540
FXUS63 KLSX 280536
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1236 AM CDT Sun Apr 28 2024

.KEY MESSAGES...

- Thunderstorms late this evening and overnight have the
  potential to produce heavy rain and localized flooding across
  northeast MO and west-central Illinois, with a limited potential
  for damaging winds and an isolated tornado as well. A Flood
  Watch has been issued for parts of northeast Missouri and west-
  central Missouri from 7 PM tonight until 10 am tomorrow morning.


- Thunderstorms are likely late tomorrow afternoon through late
  evening in many areas, with damaging winds, large hail, and an
  isolated tornado or two possible.

- A few thunderstorms will be possible mainly along and south of
  I-44 Monday afternoon and evening.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 318 PM CDT Sat Apr 27 2024

While conditions are quiet across the region as of this writing,
this will not last long as multiple rounds of thunderstorms remain
on the horizon over the next 48 to 60 hours. At 2 PM CDT, a surface
low was located across southwest Kansas and the OK/TX panhandles,
with a nearly stationary front extending northeastward into
Kansas, northern MO, and southern IA. South of this boundary lies
a broad warm sector characterized by surface dewpoints in the low
to mid 60s, and precipitable water values between 1 and 1.25
inches, the latter of which is near the 90th percentile for this
time of year. Meanwhile, a slow-moving shortwave and associated
jet streak was located across the central high plains, providing
ample upper forcing to overcome weak capping along both the
dryline and previously mentioned warm front extending into
northwest Missouri.

As the afternoon progresses, this convection is expected to expand
and gradually drift eastward and eventually into parts of
northeast/central Missouri and west central Illinois. While
scattered weak thunderstorms ahead of this activity may produce a
few heavy downpours and small hail this afternoon, the more
impactful thunderstorms are likely later in the evening and
overnight as more organized storms arrive from the west. Across
northeast Missouri and west-central Illinois, training thunderstorms
will be possible due to storm motions nearly parallel to the stalled
front. Meanwhile, anomalously high moisture content will promote
moderately efficient rainfall rates, which may fall in areas that
have already observed significant rainfall in the past couple of
days. Meanwhile, CAMs have indicated a potential for locally heavy
rainfall across a narrow swath in these areas, and confidence is
increasing in the potential for 1 to 3 inches of rain, and
potentially higher amounts possible as well. As such, we`ve issued a
Flood Watch for this evening through tomorrow morning across a few
counties in northeast Missouri and west-central Illinois, in
addition to many other counties added by neighboring offices to our
north and west.

Meanwhile, the strengthening low level jet will also cause low
level shear to increase through the evening and overnight ahead of
these storms, and while instability will wane considerably
overnight (SBCAPE falling to 500 J/kg or less), we can`t rule out
a few damaging wind gusts or even a tornado or two late this
evening and overnight. This threat will steadily diminish after
roughly 3 am as storms wane in intensity. Remnant showers and
thunderstorms will continue to gradually spread eastward through a
majority of the area through the morning hours, and eventually
diminish as they move east of the Mississippi River.

Exactly when this occurs will play a big role in the potential for
local strong/severe thunderstorms tomorrow. Considering the
potential for widespread cloud cover and remnant showers,
instability will be at a premium tomorrow afternoon locally and
there remains some question as to how efficiently the warm sector
will be able to destabilize. What is most likely is that a
corridor of moderate instability will develop across western
Missouri, and forcing from the departing shortwave and mid level
jet streak will drive thunderstorm development here during the
afternoon. This activity will then drift eastward during the late
afternoon and evening as it matures, eventually reaching our
portions of central and northeast Missouri, but encountering a
steadily less favorable thermodynamic environment. However, should
we maintain sufficient instability, 40 to 50 kt of 0-6km shear
will be present, with gradually increasing low level shear late in
the afternoon and into the evening as the low level jet
increases. In other words, while confidence is high that much of
the region will see more showers and thunderstorms late in the
day/evening tomorrow, likely in the form of a broken linear
complex, the threat of severe storms remains conditional.

Thunderstorms will move east of the area late in the evening and
overnight tomorrow, and a cold front will lag this activity and
stall across the area sometime during the day Monday. The
implications of this will be discussed in the long term section
below.

BRC

&&

.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...Flood Watch until 10 AM CDT this morning for Knox MO-Lewis MO-
     Marion MO-Monroe MO-Shelby MO.

IL...Flood Watch until 10 AM CDT this morning for Adams IL.

&&

$$

WFO LSX