Sunrise 5:47 AM   Sunset 8:12 PM Moonrise 1:43 PM   Moonset 2:01 AM

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FXUS63 KLSX 172328 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 628 PM CDT Fri May 17 2024

KEY MESSAGES

- Isolated to scattered showers and thunderstorms will continue until around sunset. Very localized, minor areal flooding is possible.

- Largely dry conditions are expected through the weekend with temperatures warming to above average.

- Indications point toward a more active weather pattern early to mid-next week, including one or more rounds of showers and thunderstorms. There is a potential for strong to severe thunderstorms across portions of the area Tuesday evening and night.



SHORT TERM
(Through Late Sunday)
Issued at 309 PM CDT Fri May 17 2024

Currently, the Mid-Mississippi River Valley is beneath weakly confluent upper-level flow to the north of a trough traversing the Southern Plains and Lower Mississippi River Valley. Despite a lack of large-scale ascent, heating of seasonably moist airmass with temperatures in the 70s to near 80 F has resulted in isolated to scattered showers and a few thunderstorms, with greatest coverage, numerous at times in southeastern MO. Although MLCAPE has reached 1000 to 1500 J/kg, deep-layer shear remains under 20 kt and there is little dry air to enhance downdrafts, precluding a severe thunderstorm threat. Slow movement of showers and thunderstorms could instead lead to very localized, minor areal flooding.

Through the weekend, an upper-level ridge will begin to amplify over the Mississippi River Valley behind the trough, supporting predominantly dry conditions through large-scale subsidence and strengthening of subsidence inversion expected to "cap" most attempts at shower or thunderstorm development. However, showers and thunderstorms may come close to northeastern MO and west-central IL Saturday evening as a broken, dissipating line of showers and thunderstorms approaches with a weak cold front and then on Sunday as a potential MCS tracks along that front as it starts to lift back north. Instead, the main story for the weekend will be predominantly dry conditions and temperatures rising above average as 850-hPa temperatures rise above the 90th climatological percentile. This warming trend includes widespread high temperatures in the 80s F, with some locations flirting with 90 F on Sunday. Light winds, radiational cooling, and the lingering moist airmass will also support development of at least areas of fog in river valleys overnight through sunrise, especially tonight/Saturday morning when fog is forecast to have the greatest coverage and possibly become locally dense. That being said, the exact coverage and lowest visibilities in the fog is still not completely clear.

Pfahler

LONG TERM
(Sunday Night through Next Friday)
Issued at 309 PM CDT Fri May 17 2024

Late Sunday through the middle of next week, the upper-level wave pattern across the CONUS will become increasing amplified as longwave trough becomes established over the western CONUS and the weekend ridge shifts eastward. Resulting strong upper to mid-level southwesterly flow across the central CONUS, navigated by a parade of shortwave troughs and overlaying a broad, moist and unstable warm sector will be favorable for rounds of strong to severe thunderstorms across portions of the Plains and Mississippi River Valley during the late Sunday through Thursday timeframe.

Unsurprisingly, there is still plenty uncertainty in the details of the aforementioned pattern, particularly the timing, amplitude, and track of each upper-level shortwave trough. There is at least a consensus in global model guidance that the most vigorous shortwave trough-driven ascent will remain largely west and northwest of the CWA through Monday night, along with the greatest threat of strong to severe thunderstorms. But this leads to northeastern MO and west- central IL with the highest probabilities of showers and thunderstorms during that time, perhaps as remnants of severe thunderstorms from the west.

Tuesday into Wednesday, the track of the upper-level shortwave troughs and strongest southwesterly flow is indicated to shift eastward toward the Mid-Mississippi River Valley, leaving portions of the CWA in a more favorable position for strong to severe thunderstorms, but many details are still unclear. The highest probabilities of showers and thunderstorms are 50 to 70 percent Tuesday evening and night ahead of an approaching cold front, along with ensemble model guidance probabilities of SBCAPE greater than 1500 J/kg at 40 to 60 percent within deep-layer shear of 35 to 50 kt- -sufficient for at least a few strong to severe thunderstorms. One of the main forecast uncertainties is the eastward progression of the cold front, helping to determine if the most likely area for strong to severe thunderstorms includes the CWA on Tuesday and if that threat (and the cold front) remains in the CWA again on Wednesday. Ensemble model guidance still demonstrate a spread in frontal timing, although there has been some increase in membership toward a slower frontal passage, remaining in the CWA on Wednesday. Aside from showers and thunderstorms, temperatures are expected to remain above average ahead of the front with the entire NBM temperature distribution above average. The NBM interquartile temperature range increases to around 10 F on Wednesday with frontal timing variability. This spread remains through the end of the week behind the front, with nearly equal distribution above and below average owing to growing uncertainties in the upper-level pattern.

Pfahler

AVIATION
(For the 00z TAFs through 00z Saturday Evening)
Issued at 622 PM CDT Fri May 17 2024

Isolated to widely scattered showers and thunderstorms will continue south of the Missouri river through sunset. Coverage and strength are already weakening, so the convection may not even last that long. Elsewhere and otherwise, VFR conditions are expected to prevail through the evening. Areas of fog will likely develop after 06Z, primarily in locations that received rain this afternoon such as the eastern Ozarks and parts of southwest Illinois, and other fog-prone areas such as river valleys. Fog should dissipate quickly after sunrise on Saturday and VFR conditions are expected to prevail thereafter.

Carney

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

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