538
FXUS63 KLSX 221911
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
211 PM CDT Sun Sep 22 2024

.KEY MESSAGES...

- A few strong to severe thunderstorms are possible this afternoon
across southern Missouri and Illinois, damaging winds are the
greatest threat.

- After another round of showers and thunderstorms Monday a
  relative dry period will settle in for the work week.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 211 PM CDT Sun Sep 22 2024

The heavy cloud cover that has been in place from this morning`s
rain has started to break up across southern Missouri and Illinois,
increasing temperatures and instability in this area. While cloud
cover is breaking up further north into the LSX forecast area and as
far north as the I-70 corridor in some areas, it remains uncertain
how much destabilization will be possible in these areas this
afternoon. Where destabilization occurs, roughly 1000-2000 J/kg of
MLCAPE will develop and thunderstorms will be able to form from a
combination of an approaching cold front and a small mid-level
disturbance. With roughly 25-30 kts of 0-6km shear a few storms
will be able to become strong to severe, with damaging winds the
main concern. Even if cloud cover doesn`t clear significantly over
our forecast area, storms could track from a more favorable
environment in southwest Missouri eastward into the southern
forecast area. The location of the SPC Marginal Risk outlines the
most favorable area for strong to severe thunderstorm development.

The risk for thunderstorms will diminish with sunset as instability
wanes and the cold front exits the forecast area. After a brief dry
period overnight, another round of rain will move northeast into the
forecast area, driven by the low-level jet and interacting with the
lingering elevated front. This shot of rain will push through the
area during the early morning to mid-morning hours. Precipitable
water values near the climatological max (1.50-1.75") will boost
rain rates despite the meager instability tomorrow.

A 20-50% chance for showers and a few thunderstorms lingers through
the day and into the evening hours as a mid-level shortwave
approaches the forecast area ahead of an advancing mid-level trough
and interacts with the persistent low-level jet. With the cold
front expected to be south of the forecast area, instability will
be at a premium. SREF guidance indicates only a 10% chance of
MUCAPE values greater than 500 J/kg across the far southern CWA
Monday afternoon. If the cold front were to stall within the
southern CWA, instability would be higher and likely be able to
generate a strong thunderstorm, but confidence in this is low.
Given this, we have chosen not to message the SPC Day 2 Marginal
at this time. Across the 24 hour period more beneficial rain is
expected, with global ensemble guidance showing a 90% chance of at
least 0.5" of rain and a 50-60% chance of 1.0" of rain south of
the I-44 corridor in Missouri. Lesser amounts are expected to the
north and east.

Cold air advection behind today`s cold front will cool 850 mb
temperatures to around normal (12-16 degrees C), and combined with
rain and heavy cloud cover, will result in another cool day tomorrow
with some portions of the forecast area struggling to exceed 70
degrees.

Delia

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 211 PM CDT Sun Sep 22 2024

The previously mentioned mid-level trough will approach the mid-
Mississippi Valley Tuesday, though moisture will be limited behind
Monday`s precipitation and significant rainfall is not expected.
Through the remainder of the week the mid-level trough will linger
over the lower-Mississippi River Valley, while a mid-level ridge
will ooze over the trough to the north, generating a Rex block that
will persist into the weekend. With the trough positioned across the
lower-Mississippi River Valley, the best moisture return, lift, and
precipitation chances will remain largely south of the forecast
area.

Some ensemble systems are indicating that a tropical system
(currently a tropical disturbance over Nicaragua) may move northward
and interact with the mid-level trough. This would bring a surge of
moisture and lift into the lower to mid-Mississippi Valley,
increasing precipitation chances for the weekend. This scenario
would be our next best chance of precipitation in the long term, and
obviously is highly contingent on the development and motion of the
tropical disturbance. We will be monitoring this over the coming
week.

Under the more stagnant Rex block, 850 mb temperatures remain near
normal, so fluctuations in daily high temperatures will be largely
tied to how much cloud cover is present. Currently high temperatures
are projected to remain near to just below normal, but increased
sunshine would result in a bump of up to 5 degrees.
Delia

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1159 AM CDT Sun Sep 22 2024

A mixed IFR to VFR ceiling persists over the area with gradual
improvements expected at the terminals to high MVFR or even VFR
flight conditions. Ceilings are expected to drop back down to MVFR
or even IFR conditions as early as the evening hours as daytime
mixes diminishes. Confidence is highest that conditions will
degrade to MVFR with a round of rain that is expected tomorrow
morning. Guidance points to IFR conditions building in overnight,
largely tied to the previously mentioned rain, however confidence
is not yet high enough to include a mention in the TAFs for the
moment.

Speaking of rain, showers are lingering across the area this
afternoon but should gradually clear ahead of shower and
thunderstorm chances mainly south of I-70 and the mid-Missouri and
St. Louis metro terminals. The next best chance (50-60%) for rain
will push into the area tomorrow morning from the southwest,
impacting the mid- Missouri and St. Louis metro terminals.

Southwesterly winds will become northwesterly to northerly over
the afternoon and into early evening as a cold front slides
through the forecast area. Wind speeds will stay largely below 10
kts.


Delia

&&

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

$$

WFO LSX