Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

016
FXUS63 KLSX 211954
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
254 PM CDT Tue Apr 21 2026

.KEY MESSAGES...

- A warmup featuring temperatures 5-15 degrees above average
  continues through Thursday.

- Confidence is high (80-90%) in widespread rainfall Thursday
  night and continuing through Friday with a chance (30-50%) of
  thunderstorms.

- An additional chance for showers and thunderstorms returns late
  in the weekend lasting into early next week.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 252 PM CDT Tue Apr 21 2026

Latest surface analysis reveals a tightened surface pressure
gradient across the area with a weak cold front stretched from
central IA to northern IL that is slowly drifting southward. This
front is being sent southward by a distant mid-level shortwave and
surface reflected low across Ontario. With the driving features far
away, and no reinforcing airmass behind the front, it is expected
to stall across far northern MO/west-central IL. The result for us
today ahead of the approaching front has been increased
southwesterly flow gusting 20-30kts. This wind direction has favored
downsloping off the Ozark Plateau which has boosted temperatures
into the upper 70s/lower 80s this afternoon.

A few high-resolution members develop scattered showers and an
isolated thunderstorms near the front. With southwesterly flow
colliding into the slow moving boundary, enhanced surface
convergence has led to low-level moisture pooling that has resulted
in a narrow swath of weak instability. Currently, the greatest
instability is across northwestern MO (500 J/kg MLCAPE), with
tapering magnitude further east. The latest RAP shows a steady
increase in instability across far northeast MO and west-central IL
until sunset when a narrow swath of 1,000 J/kg MLCAPE is resolved.
Recent CAMs have trended significantly lower over the last 12-24hrs
with the coverage of shower/thunderstorm initiation along this
front. Now, virtually every CAM keeps the development across central
IL and points further east-southeast, which seems reasonable as a
rather strong 750mb cap inversion is in place across the area.
Regardless, there remains a very low (10-20%) potential for an
isolated shower/thunderstorm after sunset for mostly west-central IL
with chances shifting southeastward near south-central IL by
sunrise. Aside from that low probability, tonight will be pleasant
as gusty southwest winds fade after sunset and temperatures slowly
lower into the 50s/60s by sunrise on Wednesday.

By Wednesday, the large-scale pattern involves a mid-level ridge
with its axis across the Great Plains. Mid/upper-level longwave
troughs are situated on either side of this ridge, with a broad
longwave out west across the West Coast and another trough swinging
into the Northeast CONUS. The continuation of mid/upper-level
northwesterly flow will help keep the area dry Wednesday into
Thursday, even though low-level southerly warm/moist advection will
be persisting as well. The surface pressure gradient slackens on
Wednesday, which results in calmer winds (5-12kts) across the area.
Slightly weaker winds (less efficient downsloping), along with a
little more cloud cover, results in temperatures that will be a tad
bit cooler, but still warm with highs in the upper 70s by the
afternoon.

Peine

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 252 PM CDT Tue Apr 21 2026

An active and wet pattern looks to emerge late on Thursday through
early next week. On Thursday, the mid/upper-level regime features a
broadened longwave trough across the western half of the CONUS, with
a ridge near the Great Lakes Region. This ridge is progged by long-
range guidance to narrow and deamplify as it gets pushed eastward
and squeezed into a nearly stationary longwave trough over New
England, allowing the broadened trough to begin influencing the
sensible weather in our area.

First, a majority of the day on Thursday is expected to be warm and
dry, with the ridge axis to the east of the area on Thursday, deep
southwesterly flow will commence resulting in efficient warm/moist
advection. As a result, afternoon highs are forecast to be in the
lower 80s across the entire area, with a gradual increase in cloud
cover from west-east throughout the afternoon. Within the broad
longwave trough mentioned above, a notable mid-level shortwave
swings around the base ejecting into the Great Plains Thursday
afternoon sparking lee-side surface cyclogenesis across the Northern
Plains. The evolution of these features sends a cold front eastward
across the Great Plains towards the area by Thursday evening,
bringing increased chances for widespread showers and thunderstorms.
For our area, the best overlap of instability and shear Thursday
evening into Friday morning will be across central/northeast MO,
where LREF mean SBCAPE reaches 1,000 J/kg with mean 0-500mb Wind
Shear around 25-30kts, which is conducive for a marginal threat of
strong to severe thunderstorms across that area.

