586
FXUS63 KLSX 220820
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
320 AM CDT Wed May 22 2024

.KEY MESSAGES...

- Much cooler temperatures across the region today behind the cold
  front. Highs will be 10 to 15 degrees cooler than yesterday.
  Some showers and thunderstorms are possible south of I-70, but
  the severe weather risk is low.

- The rest of the week and weekend will feature multiple
  opportunities for showers and thunderstorms with daytime
  temperatures in the 70s and 80s. It won`t be a total washout,
  but periods of rain are possible just about any day. The driest
  day is looking to be Saturday, while Sunday has the best chance
  of stronger storms near our area.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 313 AM CDT Wed May 22 2024

As of 3AM the cold front has just about cleared our forecast area to
the southwest, with winds shifting to the west behind it and
dewpoints falling into the 50s. The front will settle near the Ohio
River this afternoon with our area solidly on the cool side of this
boundary with high pressure building in from the west. A mid level
shortwave trough tracks east across the region this afternoon,
likely sparking some elevated showers and thunderstorms as it
does so. Instability on the cool side of the boundary is pretty
weak, though, so we are not expecting much potential for severe
weather. The better severe weather threat will be along the
surface cold front where greater instability and shear exists. The
Day1 Outlook from SPC has continued to shift the greatest threat
southeast as the frontal position becomes more clear, although we
remain in the lowest level marginal risk. If a storm does get
strong to severe today the threat would most likely be hail. This
threat is low enough that we are not messaging it aggressively.

Surface high pressure pushes to the east tonight into Thursday,
allowing a light southerly return flow to develop. The remnant front
will have lost its definition at this point especially after
convection along it Wednesday afternoon. Even so, as moisture oozes
back north it will create another opportunity for showers and
thunderstorms tonight into the day Thursday, mainly for the southern
half of our forecast area. We will be developing shortwave ridging
in the wake of Wednesday`s trough, though, so forcing to focus
lift to trigger showers and storms will get harder to come by
despite increasing instability on Thursday.

Kimble

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 313 AM CDT Wed May 22 2024

The general pattern remains fairly well agreed upon with a west to
southwest flow aloft and multiple shortwave troughs through the
weekend bringing multiple chances for showers and thunderstorms
most days. The strongest of these waves comes through around
Sunday which represents our clearest threat for showers and
thunderstorms.

On Friday the best shortwave energy passes by to the north with a
wave moving through the Northern Plains. The pull of warm, moist air
into this system will bring an unstable air mass through our area
and provide a chance of showers and thunderstorms. While instability
is maximizing, shear remains pretty weak with the best flow aloft
well to our north closer to the core of the trough. In fact, overall
lift is rather weak in our area ahead of the cold front and current
forecast POP from NBM may be too high. If storms do develop then
they will likely be weakly organized in a very unstable environment
and weak to moderate shear. The marginal risk from the SPC Day3
outlook seems appropriate at this stage with locally damaging winds
or large hail the likely threat if stronger storms develop. The
uncertainty on where the focus for storms will be provides us
enough uncertainty to delay messaging this just yet.

Friday`s trough to our north slings a cold front through our area
Friday night bringing a slightly cooler and drier air mass in behind
it. This sets Saturday up as likely the best day of the weekend with
the lowest rain chances and very comfortable temperatures.

The next trough tracks further south than the prior one, closer to
our forecast area on Sunday and deepening as it moves through.
Moisture return ahead of it Saturday night into Sunday morning
represents our best chance of showers and thunderstorms of the
weekend. How this evolves is still uncertain and could come with a
threat for strong to severe storms depending on how it plays out.
The greatest uncertainty is on the timing of the wave as well as
how far north the surface warm sector lifts ahead of it. North of
the surface warm front storms will be elevated and primarily a
threat for large hail depending on the amount of instability
available. In the warm sector where surface-based instability is
present a broader severe weather threat is possible especially if
the timing of the wave aligns well with daytime heating. At the
moment, that threat looks greater to our south.

Behind this trough we get at least a couple of days of cooler
weather again with a drying trend as well. Lingering PoPs beyond
Sunday are primarily due to the potential for a backside wave moving
through the northwest flow behind the departed trough. If this
occurs it will likely be a period of light showers Monday or
Tuesday within an otherwise largely dry pattern. There is
considerable spread in the guidance on that backside trough so
confidence is fairly low at this point.

Kimble

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1051 PM CDT Tue May 21 2024

Lingering thunderstorms may continue to impact St. Louis area
terminals near the beginning of the 06Z TAF cycle, but this will
be a short window, and VFR conditions are expected the remainder
of the night and into tomorrow morning. Heavy rain and gusty,
erratic winds will be the primary hazards with these
thunderstorms should they directly impact any individual terminal.
Scattered showers are expected to redevelop along and south of
the front tomorrow afternoon, but may only impact I-70 corridor
terminals for a brief period before moving south. VFR conditions
are expected at UIN throughout the TAF cycle.

BRC

&&

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

$$

WFO LSX