158
FXUS63 KLSX 190835 CCA
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
314 AM CDT Thu Sep 19 2024

.KEY MESSAGES...

-Several opportunities for rain tonight through next week. The
 best chances of showers and storms will be Saturday night
 through Sunday night. Otherwise, while any rain is beneficial,
 the chances for significant drought relief remains low.

-Well above normal temperatures through Saturday, then cooler
 weather for the last half of the weekend and into next week.

&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 310 AM CDT Thu Sep 19 2024

With an upper level ridge across the region, a shortwave over the
Northern Plains will continue to slowly lift northeastward into
southern Canada, thus dry and hot conditions will prevail across
our area today. Highs will be in the upper 80s to low 90s under
mostly sunny skies.

In the meantime, a cold front associated with the Northern Plains
system will move into northwestern Missouri this afternoon. As the
front begins to move into northeastern Missouri this evening, will
see increasing chances of showers and storms ahead of it. However,
the main energy with the front will stay north of the forecast area.
Also, a southwesterly low level jet will develop, but it will be
fairly weak (around 25kts) and veer to the west by Friday morning.
So precipitation coverage will be widely scattered with the best
chances over portions of northeast Missouri and west central
Illinois. The latest CAMs still have some differences in timing and
location of the rain, but confidence is high that the best period of
precipitation will be between 11 pm and 7 am. Despite MU CAPEs
around 1000-1500 J/kg over portions of central/northeast Missouri
late this afternoon, 0 to 6km bulk shear will be fairly weak (25kts)
with ML lapse rates less than 6C/km. The instability will weaken
fairly rapidly with loss of daytime heating, thus do not expect any
severe storms in our area this evening.

Otherwise, the front will gradually wash out as it sinks south
during the day on Friday. Also, the upper level ridge will remain
anchored overhead. Therefore, expect another well above normal day
with highs in the upper 80s to low 90s. Some concern that activity
could fire back up Friday afternoon. The latest HREF ensemble has
mean MU CAPEs in excess of 1000 J/kg, mainly south of I-70. However,
there is little to no convergence left along what remains of the
frontal boundary. So for now will keep dry conditions in this area.

Byrd
&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 310 AM CDT Thu Sep 19 2024

Even though the upper level ridge will remain overhead through
the weekend, the trough over the western CONUS will begin to lift
out towards the Central Plains Friday night. By early next week
the upper level ridge will begin to be suppressed to our south,
allowing a series of shortwaves to slide through the region with
rounds of showers and storms. Even though the latest cluster
analysis still has various timing, location and strength issues
with how the upper level pattern evolves through next week, the
latest deterministic models are in decent agreement that a strong
wave will move through Saturday night into Sunday. Even though the
region remains cutoff from Gulf moisture, strong southwesterly
flow will usher in monsoonal moisture into the region by Saturday.
So showers and thunderstorms are likely (60-80%), especially
along and north of I- 70. Beyond Sunday night, with such
uncertainty on how the system will develop, only expect low rain
chances (roughly 30-40%) Monday through Wednesday.

As for temperatures through the extended period, one more day of
above normal highs on Saturday despite increasing cloud cover. Then
as the ridge is suppressed to our south beginning on Sunday,
temperatures begin to cool to near normal. With some uncertainty on
how the upper level pattern will evolve, especially early next week,
the latest ensembles have a fairly wide spread between the 25th and
75th percentiles (at least 10 degrees) for temperatures Monday
through Wednesday.

Byrd
&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1043 PM CDT Wed Sep 18 2024

Mid to high clouds are moving east of the Mississippi River late
this evening with clearing from west to east. Surface observations
around most sites are showing rather large dewpoint depressions,
which denote a dry surface layer. Among the few exceptions are
sites near river valleys, including KSUS. Already light/calm
winds and narrow dewpoint depressions provide enough argument for
a brief period of MVFR fog. This has been added at KSUS from
10z-13z Thursday morning. Support for fog is otherwise limited to
southeast MO and portions of southwest IL, which will well outside
the terminals. Aside from KSUS, VFR is expected through the
remainder of the period with typical diurnal wind behavior.

Maples
&&

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

$$
WFO LSX