Sunrise 6:06 AM   Sunset 7:48 PM Moonrise 10:02 PM   Moonset 6:23 AM

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FXUS63 KLSX 261153 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 653 AM CDT Fri Apr 26 2024

KEY MESSAGES

- An active weather pattern over the central U.S. will introduce multiple days with severe thunderstorm potential, including marginally severe hail, damaging winds and an isolated tornado.
Much of the threat will remain over western portions of the forecast area until Sunday.

- Sunday remains the highest threat for severe thunderstorms ahead of an approaching cold front. Large hail, damaging wind, and tornadoes will all be a concern. Any thunderstorm will be capable of producing heavy rain with a localized flash flood threat.



SHORT TERM
(Through Late Saturday Afternoon)
Issued at 553 AM CDT Fri Apr 26 2024

A wing of mid-level warm air advection currently bisects the state of Missouri with a strong 50-60 knot low level jet pointed into western sections of the state. IR satellite shows robust cloud development in this region with regional radar mosaics mapping showers and thunderstorms from northwest Missouri to just west of Columbia as of 09z this morning. While the CAMs have all had some version of this activity, the HRRR seems to be handling this the best so far. Activity is expected to to move northeast through mid- morning underneath an area of upper level vorticity that advects northeast. Thunderstorms are elevated and while MUCAPE values creep up over eastern Missouri and western Illinois this morning, they are substantially lower than the 1500+ J/kg values that exist over the central Plains. This morning's thunderstorms are expected to remain sub-severe with small hail accompanying the strongest thunderstorms.

Severe weather potential this afternoon into tonight will rely on development that takes shape over sections of western and southwestern Missouri. Some of the potential activity is already developing over northern Texas and Oklahoma, where yet another mid/upper level disturbance continue to drive east-northeast around the parent upper level trough. CAMs all show some version of a broad mess of showers and thunderstorms moving into central Missouri by mid-afternoon. The question then is the coverage and intensity of thunderstorm activity as it runs into weaker instability (less than 1000 J/kg). Mid-level lapse rates of 6-7.5C edge into western sections of the CWA but drop dramatically to the east will little, if any, support along and east of the Mississippi River. It is likely convection will weaken over eastern sections of the forecast area. HRRR could be overselling thunderstorm intensity after 00z with no surface forcing mechanism to support such. However, stronger thunderstorms could produce marginally severe hail, damaging winds and/or an isolated tornado. The main focus remains west, where yet another round of surface based, discrete convection fires along the front over eastern Kansas and moves into western Missouri. How this activity impacts the region will largely depend on the preceding round of convection, should it keep conditions cooler and more stable east of the more discrete development to the west. It is more plausible that the later development further west will weaken as it approach central Missouri, if not die out entirely.

Trends are making Saturday look more tame than previously thought.
While high MUCAPE values cover the region, the warm front lifts to the north with warm air aloft keeping the environment capped much of the time. The upper low moves to the northeast with a void between it and the next upper low that drive into the Plains for the later half of the weekend. Warmer air aloft also keep the environment capped with the absence of a strong surface forcing mechanism. Aside from an isolate to widely scattered shower/thunderstorm along remnant boundaries produced by prior convection, much of the daytime period seems quite tame in comparison.

Maples

LONG TERM
(Saturday Night through Thursday)
Issued at 553 AM CDT Fri Apr 26 2024

The main concern in the long term period is late Saturday into Sunday morning and again late Sunday afternoon into Sunday evening.
However, there are some trends that may be signaling a slowing cold front and much later arrival than previously anticipated. This could lend some challenges to thunderstorm potential as we get into the late weekend and early next week.

This pattern makes the forecast sound more like a broken record with repeated waves of showers and thunderstorms developing from the west and tracking east into the local area. The difference by Saturday is the broader area of greater instability that encompasses the region as MUCAPES climb to around 3000 J/kg over central and northeast Missouri. A more vigorous upper low closes off over the Four Corners Region before taking a negative tilt and turning northeast later in the weekend. The cold front remains well to the west, once again triggering showers and thunderstorms over the Plains.
Deterministic guidance is not entirely aligned on the eastward extent of convective potential late Saturday into Sunday with varying strength in disturbances that eject northeast ahead of the upper low. NAM is far weaker with next to nothing, while the GFS is more aggressive and stronger with the leading shortwave providing ascent over a broad region of instability. Even considering the more aggressive GFS, thunderstorms edge into central and northeast Missouri before decaying late Saturday night into Sunday morning.
Should this slightly westward trend continue, it may be enough to keep most of the severe potential west of the forecast area. For now, the far western extent of the CWA could see a strong to severe thunderstorm late Saturday night into early Sunday, but confidence is not high.

Sunday afternoon into Sunday evening has been the primary timeframe when severe potential increases as the upper low tracks northeast and gives some eastward push in the overall pattern. However, there are now questions arising there, too. While the surface boundary does edge closer to the area, there isn't a real push of cold air and the boundary itself doesn't seem to clear the area entirely, if at all. In fact, southerly flow persists into early Monday with surface temperatures in the mid-50s to near 60 degrees and a brief westerly component to surface flow

Maples

AVIATION
(For the 12z TAFs through 12z Saturday Morning)
Issued at 645 AM CDT Fri Apr 26 2024

Conditions will gradually decline through the morning from west to east with showers and thunderstorms coinciding with IFR ceilings at central Missouri terminals this morning. Activity extends northeast through KUIN, though ceiling remain VFR but should lower as heavier precipitation moves in from the southwest.

Metro terminals are surround by showers and VFR ceilings. The main question here is how quickly MVFR ceilings move in along with thunderstorms from the west and southwest. Hi-res guidance and be more pessimistic and more consistent in recent update with showers and thunderstorms moving in late this morning into the early afternoon. While IFR cannot be ruled out in pockets of locally heavier rainfall, ceilings were capped at low end MVFR as thunderstorms weaken with eastward movement. KCPS is a bit more optimistic with the uncertainty in eastward extend of thunderstorm potential.

Rainfall chances extend through the afternoon but tried to build in a precip-free period later today, when the best potential for dry time this evening. Conditions improve back to MVFR/VFR late this evening/tonight before another round potentially move in late Friday into Saturday.

Maples

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

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