656
FXUS63 KLSX 280916
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
416 AM CDT Sun Apr 28 2024

.KEY MESSAGES...

- A line of showers and thunderstorms will move east out of
  west/southwest Missouri this morning and is expected to weaken
  in time.

- Additional showers and thunderstorms are expected along a cold
  front this afternoon into late this evening. A few thunderstorms
  could be severe with marginally severe hail, damaging winds and
  an isolated tornado or two.

- The front will stall over the region Monday with showers and
  sub-severe thunderstorms over sections of southeast Missouri
  and southwest Illinois. The front returns north with warmer than
  normal conditions continuing through the upcoming week.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 413 AM CDT Sun Apr 28 2024

Key Messages:

- A line of showers and weakening thunderstorms will move east out
of western Missouri this morning and continue to decay as the line
approaches the Mississippi River.

- Additional thunderstorms are expected along a cold front this
afternoon into late this evening. A few thunderstorms could be
severe with marginally severe hail, damaging winds and an isolated
tornado or two.

- The front will stall over the region Monday with showers and sub-
severe thunderstorms over sections of southeast Missouri and
southwest Illinois. The front returns north with warmer than normal
conditions continuing through the upcoming week.

Radar trends show precipitation over northeast Missouri and west-
central Illinois, remnant of overnight convection that quickly
decayed as it encountered a less favorable environment over eastern
sections of Missouri. In fact, very little lightning is being
detected within this area and what thunderstorms remain are further
west with the next round of approaching precipitation.

The the focus shifts to western Missouri where another line of
weakening thunderstorms are progressing eastward early this morning.
Given the diurnal minimum and thunderstorms that preceded the
incoming activity, further decay is expected as the line departs the
origin of instability and moves into air that has been largely
stabilized by preceding thunderstorms. Though this activity is
expected to be sub-severe as it enters the CWA this morning,
rainfall and cloud cover could eat into potential for instability to
recover this afternoon, when redevelopment is possible along the
approaching cold front.

This afternoon`s thunderstorm potential could very well be to be
conditional upon the survival and geographic spread of the lingering
rainfall that is ejecting out of western Missouri. The upper level
low continues to track into the northern Plains with the surface low
slightly to the southeast over western Iowa. The trailing cold front
begins to slow, presenting less of a convergent focus as it moves
into central Missouri. Upper ascent becomes more fragmented in the
vicinity of the surface front, along with marginal MUCAPE values
around 1000 J/kg encompassing much of the state of Missouri. Mid-
level lapse rates of around 7C are more respectable than in previous
days, but highest values reside over south-central Missouri around
the time convective initiation is favored between early and mid-
afternoon. RAP model sounding show skinny CAPE profiles closer to
the Mississippi River, suggesting hail may not be much of a threat,
if any at all, as thunderstorms approach the St. Louis Metro area.
The one favorable parameter are 0-6km shear values that are at 40-50
kts ahead of the front, along with a strengthening LLJ.

All of this suggests that redevelopment will once again be focused
to our west, closer to the slowing front. This leaves a limited
window of opportunity to organize behind the preceding rainfall and
prior to waning diurnal maximums with much of the surface air
already being rain-cooled over central Missouri. Should strong to
severe thunderstorms develop this afternoon, their peak intensity
may be over western portions of the CWA. Should hail be a concern,
it will be with initial discrete development, which then transitions
to a damaging wind threat as convection become linear. An isolated
tornado or two may be embedded in the line with backed winds ahead
of the approaching line. Confidence in severe weather is low given
all of the potential caveats ahead of the redevelopment.

Guidance has been consistent in stalling the front over southern
sections of the forecast area Monday. This becomes the focus for
scattered, sub-severe thunderstorms over  sections of southeast
Missouri and southwest Illinois Monday afternoon.


Maples

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 318 PM CDT Sat Apr 27 2024

By Monday morning, the bulk of the remnant shower and thunderstorm
activity from the previous night will likely have moved east of the
area. However, as previously mentioned a cold front will remain
draped across the region, with a modestly unstable warm sector
draped across southeast Missouri and west-central Illinois by mid
afternoon. Meanwhile, a belt of modest 500mb flow will remain in
place aloft (20-35kt), but the strongest winds and forcing aloft
will depart as the previously discussed shortwave and jet streak
move into the upper Midwest. While this will not be much to work
work, this may support at least some scattered convection along
and south of the cold front by mid to late afternoon Monday, with
a limited potential for some small hail or gusty downburst winds.

Beyond Monday, confidence in the day-to-day details diminishes as we
emerge into a regime of largely zonal flow aloft. Tuesday remains
the most likely day to remain dry during the work week, but the
passage of several shortwaves along the periphery of a broad upper
ridge across the central CONUS will maintain multiple chances for
rain over the latter half of the week. Meanwhile, temperatures are
likely to remain near or slightly above average overall, but the
potential for widespread cloud cover and waves of rainfall will
likely limit this potential. The NBM in particular is very warm with
regards to temperatures late in the week, and in spite of the
relatively narrow ensemble spreads, it remains to be seen whether or
not these very warm temperatures (upper 80s to low 90s) can be
realized.

BC

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1223 AM CDT Sun Apr 28 2024

A line of strong thunderstorms extends from west-central Illinois
through central Missouri as of 0530Z this morning. Observations
have shown gusts of 30-35 kts to be fairly common with localized
gusts up to 45 kts around KCOU. The line is preceded by outflow
that has briefly turned winds out of the northwest, which
consequently will lead to continued weakening with eastward
movement. Over the last 24-36 hours, guidance has been consistent
in suggesting the line will weaken considerably as it approaches
the metro terminals. Signals are that this process has already
begun with the line becoming outflow dominant. While KCOU/KJEF and
KUIN will continue to be impacted by SHRA/VCTS and MVFR ceiling,
metro terminals may be spared of similar conditions with weakening
showers and an intermittent clap of thunder early this morning.

Focus shifts to Sunday afternoon as a slowing frontal boundary
could ignite additional showers and thunderstorms around and
after 18z. The coverage and intensity will be affected by behavior
of prior convection and how well the atmosphere can recover
instability. Nonetheless, another round of thunderstorms could
impact all terminals as they progress from central Missouri from
mid-afternoon through eastern Missouri through early evening. A
few storms could be strong to severe with heavy rain, gusty winds
and frequent lightning. For now, this was handled with SHRA/VCTS
mention until details can be fine tuned in subsequent updates.

Maples

&&

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

$$

WFO LSX