St. Charles, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

267
FXUS63 KLSX 242330
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
630 PM CDT Thu Jul 24 2025

.KEY MESSAGES...

- Locally heavy rainfall is possible this tonight through Friday
  morning across northeast Missouri and west-central Illinois
  along with some gusty winds.

- While some areas (northeast Missouri and west-central Illinois)
  will see a brief break in the dangerous heat on Friday, it will
  continue for most of the rest of the area through at least
  Tuesday.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 236 PM CDT Thu Jul 24 2025

A weak cold front is currently located near a KSLN>>KDBQ line as
of 1900 UTC. Convective initiation has been focused in eastern
Kansas near or just behind this boundary where the strongest low-
level convergence is located. This activity will spread
east/northeast into early this evening, reaching parts of
northeast Missouri near or just after 0Z Friday. Cannot rule out a
few gusty winds through late this evening in these areas, but the
environment is not supportive for organized severe weather. The
deep-layer shear is only about 20 knots, which signifies that
thunderstorms should tend to be cold-pool dominant. There will
also be a bit less instability in these locations due to thicker
mid/high level cloud cover which has been in place since this
morning. In addition, there is not much in the way of dry air
aloft (for entrainment) nor steep low-level lapse rates. Downdraft
CAPE from the RAP early this evening is only expected to be in
the 400-700 J/kg range, which is quite modest for this time of
year.

Locally heavy rainfall looks to be the higher potential threat,
moreso in northwestern portions of the flash flood watch. While
individual storms/clusters should move relatively quickly, there is
the potential for training along/near the frontal boundary in an
environment characterized by deep warm cloud depths and anomalously
high precipitable water values (>2.00"; >99th percentile). The main
limiting factor is that the stronger low-level moisture convergence
is well to our west tonight (closer to the Missouri-Kansas border).
This means that the genesis region for overnight convection will
stay well to our west, with storms likely weakening as they head
east/northeast into northeast Missouri. Due to the much weaker low-
level moisture convergence in our area, the current thinking is that
flash flooding, if any occurs, would be quite isolated. Across the
watch area, pockets of 1-2" of rain is expected though cannot rule
out a spot or two with up to 3-4" if storms train for a long enough
duration. In terms of the flash flood watch, only subtle changes
were made. The main change was to delay the start to 0Z this evening
based on the latest timing of thunderstorms moving upstream, as well
as ending it at 18Z Friday. No areal changes were made, though the
chances of any flash flooding do look to wane with southeastern
extent.

Dangerous heat is the other concern for Friday. While there remains
some uncertainty on exactly how far southeast thunderstorms reach by
late morning, there is not much in the way of any veering of the low-
level jet. In addition, mid/upper level heights are neutral and may
even increase slightly Friday afternoon. This means that southern
sections of the area will feel the influence of the mid/upper level
ridge. The current thinking is that widespread morning rain/clouds
in northeast Missouri and west-central Illinois will provide a more
definitive (albeit short) break to the ongoing heat so will let the
heat advisory expire this evening up that way. Further southeast,
elected to extend both the advisory and the warning not only through
Friday, but all the way through Tuesday. Peak heat index values of
100-105+F are forecast Friday afternoon, with the hottest locations
in/near metropolitan St. Louis and southwest Illinois. Confidence in
100+ heat index values increases the further southeast you travel.

There probably will be some redevelopment of showers and
thunderstorms along the remnant outflow boundary Friday afternoon,
but exactly where is uncertain. The best guess is somewhere near the
I-44/I-55 corridors in Missouri/Illinois respectively.


