Sunrise 5:59 AM   Sunset 7:56 PM Moonrise 2:08 AM   Moonset 12:39 PM

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FXUS63 KLSX 021138 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 638 AM CDT Thu May 2 2024

KEY MESSAGES

- An active weather pattern with rounds of showers and thunderstorms will continue through next week, although a majority of the time will be dry.

- There is a potential for isolated strong to severe thunderstorms today, generally within a window from 12 to 7 pm and across central, northeastern, and east-central MO as well as west- central IL. Gusty to briefly damaging winds and marginally severe hail are the main hazards.



SHORT TERM
(Through Late Friday Afternoon)
Issued at 349 AM CDT Thu May 2 2024

A warm front just to the north of the I-70 corridor as of 08z will lift quickly northward early this morning. The strongest forcing associated with a 40 to 50 kt south-southwesterly LLJ extending over the front, sufficient for isolated to scattered showers and thunderstorms, will remain focused just west and northwest of the CWA However, a few showers and thunderstorms could develop across central, northeastern MO and west-central IL early this morning.
Patchy fog has also developed where yesterday evening's cluster of showers and thunderstorms tracked and skies have cleared across southeastern MO and southwestern IL.

Two MCSs are ongoing early this morning: one in NE and northern KS along/ahead of an approaching cold front and one in OK and northern TX associated with an MCV. These MCSs and their parent forcing mechanisms will be the drivers of showers and thunderstorms across the CWA today and tonight as they arrive. It is uncertain whether reintensification or entirely new redevelopment of thunderstorms will occur, but short-term model guidance depict 750 to locally near 2000 J/kg of MLCAPE downstream during the afternoon along with 10 to 20 kt of deep-layer wind shear. This marginal instability-wind shear parameter space supports thunderstorms to be loosely organized into one or more northeastward-advancing multicell clusters or MCSs containing sporadic gusty to briefly damaging downburst winds and marginally severe hail. Relatively greater deep-layer wind shear closer to 25 kt is expected across portions of central, northeastern MO and west-central IL which may allow for slightly greater, transient organization of thunderstorms, perhaps a short-lived supercell. Therefore, the threat of isolated strong to severe thunderstorms is relatively greatest across those areas, decreasing with southeastward extend. The associated window is from 12 to 7 pm but could end up much shorter if widespread thunderstorm development occurs and cold pools begin undercutting thunderstorms. Ahead of showers and thunderstorms, strong low-level south-southwesterly flow and deep BL mixing will lead to high temperatures reaching the low to upper 80s F. Cooler temperatures in the 70s F are expected in northeastern MO, where showers and thunderstorms will arrive earlier.

Thunderstorm coverage and intensity is anticipated to quickly decrease this evening as instability is overturned and nocturnal stabilization occurs in the absence of a strong LLJ, leaving a transition toward light rain/showers and a few rumbles of thunder overnight. The vast majority of this precip will depart to the east and/or dissipate by Friday morning. Redevelopment of isolated to scattered showers and thunderstorms is possible Friday afternoon, primarily in southeastern MO and southwestern IL. However, the aforementioned cold front will become increasingly diffuse and ill- defined by that time across those areas and large-scale ascent will be minimal, decreasing the shower and thunderstorm potential.
Instead, the highlight of Friday will be a cooler airmass arriving behind the front, although with temperatures still slightly above average.

Pfahler

LONG TERM
(Friday Night through Wednesday)
Issued at 349 AM CDT Thu May 2 2024

Saturday through early Monday, a pair of migratory upper-level shortwave troughs within time-mean southwesterly flow will traverse the Mid-Mississippi River Valley, maintaining the ongoing pattern of episodic showers and thunderstorms. There are some differences among model guidance in the timing of these troughs, but the general timeframes of highest shower/thunderstorm probabilities in ensemble model guidance are Saturday during the day and then Sunday afternoon into night. Much of the weekend will actually be dry, though, and accompanied by near to above average temperatures, given the quick- moving nature of the associated troughs.

Upstream of the upper-level shortwave troughs, global model guidance portray an upper-level closed low tracking from the West Coast and Rocky Mountains over the weekend before shifting into the Northern/High Plains next week, providing a significant strengthening of upper-level southwesterly flow over the central CONUS. There are still notable differences of the position and amplitude of the closed low captured in cluster analyses applied to the height patterns of ensemble model membership, but this overall pattern appears supportive of strong to severe thunderstorms across portions of the central CONUS sometime next week. That being said, the specifics of this pattern are nearly impossible to glean at this juncture, including the exact threat of strong to severe thunderstorms to the CWA Persistent low-level southerly flow associated with the pattern will also favor temperatures warming further above average, with exact values depending on showers/thunderstorms, cloud cover, and the position of an oscillating front.

Pfahler

AVIATION
(For the 12z TAFs through 12z Friday Morning)
Issued at 637 AM CDT Thu May 2 2024

Showers and thunderstorms currently across northwestern and western MO as well as northwestern OK and southwestern MO this morning are anticipated to redevelop and/or reintensify this afternoon while sweeping northeastward through the evening. Thunderstorms will be capable MVFR to IFR flight conditions and gusty winds, especially at KCOU, KJEF, and KUIN. At St. Louis metro terminals, thunderstorms may begin decreasing in intensity during their arrival. Following thunderstorms, a period of showers/light rain, perhaps with a few rumbles of thunder, is also expected in addition to MVFR to IFR stratus. IFR stratus is most likely at KSTL and KCPS. Flight conditions will improve Friday morning from northwest to southeast as stratus scatters and lifts.

Pfahler

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

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