Numerical model guidance reveals this cold front slowing down while
it pivots clockwise, with some guidance stalling the front across
the area on Friday, while others progress the front into southern
MO/IL by Friday afternoon/evening. The exact frontal placement on
Friday will be key in determining where the greatest threat for
lingering showers may be along with additional chances for strong to
severe thunderstorms Friday afternoon/evening. Along and south of
this boundary, instability is forecast to be near 1,000 J/kg along
with 0-500mb Wind Shear of 30-35kts, indicating there is continued
potential for continued thunderstorms in the open warm sector.
Currently, this front is expected to reach southern MO/IL by the
afternoon/evening, placing the greatest probability of continued
showers and thunderstorms south of the area. It is important to note
that guidance still varies on the placement of this feature, meaning
that the threat for thunderstorms could shift with future forecasts.
Additionally, the mean flow will be nearly parallel to the quasi-
stationary boundary, which leads to a concern for heavy rainfall.
Across the area, Thursday night into Friday, LREF mean PWATs
approach the 95th percentile, indicating that plenty of column
moisture will be in place for convection to tap into. The LREF 95th
percentile for accumulated precipitation Thursday into Saturday
reveals areas of 2"+, which more accurately represents higher end
amounts that may be realized as a result of thunderstorms.

The mid-level shortwave continues to swing cyclonically around the
longwave trough, lifting into southern Canada by Saturday. Quasi-
zonal flow will prevail for the area Saturday into Sunday with
several nebulous shortwaves being resolved within the flow on
deterministic guidance. With no clear or strong mid-level forcing at
play, the position of the stalled/semi-stationary boundary on Friday
becomes increasingly unclear Saturday into Sunday as it waivers
aimlessly under quieter flow. The most recent guidance keeps the
boundary off to the south, placing the area under a cooler and
potentially drier airmass Saturday into Sunday. This is revealed
well by the LREF 6hr probabilities for accumulated precipitation as
they stay below 20% for the entire area Saturday into Sunday.
Additionally, temperature IQRs become centered near average with
spreads of 5-7 degrees, indicating consensus in a return to near
average temperatures. If the location of this front trends northward
for the weekend, then chances for showers and thunderstorms will
also increase.

Southern stream flow also begins to pick up in activity, with a mid-
level shortwave moving across the Four Corners Region Sunday into
Monday unveiled by deterministic long-range guidance. Plenty of
variance exists regarding this mid-level feature, but the general
consensus is that it ejects into the Great Plains Sunday night into
Monday, bringing additional chances for showers and thunderstorms to
the area on Monday. Temperatures appear to be near average into
early next week as each system that progresses across the region
lacks a reinforcing colder airmass.

Peine

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1235 PM CDT Tue Apr 21 2026

Strong and gusty southwesterly winds continue this afternoon before
fading around sunset this evening. Gusts will hold on longer at
KUIN/KCOU with quicker decoupling at river-valley locations
(KSUS/KCPS/KJEF). A weak cold front stalls near northern MO/west-
central IL late this evening with a low chance (10-20%) for
scattered showers or thunderstorms near KUIN. Shower and storm
coverage is forecast to be greater across central IL and points to
the east. A large area of low stratus is positioned across
Louisiana/Texas/Oklahoma and is progged by guidance to lift
northeast towards central MO tomorrow morning. Guidance is also
keying in on possible MVFR ceilings developing near the stalled
boundary across northern MO/central IL tomorrow morning, which would
potentially threaten KUIN. Confidence is lower in MVFR ceilings
occurring near the St. Louis metro tomorrow morning, so no mention
was included in this TAF issuance.

Peine

&&

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

$$

WFO LSX