St. Peters, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

798
FXUS63 KLSX 250838
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
338 AM CDT Fri Jul 25 2025

.KEY MESSAGES...

- While northern Missouri and west-central Illinois will enjoy a
  break from the oppressive heat and humidity today, along with a
  low threat for strong to marginally-severe storms, those south
  of there will continue to observe heat index values in the
  triple digits.

- Dangerous heat and humidity continues for at least those in,
  around, and south of the I-70 corridor until at least Tuesday.
  Towards the end of next week, confidence is increasing in a
  much- needed end to the heat.

&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 337 AM CDT Fri Jul 25 2025

A weak, southwest-northeast oriented low-level jet continues to
draw moisture through the northern half of Missouri and west-
central Illinois at this hour. This LLJ, along with weak mid-level
ascent and a transient, stationary surface front, is prolonging
showers and thunderstorms over the same area. While pockets of
heavy rain and localized flooding were observed overnight, they
were fairly minor and confined to west-central and western
Missouri. While locally- heavy rain is still a limited threat
through the morning, confidence in flash flooding is lower than
previous forecasts. As such, the Flood Watch in northeast Missouri
and west-central Illinois was cancelled early.

While the heavy rain threat is waning, the stream of moisture near
the front will continue to support abundant cloud cover and 40-50%
rain chances limit the threat of dangerous heat in northeast
Missouri and west-central Illinois today. Further south, mostly
clear skies and low (less than 15%) rain chances will result in
little relief from tiple-digit heat index values. Some high-
resolution guidance draws a remnant outflow boundary from last
night`s convection back north as a mesoscale convective vortex
during the afternoon. While timing, location, and strength of this
feature vary, there is a fairly unstable environment for at least a
marginal damaging wind threat across northern Missouri and west-
central Illinois late this afternoon into the evening. Most
convection, however, will likely remain below severe limits. Copious
moisture throughout the column will keep a threat for heavy rain and
perhaps localized flooding around tonight. That said, the best
threat for organized flooding will be to our north and west.

Shower and thunderstorm chances linger through Saturday, primarily
north of the I-70 corridor, with the front wavering over the
northern third of the forecast area. For those south of here, it`s
wash-rinse-repeat with another hot and humid day in store. The mid-
level ridge will gradually build back poleward and promote broad
subsidence and more widespread warming in the region by early Sunday
morning.

MRB

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 337 AM CDT Fri Jul 25 2025

Sunday isn`t clear-cut for exact sensible weather, either, as most
guidance shows some weakening near the periphery of the ridge across
our area introducing a 20-50% chance for showers and thunderstorms
again centered near the Missouri-Iowa border. Further south, the
ridge will be more established and quash any instability while
keeping the I-70 corridor and south hot and humid again. Most
uncertainties relating to the heat exit by Monday, when the ridge
finally wins out again and advances northward. Dangerous heat and
humidity continue their relentless impact on the region through
Tuesday, with the NBM Maximum Temperature 10th percentile (near the
best-case scenario) still in the mid-90s. Coupled with very warm
(upper 70s to low 80s) overnight lows and a humid airmass, this
heatwave will continue to result in somewhat considerable impacts in
the St. Louis metropolitan area. Slightly fewer impacts can be
expected outside of the urbanized area where the urban heat island
influence is not as intense.

Like an oasis in the desert, relief appears to be on the horizon in
the form of a significant pattern change by mid-week. Confidence is
gradually increasing that this is no mirage: now a preponderance
of ensemble members show a longwave trough impinging on the
eastern flank of the ridge and shunting it southwest of us. Nearly
the entire NBM envelope draw temperatures and humidity to much
more tolerable levels starting as early as Wednesday as a result.
Some deterministic guidance pulls showers and thunderstorms along
a backdoor cold front (east-west moving) into the bi-state
Wednesday, which would make sense given the large scale pattern.
We seem to be solidly in a new airmass regime by Thursday that
sticks around for at least a few days. These solutions are further
supported by the CPC 6-10 Day and 8-14 Day Temperature Outlooks,
as well as the CIPS Extended Analog Guidance from Saint Louis
University. They all paint increasing probabilities of near-normal
to below-normal temperatures into late next week.

MRB

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1103 PM CDT Thu Jul 24 2025

Showers and embedded thunderstorms will continue through much of the
night across northeastern, north-central MO and west-central IL,
leading to bouts of MVFR flight conditions at KUIN. A few
thunderstorms are expected to come at least close to KCOU, but
confidence is lower and sufficient for a PROB30 group. Precipitation
will decrease in coverage and dissipate through and after sunrise
Friday morning. Redevelopment of thunderstorms is possible (20 to 30
percent chance) late Friday afternoon and evening; however,
confidence in their location, timing, and coverage and therefore
impacts at any one terminal is too low for explicit mention at this
juncture. Outside of thunderstorms, predominantly VFR flight
conditions are anticipated.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 7 PM CDT Tuesday for Boone MO-Callaway MO-
     Cole MO-Crawford MO-Gasconade MO-Iron MO-Lincoln MO-Madison
     MO-Moniteau MO-Montgomery MO-Osage MO-Reynolds MO-Saint
     Francois MO-Sainte Genevieve MO-Warren MO-Washington MO.

     Extreme Heat Warning until 7 PM CDT Tuesday for Franklin MO-
     Jefferson MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO.

IL...Heat Advisory until 7 PM CDT Tuesday for Bond IL-Calhoun IL-
     Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Marion IL-Montgomery IL-Randolph IL-Washington IL.

     Extreme Heat Warning until 7 PM CDT Tuesday for Madison IL-
     Monroe IL-Saint Clair IL.

&&

$$

WFO LSX