Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

584
FXUS63 KLSX 211137
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
637 AM CDT Sun Jun 21 2026

.KEY MESSAGES...

- Thunderstorms are anticipated across the area this morning
  through tonight. Some storms may be strong to severe this
  afternoon/evening with the main threats being tornadoes,
  damaging winds, and large hail.

- Locally heavy rainfall potentially leading to flash flooding is
  also expected this morning into tonight.

- Seasonably cool conditions are forecast this week.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 221 AM CDT Sun Jun 21 2026

An active period of weather is forecast from early this morning into
tonight, including the risk of severe weather and heavy rainfall
potentially leading to flash flooding in some locations. Currently,
as of 06z (1am), a large mesoscale convective vortex (MCV)
encompasses much of the state of KS, with a broad area of showers
and thunderstorms extending northeastward into northern MO. The
strongest portion of this activity, which is being maintained by a
strong (50+kt) low-level jet, is currently tracking eastward across
south-central KS, essentially following the instability gradient
that extends eastward into southern MO. All of this activity has
been initiated by a mid-level shortwave trough that is traversing
the Great Plains, which has also sparked the formation of a lee-side
surface cyclone that is positioned to the west of the MCV.

As the MCV pushes eastward into central/northeast MO around 12z
(7am) this morning, a strong LLJ will continue to provide abundant
moisture transport and low-level shear to maintain convection.
However, with marginal instability early this morning (<500 J/kg
MUCAPE) across much of the area, the greatest threat with stronger
thunderstorms is expected to be marginally severe wind gusts up to
60mph, which would be favored along any bowing/surging segments that
may form. The primary threat is still forecast to be heavy rainfall
potentially lead to flash flooding. Forecast soundings indicate that
a warm cloud layer >12,000ft will be present, favoring warm rain
processes. Also with PWATs near 2", thunderstorms will be efficient
rainfall makers, and capable of rainfall rates in the 1-2+"/hr
range. The greatest threat for flash flooding this morning appears
to be across central/northeast MO and west-central IL as the center
of the MCV should be focused in those areas.

By the early afternoon, the MCV is forecast to be sliding across
east-central MO into western IL. An intensification of thunderstorms
is possible as instability increases throughout the day. Also,
within the MCV and vicinity of the surface cyclone, low-level
shear/SRH becomes enhanced, leading to an increasing threat for
tornadoes. With rich moisture and elongated low-level hodographs,
storm clusters with embedded supercells and surges with mesovortices
will be possible, especially if low/mid-level instability increases.
As a result, the greatest threats anticipated this afternoon/evening
are tornadoes along with damaging winds. This includes a potential
for strong tornadoes if supercells remain relatively organized and
sustained across the area.

Additionally, an outflow boundary in the wake of the MCV along with
the cold front associated with the surface cyclone will serve as
localized enhancements of surface convergence. Therefore, with
sufficient instability, it is expected that additional thunderstorm
development will happen near these features, perhaps somewhere from
central-southeast MO in the afternoon. With sufficient deep-layer
shear (>30kts), there is the potential for organized development
along these boundaries. Tornadoes, damaging winds, and large hail
will all be possible with this convective activity, but remains
dependent on the magnitude of instability that is available. Lastly,
the threat for heavy rainfall will continue, with the threat area
potentially shifting to southern portions of the area where the
better convective coverage is forecast to be in the late
afternoon/evening. Also, with the mean flow becoming increasingly
parallel to the forecast boundary later tonight, training of
thunderstorms may enhance the threat for heavy rainfall and flash
flooding.

Guidance reveals the surface cyclone lifting northeastward tonight,
with the cold front clearing to the southeast by tomorrow morning,
bringing an end to the threat for severe weather and flash flooding.
An additional subtle mid-level shortwave is progged to traverse the
area on Monday and may support some lingering post-frontal showers
across southern portions of the area early Monday. With low-level
CAA taking hold on Monday along with post-frontal stratus, a cooler
day is forecast with highs in the 70s across the entire area.

Peine

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 221 AM CDT Sun Jun 21 2026

Northwesterly flow aloft will take hold over the region Tuesday into
the end of the week, resulting in a stretch of largely dry
conditions along with seasonably cool temperatures. A mid-level
shortwave near the Great Lakes Wednesday into Thursday may send a
cold front our way. This feature would bring our best chances for
rain sometime mid-week, with 20-40% chances being revealed by the
latest LREF. Generally, highs in the upper 70s/lower 80s and lows in
the low/mid 60s look to prevail, which is about 5-10 degrees below
climatological normals. This is supported well by the LREF, which
has narrow 3-5 degree temperature IQR spread through Friday.

By the end of the week, long-range deterministic guidance indicates
an additional shortwave aloft moving into the Midwest. Currently,
this appears to be the next best chance for more rainfall, sometime
from late Friday through next weekend. A signal for a warming trend
also appears next weekend, however this is also when LREF
temperatures IQR spreads increase to 10 degrees, indicating more
variability.

Peine

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 637 AM CDT Sun Jun 21 2026

A mesoscale convective vortex is currently moving from western MO
into central MO, with a bowing segment nearing central MO within the
next hour (12-12:30z). Wind gusts up to 45kts have been recently
observed within this segment, which will impact KCOU/KJEF between 12-
13z bringing gusty winds and low visibilities with brief heavy rain.
Widespread showers with embedded thunderstorms is forecast at all
terminals through the late morning into the early afternoon.
Stronger thunderstorms will be capable of IFR visibilities
associated with heavy rainfall, with potential for severe
thunderstorms brining damaging wind gusts, tornadoes, and large
hail. Southerly surface winds become gusty throughout the day with
eventual veering from north to south brining northerly winds behind
a cold front later this evening into tonight. Widespread low MVFR
ceilings are forecast along and behind this cold front with
potential continuing to increase for IFR ceilings at all terminals
as well. Low MVFR/IFR ceilings may potentially stick around until
early Monday with gradual improvement expected into the day
Monday.

Peine

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch through late tonight for Franklin MO-Saint Charles
     MO-Saint Louis City MO-Saint Louis MO.

     Flood Watch through Monday morning for Crawford MO-Iron MO-
     Jefferson MO-Madison MO-Reynolds MO-Saint Francois MO-
     Sainte Genevieve MO-Washington MO.

     Flood Watch through this evening for Audrain MO-Boone MO-
     Callaway MO-Cole MO-Gasconade MO-Knox MO-Lewis MO-Lincoln
     MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-
     Pike MO-Ralls MO-Shelby MO-Warren MO.

IL...Flood Watch through late tonight for Bond IL-Fayette IL-Greene
     IL-Jersey IL-Macoupin IL-Madison IL-Montgomery IL.

     Flood Watch through Monday morning for Clinton IL-Marion IL-
     Monroe IL-Randolph IL-Saint Clair IL-Washington IL.

     Flood Watch through this evening for Adams IL-Brown IL-Calhoun
     IL-Pike IL.

&&

$$

WFO LSX