288
FXUS63 KLSX 132030
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
330 PM CDT Tue Aug 13 2024

.KEY MESSAGES...

- An active weather pattern continues across the region, with
  locally heavy rainfall and flash flooding possible, including
  across central and southeastern MO. However, it is uncertain
  where the axis of heavy rainfall occurs.

- Additional rounds of showers and thunderstorms are forecast
  Wednesday night into Thursday morning and then again sometime
  late Thursday, which lower confidence. Confidence is low on
  whether or not thunderstorms are severe.

- Lower chances (20 to 40 percent) of showers and thunderstorms
  persists through the weekend, but dry conditions are most
  likely.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 329 PM CDT Tue Aug 13 2024

A slowly weakening MCS continues to track eastward across the
southern third of MO and into southeastern MO. Although gusty winds
may accompany this MCS, it should eventually dissipate through late
afternoon as it encounters lower instability and moisture. A few
showers could linger across central and southeastern MO through the
rest of the afternoon and early evening as an remnant MCV also
arrives.

A front, temporarily pushed into southern KS, northeastern
OK, and western AR by the ongoing MCS will lift northeastward this
evening through overnight. The cool (east) side of this front as it
becomes oriented from north-northwest to south-southeast will be the
focus of another round of showers and thunderstorms somewhere from
northwestern MO to southern MO, potentially including central and
southeastern MO, between late this evening through Wednesday morning
as a 30-kt southwesterly LLJ develops. The resulting pattern and
environment (e.g., PW >2", deep warm cloud depths, deep-layer flow
paralleling the front) are favorable for locally heavy rainfall from
an MCS with training showers and thunderstorms, but CAMs have been
struggling to decipher where the heavy rainfall axis will originate
and be placed largely due to the influence of the ongoing MCS. The
latest HREF LPMM highlights a corridor of 3 to 7+" from west-central
to southern MO, which is a slight southwest shift from the previous
run. Between dry antecedent conditions and this trend in the HREF,
confidence has not increased in locally heavy rainfall and flash
flooding occurring within the CWA in central and southeastern MO.
That being said, the variability in the CAMs suggests the the
placement of the heavy rainfall axis is not yet decided and a shift
back northeast cannot be ruled out.

Showers and thunderstorms are expected to gradually shift
northeastward toward the Mississippi River Wednesday morning into
midday as the front also attempts to shift northeastward, but
intensity and coverage will gradually decrease as the LLJ wanes and
mid-level heights rise with the arrival of an upper-level ridge axis
during the afternoon. Only a few isolated showers and thunderstorms
are possible by the evening. Uncertainties in how quickly showers
and thunderstorms dissipate and lingering clouds break also has
implications for the high temperature forecast on Wednesday with the
NBM interquartile range spanning 5 to 7 F, spread from below to near
average.

Pfahler

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 329 PM CDT Tue Aug 13 2024

Another round of showers and thunderstorms is expected again
overnight Wednesday night into Thursday morning with origins across
northeastern KS, eastern NE, and southwestern IA during the evening
as an upper-level trough approaches. A veering LLJ and a reservoir
of at least elevated instability across much of MO indicated by
model guidance, suggest that these showers and thunderstorms will
tend to move east-southeastward into the CWA along and north of I-70
overnight into Thursday morning as they grow upscale into one or
more MCSs. A few of these thunderstorms could produce gusty winds,
but there is question on how unstable the BL will be by that time
and how balanced thunderstorms will be with their cold pools. The
environment will still be favorable for heavy rainfall, but the
pattern itself will not be as favorable as tonight/Wednesday morning
due to less of a parallel component of deep-layer flow with the
frontal boundary that continues to lift northeastward to somewhere
near the Mississippi River by Thursday morning.

Concern is increasing for redevelopment of showers and
thunderstorms later in the day on Thursday to be influenced by an
MCS that will likely be ongoing during the morning. These influences
include where and how much airmass/instability recovery takes place
and if there will be any remnant outflow boundaries. Synoptically,
an upper-level trough will approach the Mid-Mississippi River
Valley, spreading broad ascent over the region, but model guidance
generally agree that an associated cold front will not arrive until
Thursday night or Friday morning. As a result, confidence is low on
where and when shower and thunderstorm development occurs and what
the coverage will be, with even lower confidence on whether or not
thunderstorms will be severe. However, with 30 to 40 kt of deep-
layer shear advertised, Thursday will need to still be monitored for
severe thunderstorms, if other factors can contribute
constructively. High temperatures warming to above average could
also be jeopardized on Thursday if morning showers and thunderstorms
linger long enough.

There is still uncertainty in how quickly the cold front will clear
the CWA on Friday, with the chance (20 to 30 percent) of showers and
thunderstorms lingering through the day. Upper-level northwesterly
flow will prevail through the weekend into early next week over the
Mid-Mississippi River Valley with the front settling just south and
southwest of the CWA. With the front possibly remaining nearby and
several upper-level shortwave troughs navigating the northwesterly
flow, some ensemble model members depict showers and thunderstorms
at times through the period. A slightly larger majority has dry
conditions. Temperatures will also depend on the proximity of the
front, with NBM interquartile temperatures ranges spanning 4 to 7 F
between near and slightly below average temperatures.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1219 PM CDT Tue Aug 13 2024

A decaying complex of showers and thunderstorms will continue to
track across southern and southeastern MO early this afternoon, with
only a short period of rain expected to reach KJEF. Otherwise,
improvement in flight conditions are expected through the afternoon,
reaching VFR this evening. Late this evening through Wednesday
morning, another elongated complex of showers and thunderstorms is
forecast to develop, but there is uncertainty in its exact
placement/track, which has the potential to include central and
southeastern MO. MVFR flight conditions are included with this TAF
package, but greater reduction in flight conditions and heavy
rainfall are possible. Elsewhere to the north and east, VFR flight
conditions are likely to persist.

Pfahler

&&

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

$$

WFO LSX