144
FXUS63 KLSX 311754
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1254 PM CDT Fri May 31 2024

.KEY MESSAGES...

- Rain returns to the region overnight tonight, and after a
  largely-dry Saturday night through Sunday, additional
  thunderstorm chances linger in the region through Wednesday.

- Temperatures will warm from the roughly-seasonable values we`ve
  recently enjoyed into the low/mid-80s starting Sunday,
  approaching 90 degrees by mid-week. Exactly how warm we get
  will depend on currently-uncertain rainfall timing and coverage.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 230 AM CDT Fri May 31 2024

Regional radar mosaic depicts a nearly stationary area of showers,
with a few thunderstorms, stretching across the Kansas/Missouri
border and north into the Missouri River Valley. This activity,
forced by a weak and sluggish mid-level shortwave, weak low-level
jet, and nebulous surface convergence, is causing nothing more than
increased high cloud cover over the forecast area. Recent high-
resolution model guidance now suggests that this mid-level wave will
be slower to translate east than originally forecast, and that dry
air will be more prominent in the mid-levels. As such precipitation
chances today were slowed from west to east. Cloud cover and weak
easterly winds from a departing surface high to our northeast will
keep temperatures in the upper 70s to low 80s today, near to
slightly-below normal once again.

By the late afternoon into the early evening, a weak surface low
will develop in the Lower Mississippi Valley as the mid-level wave
amplifies slightly. As the low tracks north and east, weak but
widespread moist isentropic ascent will allow for a shield of
stratiform rain to advance into the region from southwest to
northeast. It is worth noting that one or two CAMs do not show this
feature, which would lead to far more localized rain (under
thunderstorms) if it were to occur. That said, the synoptic pattern
seems more conducive to light, stratiform rain. While most will see
amounts around 0.50-0.75 inches or less, there are hints in high-
resolution model soundings that a few embedded thunderstorms are
possible in eastern and central Missouri into west-central Illinois
along an axis of deep moist convergence. If more convective elements
can occur, and are slow-moving or training, amidst precipitable
water values very likely (near 100% chance) exceeding the 90th
climatological percentile, locally-heavy rain would bring a threat
for nuisance flooding. That said, this threat is limited spatially
and nearly all will see unimpactful rain through Saturday morning.

Rain is slow to exit Saturday thanks to a secondary mid-level wave
of PVA forcing additional showers and thunderstorms. Chances
decrease through the day from west to east as the wave passes,
though it doesn`t look like a washout in any one spot whatsoever.
The rain and cloud cover will work to keep temperatures in check
once again, with most of the region struggling to warm out of the
upper 70s. That said, if we clear out sooner than forecast,
central Missouri has the potential (15-20% via high-resolution
ensembles) to reach 80 degrees.

MRB

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 230 AM CDT Fri May 31 2024

By late Saturday, quasi-zonal flow becomes more established aloft,
putting the region`s rain chances at the mercy of subtle shortwaves
that introduce timing and location uncertainty to the forecast.
Saturday night, and most of Sunday, will remain largely dry with mid-
level shortwaves staying to our south and west. There is a low
chance (15-20%) for a few thunderstorms Sunday afternoon in central
and southeast Missouri where better instability will exist, but a
lack of upper-level ascent will keep any activity widely-scattered
at best. Waves of mid-level forcing will continue to march through
the Midwest from Monday through Wednesday, bringing the threat for
showers and thunderstorms each day. That said, ensemble cluster
analysis varies on exactly when and where rain will fall. This
results in broad-brushed rain chances in the forecast, though the
first half of the week does not look like a washout. There is also
very little threat for stronger thunderstorms at this point, with
these shortwaves not looking very strong.

What will be more noticeable are our warming temperatures:
stronger 850mb warm-air advection amidst low-level southwest flow
will bring temperatures into at least the low to mid-80s on
Sunday. We warm further as the work week begins, with 850mb
temperatures likely (60- 80% chance) reaching the 75th
climatological percentile by then. Tuesday is forecast to be the
warmest day of the week, when the low- level thermal ridge is
strongest over the region (19-22 degrees Celsius). That said,
exactly how warm we get will be much more closely tied to where,
when, and how widespread precipitation will be on each day. While
global-scale ensembles have very low probabilities (5-10%) of
reaching 90 degrees on Tuesday, I wouldn`t rule it out if
precipitation holds off and strong southerly low- level flow
prevails. MOS guidance does nod to this potential, and is likely
closer to the high-end possibility. Any temperatures this warm
would be relegated to the metropolitan areas, most likely, though
most of the region would sit in the mid/upper 80s.

While global-scale ensembles vary on forecast details through mid-
week, they do converge on a pattern change by late Wednesday. A
rather strong upper-level ridge (near the 99th climatological
percentile in model guidance) establishes over the western CONUS,
with a well-defined wave tracking through the northern Plains into
the Great Lakes. The resultant surface low would draw a cold front
through the bi-state area and usher in a more concrete chance for
widespread showers and thunderstorms. That said, exactly when the
front passes and how strong it will be remains uncertain, leading to
lower precipitation chances overall. Following this frontal passage,
the Mississippi Valley will be squarely in deep northwest flow. This
will bring our low, but persistent rain chances to an end from
Thursday into the weekend, and cool temperatures back to
near/slightly-below normal (upper 70s to low 80s). How long we stay
in this pattern is uncertain, with some guidance bringing a ridge
into the Plains by late Saturday and others holding onto the
northwest flow through next weekend.

MRB

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1248 PM CDT Fri May 31 2024

VFR flight conditions will prevail this afternoon with gusty
southeast winds. Showers will overspread the area tonight
accompanied by gradually lowering flight conditions with all of
the terminals deteriorating to low MVFR or IFR overnight into
early Saturday morning. There could be some brief embedded thunder
accompanying the showers late this evening into Saturday but the
coverage and probability is too low to mention.

Glass

&&

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

$$

WFO LSX