868
FXUS63 KLSX 141125
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
625 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 12z TAFs through 12z Tuesday Morning)
Issued at 625 AM CDT Mon Jul 14 2025

Conditions early this morning are rather complex with a gradient
in conditions from southeast to northwest. In the southeast, mid
and high level cloud cover has made morning fog more patchy in
nature. In the center, including the St Louis metro, low level
clouds which developed overnight have prevented any fog formation
but have led to widespread IFR ceilings. Further northwest,
including much of central Missouri extending northeast past
Quincy, lack of clouds overnight led to fog formation, with most
locations near 1/4SM at times.

Over the next few hours as the sun acts to warm the atmosphere,
expect the northwest fog to disperse first, giving way to a clear
sky with eventual cumulus development in the afternoon. Toward the
center where the IFR cloud deck has developed, this cloud deck
will gradually lift and break up through the morning, but MVFR
conditions may last for several hours before break up.

Scattered afternoon thunderstorms are expected to develop mainly
over southern MO and IL, likely remaining south of all TAF sites.
St Louis area locations have the best chance of any TAF site of
being affected, but chances are too low to include in the TAF.

In areas where it`s clear again tonight (again mainly in the
northwest), it`s possible we could see fog developing again.
Current thinking is that this is less likely tonight as
temperatures will warm well into the 80s to near 90 this
afternoon, setting a much higher ceiling from which temperatures
must fall before reaching the condensation point for fog.

Kimble

&&

.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