383
FXUS63 KLSX 140704
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
204 AM CDT Mon Jul 14 2025

.KEY MESSAGES...

- Seasonably warm temperatures are expected this week, with
  occasional afternoon thunderstorms.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 204 AM CDT Mon Jul 14 2025

Nearly calm winds overnight beneath a mostly clear sky after a
relatively cool day yesterday and continued high low level moisture
have led to some areas of fog this morning. This is most prevalent
across the northwestern portion of the forecast area where the sky
has been clear most of the night. A Dense Fog Advisory is in effect
until 9AM for the areas most likely to see areas of dense fog.

The upper air pattern this morning features two ridges affecting
much of the continental US. The first is centered in the Gulf, while
the next is centered near the southern California coast. Between
these ridges exists a remnant trough which at this point is a very
subtle shear zone between the subsident northwest flow around the
California ridge and the ascent within the southwest flow around the
Gulf ridge. This elongated trough is very subtle, with only about 40
meters of height difference at 500MB between the center of the Gulf
ridge and the core of the shear zone. This makes it considerably
more difficult to decipher the positioning of waves embedded in the
broader trough axis. While these waves are subtle, they are enough
to provide lift for scattered showers and thunderstorms within the
southwesterly flow around the Gulf ridge. This lift shifts a bit
further southeast today compared to yesterday, placing the focus for
showers and thunderstorms from Texas through the Ohio Valley.
Meanwhile, the California ridge expands its influence eastward in
the wake of a shortwave trough exiting the Great Lakes and into New
England today. The result for us will be a shift toward drier and
warmer conditions today being influenced more by the subsident
northwest flow.

That`s not to say we won`t see any convection this afternoon. We
still see a very moist and unstable air mass in place beneath
relatively cooler air aloft within the elongated trough. However, we
should see less total coverage of convection than we have seen in
previous days, with the focus for more widespread thunderstorms
shifting a little further southeast. Although there will be plenty
of instability (1000-2000 SBCAPE), wind shear is very weak (less
than 10KT) favoring pulse style thunderstorms which pop up and rain
themselves out in place fairly quickly. These will be capable of
locally heavy rainfall as precipitable water remains high (near 2
inches). However, with the lack of shear or forcing to sustain
prolonged convection, we expect the threat of flash flooding or
severe weather is lower. Convection ends with the loss of daytime
heating this evening.

Kimble

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 204 AM CDT Mon Jul 14 2025

While the California ridge is initially the more dominant one, with
time this ridge will be beaten back a bit by a trough moving into
the Pacific Northwest, allowing a broader scale ridge to become
established further east into the western Atlantic. As ridging gets
a bit stronger locally, we`ll see our temperatures warm a few
degrees into the low 90s during the middle of the week. The
relatively stationary elongated trough gradually fades with time,
but it will still exist enough to keep the daily chance for
afternoon thunderstorms in the region at least through Tuesday and
perhaps Wednesday as well.

Meanwhile, the trough entering the Pacific Northwest will move out
into the Northern Plains during the middle of the week. While much
of the energy with this trough heads due east through the Great
Lakes, it is expected to drop a cold front southward Thursday into
Friday. This nearby passing trough and advancing cold front bring
our next chance for showers and thunderstorms, this time a bit more
obviously forced with the front. Thus we see our PoP rise Thursday
with the approach of the front. Since the majority of the forcing
with the trough pushes east, the front will lose its southward push
as it slides beneath a broad, flat-topped ridge. It seems likely at
this point that we`ll be dealing with the stalled front in our
vicinity on Friday leading to primarily diurnally generated showers
and thunderstorms and some cooler temperatures. Subtle strengthening
of the ridge brings the warmer temperatures back north again this
weekend, though some passing shortwaves within the broader zonal
flow atop the ridge could be the focus for additional rounds of
showers and thunderstorms at various times this weekend. This is
more likely in northern areas where the ridge is not as strong and
closer to those passing shortwaves.

Kimble

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1004 PM CDT Sun Jul 13 2025

The primary concern during the 06Z TAF period will be the
potential for dense fog and low ceilings during the first 6-10
hours of the period. While a few weak showers continue to linger
across the area, most of this activity has drifted south of local
terminals and skies are gradually clearing. As this continues,
light winds and residual humidity will likely lead to a
combination of low clouds and fog, with LIFR ceilings and at least
patchy dense fog likely. The highest probability of 1/2 mile
visibilities or less exist at COU/JEF/UIN where clouds are
clearing more quickly, but some of this is possible at St. Louis
area terminals as well. However, metro area terminals may be more
likely to see LIFR ceilings rather than persistent dense fog.

Visibilities are expected to improve by mid morning, and ceilings
will gradually lift. While there will be improvement in ceiling
heights by midday, ceilings may be slow to reach VFR as stratus
gradually transitions to scattered cumulus during the afternoon.

Otherwise, a few showers will be possible mainly at St. Louis area
terminals in the afternoon, but probabilities remain too low for
TAF inclusion at this time.

BRC

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CDT this morning for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Knox MO-Lewis MO-Marion MO-
     Moniteau MO-Monroe MO-Pike MO-Ralls MO-Shelby MO.

IL...Dense Fog Advisory until 9 AM CDT this morning for Adams IL-
     Brown IL-Pike IL.

&&

$$

WFO LSX