O'Fallon, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

463
FXUS63 KLSX 161003
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
503 AM CDT Mon Jun 16 2025

.KEY MESSAGES...

- Scattered showers and thunderstorms may produce heavy rain rates
  across parts of southeast Missouri and southwest Illinois this
  afternoon. The greatest threat of flash flooding is likely just
  to our southeast, but isolated instances are possible locally.

- Multiple rounds of thunderstorms are likely (50-80%) between
  late Tuesday evening and Wednesday night. Some of these storms
  may be capable of reaching severe levels, with all hazards
  possible. Confidence in timing and locations remains low.

- Above average temperatures are a near certainty (90% or greater)
  over the weekend, with heat impacts possible. We will almost
  certainly exceed 90 degrees through the weekend and possibly
  beyond.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 248 AM CDT Mon Jun 16 2025

The primary concern in the short term continues to be the potential
for showers and thunderstorms across parts of southeast Missouri and
Southwest Illinois throughout the day today. While severe storms are
not likely, this activity may be capable of producing heavy rain
rates and occasionally gusty downburst winds, and we can`t rule out
isolated instances of flooding.

The primary driver of this activity will be a slow-moving and weak
surface low, which is currently centered across Arkansas with a
broad surface trough extending to the northeast into the Ohio
Valley. As of 1 AM, a complex of thunderstorms is located across far
northeast Arkansas and is moving into far southeast Missouri,
producing lightning and locally heavy rainfall. An extension of
weaker convection and light stratiform rain also extends farther
northwest into our area, with some light rain as far north as the
St. Louis metro area. While we can`t rule out a few pockets of
heavier rain across our far southeastern Ozark and southwest
Illinois counties, we expect that the bulk of this overnight/early
morning activity will remain to our southeast.

However, even as this complex moves off to the northeast, moisture-
laden air will remain in place across these areas, and may even
increase slightly as we approach the afternoon. Meanwhile, the
aforementioned weak surface low will drift even closer, while a weak
mid-level trough will move overhead as well. This modest forcing,
combined with modest afternoon instability (500-1000J/kg CAPE), is
expected to result in another round of showers and thunderstorms
from early afternoon through early evening in these same areas. As
is the case currently, latest high-resolution guidance suggests that
the core of the moisture (PWATS 2 to 2.2 inches) will remain just to
our southeast. However, just enough of this moisture (1.6-2.0 inch
PWATS) will extend into our portion of the Ozarks to support locally
heavy rain rates. Meanwhile, profiles suggest that instability will
be spread across a very deep storm depth (~40kft), with deep warm
cloud depths as well. Steering flow is also expected to be quite
weak and decreasing with time, although we do expect some modest-
anvil level flow that may help with ventilation. All of this is to
say that thunderstorms are not likely to move quickly today, and
this may exacerbate the heavy rain threat. Still though, given that
the richest moisture is just off to our south, we expect that the
most significant threat of flash flooding resides across far
southeast Missouri, and we`ve opted to hold off on any Flash Flood
headlines for the time being. This activity is expected to wind down
during the evening as it moves off to the east.

Overnight, relatively dry air aloft combined with weak subsidence
and shortwave ridging should keep our area largely dry, although we
can`t completely rule out a brush with the last remnants of a
central-plains convective complex near daybreak. This is unlikely
though, and most precipitation will remain to our west.

BRC

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 248 AM CDT Mon Jun 16 2025

The two main items of interest during the extended portion of the
forecast will be the potential for multiple rounds of thunderstorms
Tuesday night and Wednesday, including possibly severe
thunderstorms, followed by a notable warming trend late in the week
and over the weekend.

By the time Tuesday afternoon arrives, all eyes will be focused on
the evolution of thunderstorms across the central Plains, which will
likely be ongoing at this point. However, most of our local area is
expected to remain dry for much of the afternoon, owing to weak
forcing and substantial convective inhibition. Still, the return of
southerly flow ahead of a deepening central plains surface low will
draw increasing heat (temperatures in the mid to upper 80s) and
humidity northward into our area, along with substantial but capped
instability (2500-3500 J/kg CAPE). While this will set the stage
ahead of convection to our west, exactly how this will evolve later
in the evening and overnight (and possibly Wednesday morning)
remains rather uncertain.

It does appear likely that a complex of thunderstorms will develop
along the nose of a strong low level jet and surface cold front
across mainly central and eastern Kansas tomorrow afternoon,
although it is unclear whether the steering flow would carry this
particular complex into our area. However, additional convection
will also be possible later in the evening farther to the northeast
as a mid-level shortwave moves across the area, and the
southwesterly low level jet shifts to the east. In any case, some
combination of these thunderstorm complexes may reach parts of
northeast/central Missouri between late Tuesday evening and early
Wednesday morning, although the exact timing of this remains rather
uncertain due to considerable spread among models. If such a complex
does get steered into our area, there will be a very unstable
airmass in place, along with sufficient wind shear for organized
updrafts and thunderstorm maintenance. As such, a threat of severe
storms will exist sometime between late Tuesday evening and early
Wednesday morning across central / northeast Missouri and west-
central Illinois, with a slight lean toward damaging straight-line
winds as the primary hazard (although large hail and tornadoes can`t
be ruled out). Not only this, but considering the currently forecast
moisture parameters (1.8-2.0 inch PWAT) there may also be a threat
of locally heavy rainfall, particularly if we see a backbuilding
MCS. There is also a modest developing signal for heavy rain in the
ECMWF Extreme Forecast Index product across northern Missouri during
this timeframe, which adds credibility to this threat.

