Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

938
FXUS63 KLSX 201049
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
549 AM CDT Sat Jun 20 2026

.KEY MESSAGES...

- A Flash Flood Watch will be in effect from late tonight into
  early Monday for the threat of heavy rainfall potentially
  leading to flash flooding.

- Some storms could be strong to severe late tonight into late
  Sunday evening, bringing the threat of tornadoes and damaging
  wind gusts.

- Below-average to seasonably cool conditions are anticipated next
  week.


&&

.SHORT TERM... (Through Sunday Night)
Issued at 220 AM CDT Sat Jun 20 2026

Regional radar imagery reveals a few isolated pockets of showers
that are percolating southeastward and are being aided by weak low-
level frontogenesis associated with a weakening boundary. Some
pockets of heavier showers featuring intermittent lightning cannot
be ruled out as pockets of MUCAPE near 250 J/kg is forecast by
guidance to linger throughout the early morning hours, but the best
chance of this is across western MO. High-resolution guidance is in
good consensus that this activity will continue through 10z mostly
across central/northeast MO and quickly fade into the mid-morning as
the previously mentioned band of frontogenesis fizzles out. The
remainder of today is expected to be dry with seasonably cool
temperatures and light surface winds continuing into tonight.

Beginning tonight, our attention turns to a developing lee-side
surface cyclone that will initiate convection across KS/NE, with
quick upscale growth into an MCS/complex with eastward extent. The
commencement of the features above are in response to a shortwave
trough that will be traversing eastward into the Great Plains within
quasi-zonal flow in the mid/upper-levels. The MCS is forecast by
CAMs to be nearing central/north-central MO shortly after midnight
as it propagates along the Corfidi upshear vectors and/or favors to
track along the eastward extending warm front. As it enters into the
area (northeast/central MO), the greatest severe threat will be
damaging winds associated largely to any bowing/surging segments
that are still present. Additionally, locally heavy rainfall is
possible as anomalous PWATs near 2" (~99th percentile of
climatology) and a strengthening LLJ leading to increased moisture
transport will all be in place. If training or back-building along
the southern flank becomes favored across the area, this will
enhance the flash flooding threat as thunderstorms will be efficient
at rainfall production with >12,000ft warm cloud layers favoring
warm rain processes. A widespread 1-3"+ is forecast from this MCS
with locations that experience training or multiple rounds of
thunderstorms may easily reach >3" of rainfall from early Sunday
morning into Sunday evening.

What remains quite uncertain is the evolution of this MCS
(intensity, speed, and track), as is typical with these systems.
Lingering cloud cover/thunderstorms associated with the remnant MCS
throughout the day would decrease instability and thus lower the
afternoon/evening severe threat, while less of the above would favor
the opposite. Another feature previously mentioned will be a surface
cyclone that some guidance resolves will traverse across the area on
Sunday. If a surface cyclone does indeed cross the area at an
optimal time (afternoon/evening), that would locally enhance the low-
level shear, ultimately leading to a greater threat for tornadoes on
Sunday, especially near the warm front/surface cyclone. Otherwise, a
cold front is progged to be entering western MO Sunday evening which
should work to initiate additional convection, particularly near any
remnant outflow boundaries left behind from the earlier MCS. With
sufficient effective bulk shear (>30kts), organization of
thunderstorms is expected with a linear mode favored as the shear
vector is forecast to be nearly parallel to the boundary. Analysis
of the LREF IQR spread for MUCAPE highlights the uncertainty as the
25th-75th spread is around 1,000-1,500 J/kg across the area.
Regardless, with potential overlap of shear and instability, strong
to severe thunderstorms will be possible Sunday afternoon/evening
with the threats being damaging winds, tornadoes, and large hail.
Also, with the mean flow becoming increasingly parallel to the
forecast boundary, training of thunderstorms leading to locally
heavy rainfall will also be a threat into Sunday night.

As a result, a Flash Flood Watch has been issued for the entire area
with a range of start/end times in a window from late tonight
into Monday morning.

Peine

&&

.LONG TERM... (Monday Morning through Friday)
Issued at 220 AM CDT Sat Jun 20 2026

By Monday, the main shortwave trough axis will be crossing over the
area which is expected to bring an end to the widespread showers and
thunderstorms from west to east Sunday night into Monday morning. A
few subtle pieces of mid-level energy are progged by long-range
guidance within the wake of the shortwave, which may lead to some
lingering shower activity across the area early on Monday. Also, as
the surface cyclone lifts to the northeast, it will drag the cold
front through the region leading to ample low-level CAA on Monday.
As a result, Monday will feature high temperatures around 10 degrees
below average.

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. 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 the end of the week.

Peine

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 542 AM CDT Sat Jun 20 2026

Dry and VFR conditions are expected with light winds through 06Z
tonight. Then there will be an increasing chance for showers and
thunderstorms from the west that will first affect UIN/COU/JEF so
have included a PROB30 group at these terminals beginning at
08-09Z, and also at STL starting at 15Z on Sunday. There will be
some MVFR restriction in visibilities, though MVFR and IFR
ceilings and visibilities in moderate to heavy rain will be more
common on Sunday as thunderstorms become more widespread.

Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch from Sunday morning through late Sunday night for
     Franklin MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO.

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

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

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

     Flood Watch from late tonight through Sunday evening for Adams
     IL-Brown IL-Calhoun IL-Pike IL.

&&

$$

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