Alton Illinois Weather Forecast Discussion

741
FXUS63 KLSX 140841 CCA
AFDLSX
Area Forecast Discussion...CORRECTED
National Weather Service Saint Louis MO
341 AM CDT Tue Oct 14 2025
.KEY MESSAGES...
- Warm and dry conditions prevail through at least Friday.
- While there remains some timing uncertainty, confidence is high
  (80-90+%) that we`ll see a relatively brief period of widespread
  beneficial rain over the weekend.
- Thunderstorms are also possible (30-40%) Saturday, with a very
  conditional threat of strong or severe storms. While the latter
  threat does exist, confidence is low that these stronger storms
  will develop in our area.
&&
.SHORT TERM... (Through Friday)
Issued at 323 AM CDT Tue Oct 14 2025
Quiet, but warm conditions can be expected for much of the work week
thanks to the influence of a stubborn upper level ridge.
Early this morning the only weather item of much significance is the
potential for some fog across mainly parts of central Missouri.
While the aforementioned upper ridge remains parked almost directly
overhead, a plume of seasonably high moisture is currently
traversing its western periphery, resulting in high humidity and
quite a bit of stratus to our west. In central Missouri, though, the
mid and high level cloud cover is a bit less prominent, but at the
same time, surface moisture is actually relatively high with dew
points in the low to mid 60s. Meanwhile, winds are nearly calm
thanks to surface high pressure. The end result is a relatively
narrow pocket of fog, which has already been observed to be as low
as 1/2 to 1/4 mile at Jefferson City (and likely other points
nearby). This is likely to fluctuate through the night and into
tomorrow morning, and will likely depend on whether some of the mid-
level cloud cover to our west and move through the area.
Otherwise though, this fog is expected to diminish after sunrise,
and temperatures are likely to warm into the low to mid 80s in most
places. Some areas in northeast Missouri may not quite make it to 80
thanks to the persistence of some of this mid-level cloud cover
during the afternoon, but well above average temperatures are nearly
certain pretty much everywhere.
For the most part, this will largely continue for the rest of the
work week, as both the upper ridge and surface high pressure is not
expected to move much at all in that span. Not only this, but the
plume of moisture currently to our west is expected to move further
north and east, and we should see less cloud cover as the week
progresses. This may change Friday as low level moisture begins to
move northward, but anomalous warmth will continue regardless.
BRC
&&
.LONG TERM... (Saturday through Monday)
Issued at 323 AM CDT Tue Oct 14 2025
The primary item of interest during this forecast period continues
to be the potential for widespread rain and thunderstorms over the
weekend, with a very conditional threat of strong or severe
thunderstorms that we will discuss as well.
1. Highest confidence: widespread rain is likely over the weekend.
The main driver of this potential will be the approach of a broad,
progressive upper level trough from the west. Confidence is high
that at some point over the weekend, likely on Saturday, this trough
will move across the region and draw anomalously rich moisture
northwards ahead of an advancing cold front. At this point, it is a
near certainty that this will produce widespread rainfall across
much of the middle and lower Mississippi Valley as it moves through
the area, and that includes our area as well. While the progressive
nature of this feature will likely limit the ceiling of our rain
amounts and prevent this system from being a drought-buster, we are
reasonably confident that almost all areas will see some much-needed
rainfall at some point between Friday night and Sunday morning.
While 6-hour precipitation probabilities are likely suppressed by
significant ensemble spread (more on that later), this is really a
question of precisely when this rain will fall, rather than if it
will fall. Likewise, 24 hour rain probabilities in the LREF and NBM
of any rain are 90+ percent across the entire area Saturday,
dropping to roughly 40 to 60% for total amounts of 1/2 inch or more.
While we have high confidence in the above, there remains
considerable ensemble spread that limits our confidence in the
timing of this rain, and also the potential for strong or severe
thunderstorms locally. While the latter is possible, this threat
remains extremely conditional, and this continues to limit our
confidence that these storms will occur this far north.
2. Severe storms are possible, but there is still a lot working
against this.
Firstly, LREF cluster analysis continues to reveal considerable
ensemble spread regarding the timing of the approaching trough, and
likewise, the progression of the surface front. Not only this but
this spread seems to be heavily weighted towards individual model
suites, with the GEFS (GFS) ensemble members favoring a much more
progressive solution, and ENS (ECMWF) members favoring a slower
solution. This division is not particularly subtle either, with a
difference of 200+ miles separating the position of the front
between respective clusters by 00Z Sunday. This has massive
implications for our area, as our CWA sits almost perfectly in
between these two frontal positions. As depicted, the faster
solutions would likely push the front clear of our area before we
could ever destabilize, and essentially end our severe threat before
it materializes. On the other hand, the slower solution would allow
for a much longer destabilization window, and given the widespread
availability of strong wind shear (40-60kt of 0-6km bulk shear),
strong or severe storms would be more likely (pending other
mesoscale features that are difficult to resolve at this time
range).
Meanwhile, there are other factors that may limit our potential, not
least of all the potential for stratiform rain early in the day that
would limit our afternoon heating. This would be more likely to be a
factor with the faster solutions, but may influence the environment
regardless. As mentioned previously, the storm mode may also play a
role, which will depend heavily on the orientation of the bulk shear
vectors with the approaching front. A combination of a faster front,
limited destabilization, and parallel shear vectors would
significantly reduce our potential for severe weather.
3. There remains a valid worst-case scenario that should not be
ignored, either.
On the other edge of the sword, the aforementioned slower solutions
favored by the ECMWF do carry a threat of severe thunderstorms, as
this would provide a wider window for moisture return and
destabilization within a strongly sheared environment. While at this
time range we are still working with a more limited number of
ensemble members, there is still a non-trivial percentage of "slow
enough" solutions that keep stronger storms in play, and as such we
can`t discount it. In the worst case scenario, these slower
solutions draw very rich low level moisture into the region (60-65+
degree dewpoints), yielding CAPE values nearing 1000 J/kg coupled
with the previously mentioned shear values. While this overlap of
parameters appears more likely to our south given all of the
uncertainty discussed previously, strong/severe thunderstorms
would be possible locally should reality end up closer to the
slower solutions in the current ensemble envelope.
We should also keep in mind, also, that while current ensemble data
appears to be separated into two scenarios, that doesn`t necessarily
mean that one of them will "win". Reality may very well end up
somewhere in between, and this data simply offers a reasonable range
of possibilities at this time scale, and helps to illustrate some of
the factors that are limiting our forecast confidence. At this stage
many possibilities remain in play, and it will be prudent to
continue to monitor the forecast in the coming days as model data
reaches better agreement.
BRC
&&
.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 100 AM CDT Tue Oct 14 2025
Due to a combination of an early break in the clouds and calm
winds, LIFR fog has developed earlier than expected at JEF.
Visibilities are likely to fluctuate here through early morning,
and there is some VFR stratus to the west that may also move into
the area during the morning hours and affect the coverage of fog.
We may also see some brief visibility drops and/or low stratus at
COU overnight and early this morning.
Otherwise, VFR conditions are expected at all other terminals
through the period, aside from a low probability of patchy fog at
SUS/CPS early this morning.
BRC
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion