766
FXUS63 KLSX 251852
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
152 PM CDT Wed Jun 25 2025

.KEY MESSAGES...

- Heat and humidity will remain largely unchanged tomorrow, and
  potentially into the weekend in places that do not see afternoon
  rain or cloud cover. Expect heat index vales to reach between
  100 and 105 once again.

- While we don`t expect many showers tomorrow, more substantial
  chances for showers (30 to 60%) exist each day from Friday
  through Monday.

- While heat relief over the weekend is tenuous at best, more
  widespread relief is increasingly likely Monday through
  Wednesday.

&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 147 PM CDT Wed Jun 25 2025

Heat continues to be the main focus over the next several days,
along with chances for showers and thunderstorms that will increase
Friday onward.

Before we get there though, we can add yet another day to the list
of consecutive days with a heat index of 100 degrees or more, as the
heat index at St. Louis has already hit 101 degrees as of 12:00 PM.
This is consistent across much of the area, with perhaps slightly
lower values across northeast Missouri. With very little change
expected in the airmass tomorrow, aside from perhaps even a bit less
cloud cover that may actually allow us to warm up another degree or
two, we can expect yet another say of similar heat index values.

As a result, the ongoing Heat Advisory will remain in place as-is
for the time being, as it is a near certainty that it will be needed
through at least tomorrow evening. What is less certain is whether
or not some or all of this advisory will need to be extended into
Friday and beyond, which will be discussed in the extended portion
of the forecast.

Meanwhile, widely scattered showers and even a few weak
thunderstorms have been observed already this afternoon, with much
earlier initiation and increased coverage compared to the same time
yesterday. Showers are forming within a weakly capped and unstable
environment, complete with 2000 J/kg of MLCAPE. However, given the
persistence of the ridge overhead, very little if any wind shear
exists, and storms have lacked organization as a result. As such,
while coverage will likely increase slightly over the next few
hours, bursts of heavy rain and perhaps an occasional modest
downburst (less than 40 mph) are about all that should be expected
with these storms. Given the lack of kinematic support, these storms
should quickly diminish near or after sunset as instability wanes.

Quiet conditions are expected overnight after showers wind down, and
lows will once again settle into the 70s. During the afternoon
tomorrow, while hot and humid conditions will persist as previously
mentioned, shower coverage is expected to be much more sparse, and
most areas are likely to remain dry.


19

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 147 PM CDT Wed Jun 25 2025

Beginning Friday, the large upper ridge across the Ohio Valley
will finally begin to lose amplitude, and weak zonal flow will
prevail across the CONUS. While we will still be under the
influence of elevated 500 mb heights, and likewise a warmer than
average airmass, this opens the door for weak cold fronts, reduced
convective inhibition, and a corresponding increase in chances
for showers and thunderstorms.

In addition to providing an opportunity for meaningful rainfall,
this does offer a glimmer of hope for at least some localized heat
relief. However, I would caution that ensemble guidance continues to
forecast temperatures in the low to mid 90s Friday through Sunday
across the current advisory area, with equivalent and perhaps even
higher humidity. As such, any heat relief would have to arrive in
the form of precipitation or cloud cover, and this would likely be
very modest and limited in coverage. As such, it remains possible
that some or all of the Heat Advisory may need to be extended for
one or more days depending on how precipitation trends evolve.

Regarding rain chances, in previous forecasts ensemble guidance had
begun to focus on Frday and Sunday as the most likely days for
precipitation as separate weak fronts sank into the area.
However, latest guidance has reversed this trend, and now
precipitation chances are more or less the same each day from
Friday through Sunday, with a heavy diurnal coverage pattern. This
is reflective of the inherent difficulty in trying to pinpoint
precipitation chances during weakly forced summertime flow regimes
like this, and we`ve opted to maintain these evenly distributed
chances until we can get a better handle on the smaller scale
features. However, it should be noted that we do not expect
constant rain through the weekend, and there will likely be
extended dry periods in between bouts of showers and
thunderstorms. Meanwhile, in spite of the flattening ridge and
closer proximity of the upper jet, wind shear projections remain
very limited, and likewise the chance for strong or severe storms
does not appear to be particularly high. We will need to keep an
eye on the potential for bursts of heavy rain, though.

By Sunday night and into Monday, ensemble guidance continues to
suggest that a more substantial upper trough will move into the
Great Lakes and send a more significant cold front into our area.
This adds more confidence that more widespread heat relief will
arrive Monday and continue through early next week, with
temperatures likely dropping to at least closer to seasonal
averages. Meanwhile, the increased forcing from the cold front and
shortwave combined with the very humid airmass already in place
should also result in another round of showers and thunderstorms.
This round also has the potential to produce more widespread rain,
and a still-limited but slightly higher chance for stronger
thunderstorms.


19

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1155 AM CDT Wed Jun 25 2025

Primary concern during the 18Z TAF cycle is the potential for
isolated to widely scattered shower and thunderstorm development
this afternoon. A few showers have already developed as of 17Z,
and while lightning has not yet been observed as of this writing,
the potential for a few strikes exists, along with bursts of
heavy rain. Coverage will be somewhat sparse, but just enough to
support a PROB30 mention in the TAF. Showers will diminish this
evening, and VFR conditions are expected the remainder of the
period.

BRC

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 9 PM CDT Thursday for Callaway MO-Cole MO-
     Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO-
     Lincoln MO-Madison MO-Montgomery MO-Osage MO-Pike MO-
     Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
     City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-
     Washington MO.

IL...Heat Advisory until 9 PM CDT Thursday for Adams IL-Bond IL-Brown
     IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-
     Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Pike IL-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX