536
FXUS63 KLSX 171658
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1158 AM CDT Tue Jun 17 2025

.UPDATE...
Issued at 456 AM CDT Tue Jun 17 2025

The number of observations of fog reaching 1/4 mile or less has
increased early this morning, and as such we have opted to change
course and issue a Dense Fog Advisory for much of southwest
Illinois and southeast Missouri. Motorists should use caution
during the morning commute and be prepared for rapidly changing
visibilities, and use low-bean headlights.

&&

.KEY MESSAGES...

- While the threat remains somewhat conditional, strong to severe
  thunderstorms are increasingly likely in parts of the area
  Wednesday afternoon through the evening and early overnight
  hours. A much lower threat for a few strong/severe storms also
  exists this evening and overnight.

- Building heat is expected over the weekend, with temperatures
  almost certainly climbing into the low to mid 90s, and heat
  index values near or above 100 degrees. At least minor heat
  impacts are likely.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 348 AM CDT Tue Jun 17 2025

The primary concern during the short term period remains the
potential for severe thunderstorms, with a limited potential this
evening and overnight, and a more substantial risk tomorrow
afternoon and evening.

Before we get there, though, we do have some river valley fog
developing early this morning, generally along and south of I-70 in
a majority of our Ozark river valleys. Satellite imagery suggests
that this fog has fairly broad coverage, but is largely limited to
low lying areas and is not ubiquitous. Dense fog has also been noted
in a few locations (1/4 mile visibility), but this has not been
widespread enough to prompt the issuance of a dense fog advisory.
Still, motorists should use caution during the morning commute, as
visibility reductions can be expected when driving in and out of
river valleys.

Otherwise, attention shifts to the potential for upcoming severe
weather later this evening and again tomorrow afternoon. As we begin
this active two-day period, an ongoing severe MCS is currently
churning southeastward through central Kansas, but this early
morning activity is not expected to reach our area. Instead, its
remnants may brush up against the southwestern corner of Missouri
before either dissipating or continuing southeast into Arkansas.
Later in the afternoon, a shortwave will begin to move through the
central plains, triggering surface pressure falls and robust
southerly return flow. As such, strong destabilization is
expected to occur, and another round of strong to severe
thunderstorms is expected to develop across parts of Kansas, with
additional development possible farther northwest across the Rocky
Mountain front range.

While there remains some uncertainty regarding how this
convection will evolve, due largely to the potential for multiple
complexes interacting with each other, a majority of high resolution
models now project that a convective complex is most likely to move
into southwest Missouri overnight, with reduced coverage farther
northeast in central and northeast Missouri (i.e., our forecast
area). This is not a sure thing, though, as there remains some
variability among CAMS, and this may change as guidance gets a
better handle on the convective evolution. Meanwhile, there is also
a modest signal for some earlier shower/thunderstorm development
across our northern areas during the afternoon and evening,
separate from the overnight convection. However, while there will
likely be substantial instability in place, forcing will be
weaker overall, and the potential for severe storms with this
initial activity appears low.

All of that being said, while we can`t rule out a strong or severe
storm or two from late this afternoon through the overnight hours across
parts of central / northeast Missouri or west-central Illinois,
this threat potential appears to have decreased slightly.

By Wednesday afternoon, the aforementioned shortwave is expected to
move into our area, along with a modest cold front and/or composite
outflow boundary emanating from overnight/morning convection.
Convective debris may still also be in the vicinity, but most if
this is likely to largely clear from our area by the time peak
heating arrives. While many mesoscale (and smaller) features remain
unresolved and will depend on how morning activity evolves,
confidence continues to grow that substantial instability will
develop along and south of the aforementioned cold front, with
HREF mean SBCAPE values of 2000-2500 J/kg, and mean 0-6km bulk
shear of 35 to 45 kt.

These base parameters would be more than sufficient to support
robust updrafts and organized severe storms, pending the previously
mentioned small scale features. Storms would likely develop along
the stalled cold front sometime in the early to mid afternoon (both
new storms and possibly strengthening storms from earlier), and
drift east over the course of the evening and overnight hours. While
the coverage of storms remains uncertain, shear vectors parallel to
the stalled front suggest that storms will likely grow upscale into
linear modes, although some initial supercells will be possible.
This suggests that damaging straight-line winds would be the most
likely hazard, although moderately favorable mid-level lapse rates
and large CAPE values also support at least some marginally severe
hail, particularly with initial discrete activity. Meanwhile, low
level shear (30-35kt 0-3km) and SRH (~150-200) values are just high
enough to support a tornado threat as well, either with initial
supercells or an MCS. In the case of the latter, this threat may be
boosted by a stronger low level jet in the evening.

At this point, it is clear that strong to severe storms are very
likely to develop somewhere along the cold front. What is less
certain is how much of this activity will actually impact our local
area, as there may be gaps between complexes of storms along its
length where destabilization does not occur in time for initiation.
As such, our local threat is somewhat conditional, and there is a
"best-case" scenario out there that sees most of the storms either
skirting or missing our CWA, either to the northeast or southwest.
On the other hand, the "worst-case" scenario (and probably a more
likely one) would feature an extensive line of strong to severe
storms stretching across a large portion of the area, beginning
roughly along a line from central Missouri into southwest or west-
central Illinois and continuing southeast from there, capable of
producing damaging wind across a rather broad swath. It also would
not be a shock to see a few gusts of 70 mph or more considering the
potential for significant instability, along with a few tornadoes.
These storms would likely exit the area to the east after midnight.

BRC

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 348 AM CDT Tue Jun 17 2025

After storms exit the area tomorrow night, attention then turns to
the potential for building heat over the weekend. While Thursday is
expected to be warm but comfortable behind the modest cold front and
departing storms, a highly amplified upper ridge and quickly
returning southwesterly low level flow will drive a notable warmup
Friday through Monday, and possibly longer. Ensemble mean 850 mb
temperatures continue to be projected locally to near the 90th
percentile, with the more significant anomalies (99th percentile or
higher) across the upper Midwest and Great Lakes. While this may
take the most "abnormal" heat farther to the north, it will
nonetheless warm into at least the low to mid 90s locally, with heat
indexes likely reaching the low triple digits. This is most likely
in the St. Louis metro area, likely with at least some contribution
from the urban heat island effect. Considering that this is the
first such heat that we have seen this year, it`s likely that we
will see at least minor heat impacts in some areas over the
weekend.

BRC

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1150 AM CDT Tue Jun 17 2025

Multiple rounds of showers and thunderstorms are expected over the
next 24-36 hours. The first is this afternoon mainly across
central and northeast Missouri and west-central Illinois.
Confidence is low on the development of showers and thunderstorms,
as well as where they will be, so have held off mention in the
TAFs for now.

The next chance will be tonight, as a complex of thunderstorms
moves west to east into the region, mainly impacting the central
Missouri (KCOU, KJEF) and west-central Illinois (KUIN) terminals.
Uncertainty remains in how widespread rain will be when it moves
into the area, but confidence is high enough to include a mention
in the TAFs. Ceilings are expected to drop to MVFR conditions with these
thunderstorms that move through.

The last chance will be during the afternoon Wednesday into
Wednesday evening when a cold front will drop through the region
bringing the last chance for showers and thunderstorms. Again,
there is uncertainty in how widespread these storms will be, and
that will depend on the behavior of thunderstorms earlier in the
day. Have used a PROB30 group to block this time in the KSTL TAF.
Ceilings are expected to drop to MVFR conditions with any
thunderstorms that develop.


Winds will prevail from the south to southwest today through much
of the period.

Delia

&&

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

$$

WFO LSX