Gosselin

&&

.LONG TERM...  (Friday Night through Next Thursday)
Issued at 236 PM CDT Thu Jul 24 2025

(Friday Night - Tuesday Night)

Heat, heat, heat is the main story this weekend through at least
Tuesday of next week. As alluded to in the paragraph above, heat
headlines were extended through Tuesday over most of the region. For
portions of northeast Missouri and west-central Illinois who will
get a definitive break on Friday, the heat may return as early as
Saturday. Once it returns, it will also last through at least
Tuesday. There likely will be another round of showers and
thunderstorms Friday night into Saturday morning, but this round is
expected to be further north and east compared to tonight`s
activity. The mid/upper level anticyclone really builds in nearly
directly overhead Monday night into Tuesday. The strength of this
anticyclone is very impressive, with the center of the high
approaching 600 dm at 500 hPa! To put that in perspective, that
would be record-breaking for the lower/mid Mississippi Valley. This
type of anomalous midlevel high is a bit less rare in summer in the
central Plains, but is something not really ever seen further east.

Given that the 850-hPa thermal ridge is expected to be oriented west-
east across the area on Tuesday, the expectation is for Tuesday to
be the hottest day of this entire stretch with widespread upper 90s
forecast. Some readings topping out at or slightly above the century
mark are likely given the pattern, particularly in/around the urban
heat island of metropolitan St. Louis. In terms of dewpoints, they
should gradually decrease day by day as you lose any semblance of
near-surface moisture from recent rainfall and mixing increases into
early next week. However, slightly less humid air will be offset by
increasing temperatures. The net result is for peak heat index
values of 105-110F for much of the area through Tuesday.


(Wednesday - Next Thursday)

There is increasing confidence in a major pattern change across the
CONUS midweek next week as a very anomalous mid/upper level trough
digs into southeast Canada. The air mass associated with this trough
is also very noteworthy, with both the GEFS/EPS showing 850-hPa
temperature anomalies of -2 to -3 sigma. While the brunt of this air
mass will plunge into the Great Lakes/northeast US, guidance has
shifted steadily cooler behind the surface cold front in our neck of
the woods. The pattern is also very amplified for mid summer
standards, with strong mid/upper confluence between the western
ridge and aforementioned southeastern Canada trough. This strong
confluence portends to a seasonably strong surface high (~1024 hPa -
>90th percentile of climatology on the NAEFS) sliding southward into
the Mississippi Valley. This should allow for much cooler and less
humid air to be ushered in behind the cold front; it is just a
matter of exactly when and how cool. Speaking of which, there
remains some spread on the timing of the frontal passage, with
Wednesday being the day of most uncertainty for temperatures. Spread
between the 25th/75th percentile from the NBM for Wednesday`s highs
are in the 5-8+F range. A faster front would put an end to the heat
Tuesday evening, while a slower passage would likely mean that
Wednesday would the last of this stretch of dangerous heat.

In terms of precipitation chances, showers and thunderstorms are
possible along and possibly behind the frontal passage. Chances
are not particularly high currently, peaking out in the 20-30%.
However, as the timing of the frontal boundary decreases, look for
PoPs to increase sometime in that window.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 630 PM CDT Thu Jul 24 2025

A complex of thunderstorms has developed across western and northern
MO, which is expected to track eastward this evening, mainly north
of the I-70 corridor. Therefore, confidence is highest in impacts at
KUIN, which include IFR flight conditions and gusty winds, with
lower confidence at KCOU and KJEF where thunderstorms will likely
come close but may pass just to the north. The probability of
thunderstorms is even lower at St. Louis metro terminals this
evening. Overnight, showers and thunderstorms will continue at
times across northeastern MO and west-central IL including KUIN.

It is uncertain how long showers and thunderstorms will linger into
Friday morning as well as the timing, location, and location of
redeveloping thunderstorms later Friday. Therefore, confidence on if
and when impacts at any given terminal occur, no explicit mention of
showers and thunderstorms has been included with this TAF package.

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.

     Heat Advisory until 7 PM CDT this evening for Audrain MO-Marion
     MO-Monroe MO-Pike MO-Ralls MO.

     Flood Watch through Friday morning for Audrain MO-Knox MO-Lewis
     MO-Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby 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-
     Madison IL-Marion IL-Montgomery IL-Randolph IL-Washington
     IL.

     Heat Advisory until 7 PM CDT this evening for Adams IL-Brown IL-
     Pike IL.

     Flood Watch through Friday morning for Adams IL-Brown IL-Pike IL.

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

&&

$$

WFO LSX