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FXUS63 KLSX 060805 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 305 AM CDT Mon May 6 2024

KEY MESSAGES

- Scattered strong to severe thunderstorms are expected late tonight into early Tuesday morning. Damaging wind gusts and tornadoes are the main threats.

- Strong to severe thunderstorms are possible both Tuesday and Wednesday for portions of the area. The placement and strength of storms each of these days depends on what occurs the day prior, though the greatest chance for severe weather is on Wednesday.



SHORT TERM
(Through Tuesday Night)
Issued at 255 AM CDT Mon May 6 2024

Current water vapor imagery shows a shortwave moving across the Mid- South toward the Ohio Valley. This shortwave is driving the scattered light showers across the area currently. As this shortwave gradually moves eastward through today, rain chances will decrease from west to east this morning into the early afternoon, and remain generally along and south of the I-70 corridor.

Through today, a trough will deepen across the western CONUS as a warm front lifts northward through the CWA, causing cloud cover to linger and offset the warmer air advecting into the CWA Thusly, afternoon temperatures have been dialed down by a couple of degrees from the previous forecast, but today will still be about 10 degrees warmer than yesterday.

With warm, moist air moving into the CWA via deep south- southwesterly flow, the airmass over the CWA and points west will grow increasingly unstable ahead of a squall line that will develop across the Central Plains this afternoon associated with a cold front and a shortwave rounding the base of the trough. As this squall line enters western portions of the CWA around 2am, approximately 1,000 J/kg of MUCAPE will be available over the area.
This, and at least 50 kts of 0-6 km bulk shear, will maintain the squall line as it passes through the CWA 30-40 kts of 0-3 km bulk shear with vectors oriented from the southwest to the northeast will allow for line segments that are oriented orthogonal to the shear vector to produce mesovortices that will pair with bowing segments to produce scattered damaging wind gusts, as well as a tornado threat. This line will gradually weaken as it moves eastward across the CWA, but to what extent remains uncertain. Regardless, it may be a busy night for strong to severe thunderstorms, with the line not expected to exit the CWA until about 8am Tuesday.

The strength and timing of this squall line complicates the forecast for an additional round of strong to severe thunderstorms during the day on Tuesday. Hi-res guidance shows a variety of solutions that fall into roughly two possible outcomes. One is where the early morning squall line completely scrubs the atmosphere over the CWA of instability, leading to calm remainder of the day. The other is that the squall line leaves an outflow boundary somewhere over the CWA that allows for the atmosphere ahead of the boundary and an advancing cold front to destabilize with 3,000+ J/kg of SBCAPE among 50-60 kts of 0-6 km bulk shear. Given the shear vector's orientation to the front and/or outflow boundary, discrete strong to severe storms growing quickly upscale into clusters and line segments would be expected in the warm sector and would be capable of all hazards.
If this scenario is realized, it would most likely happen over southeastern portions of the CWA

Elmore

LONG TERM
(Wednesday through Sunday)
Issued at 255 AM CDT Mon May 6 2024

Through the day Wednesday, guidance consensus is that the upper- level trough over the western CONUS will expand eastward as it reloads with another shortwave over the Central Plains. This shortwave will move eastward through the day into the Middle Mississippi Valley - its corresponding surface low following a similar trajectory. In response, a warm front will lift northward through the Middle Mississippi Valley during the day. South of the warm front and ahead of the low's cold front within the warm sector, ensemble-based probabilities of greater than 1,000 J/kg of SBCAPE are at least 80%, with deterministic guidance showing 3,000+ J/kg.
Guidance consensus is that at least 50 kts of 0-6 km bulk shear will coincide with this instability, creating an environment conducive for severe thunderstorms capable of all hazards, including very large hail (up to baseballs) and strong tornadoes.

There still remains differences in the northward progress of the warm front among deterministic guidance and ensemble clusters, leading to uncertainty in how much of the CWA will be in the warm sector. This will likely not be exactly clear until sometime on Tuesday. If the CWA is not able to sample much of the warm sector, the severe threat will be limited and/or remain south of the area.

Behind the cold front on Thursday, ensemble clusters show the upper- level trough expanding further and shearing apart, with a cutoff forming over the western CONUS and the main trough moving eastward, placing the Middle Mississippi Valley beneath deep northwesterly flow. This flow will advect cooler air into the CWA, with ensembles clustering around temperatures at or below climatological normals to end the week and through the weekend. As for rain chances, guidance shows shortwaves digging southward along the backside of the trough through the Midwest during the weekend. This will lead to rain chances for portions of the region, but exactly when and for how long is uncertain at this lead time.

Elmore

AVIATION
(For the 06z TAFs through 06z Monday Night)
Issued at 1042 PM CDT Sun May 5 2024

Light rain will move through the St. Louis area airports through 12Z with possible MVFR visibilities. There will be MVFR/possible IFR ceilings at COU/JEF/STL/SUS/CPS through Monday afternoon. MVFR ceilings will also move into UIN on Monday morning and last through afternoon. East to southeasterly winds will continue at the terminals below 10 knots. There will be an increasing chance for a line of thunderstorms to move into the terminals from the west after 06Z on Monday night.

Britt

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

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