Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

327
FXUS63 KLSX 191103
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
603 AM CDT Sun Jul 19 2026

.KEY MESSAGES...

- Hot, humid conditions and showers and thunderstorms are
  expected today, mainly across central, east-central,
  southeastern MO and southwestern IL. The strongest storms may
  contain gusty winds and locally heavy rainfall.

- Across northeastern MO and west-central/south-central IL,
  briefly cooler conditions are forecast today as well as some
  wildfire smoke infiltrating the area.

- A cold front on Tuesday will bring an end to hot and humid
  conditions by Wednesday along with a period of drier weather.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 313 AM CDT Sun Jul 19 2026

Showers and thunderstorms along a slow-moving cold front located
immediately north of the I-70 corridor as of 08z have waned, but
additional showers and thunderstorms in southeastern MO have
persisted longer within a zone of weak low-level confluence. The
cold front will stall across central, east-central MO into
southwestern IL providing a broad focus for redeveloping scattered
to perhaps numerous showers and thunderstorms late this morning
through the afternoon and evening, mainly through weak confluence
and a subtle, easily overcome capping inversion. Although weak deep-
layer shear and limited thunderstorm organization will keep the
threat of severe weather and flash flooding low, a few stronger
thunderstorms could be capable of gusty winds and locally heavy
rainfall given 2000 to 3000 J/kg MLCAPE, modest "inverted-V"
sounding signatures, and PW around 2". Along/ahead of the front,
heat index values will also straddle 100 F during the afternoon
where high temperature reach near 90 F and pooled, higher dewpoints
exist. However, locations that experience more numerous showers and
thunderstorms will see slightly lower temperatures/heat index values.

Behind the front across northeastern MO and west-central/south-
central IL, easterly low-level low will filter a cooler airmass into
the area with high temperatures generally in the mid-80s F. Although
conditions will be less favorable for shower/thunderstorm
development in these areas, HRRR and RAP smoke dispersion models
project some near-surface Canadian wildfire smoke arriving during
the afternoon and evening. This smoke will mainly lead to reduced
air quality (see Air Quality Alert) and slight visibility reductions.

Depending on cloud cover tonight into Monday morning, conditions
will be favorable for the development of fog near the stalled front
in central and southeastern MO and especially in locations that
receive more substantial rainfall today. Further to the northeast,
smoke particulates may also serve as cloud condensation nuclei and
enable fog to develop more readily at lower RH/higher dewpoint
depressions than normal, which has been observed in past smoke
events across the CWA.

On Monday, an approaching upper-level trough in the Northern Plains
and leading, downstream shortwave trough within northwesterly flow
will allow the stalled cold front to lift back northeastward as a
warm front through the day. With the warm sector enveloping more of
the CWA again, high temperatures around 90 to the mid-90s F and
afternoon heat index values of 100 to 105 F are forecast. However,
guidance has trended toward a slower progression of the warm front
across south-central IL, potentially limiting high temperatures to
the upper 80s F with lower heat index values. CAMs indicate diurnal
development of isolated to scattered showers and thunderstorms again
on Monday, mainly across southeastern MO, but a remnant shower or
thunderstorm cannot be ruled out across northeastern MO/west-central
IL from overnight convection that develops to the northwest.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 313 AM CDT Sun Jul 19 2026

The upper-level trough will continue southeastward into the Great
Lakes on Tuesday, forcing a cold front through the CWA. Extended
CAMs have varying depictions of remnant showers and thunderstorms
tracking through IA into IL in a veering LLJ during the morning,
which may reach northeastern MO and west-central IL. Additionally,
20 to 40 percent of ensemble model membership has showers and
thunderstorms developing near and ahead of the cold front during the
afternoon/evening along/south of the I-70. However, mixed depictions
of a capping inversion and neutral mid-level height tendencies
decreases confidence in how much development will actually occur
with more favorable large-scale forcing passing just east/northeast
of the CWA. If thunderstorms can develop, there is a threat of a few
severe thunderstorms containing damaging winds with around 25 to 35
kt of deep-layer wind shear advertised. High temperatures will once
again warm into the 90s F with afternoon heat index values of 100 to
105 F along/ahead of the front, generally along/south of I-70. To
the north, it looks like post-frontal CAA will be delayed from
actual frontal passage and still permit high temperatures near 90 F
even behind the front. However, incoming, lower dewpoints will keep
heat index values from rising above 100 F.

By Wednesday, a much cooler and drier airmass will finally settle
into the Mid-Mississippi River Valley with below average high
temperatures in the 80s F and seasonably low dewpoints in the 50s
and low-60s F. A surface high pressure center will promote dry
conditions at least on Wednesday, but a series of upper-level
shortwave troughs navigating the northwesterly flow overhead will
encourage periods of showers and a few thunderstorms along/north of
a warm front that gradually lifts northeastward through Central
Plains during the second half of the week. Initially, these showers
and thunderstorms will be west of the CWA through Thursday, but the
amount of ensemble model membership indicating showers and
thunderstorms in the CWA increases Friday into the upcoming weekend.
It is uncertain how far northeastward the front reaches, although at
least the combination of in-situ airmass modification and eventual
low-level WAA supports moderating temperatures late in the week.

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 602 AM CDT Sun Jul 19 2026

Aside from an area of IFR ceilings may impact KUIN this morning, a
brief period of MVFR ceilings cannot be ruled out as cumulus begins
developing later this morning. Otherwise, the main concern today is
the development of showers and thunderstorms late this morning
through afternoon/evening mainly across central, east-central,
southeastern MO and southwestern IL. Confidence in impacts at KCOU,
KJEF, KSUS, and KCPS has increased enough to introduce PROB30
groups, but it is entirely possible that most thunderstorms remain
just southwest of KSTL.

Smoke will also spread through IL, including KUIN but it is
uncertain if it will reduce visibilities below 6SM. Conditions will
become favorable for the development of fog tonight into Monday
morning, potentially aided by some smoke closer to the Mississippi
River. With the exception of KSTL, MVFR visibilities have been
included in the TAF, but lower visibilities may need to eventually
be included if confidence increases.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 7 PM CDT Tuesday for MOZ041-047>051-
     059>065-072>075-084-085-099.
IL...Air Quality Alert until midnight CDT tonight for ILZ058>060-
     065-095>097.
     Heat Advisory until 7 PM CDT Tuesday for ILZ069-074-079-
     100>102.

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Pfahler