Exactly how long this activity can maintain itself as it propagates
farther southeast is also highly uncertain, and this will have a
direct effect on how things evolve during the day Wednesday. Some
combination of lingering convection and/or outflow, along with a
weak cold front, will likely be draped across our area as Wednesday
afternoon arrives. Along and south of this activity and composite
boundary, strong instability is expected to develop once again in
the afternoon, locally augmented by lingering cloud cover and/or
showers. Meanwhile, enhanced mid-level low associated with the
previously mentioned shortwave, along with the lingering
southwesterly low level jet, should result in sufficient wind shear
for another round of severe storms.

Exactly how all of these factors will evolve remains to be seen, as
there are many moving parts that are all linked. However, the
forecast parameter space suggests a legitimate threat for severe
thunderstorms, with most of the area seeing at least some potential.
As a "best-case" scenario, early morning clouds and lingering
thunderstorms could limit our available instability, and while
severe storms would still be possible, this would likely at least
reduce their coverage and strength. On the other hand, a "worst-
case" scenario would be for fewer morning showers and storms and
clearer skies, leading to more afternoon instability (2000-3000 J/kg
of CAPE) and 35-45kt of effective shear by the time new storms
develop. This latter scenario would be more likely to produce more
numerous severe storms with all-hazards possible. Again, the
locations most likely to be affected will depend on where the cold
front/composite outflow will be located by early/mid afternoon,
but all areas will see at least some potential.

Thunderstorms will move east of our area sometime late Wednesday or
early Thursday, replaced temporarily by drier air behind the
weakening cold front. This, along with northwest flow aloft, should
result in a seasonably warm, dry, and relatively comfortable
afternoon Thursday as we await a late-week warmup.

Friday through the weekend, ensemble guidance continues to project a
substantial ridge developing across the Mississippi and Ohio River
Valleys, resulting in steadily building heat across a substantial
portion of the Midwest. Most ensemble guidance develops the most
significant heat anomalies (relative to average) just to the north
of our area, with 850 mb ensemble mean temperatures reaching the
99th percentile across the upper Mississippi River Valley, and to
near the 90th percentile locally. While this may keep the most
"abnormal" heat to our north, we will nonetheless see a significant
warmup locally, and will almost certainly exceed 90 degrees over the
weekend. Current NBM and MOS forecasts suggest that temperatures in
the low to mid 90s will be attainable through the weekend, with the
potential to reach the upper 90s as well. There is also an
increasing signal for heat in the ECMWF EFI as well, particularly
across northern Missouri. While it`s a bit early to say just how
impactful this heat will be, it will very likely be the warmest
period we`ve seen all year, with a reasonable potential to reach
headline-level heat index values in some areas (particularly St.
Louis Metro). Meanwhile, precipitation chances continue to look
rather low Thursday through the weekend, with only a smattering of
ensemble members producing rain during this period.

BRC

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 457 AM CDT Mon Jun 16 2025

While VFR conditions will be most common during the 12Z TAF cycle,
there are a few potential weather hazards. First, some patchy fog
will be possible near the start of the period, particularly
COU/JEF/UIN, with a lower chance elsewhere. This has not
materialized as of 10Z, but some fog is evident at nearby sites
(KIRK, KMBY, KVER). If it does develop locally, it will likely be
brief.

From mid morning through mid afternoon, scattered showers and an
occasional thunderstorm may move through St. Louis area terminals.
Higher probabilities exist just to the south of the terminals, but
brief category reductions are possible due to drops in visibility
and/or ceilings. Dry conditions and VFR conditions are expected
elsewhere.

Overnight, low ceilings and fog will be possible again, this time
with better chances at St. Louis area terminals (SUS, CPS
especially).

BRC

&&

.CLIMATE...
Issued at 320 AM CDT Sun Jun 15 2025

St Louis still hasn`t reached 90 yet in 2025. This puts 2025 in
the Top 20 all time latest dates of the first 90 degree
temperature, and the latest in at least 30 years (Jun 19, 1995).
The all time latest first 90 degrees was July 4 in both 1961 and
1912. Records began in St Louis in 1874.

Columbia has also not yet seen a 90 degree reading in 2025. The
latest date of first 90 degrees on record was July 14, 1904.
Records began in Columbia in 1890.

Quincy reached 90 degrees on May 15 of this year.

Based on the current forecast, our next best chance to see 90
degrees is Tuesday, June 17.

Kimble/Pfahler


&&

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

$$

WFO LSX