505
FXUS63 KLSX 172059
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
359 PM CDT Sun May 17 2026

.KEY MESSAGES...

- At least 3 rounds of thunderstorms are expected in the short
  term: one late tonight/into Monday morning, another late Monday
  night into Tuesday morning, and finally Tuesday afternoon and
  evening. Each round has the potential to produce severe weather.

- Cooler and drier weather returns midweek.

&&

.SHORT TERM... (Through Late Tuesday Night)
Issued at 352 PM CDT Sun May 17 2026

A deepening long wave trough over the Rockies is forcing
cyclogenesis over the Great Plains this afternoon. The deepening low
level cyclone is tightening the pressure gradient across the
Mississippi Valley producing southerly flow and bringing warm humid
air north from The Gulf.  This will set the stage for several rounds
of thunderstorms through Tuesday.  The first round of storms is
expected to develop late this afternoon/early this evening as a
short wave moves into the central Plains which lifts a lobe of the
surface low northeast.  This increases convergence along the cold
front over Kansas and Nebraska and fires off convection.  Discrete
storms are expected to grow upscale into an MCS which moves east
overnight.  Most CAMs show a broken line of weakening thunderstorms
moving into our northeast and central Missouri counties between 09-
12Z.  However a few (most notably the 12Z 4km NAM NEST) have the
storms moving into Knox County as early as 06Z.  Regardless of when
they arrive, the GFS and RAP are showing enough MUCAPE across our
area to maintain the convection as it continues east.  Deep-layer
shear is lacking at 20-25kts across most of the area, and forecast
soundings indicate a fairly deep stable layer below the instability.
 While some wind gusts to 50-60 mph will be possible initially
across northeast Missouri, thing sub-severe gusts become more and
more likely as the line continues east into Monday morning.

Still a good deal of uncertainty for Monday`s forecast as some
guidance moves the weakening showers and storms through quickly or
dissipates them entirely by 18Z and others slow the storms down, but
continue to march them southeast through the forecast area all day.
The lingering precip scenario looks less likely, but current high
temperature forecasts in the mid to upper 80s will likely not be
realized if clouds and showers linger through the day.  Regardless,
another upper level short wave breaks off the long wave over the
Rockies and moves into the Great Plains during the afternoon and
evening.  This wave forces another low to develop on the surface
front over the Great Plains, and it races northeast through Monday
night.  Another round of thunderstorms develops on the front over
Kansas and Nebraska as the front begins to move, and they quickly
grow upscale into QLCS.  Much like tonight`s line, this line will
lose steam as it moves east, reaching our area between 06-09Z.  The
presentation of this line is not very impressive in most CAMs by the
time it reaches our area.  The most likely area for severe weather
in our area should be northeast Missouri into west central Illinois
where MUCAPE ranges from about 1800 J/Kg in the RAP to around 1200
J/Kg in the GFS. 0-6km shear isn`t terribly impressive at around
25kts and there is a stable layer below the instability once again.

The final round of storms comes Tuesday as the cold front drifts
into the area.  Think the severe risk is conditional on how the
overnight convection plays out as it moves through our area.  A
strong enough outflow boundary would wash out the instability ahead
of the front and lingering clouds/showers could limit recovery.  As
it stands, the GFS develops around 2200 J/Kg MLCAPE ahead of the
front during the afternoon and forecast soundings only show the
tiniest bit of CINH.  However the shear is still unimpressive at 20-
25kts.  These parameters favor multi-cell clusters with the
potential for damaging winds and large hail during the afternoon.
Guidance shows the front moving south of the CWA by 03Z Wednesday.

Carney

&&

.LONG TERM... (Wednesday through Next Sunday)
Issued at 352 PM CDT Sun May 17 2026

Briefly in the long term...cool high pressure builds into the
Midwest Wednesday which brings drier weather to the region and
temperatures fall back below normal.  The deterministic NBM is
forecasting highs in the mid 60s to low 70s Wednesday which is above
the upper quartile of the LREF temperatures.  However strong May
sunshine may win out over cold advection so these temperatures are
not unreasonable.  Below normal temperatures continue Thursday with
morning lows dropping into the mid 40s to around 50 and highs of 70-
75. Southerly flow and near to above normal temperatures with a
chance for rain return Friday and Saturday as the surface high moves
into southeast Canada and another trough moves into the Plains and
Midwest.

Carney

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1240 PM CDT Sun May 17 2026

VFR flight conditions and gusty southerly flow is expected to
prevail across the area through the evening. A line of
thunderstorms will develop over Kansas and Nebraska this evening
and move east through the night. It will eventually move into
central and northwest Missouri late tonight into Monday morning.
Guidance varies on the speed of the line, and therefore the
arrival time into our area. However the general consensus is that
the line will be moving into central and northeast Missouri
between 08Z and 12Z. The line will continue moving east into our
area through the morning. Gusty winds and IFR visibilities in
heavy rain will be possible.

Carney

&&

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

$$

WFO LSX