703
FXUS63 KLSX 261129
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
629 AM CDT Sat Jul 26 2025

.KEY MESSAGES...

- Dangerous heat and humidity will persist for most of the region
  into early next week, particularly on Monday and Tuesday.
  Relief begins Wednesday north of I-70, and spreads areawide
  Thursday.

- Scattered thunderstorms will threaten most of the region today
  and tomorrow intermittently, with gusty winds and heavy
  downpours being the main threat. While unlikely, there is a low
  (10%) chance for damaging wind today north of I-70 from the
  strongest thunderstorms.

&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 342 AM CDT Sat Jul 26 2025

Showers and thunderstorms continue to percolate along a
stationary frontal boundary and amidst near-record precipitable
water values from northwest Missouri to southeast Iowa. Most of
the activity is evading the northern fringes of the forecast area,
focusing where SPC Mesoscale Analysis paints an axis of deep
moist convergence. Further removed from the forcing mechanism and
moisture axis, little to no convective activity exists at this
hour.

The evolution of the ongoing convection will have a somewhat
important role in our rain and thunderstorm chances today. Some high-
resolution guidance grows the thunderstorms in southern Iowa upscale
closer to sunrise, which drops a composite outflow boundary through
northern Missouri and west-central Illinois. That feature, amidst
MLCAPE values of 1500-2000J/kg that are uniformly depicted in all
available guidance, would lead to thunderstorm development in the
afternoon through the evening along and north of the I-70 corridor.
If this occurs, one or two thunderstorms could threaten 60mph winds,
but most will stay sub-severe. Without this forcing mechanism, which
seems a bit more realistic given the elevated nature of the current
convection, any activity would be more scattered in nature over this
same area. Any of these storms will be efficient rain-producers, so
localized flooding can`t be ruled out. Further south, other than a
low threat for isolated thunderstorms, dangerous heat and humidity
will continue to impact the St. Louis metropolitan area and parts of
southeast Missouri and southwest Illinois. Even beyond here, the
ongoing heat wave will continue to warrant care during the peak
heating of the day by those susceptible to heat-related illnesses.

Convective activity gradually wanes during the evening, though a
developing low-level jet may interact with another composite outflow
(in the scenario of more widespread convection during the day) to
keep thunderstorms percolating along the I-70 corridor. Chances
reinvigorate with diurnal heating on Sunday, primarily across
southeast Missouri and southwest Illinois where another transient
boundary may exist amidst strong instability. Rain chances again
drop, this time for a few days, as the upper-level ridge becomes
more pronounced over the region and more dangerous heat spreads
north.

MRB

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 342 AM CDT Sat Jul 26 2025

The end of the heatwave is in our sights now, but not before one
more pronounced stretch of hot, humid conditions Monday and Tuesday.
By this point, the upper-level ridge will be well-established
overhead and any threat for thunderstorms (which would make
temperatures more uncertain) becomes very low (less than 10%). Not
much can be said about this that hasn`t already been said: heat
index values of 105-110F are likely across almost the entire region,
with the worst conditions being felt in urbanized, metropolitan
areas. There continues to be a slight high bias in the NBM dewpoints
at this lead time that is accounted for in this forecast, but the
heat and humidity will still combine to create dangerous conditions
for anyone outside for prolonged periods of time.

The ridge begins eroding Wednesday with the slow equatorward
advancement of a longwave trough in eastern Canada, and with it our
heatwave will come to its long-awaited conclusion. The trough will
advance a stronger cold front through the region slowly during the
day on Wednesday, introducing uncertainty as to exact temperatures
at that point surrounding the front`s location and any rain chances.
However, nearly all available guidance pushes the boundary south of
the region by Thursday. In its wake, the bi-state will be at the
southern fringe of a fairly impressive surface high (for late
July/early August) and 850mb temperatures approaching the 10th
climatological percentile. All of this spells a fairly comfortable,
albeit not necessarily dry, end of the week. While some
understandable uncertainty as to how cool it will be still exists,
the high-end temperatures from the NBM are still 5-10 cooler than
normal. While a modest warmup back to near-normal is evident in the
available ensemble data by early next week, the cooler weather stays
in place through Saturday.

MRB

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 616 AM CDT Sat Jul 26 2025

Dry, VFR conditions will prevail for most of the valid TAF period.
Persistent showers and thunderstorms are draped across northern
Missouri and are currently avoiding the regional TAF sites.
Thunderstorm chances eventually drift south this afternoon and
threaten KUIN and the central Missouri terminals. St. Louis may
also see this threat near sunset, but the chances are far lower
and were not included in the TAFs. Otherwise, weak southerly winds
prevail and VFR conditions are forecast.

MRB

&&

.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-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-Sainte Genevieve MO.

     Heat Advisory from noon today to 7 PM CDT Tuesday for Audrain MO-
     Monroe MO-Pike MO-Ralls MO.

IL...Heat Advisory until 7 PM CDT Tuesday for Calhoun IL-Fayette IL-
     Greene IL-Jersey IL-Macoupin IL-Montgomery IL.

     Extreme Heat Warning until 7 PM CDT Tuesday for Bond IL-Clinton
     IL-Madison IL-Marion IL-Monroe IL-Randolph IL-Saint Clair
     IL-Washington IL.

     Heat Advisory from noon today to 7 PM CDT Tuesday for Pike IL.

&&

$$

WFO LSX