660
FXUS63 KLSX 041747
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1247 PM CDT Tue Jun 4 2024

.KEY MESSAGES...

- Isolated severe thunderstorms are possible again this afternoon
  into evening across the region. Primary hazards in the
  strongest thunderstorms include damaging winds (up to 60mph) and
  large hail (up to 1 inch).

- Our greatest chance at seeing thunderstorms will be tonight
  along a cold front. If we see any severe weather from these
  storms, the primary hazard will be damaging winds (up to 60
  mph).

- Largely dry conditions and near average temperatures will follow
  the cold front. A low chance of thunderstorms (up to 20%)
  returns this weekend.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 338 AM CDT Tue Jun 4 2024

A mid-level shortwave trough and surface MCV are located over
eastern Kansas/western Missouri as of 07z and slowly moving east-
northeast. The MCV has only been able to produce generally short-
lived showers along a wing of weak moisture convergence and low-
level warm air advection. With no SBCAPE and limited MUCAPE (up to
500 J/kg), it`s unlikely that this feature will materialize into
anything more than it currently is, and confidence in this feature
drastically impacting this afternoon`s forecast is low. Meanwhile,
ongoing convection in eastern Oklahoma is expected to spark
additional northeastward moving convection along an outflow boundary
early this morning. A weak (20 to 30kt) LLJ along with warm air and
moisture advection will keep convection alive as it reaches south-
central Missouri late this morning. We have confidence that limited
surface based instability (500 - 1000 J/kg) and weak bulk shear (up
to 20 kts) will keep these thunderstorms below severe criteria.
However, confidence begins to wane regarding how they will impact
afternoon convection.

This afternoon`s near storm environment will be similar to that of
yesterday`s: high SBCAPE (2000 to 3000 J/kg) with low bulk shear (20
to 25 kts). Although a mid-level trough will crawl through the
region, forcing for ascent will be weak, leaving the heavy lifting
to warm air advection to spark disorganized, pulse convection.
Thunderstorm coverage will increase from isolated to scattered
through the afternoon as instability increases. However, the
coverage of severe thunderstorms specifically will remain isolated
at best. Deterministic soundings show long, skinny CAPE this
afternoon, translating to slow building updrafts with a lack of
substantial hail CAPE. Largely saturated profiles, low shear values,
and weak low-level lapse rates indicate that downdraft potential and
momentum transfer will be minimal as well. These factors are
expected to keep thunderstorms largely sub-severe this afternoon. If
a thunderstorm is able to become severe damaging winds of up to 60
mph and large hail of up to 1 inch will be the primary hazards.
Thunderstorms may have a greater potential at becoming severe if
this morning`s convection establishes outflow boundaries in the
area.

The finale of our early week thunderstorm chances will come late
this evening along a cold front. Upper level ascent will maximize
during the frontal passage and MUCAPE values are expected to still
range from 1500 to 2000 J/kg. Thunderstorms are thus expected along
the front as it enters the forecast area from the northwest. Low
bulk shear (20 to 30kts) and 0 to 3km shear (10 to 20 kts) in
addition to diminishing daytime instability and mid-level forcing
decrease confidence in the front`s severe threat. If any segment
does become severe, damaging winds of up to 60 mph will be the
primary hazard. The best potential of this happening would be in
northeast Missouri and west-central Illinois, as the line is
expected to weaken and break with time due to the aforementioned
factors. The threat for flash flooding has been previously discussed
with this frontal passage, but the progressive, southwest to
northeast oriented, weakening line of showers and thunderstorms
should preclude this in most locations. There`s no substantial
signal for backbuilding or training, either, further lowering the
potential for flooding. The latest LPMM reflects this, with very
localized spots of 2 to 3 inches of rain, decreased from days past.
Again, northeast Missouri and west-central Illinois have the
greatest chance at seeing flash flooding given the strength of the
line as it passes through those areas. Of course, any area with poor
drainage and/or areas that have been impacted by recent heavy rain
there is still a have the potential for very localized flash
flooding.

The cold front is expected to clear the area by Wednesday morning
with all precipitation exiting shortly after. High temperatures
Wednesday will land in the low to mid-80s despite the frontal
passage, but it will be notably less humid as moisture is scoured
from the region. A dry secondary cold front will bring cooler air to
the region Wednesday night, dropping lows into the low to mid-60s.

Jaja

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 338 AM CDT Tue Jun 4 2024

A deep mid-level closed low will spin over the Great Lakes region
late this week, keeping the region in northwest flow aloft. There is
signaling in deterministic guidance of two shortwaves passing
Saturday and Sunday, but given the expected flow pattern not being
favorable for thunderstorms and the differences in the troughs
among models, confidence is low in what these features will bring
us this weekend. The NBM interquartile range reflects this as
well, with 9+ degree differences between the 25th and 75th
percentiles beginning on Saturday. What we are confident in is a
cooler, seasonable weekend ahead.

Jaja

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1230 PM CDT Tue Jun 4 2024

Scattered showers and thunderstorms will be in the vicinity of
the airports early-mid afternoon. There may be the potential for
additional development later this afternoon into the early this
evening, but confidence in this trend is too low to include in the
TAFs at this time. There is higher confidence with thunderstorms
chances late this evening and overnight ahead of a cold front that
will pass southeast through the area. Any thunderstorms that
occur through tonight have the potential to reduce ceilings and
visibilities to MVFR (possible IFR) conditions in brief downpours
with gusty winds. Otherwise, drier conditions are expected behind
the front on Wednesday with VFR conditions.

Britt

&&

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

$$

WFO LSX