012
FXUS63 KLSX 040826
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
326 AM CDT Sat May 4 2024

.KEY MESSAGES...

- Isolated strong to severe thunderstorms will be capable of
  quarter- size hail and damaging wind gusts today from roughly
  2-8pm.

- An active weather pattern will bring multiple rounds of severe
  thunderstorms to portions of the Midwest early to mid next
  week. Specifics on exact timing, location, and magnitude are
  still unclear, but confidence continues to increase that the
  area will experience at least a round of severe weather.


&&

.SHORT TERM... (Through Sunday Night)
Issued at 319 AM CDT Sat May4 2024

A shortwave is digging into the Central Plains per recent water
vapor imagery, and is aiding in driving ongoing convection along a
cold front stretched across that portion of the CONUS. Ahead of this
front across the Midwest, surface high pressure is moving eastward,
with low-level winds becoming increasingly southerly. However,
winds have been calm enough for patchy fog, some locally dense to
develop where skies have been mostly clear. This fog will
dissipate with the rising sun this morning.

This morning, these southerly winds will begin advecting deeper
moisture into the CWA ahead of the decaying Plains convection as it
enters the area. This decaying convection is not expected to pose a
severe thunderstorm threat when it enters portions of central
Missouri during the late morning. This afternoon, the increasing
boundary layer moisture will yield roughly 1,500-2,000 J/kg of
SBCAPE per 00z deterministic guidance and the HREF mean. If
convective debris from the morning storms is denser and more
widespread, then surface heating and thusly instability will be
limited to some degree. Given 0-6 km bulk shear is expected to only
be around 20 kts at best, lower instability would greatly reduce the
already isolated severe thunderstorm threat. If the previously
mentioned levels of instability are realized, then an isolated
threat of severe thunderstorms can be expected along the cold front
and any lingering outflow boundaries this afternoon as a shortwave
passes overhead. Hail up to around the size of quarters and damaging
wind gusts are the main threats from roughly 2-8pm.

Another shortwave within the southwesterly flow aloft will move out
of the Southern Plains and toward the Middle Mississippi Valley
tomorrow. Guidance consensus has slowed this shortwave and its
surface reflection, leading to much of the area being dry through
the day tomorrow. During the afternoon and into the evening,
scattered showers and thunderstorms will begin moving into the CWA
from the southwest, with the best rain chances (50-70%) remaining
along and south of the I-70 corridor through Sunday night.

Elmore

&&

.LONG TERM... (Monday through Friday)
Issued at 319 AM CDT Sat May 4 2024

Monday will be the start of a multi-day stretch of severe weather
potential across the Great Plains and Midwest. Aloft, a trough will
broaden over the western CONUS as a shortwave pivots through the
Great Plains and into the Upper-Midwest. At the surface, a low will
occlude across the Northern Plains as it sweeps a cold front through
the Central Plains and lifts a warm front through the Midwest.
Guidance consensus is that the cold front will stay west of the CWA
during the day Monday, but the warm front will lift northward
through the area. Scattered showers and thunderstorms are possible
(40-70% chance) with the warm front, but 0-6 km bulk shear of 20 kts
or less leads to low confidence in severe weather occurring with the
warm front. If severe weather does impact the CWA Monday, it would
likely do so late Monday night as decaying convection associated
with the Plains cold front moves into the area.

The cold front moves into the CWA Tuesday as another shortwave
passes through the Midwest along the southeastern periphery of the
trough. This will increase 0-6 km shear across the warm sector to
roughly 50 kts among SBCAPE peaking around 1,500 J/kg, setting the
stage for a potent environment for severe thunderstorms. Given the
current lead time, there`s still plenty of time for details to
chance, such as the phasing of the shortwave, timing of the cold
front, and ability of the atmosphere to sufficiently destabilize if
there`s residual cloud cover from overnight/early morning
convection.

For Wednesday, the trough reloads as another shortwave moves through
the Central Plains and into the Midwest, spawning another surface
low that will follow a similar trajectory. In turn, the previous
day`s cold front will lift northward as a warm front, placing
portions of the Middle Mississippi Valley in the warm sector
similarly characterized as the one the day prior and leading to
another potential round of severe weather. The placement of this
threat will be dependent on how far northward the warm front can
travel and the timing of the cold front.

In the wake of the front to end the week, the upper-level trough
will move eastward and place the CWA beneath deep northwesterly flow
per ensemble clusters and deterministic guidance. This flow will
advect cooler, more seasonable to slightly cooler than normal air
into the region. The 75th percentile of ensemble guidance continues
to hover right around climatology, building confidence in this
outcome.

Elmore

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1036 PM CDT Fri May 3 2024

The only change since the last TAF update is the increasing
potential for fog development early Saturday morning. The
potential for dense fog should remain over southeast Missouri and
far southwest Illinois. However, patchy fog could develop as far
north as I-70, including the metro terminals. The sites to monitor
will be KSUS/KCPS, as recent observations show winds have gone
calm at KSUS. TAFs do not include the mention of fog, but may
later be amended if trends continue in this direction.

The main impact will stem from thunderstorm potential Saturday,
progressing west to east along and ahead of an advancing cold
front. Thunderstorm potential still look highest after 18z through
about 00z. Cloud bases remain at VFR level through much of the
period until the cold front pushed through Saturday evening, at
which time a brief period of MVFR ceilings are likely to
accompany the front. Outside of this, any conditions lower than
VFR will depend on direct impacts from thunderstorms.

Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CDT this morning for Iron MO-
     Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve
     MO.

IL...Dense Fog Advisory until 9 AM CDT this morning for Randolph IL-
     Washington IL.

&&

$$

WFO LSX