139
FXUS63 KLSX 052343
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
643 PM CDT Sat Jul 5 2025

.KEY MESSAGES...

- Widely scattered showers (20-30% chance) and a few sporadic
  thunderstorms (10% chance or less) will persist the remainder
  of the afternoon, with a few possible overnight as well.

- Increased chances (30 to 60%) for showers and thunderstorms
  exist tomorrow afternoon, with a few microbursts and heavy
  downpours possible. While we don`t expect those hazards to be
  widespread, some minor wind damage and localized flooding can`t
  be ruled out.

- A typical summertime pattern will persist for the next week,
  with repeated chances for scattered, mostly weak showers and
  thunderstorms almost daily and near average temperatures.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 120 PM CDT Sat Jul 5 2025

The primary concern for the remainder of the day and into
tomorrow continues to be the potential for showers and
thunderstorms, along with potentially some isolated downbursts and
heavy rain rates tomorrow afternoon.

As we move past midday, some weak showers have already broken
through a very weak cap across parts of central Missouri, with
some more robust thunderstorm activity just to the northwest of
there. These storms are likely being triggered by a combination of
afternoon heating and cooling aloft from a very subtle mid/upper
level shortwave evident on water vapor imagery. Instability is
already moderate (1000-1500 J/kg ML CAPE) and will likely increase
another 500 J/kg or so over the next few hours. However, wind
shear is very meager, with effective shear values of only around
15 kt or so. As a result, this activity has not been particularly
strong yet, and our local cells have not produced lightning as of
12:30. However, it should be noted that a slightly more
substantial bowing segment has produced a bit of lightning just to
the northwest, and we can`t rule out a few strikes over the next
several hours. Otherwise though, strong/severe storms are not
expected, aside from a limited potential for a microburst.

This initial activity is likely to diminish as the sun sets, but
another round of decaying showers and thunderstorms will be possible
overnight as activity forming along an upstream cold front sinks
into our area. Latest model trends suggest that this activity will
be diminishing as it arrives, likely due to the loss of
instability and continued very weak shear. While we will likely
see at least some semblance of showers/storms overnight, impacts
are expected to be minimal.

By tomorrow afternoon, the aforementioned weak "cold" front will
settle somewhere across the area, likely near or just north of the I-
70 corridor by mid afternoon. This boundary is likely to provide a
more focused forcing mechanism for convection in a very weakly
capped environment, although there may be even less upper level
support than today. Meanwhile, moisture content is likely to
increase, with precipitable water values surging to between 1.8 and
2.2 inches, per the HREF. Meanwhile, moderate instability will
remain in place, perhaps reaching as high as 2000 J/kg of SBCAPE
along the front by late afternoon. Model soundings also indicate
shallow inverted V soundings with steep low level lapse rates, along
with some potential for mid-level dry air entrainment that could
enhance the downdraft potential. As such, it remains possible that
we could see a few microbursts with this activity, although the even
weaker effective shear (10kt or even less) will likely prevent
storms from becoming organized. Meanwhile, bursts of heavy rain will
also be possible, and we can`t rule out very localized flooding if
cells can last for more than an hour or so in one spot. They won`t
be moving very quickly and rain rates may be high, but with such
weak wind shear it remains to be seen whether updrafts will be able
to maintain themselves before they get cut off by their own outflow.

This activity is likely to slowly drift to the southeast with the
front during the evening and diminish in coverage, largely driven
by their own outflow, but a few showers may linger through the night.

BRC

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 120 PM CDT Sat Jul 5 2025

From Monday onwards, a stagnant pattern and the previously mentioned
weak frontal boundary are likely to result in additional chances for
scattered showers and weak thunderstorms almost every day through
the end of the work week. While the afternoons will likely be
favored on most days, that may not always be the case every day.

First, the stalled front is likely to remain across the area Monday,
although there is a lot of uncertainty regarding how this boundary
will be augmented by outflow from Sunday`s storms. In any case,
another round of scattered showers and storms is likely to develop
Monday afternoon wherever it settles, with similar potential
hazards. Ensemble guidance suggests that this is most likely to
occur along and south of I-70.

Between Tuesday and Wednesday, a slightly more substantial
shortwave may move across the upper Mississippi Valley region,
triggering more widespread and slightly more organized storms to
our northwest Tuesday afternoon. While we may see another round of
weak afternoon pop-up showers locally during the afternoon, the
better chances may actually be overnight through Wednesday
morning. Given that this is still several days away and still
rather weakly forced environment, this timing remains very
tenuous.

Beyond Wednesday morning, confidence in the day-to-day details
remains low, but there is not much to suggest that the pattern will
significantly change over the next week. LREF ensemble mean 500mb
forecasts suggests that there may be a slight trend toward lowering
heights and weak troughing late in the week, which may open the door
for additional subtle shortwaves to move through the area, and these
may provide some modest forcing enhancements for precipitation. As
such, we will maintain the potential for scattered showers and
thunderstorms each day through Saturday.

Meanwhile, with little in the way of synoptic cold fronts or
substantial changes in the overall pattern for the foreseeable
future, the temperature forecast remains relatively steady for the
next week, and right around seasonal averages. This will vary likely
vary from day to day and from location to location depending on how
showers and cloud cover evolve, but most should expect to see
typical early July temperatures and moderate to high humidity for
the next week.

BRC

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 636 PM CDT Sat Jul 5 2025

An area of showers and thunderstorms is moving away from KUIN,
with some other activity south of KJEF/KCOU. All activity should
gradually weaken and then dissipate by late evening with loss of
instability. Additional storms are expected to move southeast out
of northwest Missouri/southern Iowa toward northeast Missouri
after midnight. These storms should weaken overnight but may make
it to KUIN before dissipating. Additional storms are forecast to
develop around or just after midday on Sunday, mainly impacting
in/around metro St. Louis.

Gosselin

&&

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

$$

WFO LSX