913
FXUS63 KLSX 150825
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
325 AM CDT Sun Sep 15 2024

.KEY MESSAGES...

- Today will be another warm day with scattered afternoon showers
  and thunderstorms.

- Warm and dry conditions are expected through the upcoming week.

&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 322 AM CDT Sun Sep 15 2024

A large ridge of high pressure is centered over the Great Lakes
today while an elongated trough related to the remnants of Francine
lingers across the southern US extending into southern Missouri.
While the trough continues to weaken and provide only localized
support for ongoing showers across the Midsouth, its broader axis
extending up into Missouri does represent an area of cooler
temperatures aloft over the top of a continued warm and moist
surface air mass. So while we don`t have a persistent area of thick
cloud cover or showers to contend with this morning, as daytime
heating begins and we warm well into the 80s, we`ll see cumulus
bubble up and eventually sprout into showers and thunderstorms this
afternoon in the absence of any appreciable capping. This is similar
to what occurred yesterday when temperatures warmed a bit more than
expected and air mass showers and a few thunderstorms developed for
a brief few hours during the mid to late afternoon. These pop up
showers and storms may occur anywhere across the forecast area this
afternoon, but we believe the favored areas will be west of the
Mississippi River, especially toward central and southwest Missouri
where instability is likely to be greater. Some low level dry
advection on the easterly flow will limit instability further east
at least somewhat.

Models vary substantially on just how much instability develops this
afternoon, but the consensus is for surface based CAPE to exceed
1000, perhaps even 2000. Winds aloft are weak and shear is almost
nonexistent, so thunderstorms that form today will be of the pulse
variety, going up and down fairly quickly, possibly organizing
around a common cold pool. If we do manage to realize some of the
higher end instability, it wouldn`t be out of the question that a
strong to severe storm could occur with downburst winds and hail the
threats. But due to the lack of shear to organize the thunderstorms
we do not anticipate messaging a severe weather threat today. Storm
motion will be from east to west, the opposite of what we typically
see, a factor of the easterly flow through the low and mid levels.

As for Monday, afternoon shower and thunderstorm chances are even
lower and shifted further west as the low level dry advection on
easterly flow continues. Dewpoints drop into the 50s along and east
of the Mississippi River, but there remains just enough low level
moisture to keep a low (20 percent) chance for afternoon showers and
thunderstorms mainly in central Missouri. Temperatures continue to
run 5 to 10 degrees above normal for mid September, in the mid to
upper 80s.

Kimble

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 322 AM CDT Sun Sep 15 2024

Our weather this week will be dominated by the broader scale ridge
across the eastern US leading to dry and warm conditions through the
week. Highs stay within a relatively small range from 85 to 90 each
day. The trough over the South eventually reorganizes around a low
developing off the Southeast coast which then gets pulled back to
the northwest through the Mid Atlantic into the Appalachians where
it stalls and weakens. While there is some variability on how far
west the remnant low wobbles through midweek, it stays far enough
east of us to avoid any rain from it.

Looking to the west, persistent troughing over the western US
maintains southwest flow over the central and northern Plains with
multiple embedded troughs ejecting northeastward through the week
and into the weekend. Over time there is a gradual drift eastward of
this trough with the southwesterly mid level flow getting close to
our area by week`s end. It`s this southwest flow with embedded
troughs and moisture pumping northward that represents our only real
chance of rain in the next week or so.

Uncertainty increases in the overall flow pattern as we get toward
this weekend and into next week. That`s understandable considering
the retrograding low over the Appalachians, its effect on the ridge
to the north and west of it, and the cumulative effect of many
troughs emanating out of the western US through the week. As
such, low rain chances (20 to 30 percent) do begin to sneak into
the western part of the forecast area Friday into Saturday.
Although there is more uncertainty in the broader flow regime,
there is general consensus that ridging in some form continues
across our area which would lead us to favor the persistent dry
and warm forecast. Even among the guidance sources that do depict
some rain scraping our area, it doesn`t come in a particularly
widespread or heavy form. Overall ensemble probability of 0.25
inch of rain in any 24 hour period never gets much above 20
percent anywhere in our forecast area right through the weekend.
Because of this we do not expect any improvement in the developing
drought conditions across the region.

Kimble

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1023 PM CDT Sat Sep 14 2024

Main concern for overnight tonight is on fog as earlier
shower/thunderstorm activity has diminished. River valley fog is
expected with the worst conditions (IFR visibilites) expected at
KSUS. Also added some MVFR fog at KJEF and KCPS. Any fog should
quickly lift and dissipate within 1-2 hours after sunset.
Additional scattered showers/weak thunderstorms are forecast on
Sunday, mainly across portions of southeast Missouri during the
afternoon hours and south of the terminals.

Gosselin

&&

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

$$

WFO LSX