316
FXUS63 KLSX 191949
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
249 PM CDT Thu Sep 19 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms (20 to 40% chance) will be
  possible overnight and tomorrow morning. Widespread significant
  rainfall is not expected, and storms are not likely to be
  severe. A few thunderstorms may redevelop tomorrow afternoon,
  but chances for this are lower (20% or less) and most areas will
  stay dry.

- The potential for meaningful and even locally heavy rainfall is
  increasing over the weekend (50 to 80%), with best chances north
  of I-70 between Saturday night and Monday morning. There will
  also be at least some potential for strong or severe storms,
  particularly Sunday, but this potential remains highly
  uncertain..

- Temperatures will to drop back to near or even slightly below
  seasonal averages Monday onward. Precipitation chances also
  diminish overall Tuesday onward, but episodes of showers will
  remain possible.

&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 247 PM CDT Thu Sep 19 2024

While a more active weather pattern is shaping up for the next
several days, quiet conditions have persisted this afternoon as we
await this change. As expected, high pressure continues to hold sway
over the area this afternoon, with mostly sunny skies (aside from a
few afternoon cumulus across the Ozarks) and temperatures already
climbing well into the 80s to even near 90 degrees shortly after the
noon hour.

However, hints of the change on the horizon can be found just to our
west, where widely scattered thunderstorms have percolated near the
KS/MO border for much of the day. While this activity is moving very
slowly and not likely to reach our forecast area, another round of
scattered thunderstorms is expected to develop in the same area
later this evening along a weak cold front. This activity is more
likely to hold together long enough to impact parts of our area
overnight and into Friday morning, likely in its weakening phase.
While instability will be much reduced overnight, enough MUCAPE
may persist through the night (roughly 1000 J/kg) to maintain
mostly elevated convection and a few weak thunderstorms into the
morning. Best chances for beneficial rain with this round will
likely be along and north of the I-44 corridor, as showers are
likely to weaken as they move farther south into the Ozarks and
southwest Illinois.

While it`s likely that these showers will largely diminish before
noon, the parent cold front will likely sink into the area and stall
somewhere in the vicinity of I-44. This boundary is expected to
become much more diffuse and likely a poor source for convective
initiation, this will keep an unstable airmass draped across parts
of southeast Missouri and southwest Illinois through the afternoon.
As such, we can`t rule out a few additional showers and
thunderstorms tomorrow afternoon and evening in these areas,
although chances for rain at any given location remain low (20% or
less).

Otherwise, temperatures tomorrow are likely to be similar today, as
this cold front is very weak and is not expected to put a
significant dent in the very warm airmass currently in place. Some
areas that see persistent could cover through the afternoon may get
a bit of relief, but otherwise expect another day in the upper 80s
to near 90 for most.

BRC

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 247 PM CDT Thu Sep 19 2024

By Saturday, an upper ridge is expected to slowly build back into
the central U.S. and lower Missouri Valley, along with deep
southwesterly flow. However, while this typically would bring to
mind warmer and drier conditions, an unusual combination of
ingredients will actually lead to increasing chances for
showers and thunderstorms over the weekend and into early next week.

What separates this pattern from a typical southwest flow regime
is an unusual abundance of moisture. This is likely a combination
of "monsoonal" mid-level moisture originating from the subtropical
eastern Pacific, and Gulf of Mexico moisture in the low levels
wrapping around the western periphery of surface high pressure and
eventually pooling along an advancing cold front. While this is
atypical for this flow regime, there is a clear signal for
anomalously high precipitable water content arriving in the lower
Missouri Valley by early Saturday, and steadily increasing to
near the 98th percentile or even higher by Sunday morning (per
GEFS/NAEFS/ECM ensemble means). Meanwhile, subtle shortwaves will
likely traverse the northern periphery of the ridge over the
weekend, followed by a more significant shortwave and cold front
sometime between late Sunday and early Monday.

While the timing of these latter forcing mechanisms is somewhat
uncertain, their presence along with the anomalously high
moisture content is leading to an increasing potential for
significant rainfall over the weekend and into Monday. While the
potential for scattered showers and thunderstorms will exist
Saturday, the stronger forcing and increased moisture content
between Saturday night and early Monday makes this the favored time
frame for significant rainfall, with at least some potential for
heavy rain as well. At the present time the potential for heavy rain
is maximized across northwest Missouri where a stalled frontal
boundary and the highest moisture content are both likely to reside
for a longer period of time. This area is also highlighted with
reasonably high values (.7 to .8) in the ECMWF Extreme Forecast
Index for precipitation. While this bullseye is just outside of our
forecast area, there is also a reasonable potential for locally
heavy rain to extend into our area as well, primarily as far south
as the I-70 corridor. Again, the best potential for heavy rain is
likely to occur sometime between late Saturday night and early
Monday, and may occur across multiple waves.

Meanwhile, the potential for strong or severe thunderstorms will
also exist during this timeframe, although there remains a great
deal of uncertainty regarding this threat. While instability is
expected to steadily increase over the weekend along with the
increasing moisture content, wind shear is expected to remain
relatively weak until the more significant shortwave arrives
sometime late in the weekend. The strength and timing of this
feature remain somewhat uncertain, but it will likely bring enhanced
southwesterly mid and upper level flow with it, contributing to
increasing wind shear. This wave will also drive a cold front
southeast through the area sometime between late Sunday and early
Tuesday, which will also provide a forcing mechanism for convective
initiation. Both of these factors will lead to an increased
potential for stronger, potentially severe thunderstorms, but there
remain many timing and strength details that are unresolved.
LREF cluster analysis reveals that there remains a wide range of
reasonable solutions regarding the timing and strength of the
arriving shortwave, and these differences have a dramatic impact on
the distribution of precipitation.

Once the cold front exits the area to the southeast sometime between
Monday and Tuesday, northwesterly flow is likely to develop, and a
cooler airmass will settle into the area. Temperatures are likely to
eventually settle back to near or even slightly below seasonal
averages as a result. Precipitation chances also appear to diminish
overall, but there remains a considerable amount of variability
among ensemble members regarding precipitation throughout the work-
week. Operational NBM precipitation chances have diminished to only
around 20-30% throughout the work week, but there remains a
relatively high percentage of LREF ensemble members that produce
precipitation through the week...albeit with generally low amounts.
As such, it would not be a surprised to see unsettled conditions
persisting into the work week, but with a low potential for
significant rain amounts.

BRC

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1106 AM CDT Thu Sep 19 2024

Primarily VFR conditions are expected through the 18Z TAF cycle,
with a few possible exceptions. During the overnight hours, a
decaying showers and thunderstorms are likely to move through the
area, first impacting UIN/COU/JEF during the late evening and
early overnight hours, followed by STL/SUS/CPS a few hours later
and into early tomorrow morning. Confidence is low that any
individual terminal will be directly impacted, but should a shower
move overhead, brief visibility and ceiling reductions to MVFR
levels will be possible. There is an even lower chance for
redeveloping showers/thunderstorms at STL/SUS/CPS again between
18Z and 00Z tomorrow.

BRC

&&

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

$$

WFO LSX