St. Charles, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

611
FXUS63 KLSX 051712
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1212 PM CDT Sat Jul 5 2025

.KEY MESSAGES...

- Near to slightly above normal temperatures in the upper 80s to
  low and mid 90s are expected again today. Isolated to scattered
  thunderstorms are possible ahead of a cold front mainly during
  the afternoon and evening across central and northeast Missouri
  into west central Illinois.

- Slightly cooler temperatures are expected Sunday into next week
  due to the possibility of isolated to scattered thunderstorms
  each day.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 413 AM CDT Sat Jul 5 2025

The primary concern in the short term is convective trends today
through Sunday.  Surface analysis shows low pressure moving through
the northern Plains into the Upper Midwest this morning.  The low is
dragging a cold front behind it which is moving through central
Nebraska into western Kansas, and this front will continue drifting
east today to near Omaha/St. Joseph by around 00Z this evening.
Guidance has been pretty consistently producing scattered convection
ahead of the front as far east as central and northeast Missouri
into west central Illinois today, potentially getting as far as east
central Missouri by late this afternoon/early this evening.  There
should be a generally decrease in coverage this evening with the
loss of daytime heating, though isolated to widely scattered storms
could linger all night tonight.  Highs this afternoon will be
dependent upon cloud cover and convection, and therefore be a little
cooler in the mid and upper 80s over central and northeast Missouri
than points farther east in the 90s.

The front continues to drift southeast to near the I-70 corridor by
late Sunday afternoon/early evening.  This will provide a focusing
mechanism for convection on Sunday leading to more widespread
thunderstorms. Guidance PoPs have increased slightly to around 60%
in most locations Sunday afternoon, and this looks reasonable given
the position of the front and warm humid air producing MLCAPE values
between 2000-2500 J/K.  Luckily the 0-6km shear is very weak at only
10-15kts so widespread severe storms are not expected, though a few
wet microbursts are possible given forecast inverted V soundings.
Temperatures Sunday should be a few degrees cooler in the mid to
upper 80s in most locations due to more cloud cover and convection.

Carney

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 413 AM CDT Sat Jul 5 2025

The short wave trough which was pushing the cold front south
attenuates and lifts northeast Sunday night.  This leaves the front
near the I-70 corridor where it will serve as a focus for convection
on Monday.  Another weak wave moves across the Midwest Monday night
which, along with outflows from convection, may give the front a
little push south into Tuesday morning.  However, the position of the
front will likely depend more on convection than the short wave
making it very low confidence.

Another stronger wave moves through the Plains Tuesday
afternoon/evening and into the Mississippi Valley by 00Z Thursday.
The southwest flow ahead of the wave on Tuesday should push the
effective front back to the north to near the Missouri/Iowa border,
and the deterministic GFS develops convection on the front near
Omaha by 00Z Wednesday.  The convection spreads east-southeast
across most of the area through Wednesday night. While this time
period has the potential to be fairly wet and stormy, the GFS looks
suspiciously like it`s contaminated by grid-scale convective
feedback.  Looking at the LREF mean, the general pattern is similar
to the GFS with an amplifying long wave trough over the Mississippi
Valley and Great Lakes, and a ridge near the Four Corners Region.
The deterministic ECMWF is actually much closer to the ensemble
mean, and it may be a better representation of what will happen than
the GFS. However the cluster analysis is showing some uncertainty in
the amplitude of the trough which likely indicates latitudinal
differences in the track of the short wave as it passes over the
Mississippi Valley. P-Wats are forecast to be 1.8+ in the majority
of the ensemble members and even the 25th percentile is 1.5+. Mean
warm-cloud depths are forecast to be well in excess of 10,000ft.
This suggests that any thunderstorms that do affect the region
Tuesday night through Wednesday night could be efficient rain
producers.

Guidance shows weak northwest flow aloft Thursday and Friday as the
long wave drifts into the Ohio Valley.  Low level flow is generally
weak and variable, lacking any real synoptic scale boundaries to
focus convection.  Convection will likely be produced through
diurnal processes and outflow boundary interactions.  With all of
this said, next week`s temperatures will be highly dependent upon
convective trends each day, particularly during the latter half with
no synoptic boundaries to hint toward warming or cooling trends. The
going forecast of near normal temperatures looks reasonable given
the pattern and uncertainties in convective trends.

Carney

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1207 PM CDT Sat Jul 5 2025

Primary concern is the potential for showers and thunderstorms,
which may impact terminals at various times throughout the 18Z
period. Initial shower activity has already developed near COU/JEF
at the start of the period, and this may briefly impact each of
these terminals for the rest of the afternoon with bursts of
heavy rain, lightning, and ceiling/visibility reductions generally
to MVFR levels. A few of these storms may also impact UIN a few
hours later in the afternoon as well. A brief lull in the activity
is expected in the evening, but another round of decaying showers
and storms will be possible at all terminals (including St. Louis
area terminals) overnight, with similar hazards as well.

Another round of showers and thunderstorms is expected tomorrow
afternoon at all terminals, which is just beyond this TAF period
for all sites except STL. Here we opted to add a mention of
prevailing thunderstorms from 20Z tomorrow through the end of the
period, with around a 60% chance of impacts to the terminal.

BRC

&&

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

$$

WFO LSX