Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

498
FXUS63 KLSX 182029
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
329 PM CDT Tue Aug 18 2026

.KEY MESSAGES...

- Widespread showers and thunderstorms are forecast late tonight
  into Wednesday morning. There is a low threat of damaging
  winds, mainly in central MO. Otherwise, thunderstorms will be
  capable of gusty winds and locally heavy rainfall/isolated
  flash flooding.

- Additional showers and thunderstorms are possible (30 to 40
  percent chance) Wednesday afternoon and evening. There is a
  conditional threat of severe thunderstorms across at least
  southeastern MO, but overall forecast confidence is low.

- Near to slightly below average temperatures will prevail
  through this weekend.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 328 PM CDT Tue Aug 18 2026

The MCS across northeastern KS and western MO has been slow to
weaken today with a few sprinkles or light showers reaching central
MO. Cloud debris from this MCS has also limited heating across
northeastern MO and west-central IL with high temperatures in the
low-80s F, mid-80s to around 90 F elsewhere amidst greater
insolation.

A remnant outflow boundary from the aforementioned MCS and an
incoming cold front will serve as focii for shower and thunderstorm
development tonight across eastern KS into western MO at the nose of
a seasonably strong (30 to 40-kt) nocturnal LLJ. CAMs are in general
agreement that upscale growth into a forward-propagating MCS will
quickly take place and track east-southeastward through central,
east-central, southeastern MO late tonight (after midnight).
Effective wind shear of 20 to 30 kt and MUCAPE projected around 1000
to 1500 J/kg will be marginally favorable for organization of
thunderstorms, but a nocturnal temperature inversion will be
strengthening by the time thunderstorms reach the CWA, limiting
confidence in severe thunderstorms with damaging winds. That being
said, the inversion will be more shallow and beneath a layer of dry
air and steeper lapse rates during earlier arrival across central
MO, where the threat appears locally greatest. Otherwise,
thunderstorms will be capable of sub-severe gusty winds.

Another threat late tonight into early Wednesday morning will be
locally heavy rainfall and isolated flash flooding. PW increasing to
around 2" (95th climatological percentile), deep-warm cloud depths,
and strong LLJ are all favorable ingredients for heavy rainfall, but
it is uncertain how much trailing/training thunderstorms there will
be with the LLJ quickly veering. The latest HREF and REFS QPF PMM
and LPMM indicate a corridor of localized rainfall amounts up to 3
or 4" mainly across central/southeastern MO, which is just above 3-
hr FFG. With uncertainties in how sustained heavy rainfall rates
will be, confidence in flash flooding beyond a localized basis is
too low to warrant a Flood Watch at this time.

Showers and weak thunderstorms could be widespread along/south of I-
70 through Wednesday morning, gradually decreasing in coverage as
the LLJ also gradually weakens. However, it is uncertain how quickly
showers will dissipate, or even if they do entirely, during the
afternoon, which lowers confidence in the temperature forecast and
threat of severe thunderstorms during the afternoon/evening. If
clouds and precipitation linger through the afternoon, high
temperatures could be limited to the upper 70s to low-80s F and the
effective front shunted to southeastern MO or even further south.
This boundary and locations to the south will be the most likely
area for surface-based thunderstorm development including supercells
initially with 30 to 40-kt of deep-layer shear. Therefore, damaging
winds, large hail, and even a brief tornado, where low-level
shear/helicity are enhanced near the front, are potential hazards.
Otherwise, there should be some uptick in overall shower and
thunderstorm coverage as an upper-level trough arrives, but other
than in southeastern MO thunderstorms will be elevated and dependent
on how much instability can be regenerated. Showers and
thunderstorms will become more focused to the south and southeast of
the CWA Wednesday night in response to where LLJ forcing is greatest.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 328 PM CDT Tue Aug 18 2026

The front will advance further south of the CWA by Thursday with a
high pressure center arriving in its wake with dry and seasonably
cool conditions persisting into Friday. In prevailing upper-level
northwesterly flow, a shortwave trough and accompanying cold front
will pass sometime in the Friday evening through Saturday afternoon
timeframe, but the timing is uncertain. Given that moisture return
will be limited, only 20 to 30 percent of ensemble model membership
has showers and thunderstorms. Without strong/persistent low-level
WAA, temperatures will be near to slightly below average, cooling
further below average on Sunday as seasonably cool airmass and
strong surface high pressure center skirts the Mid-Mississippi River
Valley.

Early next week, global model guidance agrees that northwesterly
flow will continue but vary on whether or not heights rise as a
ridge across the Rocky Mountains amplifies or heights fall as
longwave troughing develops across the Great Lakes. The former would
favor warming temperatures to above average and the latter would
favor a slow moderation of temperatures around average. There is not
a clear preference for either scenario in ensemble model guidance at
this time. Additionally, there is a signal for passing shortwave
troughs to provide rounds of showers and thunderstorms during that
time, further complicating the pattern.

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1234 PM CDT Tue Aug 18 2026

Dry and VFR flight conditions will continue through at least this
evening, until showers and thunderstorms develop and track eastward
through the area overnight, mainly along and south of the I-70
corridor. Therefore, confidence in impacts is highest at KCOU, KJEF,
and St. Louis metro terminals; however, KCOU/KJEF is preferred for
relatively stronger thunderstorms that could contain gusty winds.
Thunderstorms will also be capable of MVFR to IFR flight conditions
but flight conditions will generally be improving Wednesday morning
into afternoon as showers and rumbles of thunder gradually decrease
in coverage. Winds will be light outside of thunderstorms.

&&

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

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Pfahler