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000
FXUS63 KLSX 260429
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1129 PM CDT Thu Apr 25 2024

.KEY MESSAGES...

- The central United States is on the verge of a multi-day severe
  weather event beginning tonight. The bi-state region will be on
  the eastern edge of this threat initially, with glancing
  threats tonight and late Saturday night, before the threat
  shifts east on Sunday.

- Sunday evening and night stand out as our highest threat for
  severe thunderstorms ahead of a cold front. Large hail, damaging
  wind, and tornadoes will all be a concern. Heavy rain may also
  result in localized flash flooding.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 247 PM CDT Thu Apr 25 2024

High clouds continue to stream into the region from ongoing
convection along and ahead of a warm front, currently stalled from
the central Great Plains down through the southern Ozark
Mountains. The majority of the forecast area is dry thanks to a
stalled surface high centered over the Great Lakes promoting cool
and dry east winds. Overhead, a ridge is building over the central
CONUS with a trough digging in the Intermountain West that will
be the focus for our active weather through the weekend. The warm
front is forecast to lift north tonight as a low-level jet
intensifies, which will continue to promote elevated convection
overnight into Friday morning. With meager forcing and instability
in place, these thunderstorms will very likely (80%+ chance)
possess an isolated threat for marginally-severe hail. As the LLJ
weakens after sunrise, the intensity for these thunderstorms will
drop even further as the convection spreads north and east.
Depending on the speed of the warm front and these thunderstorms,
some areas have a low (20% chance) threat of heavy rain and
localized flooding. I`m not confident that any one area is under a
greater threat given the widespread nature of the thunderstorms,
however with highly- anomalous (above the 99th climatological
percentile) precipitable water values in place and relatively-slow
motion, heavy rain will be closely monitored.

The greater impacts from this convection will be its influence on
available instability for the afternoon and evening, when a
shortwave glances the Mid-Mississippi Valley to its northwest. While
forcing is marginally more impressive, and deep-layer shear
increases a bit, available instability will be in question if
morning thunderstorms linger into the early/mid-afternoon. There`s
also no discernible surface or low-level lifting mechanism
outside of a possible remnant outflow boundary from the earlier
convection. All of this is to say the threat for any
thunderstorms, much less severe thunderstorms, is highly
conditional on favorable instability and forcing existing at the
same time. The best threat for this now seems to be further south,
closer to the I-70 corridor and south, but that area is also more
divorced from the best mid/upper-level ascent. With better
kinematics to our north and west, this round of severe weather
will largely stay confined to western Missouri and the central
Great Plains. Regardless, if a severe thunderstorm can develop in
our area, it will threaten primarily large hail and damaging wind.
If the boundary on which convection develops is still somewhat
defined, I can`t rule out a brief tornado as well.

MRB

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 247 PM CDT Thu Apr 25 2024

Any lingering convection gradually diminishes overnight, though with
the entire region within an open warm sector and the warm front to
our north, instability will be more appreciable by the hour as we
enter Saturday. Instability increases markedly during the day under
strong warm air advection within a moisture-rich airmass,
particularly across the eastern Plains and western Missouri where
that 925/850mb WAA is strongest. However, some degree of ridging
overhead will act to inhibit deep convection in a widespread sense
during the day. It`s worth noting that the amplitude of the ridge
isn`t as strong in the guidance as it was 24 hours ago, which gives
me pause in saying Saturday will be largely dry. However, with very
little forcing or lift to focus the instability, any convection that
develops will be more nebulous and not very intense as a result.

The second of two amplified shortwaves approach the region Saturday
evening, helping to advance the synoptic system further east. There
is potential for a round of nocturnal convection to develop
overnight Saturday into Sunday morning in western in northern
Missouri on the nose of the low-level jet, but most indication are
that the threat area will once again be confined to central/northern
Missouri and west-central Illinois at the worst. This round would
likely grow into some convective complex and threaten damaging wind
and a few tornadoes. The threat for severe weather across the bi-
state region becomes more widespread Sunday afternoon and night. Our
airmass will once more destabilize, but now amidst much stronger
ascent and forcing. The second mid-level wave will cross over much
of the region, and while the speeds themselves aren`t very strong,
there is some upper-level jet streak support for synoptic ascent.
Instability is still somewhat appreciable, but not as impressive as
in days past. That said, the shear and kinematic fields along with
subtle pre-frontal surface convergence may be able to compensate and
initiate strong to severe thunderstorms in the afternoon across
western Missouri. Thunderstorms look to start discrete with strong
deep-layer shear, but storm vectors will favor upscale growth at
some point in the late afternoon. Exactly when is unclear, which
affects the evolution of the convection and overall threats. That
said, all severe hazards (and heavy rain) are realistic on Sunday.

There continues to be a gradual slowing of the cold front that would
essentially end the severe weather threat compared to previous
forecasts. It now looks like the front will still be either west of
the region or just entering central Missouri Monday morning. While
this would lead to a buildup of instability ahead of the boundary,
leading to a threat for more showers and thunderstorms, the shear
parameter space becomes less favorable for severe weather in the
wake of the strong shortwave on Sunday. While the threat for any
strong convection is low on Monday, subtle changes in the pattern
may result in large differences in the potential. The front and any
ascent aloft finally exits late Monday, but the boundary itself may
waver north and south amidst near-zonal upper-level flow. As such,
persistent 15-40% PoPs exist through the rest of the forecast
period. I don`t expect it to be raining that entire time, but there
is very little certainty this far out regarding exactly when rain
chances peak in a given location.

MRB

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1115 PM CDT Thu Apr 25 2024

A broad area of MVFR ceilings will overspread the area through the
overnight hours ahead of a warm front. Some IFR is possible,
however ensemble guidance is showing IFR probabilities mostly
below 50 percent, so confidence in IFR is low and will therefore
not go lower than 1000ft in the 06Z TAFs. Winds will increase
from the southeast and become gusty through the early morning.
Widely scattered showers and likely a few thunderstorms are
expected to develop near the Mississippi River between 09-12Z and
move northeast through the morning. Another, more widespread area
of showers and thunderstorms is expected to develop over central
or west central Missouri between 12-15Z, and this area will move
east to affect much of the area through 18-20Z. Ceilings behind
this second area of convection should rise to between 2500-4000ft.

SPECIFICS FOR KSTL:

Latest indications are that widely scattered showers will develop
in the vicinity of the terminal between 10-13Z. I do not expect
much thunder in this first batch of convection, although it`s not
a zero chance at Lambert. The second batch of convection looks
likely to affect the terminal between 18Z-21Z. This looks more
organized and should have more coverage as it moves in from the
west.

Carney

&&

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

$$

WFO LSX