771
FXUS63 KLSX 172011
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
311 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 18z TAFs through 18z Saturday Afternoon)
Issued at 1230 PM CDT Fri May 17 2024

Isolated to scattered showers and thunderstorms will continue to
develop through the afternoon, with the greatest coverage across
southeastern MO and southwestern IL. Confidence in direct impacts at
any of the terminals is low, warranting only a mention of VCSH for
now. Showers and thunderstorms will dissipate as the sun sets this
evening. Otherwise, VFR flight conditions are anticipated the
rest of today.

Conditions will be favorable for areas of fog to develop again
overnight into Saturday morning, especially in river valleys and
including at KJEF, KSUS, and KCPS with MVFR to IFR flight
conditions. Lower visibilities than currently indicated are
possible. Fog will lift an hour or two after sunrise on Saturday
with area-wide VFR flight conditions returning.

Pfahler

&&

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

$$

WFO LSX