Sunrise 6:05 AM   Sunset 7:51 PM Moonrise 11:09 PM   Moonset 7:05 AM

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FXUS63 KLSX 270947 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 447 AM CDT Sat Apr 27 2024

KEY MESSAGES

-Widely scattered showers and sub-severe thunderstorms are possible during the day today. There will be ample dry time in between, but any given location impacted by thunderstorm could receive a brief downpour.

-Two more distinct rounds of showers and thunderstorm arrive late tonight into early Sunday and late Sunday morning into Sunday night. The strongest storms could be severe with large hail, damaging winds, and isolated tornadoes. Heavy rainfall could produce localized flash flooding. .



SHORT TERM
(Through Late Sunday Afternoon)
Issued at 446 AM CDT Sat Apr 27 2024

The CONUS view on water vapor shows yesterday's upper level low continuing to depart to northeast through the Upper Midwest with the next upper level low rotating east-northeast into the Four Corners Region. The approaching upper level low will bring additional potential for more significant shower and thunderstorm activity to the central U.S. Until then, the local area gets a brief reprieve from widespread rainfall.

The last bit of shower and thunderstorm activity weakened and moved out of the forecast area just prior to midnight. All has been quiet since then. However, another weak mid-level impulse is expected to pass north-northeast over an area of weak surface convergence that resides at the northwest edge of the southeastern ridge. MUCAPE will gradually build overhead, eventually shrouding much of the CWA with 500-800 J/kg of instability through peak heating this afternoon.
While this isn't much, it'll be enough for widely scattered, disorganized showers and thunderstorms to develop in a region of relatively weak 0-6km shear (30-35 kts). There will be ample dry time between showers and thunderstorms, but any given area that gets under one a thunderstorm could see a brief, localize downpour. It seems there is enough of a northward shift in the warm front near the MO/IA border that better potential over northeast Missouri and west-central will shift into Iowa this evening. This should be watched, however, as some CAMs have shown the potential for intensifying thunderstorms in an area of relatively higher MUCAPE (1000-1200 J/kg) and 0-6km shear (around 40 kts).

Widespread showers and thunderstorms are expected develop again over the central Plains this evening as a N/S oriented LLJ jet strengthens to 50-60 kts this evening into tonight. CAMs are in fairly decent agreement showing showers and thunderstorms growing upscale and organizing into a linear structure as they track eastward into western Missouri around and after 00z this evening.
While 2000 to as much as 3500 J/kg of MUCAPE extends into northeast Missouri and west-central Illinois late this afternoon, it begins to wane through the evening into late tonight. This, along with mid- level lapse rates (5.5-6C) along and east of the Mississippi River, will likely lead to weakening in the line as it tracks into east central Missouri late this evening. How long it survives may depend on development of a cold pool as some CAMs show the line beginning to bow out from west to central Missouri. The hail threat, while not zero, is likely to be lower as it transitions to a wind and tornado threat as soundings showing surface winds being backed ahead of the line over central Missouri. The evolution of the line will need to be watched later today into this evening to more accurately assess the overall threat over western sections of the CWA It is expected that the line will continue to weaken as it approaches the Mississippi River with some indications that it may even collapse entirely before reaching the St. Louis Metro Area.

Ongoing strong to severe thunderstorms eject northeastward out of Oklahoma early Sunday morning, bringing the potential for widespread activity to the local area late Sunday morning into Sunday afternoon. How this evolves will have a domino effect on severe potential heading into Sunday afternoon and Sunday night. The upper level low tracks northeast through the central U.S. in a similar manner as the prior upper low. This motivates some eastward movement in the cold front, but begins to slow and eventually become more east-west oriented in time. Prior waves of showers and thunderstorms, along with their coverage, could result in a relatively more stable environment by the time any surface convergence reaches central Missouri. Given the latest trends, confidence is not high in severe potential Sunday afternoon into Sunday night.

Maples

LONG TERM
(Sunday Night through Friday)
Issued at 446 AM CDT Sat Apr 27 2024

Sunday's upper low will continue to exit to the northeast Monday as a surface cold front becomes more diffuse and stalls over southern sections of the forecast area. This suggests additional chances for showers and thunderstorms mainly over southeast Missouri and southwest Illinois, especially as better upper ascent remains well south in relation to an upper shortwave that traverses Arkansas.
This coincides with better instability, where MUCAPES might climb above 1000 J/kg. Further north and west, surface high pressure attempts to build into the area with modest mid-level height rises behind the front. Instability never fully recovers, limiting the northward extent of rainfall potential as surface flow briefly turns westerly.

The best potential for dry time is Tuesday as we get between the departing system and any additional systems that track across the central U.S. The quasi-zonal flow remains active with multiple system tracking through the central Plains into the Midwest. Among the main topics could very well be the well above normal temperatures. There isn't a whole lot of spread (5-8 degrees) in NBM temperature data through midweek with the 75th percentile hinting at the potential for low to mid-90s by Tuesday and Wednesday. The southwest flow component may be in play, which is climatologically supportive for above normal warmth. Active weather will be the key with any rainfall or resulting cloud cover inhibiting diurnal heating in this period. Nonetheless, even the 25th percentile is suggesting mid to upper 80s, which gets within reach of record highs. Cooler looks to hold off until late week, as a cold front approaches the region from the northwest.

Maples

AVIATION
(For the 06z TAFs through 06z Saturday Night)
Issued at 1119 PM CDT Fri Apr 26 2024

Isolated showers and thunderstorms will continue to be possible across much of the area through the period. The most likely times will be right at the beginning of the period from 06Z to around 10Z, then again on Saturday afternoon/early evening. Did not mention thunder in this TAF cycle due to the timing uncertainty and likely low coverage of storms. Otherwise, VFR ceilings which will occasionally scattered out at times and southerly flow gusting up to 25-30kts is expected to prevail. There will also be a few patches of MVFR ceilings early in the period, mainly across central and northeast Missouri.

Carney

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

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