999
FXUS63 KLSX 062322
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
622 PM CDT Sun Jul 6 2025

.KEY MESSAGES...

- Scattered to numerous showers and thunderstorms can be expected
  the rest of the afternoon and into the evening. While most will
  not be strong or severe, occasional strong downburst winds and
  bursts of heavy rain will be possible.

- Scattered showers and thunderstorms remain possible (20-40%)
  across the Ozarks and southwest Illinois tomorrow, but better
  chances (30-60%) exist area-wide Tuesday.

- Seasonably warm temperatures with moderate to high humidity can
  be expected for the foreseeable future.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 202 PM CDT Sun Jul 6 2025

The primary concern for this afternoon remains the potential for
scattered thunderstorms, which will have the potential to produce
bursts of heavy rain and isolated strong downburst winds.

As of 1:00 PM, a weak cold front has moved into northern Missouri,
and continues to inch southward late this morning. Along and south
of this boundary, moderate instability is slowly building as we
approach midday, with around 2000 J/kg of MLCAPE already present per
SPC (RUC) mesoanalysis and the potential to reach 2500 J/kg by peak
heating. Meanwhile, low level lapse rates are expected to be
moderate to steep by peak heating (7.5 to 8.5 C/km), with moderate
downdraft CAPE (800-1000 J/kg) also projected. Effective wind shear
is expected to remain generally weak, with deep southwesterly flow
of only around 10 to 20 kt at almost all levels, and this will
likely limit the ability of storms to organize and for individual
updrafts to persist.

Meanwhile, latest high resolution models continue to develop
scattered to numerous thunderstorms along and south the slowly
advancing front by mid afternoon, continuing into the early evening
before coverage diminishes after sunset. We`ve already seen the
early stages of this development, and coverage is likely to continue
to increase over the next few hours. While forcing is weak, there is
at least some modest convergence along the front, and also perhaps a
bit of upper support from a very weak mid level trough evident on
water vapor imagery. Not only this, but several consecutive runs of
the HRRR consistently forecast 30 to 60 mph convective wind gusts
associated with these storms, which makes sense considering the
thermodynamic environment described above. As such, while storms may
struggle to organize beyond much more than clusters or possibly even
a few broken line segments, the potential for isolated strong wind
gusts in that 30 to 60 mph range appears to be reasonable.

Not only this, but moisture content is also rather high along and
south of the cold front, with precipitable water values ranging
from 1.8 to 2.1 inches. While soundings do show perhaps some
modest mid-level dry air entrainment impinging on updrafts, this
high moisture content and very deep warm cloud depths support at
least briefly intense rain rates of 1 to 2 inches per hour. Storms
will have some forward motion but will be moving slowly (10 to 20
mph), and this may lead to a few pockets where 1 to 3 inches of
rain accumulates in a very short period of time. This is also
corroborated by HREF LPMM analysis and individual high resolution
model members. Some localized flooding will be possible as a
result, although we don`t expect this to be common or widespread,
and may require some training of storms in order to occur.

The most likely window for the most significant hazards appears to
be between roughly 2 PM and 8 PM, although a few earlier storms are
possible, as well as a few stragglers that linger into the evening
and overnight before completely diminishing.


19

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 202 PM CDT Sun Jul 6 2025

By tomorrow morning, the cold front is likely to settle somewhere
across southern Missouri, and may almost completely clear our area.
North of the front, drier and and more stable air is likely filter
into much of our area, and as a result of these trends, our
precipitation chances have dropped considerably from previous
forecasts. In fact, much of our local area is likely to remain dry
tomorrow, with the exception of parts of the Ozarks and southwest
Illinois in the vicinity of the stalled front. In this limited
area, instability is likely to climb once again to around 1500 to
2000 J/kg of SBCAPE, with only around 10-20kt of 0-6km shear and
precipitable water values reaching near 1.8 inches. As such,
scattered thunderstorms with occasional strong downbursts and
heavy rain rates remain possible tomorrow afternoon along and
south of wherever this boundary settles, although this may not
include much of our forecast area at all.

On Tuesday, modest southerly flow is expected to return to the area,
bringing moisture-rich air back to the north. Over the course of the
day, a mid-level shortwave will move across the region, sending
another weak front southward. As this occurs it will likely trigger
another round of showers and thunderstorms across the region,
although confidence remains low regarding the timing of the
boundary. Trends over the last 24 hours have shifted toward an
earlier arrival of the front as opposed to overnight, which aligns
the timing of local storms with peak afternoon heating and at least
increases the potential for stronger activity as well. Projected
wind shear remains weak though - roughly 15 to 20kt of 0-6km bulk
shear - which continues to limit the upper potential for strong or
severe storms in spite of robust instability projections of 2000-
3000 J/kg of SBCAPE. Still, we will need to keep an eye on the
potential for damaging downbursts once again, along with heavy rain
rates and localized flooding.

Following Tuesday`s weak front and showers/storms, slightly drier
air is likely to move into the area, perhaps with increasing
subsidence aloft. As such, precipitation chances Wednesday and
Thursday have dropped slightly from previous forecasts, especially
in areas north of the I-44 corridor. While some showers and
thunderstorms remain possible, latest trends have shifted toward
this period being more dry than wet. Later in the week though,
another, stronger shortwave remains likely to move across the middle
and upper Mississippi Valley region, and this may bring yet another
round of showers and thunderstorms to the region.

As for temperatures, little has changed regarding the forecast.
Afternoon maximums continue to be forecast near seasonal averages
for the foreseeable future, with perhaps a very slight increasing
trend late in the week thanks to modest height rises and lowering
precipitation chances.

BRC

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 620 PM CDT Sun Jul 6 2025

Storms this afternoon have left a rather thick blanket of mid and
high level cloud cover across the region. While this is VFR, these
clouds may be helpful in preventing low level fog developing
overnight. If the clouds disperse, then the chances of fog
increase. Winds remain light through tomorrow, with another round
of thunderstorms possible but likely remaining south of all the
TAF sites.

Kimble

&&

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

$$

WFO LSX