly northeastward across the Southern Plains within southwesterly flow that extends across the Midwest. At the surface, high pressure extends southward out of the Northern Plains and Upper-Midwest into the Middle Mississippi Valley. Through the day today, this high will gradually shift eastward as the shortwave and its surface reflection move toward the area. Guidance has been trending this system slower and further south in recent initializations, leading to most of the day today being dry for much of the area. Rain chances will increase this evening mainly over southeastern Missouri and southwestern Illinois and continue through the overnight hours as the shortwave slowly pivots across the region.

Tomorrow, the shortwave will continue east-northeastward into the Ohio Valley as southwesterly flow aloft resumes over the CWA A warm front will lift northward through the area as southerly low-level flow ushers in warm, moist air. As a result, afternoon temperatures across the CWA will see 5-10 degree bump over what is expected today. The increase in moisture will also increase instability over the CWA during the day into the overnight hours. Subtle ridging in the wake of the shortwave is expected to keep convection mostly at bay across much of the area. However, I can't rule out some isolated afternoon showers and weak thunderstorms mainly over portions of southeastern Missouri and southwestern Illinois given the closer proximity to the slowly departing shortwave.

Lapse rates become increasingly conducive for convection Monday night, and MUCAPE will hover around 1,000 J/kg across the area.
Afternoon convection from the Plains is expected to congeal into multiple line segments as it moves eastward toward our CWA during the overnight hours. Given aforementioned MUCAPE and 0-6 km of around 30 kts, this convection will be able to sustain itself at least through central Missouri and toward the Mississippi River during the overnight hours. Damaging wind gusts and a tornado or two are possible before convection weakens.

Elmore

LONG TERM
(Tuesday through Saturday)
Issued at 319 AM CDT Sun May 5 2024

At the start of the period on Tuesday, southwesterly flow will reside over the Middle Mississippi Valley as a shortwave pivots over the region. Guidance has been trending more aggressive with the Monday night/Tuesday morning convection rolling into the area from the Plains, which raises some questions as to how much we will be able to destabilize Tuesday afternoon. Where the early morning convection doesn't stabilize the atmosphere, 50+ kts of 0-6 km shear and around 2,000 J/kg of SBCAPE will create an environment conducive for severe thunderstorms. However, the early morning convection may cause this environment to be located east and/or south of the CWA

On Wednesday, the trough will reload as another shortwave pivots through the Central Plains and into the Midwest. In turn, a warm front will lift through the Middle Mississippi Valley as a surface low moves toward the region. Within the warm sector ahead of the low, SBCAPE of at least 1,000-1,500 J/kg per deterministic guidance and ensemble means will reside among about 50 kts of 0-6 km bulk shear. Assuming this environment is realized, severe thunderstorms capable of all hazards will be possible. However, where exactly this threat occurs depends on how far north the warm front reaches and the timing of the cold front.

In the wake of Wednesday's cold front on Thursday, ensemble clusters show flow aloft over the Middle Mississippi Valley will become increasingly northwesterly as the upper-level trough expands eastward and shears apart, forming a cutoff over the western CONUS.
Beneath the western periphery of the main trough now over the eastern CONUS, deep northwesterly flow will advect cooler air into the region, with confidence increasing that temperatures will run below normal to end the week into next weekend, as ensemble means have been trending cooler.

Elmore

AVIATION
(For the 18z TAFs through 18z Monday Afternoon)
Issued at 1223 PM CDT Sun May 5 2024

Very light winds today with mostly VFR conditions. Main area of concern is rain and lower ceilings moving in from the south. This is most likely to affect the central Missouri terminals this evening and overnight into tomorrow morning, but may impact the St Louis area as well. MVFR is likely, but IFR is also possible.
Conditions improve during the day on Monday.

Kimble

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

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