265
FXUS63 KLSX 291947
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
247 PM CDT Sun Jun 29 2025

.KEY MESSAGES...

- Periods of showers and thunderstorms are expected through Monday.

- A cold front Monday brings less humid conditions for the middle
  of the week with pleasant temperatures. Temperatures warm up
  again later in the week.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 246 PM CDT Sun Jun 29 2025

Multiple clusters of showers and thunderstorms have formed and moved
slowly across our general region today, all beneath a subtle
shortwave trough aloft. CAMs have been hit or miss with these
pockets of thunderstorms today, leading to relatively low confidence
in their output going forward over he next 24-30 hours. That makes
for a rather difficult forecast when it comes to timing and location
of thunderstorm chances. While there is considerable uncertainty on
the local level, at a bigger scale we see we remain in a humid and
generally unstable air mass beneath a subtle shortwave trough aloft.
In that environment we can expect occasional rounds of
thunderstorms. Wind shear remains quite weak so the severe weather
threat is low and focused to our west in the next several hours
where there has actually been some sun today to generate better
instability.

Guidance is in fairly good agreement that the next round of
thunderstorms gets generated across western to central Missouri
overnight. Considerable timing and location variance exists among
the CAMs on this, but the general idea has good support. Despite
some agreement here, confidence remains somewhat lower as most
guidance has not handled this afternoon`s storm clusters very well
which may play a role in what happens tonight. The greatest
thunderstorm chances will be in the western part of the forecast
area tonight.

A stronger trough is moving out of Montana and into the Northern
Plains this afternoon, pushing southeast through the Midwest
tomorrow. This brings with it a cold front which ushers in a change
in air mass behind it. As the front and associated trough move
through our region on Monday, an additional round of showers and
storms may develop in the vicinity of this front. The uncertainty on
this round centers on two things: the availability of instability
and the timing of the front. On the first point, tonight`s round of
storms and any lingering showers and clouds will be a limiting
factor on the development of more robust instability ahead of the
front. With a little lower confidence on tonight`s storms, that in
turn gives us a little better confidence that there will be enough
sun to generate instability ahead of the front on Monday. On the
second point, guidance still varies by several hours on how fast the
front moves through and thus where it will be during peak heating.
If the front is a bit slower, then convection may begin further
north, though it remains possible that the front will be south of
the forecast area by the time it is able to take advantage of peak
instability.

With the arrival of the trough aloft we will have a bit better wind
shear available on Monday. So if storms develop there`s a better
chance that they organize into clusters with the potential for
damaging winds or localized downbursts. This is particularly true if
strong instability is able to develop. While this threat exists, the
considerable uncertainty regarding Monday`s convection limits
confidence and we are currently not messaging this threat.

Kimble

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 246 PM CDT Sun Jun 29 2025

The air mass behind Monday`s front is cooler and much less humid,
with its origins in the Canadian Rockies. With drier air and more
sunshine, though, the sun will act effectively on this air mass and
warm temperatures up into the mid to upper 80s. With dewpoints as
much as 10 degrees lower than prior days, this will feel notably
less humid even if daytime temperatures are similar. Nighttime lows
will fall back into the 60s for the first time in a while due to
those lower dewpoints. This mild and less humid air mass lasts
through at least Wednesday when surface high pressure moves across
the region.

Long range guidance is in good agreement that ridging spills out of
the western US and into the Plains later this week while a broader
trough develops over the Northeast. This will tend to keep things
dry within a northwesterly upper flow over our region, though we`ll
see temperatures gradually warming as the influence of the ridge
becomes more dominant. Low level flow turning southerly will also
advect more humidity back into the region just in time for
Independence Day. The combination of gradually increasing heat and
humidity will cause heat index values to rise through the 90s to
lower 100s, closing in on more dangerous levels. Rain chances
continue to tick downward as confidence has increased in the
subsident northwest flow being present to suppress convection.

Kimble

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1215 PM CDT Sun Jun 29 2025

Fairly low confidence forecast with respect to thunderstorms in
the near term and extending into Monday. A few different
thunderstorm complexes are currently ongoing across the region,
with Quincy actually the most likely to be affected in the next
few hours. Otherwise persistent rain from a dying thunderstorm
complex has led to MVFR to IFR ceilings in and out from central
Missouri to the St Louis metro. Expect these conditions to
gradually improve with at least some heating this afternoon,
although even that is limited by extensive cloud cover.

There`s somewhat good agreement in the high resolution guidance
that a new thunderstorm complex develops across western/central
Missouri overnight, though most of this high resolution guidance
also doesn`t have a good handle on current conditions so
confidence in the consensus output is not as high as it would
seem. The most likely area impacted by this would be in central
Missouri between 6-12Z.

A cold front gradually pushes southward through the region
tomorrow. Additional thunderstorms may develop along this front as
it moves through, though this depends in part on earlier storms
and clouds which may prevent the development of instability ahead
of the front. That said, while confidence in overnight convection
is lower, confidence in convection near the front on Monday is a
little higher. As a result, this is the time period in which the
St Louis metro is most likely to see a thunderstorm, although that
chance is still low enough at this point that only a PROB30
designation will be used in the extended portion of the STL TAF.

Kimble

&&

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

$$

WFO LSX