Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

085
FXUS63 KLSX 031030
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
530 AM CDT Fri Apr 3 2026

.KEY MESSAGES...

- Severe thunderstorms are expected this evening into tonight
  with damaging winds and brief tornadoes. A lower threat of large
  hail and a brief tornado also exists this afternoon.

- Much cooler, below average temperatures are forecast late
  Saturday through Tuesday including a 30 to 60 percent chance of
  a freeze Monday night north of I-70.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 337 AM CDT Fri Apr 3 2026

Thursday`s cold front has stalled near the I-44 (MO) and I-70 (IL)
corridors early this morning, supporting nearby isolated to
scattered showers and weak thunderstorms at times as a modest LLJ
interacts with the front. However, these showers and thunderstorms
will become increasingly suppressed after sunrise by rising mid-
level heights as the LLJ weakens.

With a handoff of the front to an approaching surface cyclone across
the Central Plains, the front will begin lifting northward later
this morning, reaching northeastern MO/west-central IL during early
afternoon and departing to the north by this evening. As a result,
the majority of the CWA will be in the warm sector with high
temperatures warming into the mid-70s to low-80s F as clouds scatter
through the day. This heating and associated mixing will help to
weaken a low-level capping inversion, but model guidance is mixed on
whether or not broad, modest low/mid-level WAA/isentropic ascent
will then be enough to trigger showers and thunderstorms during the
afternoon. As such, 1-hour probabilities of measurable rainfall in
the HREF are anywhere from 10 to 30 percent. If thunderstorms
develop, between 1000 and 2000 J/kg of MLCAPE is generally depicted
by deterministic model guidance this afternoon along with 30 to 40
kt of deep-layer wind shear, marginally supportive of a few
supercells with large hail and a brief tornado where low-level wind
shear is maximized near the warm front while it is still in the CWA.
That being said, forecast soundings do indicate a warm layer around
600 hPa that could impede growing updrafts in the absence of more
robust large-scale ascent, which reduces confidence in a more
appreciable severe threat this afternoon into early evening.

Meanwhile, as an upper-level trough deepens across the Northern and
Central Plains later today, thunderstorms will also develop across
eastern KS/western MO this afternoon and evening along a cold front.
Despite some convective mode discrepancies shortly after initiation,
CAMs are in very good agreement on eventual upscale growth into a
QLCS that tracks eastward and enters northeastern/central MO and
west-central IL around 8 to 10 pm. This part of the CWA has the
greatest or most confident severe threat, which includes damaging
winds and brief tornadoes given 0-3 km wind shear vectors of 25 to
40 kt that could permit mesovortex generation with any northeastward-
surging line segments. Gradual weakening is expected as the QLCS
continues eastward across the remainder of the CWA overnight and
encounters lower instability; however, forecast soundings depict
only a shallow inversion in the boundary layer and MLCAPE
projections are near 1000 J/kg, which suggest that at least a
sporadic severe threat could continue late into the night. With the
QLCS and the parent cold front being progressive, the threat of
flash flooding is low tonight.

The QLCS and cold front should be exiting southeastern
MO/southwestern IL around or shortly after daybreak Saturday
morning, but the passage of an equatorward upper-level jet entrance
region supports a flare up of stratiform rain that lingers much of
the morning along/east of I-44 in MO and I-55 in IL. This evolution
is supported by nearly all of HREF membership, but incoming drier
air and waning ascent will allow rain to end more quickly from west
to east again during the afternoon. With the much cooler, post-
frontal airmass; temperatures will be in the 40s to 50s F by the
afternoon as strong low-level CAA dominates.

Pfahler

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 337 AM CDT Fri Apr 3 2026

Global model guidance is in agreement that upper-level northwesterly
flow will prevail across the Mid-Mississippi River Valley late
Saturday through Tuesday and longwave troughing ensues over the
Great Lakes. This pattern favors below average temperatures with
highs in the 50s and 60s F and lows in the 30s and 40s F. Another
cold front on Monday will provide an even cooler airmass in its wake
leading to Monday night being the coldest night with 30 to 60
percent probabilities of temperatures below 32 F north of I-70. This
part of the CWA will be closest to a nearby anomalous surface
anticyclone and core of the cold airmass, although it is uncertain
what the contribution of radiational cooling will be with possible
cloud cover. Around 20 to 40 percent of ensemble model membership
also have light measurable precipitation at times between Monday and
Tuesday in northeastern/central MO and west-central IL with
transient mid-level frontogenesis synonymous with passing shortwave
troughs. Additionally, 10 to 20 percent of membership even has
precipitation falling as light snow Monday night when temperatures
are coldest.

Upper-level flow will become more zonal around the middle of next
week, opening the Mid-Mississippi River Valley to a quick warmup
back to above average on Wednesday as low-level southerly flow
returns.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 530 AM CDT Fri Apr 3 2026

A period of MVFR stratocumulus/cumulus is expected this morning
across the region, until daytime heating and a warm front lifting
northward lifts and scatters these ceilings. Behind the front, winds
will become southerly and gust 20 to 25 kt at times. Isolated to
scattered showers and thunderstorms will be possible this afternoon,
with highest probabilities in northeastern MO/west-central IL
including KUIN. However, a line of more widespread thunderstorms
will track through the region this evening into tonight, accompanied
by IFR flight conditions. A few strong to severe thunderstorms are
also possible with strong winds and brief tornadoes. Rain will
linger behind these thunderstorms into Saturday, especially at St.
Louis metro terminals. At the same time, a cold front is anticipated
to sweep through the area, with highest confidence in impacts from
post-frontal MVFR stratocumulus at KUIN.

Pfahler

&&

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

$$

WFO LSX