Arnold, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

487
FXUS63 KLSX 182331
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
631 PM CDT Thu Jun 18 2026

.KEY MESSAGES...

- Conditions subtly waver between seasonably cool and dry
  conditions to near normal warmth and humidity through the middle
  of next week.

- Widespread showers and thunderstorms are possible from late
  Saturday night into early Monday. Locally heavy rain may
  accompany thunderstorms.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 106 PM CDT Thu Jun 18 2026

Light rain and isolated thunderstorms are expected to continue
moving southeast along a slow-moving, east-west oriented cold front
through 00z. Rain chances (20-40%) will mainly lead to low impacts
to locations south of I-70 through 00z. Isolated thunderstorms are
possible along the front, leading to very brief downpours.

The region is positioned at the southern edge of a longwave,
upper level trough this afternoon with a secondary surface cold
front following close behind last night`s front. Radar trends from
this morning through early afternoon show precipitation tracking
southeast around 20-25 mph with isolated thunderstorms
occasionally developing within the east-west oriented band of
precipitation. Hi- res and medium range guidance has had
difficulty in placement and overall intensity with the NAM
seemingly best suited for timing and hi-res guidance more
representative of intensity. None of the hi-res guidance seems to
be picking up on the limited convective potential, while the
NAM/GFS show low MUCAPE values (100-300 J/kg) along and south of
I-70 through approximately 00z. Thunderstorms will be very short-
lived at any one location with a brief downpour. Thunderstorms
remain isolated with precipitation duration lasting approximately
2 hours from start to finish, give or take 30 minutes. Rainfall
amounts only amount to around 0.10" or less.

Cooler air and lower dewpoints have already begun to infiltrate
the region behind last night`s front. Where rain falls this
afternoon, temperatures will be relatively cooler (mid to upper
70s) than dry locations (low-80s). Surface high pressure builds in
behind the front from the northwest tonight through Friday. Quiet
and dry conditions hold through the end of the short term period
with seasonably cool temperatures (Highs: low/mid-80s; Low: Upper
50s to mid-60s).

Maples

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 259 PM CDT Thu Jun 18 2026

Mostly dry conditions persist through the day Saturday before
another round of showers and thunderstorms impact the region late
Saturday night through early Monday morning. This round has the
potential to be widespread with locally heavy rainfall with impacts
largely centered on Sunday.

As we enter the weekend, the upper level pattern remains loaded with
multiple shortwaves lining up from the central Plains to the Pacific
Northwest. Northwesterly upper level flow gradually becomes zonal
over the later half of the weekend, initially orienting the active
storm track northwest to southeast, then more west to east into
early next week. Surface high pressure staves off precipitation
potential Saturday, but by late Saturday, the lead shortwave induces
surface low pressure near the CO/KS/NE state lines and approaches
the Mid-Misssippi Valley Saturday night. Considering the evolution
of the upper level pattern, it`s no surprise that temporal and
spatial spread increases at this distance in time. Trends support
increasing southerly flow that draws Gulf moisture northward into
the Plains ahead of the surface low before it kicks eastward.
There`s a strong indication that an MCS develops over eastern NE/KS
and dives southeast, while the parent surface low track eastward
along the MO/IA border. This focuses precipitation across the
eastern Plains into western MO with the complex, while another
boundary lines up along the southern periphery of the upper level
longwave trough across the Upper Midwest.

Though specifics are not quite clear, showers and thunderstorms are
favored after midnight Saturday night through at least early Monday
morning around sunrise (subject to change). The key message here is
that there is the potential for locally heavy rain as dewpoints
climb back into the 60s/70s with a broad breadth of PWATs above 2
inches (locally near 2.6"). The pool of moisture is accompanied by
instability in excess of 3000 J/kg over western MO, trending
downward to around 1000 J/kg near the Mississippi River.
Additionally, a strong LLJ of 50-60 kts is pointed into the central
Plains with westerly shear components of 50-60 knots. This creates a
broad area of rain potential with less confidence on details given
the chaotic setup. While this setup is currently a little too far
west for severe locally, adjustments could prove otherwise. At the
very least, it look like a good shot at rainfall, some locally
heavy. Temperatures remain near or slightly below seasonal norms
through the period.

Maples

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 629 PM CDT Thu Jun 18 2026

Dry/VFR conditions are expected through the day on Friday with
light/variable winds as a surface high approaches. There is a low
chance for some river valley fog late tonight into early Friday
morning, but confidence was too low to add to KJEF/KSUS/KCPS.

Gosselin

&&

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

$$

WFO LSX