166
FXUS63 KLSX 161150
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
Issued by National Weather Service Kansas City/Pleasant Hill MO
650 AM CDT Wed Jul 16 2025

.KEY MESSAGES...

- Today will be hot and humid, with heat index values over 100.
  Thunderstorms move in from the north tonight with a low
  threat for severe storms and flash flooding.

- A stalled front keeps showers and storms around Thursday and
  Friday.

- Heat builds this weekend into next week, with a prolonged
  stretch of hot and humid weather likely.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 310 AM CDT Wed Jul 16 2025

Ridging becomes more dominant today leading to hotter
temperatures than we have seen in a while. Highs area wide
reach up into the 90s. With continued humidity, the heat index
rises into the 100 to 105 degree range this afternoon. While
this is closing in on dangerous levels, the duration of this
heat is expected to only be one day, so we have opted not to
issue a Heat Advisory.

With the ridge more dominant, we wont see nearly as much pop
up shower and thunderstorm activity this afternoon. A few pulse
storms may develop, but they will be few and far between. A
remnant convective complex moves by to our north, strengthening
as it nears the Great Lakes, but it is expected to remain north
of our forecast area today. The greater threat for thunderstorms
comes tonight when thunderstorms form along a southward moving
cold front. This activity is expected to initiate in the evening,
pushing southward into the overnight hours. Shear does increase
as the front approaches, with just enough combination of
instability and shear to produce a threat for some storm
organization during the evening, but storms are expected to
weaken as the night goes on and instability wanes. Thus the
greatest threat for severe thunderstorms in our area is in the
far northern portion of the forecast area from about 8PM to
midnight. Damaging winds are the primary severe weather threat,
though slow moving storms in a moisture-rich environment may also
pose a threat for flash flooding. Both of these threats wane as
the night goes on and as storms shift southward.

The front stalls in the region on Thursday, with significantly
cooler weather north of the front. South of the front, the heat
continues. The front serves as a focus for another round of
showers and storms during the afternoon and evening. While there
may be just enough shear for a low severe weather threat, the
greater threat is likely locally heavy rainfall and flash
flooding. There is still considerable uncertainty on where the
front stalls and thus where the greatest thunderstorm threats
will exist.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 308 AM CDT Wed Jul 16 2025

The front is likely still with us on Friday, but it will be
losing its definition with time and gradually lifting back
to the north. So while we are likely to see another round of
showers and storms during the afternoon and evening, they may
not be quite as widespread as they are on Thursday.

As we get into the weekend, ridging builds even more over the
south central US, bringing the hot and humid weather back into
the region. Temperatures in the mid 90s and dewpoints in the
mid 70s will combine to push the heat index back over 100 again
as early as Saturday. There`s more uncertainty on this across
the northern and eastern portions of the area this weekend as
shortwave troughs rounding the top of the ridge may bring
rounds of clouds and thunderstorms. If there are more extensive
clouds then the higher temperatures may not be reached. However,
confidence is greater in southern areas that the heat will be
building back in again this weekend. The ridge continues to
build through next week, with greater confidence that the
whole forecast area will get in on the hot and humid weather.
An extended period of heat index values over 100 is
increasingly likely as we go into next week, which would
necessitate a Heat Advisory or an Extreme Heat Warning
depending on how extreme those heat index values get. While
there may be some daily variation, the position and strength
of the ridge next week gives us the stronger confidence that
we will see a long duration period of heat.

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 629 AM CDT Wed Jul 16 2025

Some localized patchy fog across the area this morning with 1/4
mile to 2 miles visibilities in Litchfield (K3LF), St Charles
KSET. particularly at river valley locations. However, the main
focus will be in the morning thunderstorms and showers moving
east across the northern portions particularly at TAF sites
KUIN/KCOU/KJEF. Expect the showers and thunderstorms to remain
mainly north of the ST Louis area and do not have any mention
of thunderstorms or showers this morning. Once these showers and
thunderstorms move across this morning there is a potential for
afternoon thunderstorms to develop after 02Z for KUIN in which
have provided a PROB30 with -TSRA and gust 20kts.

The better chance of showers and thunderstorms will likely be
overnight with as continue with KUIN but PROB30 group for KCOU
and KJEF after 06z. Again confidence is higher for storms to
remain north of the St Louis area and KSTL, KSUS and KCPS expect
to see OVC040 after 09z.

&&

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

&&

$$

SHORT TERM...Kimble
LONG TERM...Kimble
AVIATION...WFO EAX