Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

897
FXUS63 KLSX 211735
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1235 PM CDT Tue Apr 21 2026

.KEY MESSAGES...

- High temperatures of 5 to 15 degrees above average are expected
today through Thursday, with breezy winds this afternoon.

- Widespread rain is expected (80-90+%) Thursday night through
sometime Friday, with a reasonable chance (30-50%) for thunderstorms
also.

- An active pattern continues beyond Friday, with additional chances
for showers and thunderstorms late in the weekend and early next
week.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 301 AM CDT Tue Apr 21 2026

While largely non-hazardous weather is expected in the short term
(today and tomorrow), there are a few weather items to discuss,
including the continuation of a warming trend, gusty southwest
winds, and a very limited potential for some showers in our
northern areas this afternoon/overnight.

Firstly, the resumption of southwesterly low level flow has finally
put an end to the potential for patchy frost this morning, thanks to
a combination of warm air advection and mechanical mixing.
Temperatures as of 1 am had only dropped into the upper 40s to mid
50s, and are only expected to drop perhaps another 5 degrees or
so by sunrise. As such, this should easily take frost off the
table, and set the stage for an even warmer afternoon than
yesterday.

As the morning progresses, a deepening boundary layer will begin to
tap into a robust southwesterly low level jet just above the
surface, and surface winds will begin to increase quickly as a
result. While we do not expect gust speeds this afternoon to reach
Advisory levels, sustained speeds of 15 to 20 mph with gusts to 30
or 35 mph will not be difficult to achieve, especially north of the
I-44 corridor. While a nuisance for some outdoor activities, these
speeds should not be otherwise impactful, considering that green/wet
fuels and moderate humidity will mitigate the fire threat.

Meanwhile, these persistent southwesterlies will also drive
continued warm air advection, resulting in another boost in
temperatures today. Afternoon maximums are likely to reach near 80
degrees area-wide, and perhaps slightly warmer along the Missouri
River and St. Louis thanks to both downsloping and the urban heat
island.

Later in the afternoon, a subtle shortwave is expected to drive a
weak cold front through Iowa, and it is expected to stall
somewhere along our northern periphery. Moisture pooling along
this boundary combined with ample heating and steep mid level
lapse rates will boost instability. However, a substantial
capping inversion is expected to render this instability largely
inaccessible for surface parcels, especially considering that the
weakening front does not appear to be a strong initiating
mechanism. While we may see a few elevated showers or even weak
thunderstorms make a run at parts of west central Illinois and
far northeast Missouri late this afternoon and overnight, we do
not expect this activity to be significant. Most high resolution
guidance also supports this, although we do note a few outliers
that carry a few small cells into our area.

By tomorrow, a weakening surface pressure gradient will allow winds
to weaken, although we will maintain southwesterlies. Meanwhile, a
highly amplified upper ridge will move directly overhead, and as a
result we will maintain similar temperatures and largely dry
conditions Wednesday. In fact, this should produce the most pleasant
weather of the week, as more rain is on the way late Thursday onward.

BRC

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 301 AM CDT Tue Apr 21 2026

The biggest story during the extended period continues to be the
return of an active, wet pattern, which is expected to bring another
round of soaking rain and at least a few thunderstorms between late
Thursday and late Friday, followed by additional rounds of rain
late in the weekend and early next week.

Driving this change is a rather complex upper flow pattern, which
features the phasing of a weakening Pacific low (currently producing
rain in the California Coast) with both a northern stream trough and
a southern-stream subtropical jet streak. If that sounds like a lot
of things happening at once, you would be correct in your
assumption. The surface response to all of this is expected to
manifest as a broad, deep, low pressure system across the northern
high Plains by Wednesday afternoon, which will slowly move east by
Thursday evening. This will drive strong southerly return flow and a
narrow plume of moisture into the central Plains and Mississippi
river Valley ahead of a slow-moving cold front.

Within this moisture plume, building instability and surface
convergence from the front will drive widespread showers and
thunderstorms during the day Thursday, initially to our west where
strong/severe storms are increasingly likely. This area of
showers and storms will move east and into our area sometime late
Thursday and through the day Friday, and while there remains some
timing uncertainty (particularly later in the event), most
ensembles/clusters have converged on Thursday night/Friday morning
as the window of highest probability for our area, with largely
warm, dry, and breezy conditions earlier in the day. Whether or
not this timing fluctuates slightly, there is high confidence
(80-90%+) that most of our area will see rain sometime in this
window. Meanwhile, ensemble mean PWAT values approach the 90th
percentile within a narrow moisture plume, and when combined with
the slow moving front that is nearly parallel to the mean flow,
soaking rain is well within reason (50-70% chance for at least 1/2
inch per the NBM). While these probabilities don`t immediately
suggest a significant flooding threat, many places saw significant
rains over the last week, and higher rain totals due to
convective elements may be getting washed out a bit by ensemble
spread (90th percentile NBM amounts range from 1.5 to 2.5 inches).
This will need to be monitored as the event gets closer.

As for strong/severe storms, the increasing confidence in a
nocturnal event does limit our potential somewhat due to the reduced
instability, not to mention our area may also be displaced from the
strongest wind shear. Still, this will need to be watched,
particularly in central/northeast Missouri Thursday evening at the
onset of the event, and Friday afternoon/evening in the Ozarks if
the front stalls and instability can redevelop in our area.

On the latter point, confidence in the evolution of this event
decreases significantly beyond Friday morning, as ensembles/clusters
start to diverge regarding the southward progression of the front
and also the next approaching storm system later in the weekend and
early next week. There are some indications that the front may stall
or even lift back northward Friday thanks to the influence of the
subtropical jet (and another developing surface low to the west),
but this is far from unanimous, and we are now getting very far into
the weeds at this time range. This slower solution would prolong
shower/thunderstorm chances Friday, although thunderstorm chances
especially would also depend on lingering cloud
cover/precipitation/cold pools from the morning activity.

As the weekend progresses into early next week, ensembles suggest
that the upper flow pattern may briefly transition into an omega
block across the Midwest, followed quickly by another southern-
stream trough undercutting it. Again, there is considerable model
disagreement on many of these details at this stage, but in spite of
this, ensemble guidance also depicts a relatively high probability
(50-70%) of being impacted by another round of precipitation
sometime in the Sunday/Monday timeframe.

BRC

&&

.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