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FXUS63 KLSX 231125 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 625 AM CDT Sat Sep 23 2023

SHORT TERM
(Through Late Sunday Afternoon)
Issued at 334 AM CDT Sat Sep 23 2023

Key Messages:

1.) Dry conditions and above average temps are anticipated today across the area with showers and thunderstorms not expected to enter central and northeastern MO until this evening. The threat of any thunderstorms being severe is low and thunderstorms could actually struggle to advance much further east tonight.

2.) Isolated to scattered showers and thunderstorms are possible across the area Sunday afternoon and evening, depending on how much clouds break.

3.) Confidence is increasing that above average temps will prevail next week along with largely dry conditions.

An upper-level shortwave ridge will remain in place over the CWA through the day today, providing dry conditions via large-scale subsidence and a subsidence inversion. A few showers and thunderstorms may form this morning across western MO within weak WAA along weakening LLJ but are unlikely to reach central and northeastern MO. Scattered mid-level clouds and only gradually thickening upper-level clouds will permit strong insolation and deep BL mixing through the day with high temps expected to warm to above average and in the mid to upper 80s F. Tightening of the MSLP gradient and the mixing will also lead to slightly breezy conditions across central/northeastern MO and west-central IL with occasional wind gusts of 20 to 25 mph.

Upstream of the shortwave ridge, an upper-level closed low will eject eastward from the Rocky Mountains to the Northern Plains today, facilitating genesis of a surface cyclone tracking from the Central to Northern Plains that is anticipated to occlude by this evening. Within the cyclone's warm sector, including across eastern KS and western MO, numerous thunderstorms are forecast to develop this afternoon and early evening as a mid-level shortwave trough (embedded in the base of the closed low) arrives. There is concern for these thunderstorms to be severe during that time, but as these thunderstorms track eastward into central and northeastern MO this evening they will likely become increasingly elevated as they depart the warm sector and nocturnal stabilization also takes place with HRRR probabilities of 1000 J/kg or more of SBCAPE dropping sharply.
Additionally, CAMs indicate that there could be a tendency for many thunderstorms to propagate more southeastward into southwestern and southern MO with time as they attempt to remain tethered to the warm sector and a northwest-to-southeast warm front. Therefore, the threat of any thunderstorms to remain severe as they enter the CWA still appears low. In fact, even elevated instability progs decreases enough with eastward extend that it is unclear if thunderstorms will be able to reach locations to the east of central/northeastern MO.

On Sunday, a slowing, dissipating cold front is finally anticipated to reach the CWA. Isolated to scattered showers and thunderstorms are possible during afternoon and evening along and ahead of this cold front where any breaks in cloud cover allow some destabilization and another mid-level shortwave trough approaches the CWA. However, is uncertainty in how much clouds will break and destabilization will occur, which is projected onto NBM interquartile high temp ranges spanning 5 to 8 F, between the mid- 70s and the mid-80s F. The amount of destabilization is anticipated to impact the coverage of showers and thunderstorms.

Pfahler

LONG TERM
(Sunday Night through Friday)
Issued at 334 AM CDT Sat Sep 23 2023

Global model guidance continue to indicate that the upper-level pattern over North America will remain favorable for mid-latitude blocking to develop. The clusters produced by cluster analyses applied to ensemble model 500-hPa height fields all support some semblance of a Rex block, or at least blocking in general, developing across eastern-half of Canada and eastern-half of the CONUS, with contribution from the Northern Plains closed low. The individual clusters capture the variability in the strength and position of this potential block, but this pattern would prevent any significant changes to sensible weather from day to day. There has been a slight shift toward warmer temps through this period with NBM interquartile temp ranges now shifted entirely above average, possibly from a re-adjustment in the NBM's rolling bias correction.
Therefore, confidence is quickly increasing in above average temps next week. A few primarily diurnal showers will be possible next week as well, depending on how quickly the cold front dissipates and if the closed low or any shortwave troughs can traverse the region, but ensemble model guidance indicate that most of this period and the CWA will be dry.

Pfahler

AVIATION
(For the 12z TAFs through 12z Sunday Morning)
Issued at 624 AM CDT Sat Sep 23 2023

River valley fog with MVFR to IFR flight conditions will dissipate by 14z at KSUS and KCPS, leaving dry and VFR flight conditions prevailing at all terminals into this evening. Showers and thunderstorms are expected to move into central and northeastern MO this evening, likely impacting KCOU and KJEF. It is uncertain how much further east this showers and thunderstorms will survive and if there are any impacts to KUIN and St. Louis metro terminals.
Therefore, only VCSH has been included at KUIN at this juncture.
Otherwise, southeasterly winds are forecast through the TAF period with occasional gusts of 17 to 25 kt at KCOU, KJEF, and KUIN from late morning through afternoon.

Pfahler

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

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