Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

283
FXUS63 KLSX 260955
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
455 AM CDT Sun Apr 26 2026

.KEY MESSAGES...

- A few thunderstorms tonight into Monday morning could produce
  large hail and lead to locally heavy rainfall.

- A much greater severe threat will exist Monday afternoon and
  evening including very large hail, tornadoes (some strong), and
  damaging winds.

- Temperatures will cool to below average by Wednesday,
  accompanied by largely dry conditions.


&&

.SHORT TERM...  (Through Late Monday Morning)
Issued at 409 AM CDT Sun Apr 26 2026

Upper-level southwesterly flow will increase across the Mid-
Mississippi River Valley today, leading to increasing upper and mid-
level moisture and clouds through the day. Most of the daytime hours
will be dry, but a few showers and weak thunderstorms may develop
this afternoon and evening across central and northeastern MO in
response to increasing mid-level WAA. Weak, southeasterly low-level
WAA will also persist today with high temperatures slightly warmer
than Saturday and in the mid-70s to around 80s F, limited somewhat
by the increasing clouds.

Later this evening, an upper-level trough will cross the Rocky
Mountains, inducing surface cyclogenesis in the lee of the Rocky
Mountains with a warm front lifting northeastward into western,
central, and southern MO tonight. Thunderstorms, potentially severe,
are expected to eventually develop across eastern KS/western MO this
afternoon and evening in response to broad large-scale ascent,
before progressing eastward into the CWA overnight/early Monday
morning as a 50-kt LLJ develops and translates eastward. These
thunderstorms will be increasingly elevated as they reach the CWA as
nocturnal stabilization occurs and they move further into the cool
side of the warm front. However, deterministic model guidance
indicates MUCAPE will also be increasing to around 750 to 1500 J/kg
from west to east overnight/early Monday morning as an EML arrives.
Therefore, large hail is supported with any discrete thunderstorms,
but most CAMs indicate a less favorable, predominantly linear or
multi-cellular thunderstorm mode by the time they reach the CWA.
Given anomalous PW, deep warm cloud depths, and the strong LLJ, a
conditional threat of heavy rainfall is apparent, relying on
thunderstorms laying out a west-east boundary interacting with the
LLJ and aligning with the deep-layer flow that permits training of
otherwise fast-moving thunderstorms. Although coverage and location
of showers and thunderstorms tonight through Monday morning is not
completely certain, the majority (60 to 90 percent) of HREF
membership has showers and thunderstorms tracking north of I-70.

Pfahler

&&

.LONG TERM...  (Monday Afternoon through Saturday)
Issued at 409 AM CDT Sun Apr 26 2026

On Monday, the upper-level trough will negatively tilt as it
propagates across the Mid-Mississippi River Valley/Midwest,
promoting a deepening surface cyclone that tracks across
northeastern MO into IA with the aforementioned warm front advancing
far enough northeastward that the CWA is fully in the warm sector by
afternoon. Conceptually, this pattern still looks favorable for
severe weather across the region including strong deep-layer wind
shear of 45 to 55 kt, but there is uncertainty in how morning
showers and thunderstorms impact subsequent instability/airmass
recovery.

The incoming EML along with strong southerly low-level WAA and
moisture transport should facilitate destabilization once
precipitation ends and clouds break, but a few CAMs now even linger
showers and thunderstorms into the afternoon north of I-70.
Accordingly, HREF probabilities of 2000+ J/kg are highest (70 to 90+
percent) along/south of I-70, decreasing to the north as confidence
decreases in more substantial airmass recovery required for a
significant severe weather threat. The capping inversion associated
with the EML will be lifted and cooled by the trough as it arrives
during the late afternoon and evening, allowing redevelopment of
thunderstorms primarily along a Pacific cold front/dryline and
perhaps with reintensification of any lingering early day
thunderstorms. Supercells are favored, at least initially, but an
approximately 45 degree orientation of deep-layer wind shear with
the front/dryline suggests thunderstorms could evolve into a mixed
mode of supercells and line segments with time. Therefore, very
large hail and tornadoes, some of which could be strong (EF2+), will
be the main initial hazards with some shift toward damaging winds
and brief tornadoes in any line segments during the evening. The
tornado threat will be maximized near any remnant outflow boundary
or modified cold pool that coincides with locally enhanced low-level
wind shear and SRH, but given sufficient low-level wind shear across
most of the warm sector, the tornado threat will not be exclusive to
these features. Thunderstorms are forecast to exit the CWA to the
east by the overnight hours.

A secondary cold front will pass Monday night, marking the arrival
of a cool, seasonable airmass. However, the front will stall
briefly, if not lift back northward to near southeastern MO, on
Tuesday as another upper-level trough passes. This trough will track
further south and be less potent than its predecessor, but 50 to 80
percent of ensemble model membership have another round of showers
and thunderstorms along/south of I-44 (MO) and I-70 (IL) Tuesday
evening into tonight in response to a period low to mid-level
isentropic ascent extending northward over the cool side of the
front. Model guidance is mixed on how far north greater elevated
instability extends, but the threat of marginally severe hail cannot
be ruled out across southeastern MO if higher projects verify. After
Tuesday night, predominantly dry conditions are anticipated as upper-
level northwesterly flow prevails through the rest of the week,
although probabilities of measurable rainfall do reach 20 to 30
percent at times Thursday into Friday as a shortwave trough and cold
front passes. Temperatures will cool to below average on Wednesday,
persisting the the remainder of the week as low-level northerly flow
and post-frontal CAA reinforces a seasonably cool airmass.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 455 AM CDT Sun Apr 26 2026

Dry and VFR flight conditions are most likely to persist through the
daytime hours today. However, showers and thunderstorms are expected
to move into the region tonight into Monday morning, impacting all
terminals at times. There is uncertainty in the timing and coverage
of thunderstorms, with the current TAF focused on capturing the most
likely time periods of impacts with PROB30 groups. The timing and
duration of these groups will need to be refined as confidence
increases in how thunderstorms will evolve. Otherwise, easterly
winds will become southeasterly this morning, persisting through
tonight.

Pfahler

&&

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

$$

WFO LSX