Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

349
FXUS63 KLSX 181137
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
637 AM CDT Sat Jul 18 2026

.KEY MESSAGES...

- Seasonably hot and humid conditions will be most persistent
  in the Heat Advisory area through Tuesday including peak heat
  index values up to 105 F.

- Daily chances of at least isolated showers and thunderstorms
  will also exist through Tuesday. A strong thunderstorm or two
  could contain gusty winds and locally heavy rainfall today and
  Sunday.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 322 AM CDT Sat Jul 18 2026

An upper-level trough will traverse the Great Lakes today, but the
Mid-Mississippi River Valley will be displaced from associated large-
scale forcing and stronger mid-level flow. Despite this fact, strong
heating of the moist airmass in place suggests that isolated to
perhaps scattered showers and thunderstorms will develop late this
morning through afternoon along/south of I-70 as a weak capping
inversion is breached in some locations. Additionally, a cold front
trailing the aforementioned trough will progress southwestward into
northeastern MO and west-central/south-central IL this evening
providing greater coverage of showers and thunderstorms, although
this timing is out of phase with peak heating, suggesting that
thunderstorms will generally be weakening by that time. Gusty winds
will be possible with a stronger thunderstorm or two today given
1500 to 2500 J/kg MLCAPE and "inverted-V" sounding signatures, but
weak deep-layer wind shear of 10 to 15 kt and later timing of the
cold front keeps confidence in severe weather (damaging winds) still
too low to message. Considering PW near 2" (>90th climatological
percentile) and slow storm motions, thunderstorms may also contain
locally heavy rainfall but the threat of flash flooding is low. With
high temperatures around 90 to the mid-90s F and the moist airmass,
many areas will see afternoon heat index values rise to near or just
above 100 F.

As it becomes well-detached from large-scale forcing, the cold front
will slow and then stall near or just west of the Mississippi River
late tonight through Sunday. Following a lingering shower or
thunderstorm near the front overnight, diurnal redevelopment of
showers and thunderstorms is expected again during the late morning
through afternoon in the vicinity of the front. Exact coverage
showers and thunderstorms is uncertain on Sunday, but CAMs indicate
some thunderstorm aggregation that may allow for periods of greater
coverage. Ingredients for gusty winds with the strongest
thunderstorms will once again be in place along with locally heavy
rainfall, but the threat of severe weather and flash flooding is
still diminished by weak deep-layer wind shear and a lack of
thunderstorm organization. Along/west of the front, the net effect
of slightly cooler high temperatures but higher dewpoints pooled
near the front will result in afternoon heat index values again
reaching 100 to 105 F, increasing confidence in four or more days
with 100+ F generally in the Heat Advisory area. However, there is a
low concern for early development and greater coverage of showers
and thunderstorms limiting heat index values. East of the front
(west-central/south-central IL), a cooler airmass will arrive with
high temperatures in the 80s F and high confidence in heat index
values staying below 100 F on Sunday.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 322 AM CDT Sat Jul 18 2026

Another upper-level trough is progged to traverse the Northern
Plains toward the Great Lakes Monday through Tuesday, helping to
lift the front back northward out of the CWA on Monday and then
forcing passage of a cold front on Tuesday. This evolution leads to
higher confidence in Monday being the hottest day for most of the
CWA as 850-hPa temperatures rise to around 21 to 22 C (95th
climatological percentile) with high temperatures firmly in the low
to mid-90s F, accompanied by afternoon heat index values of 100 to
107 F. Cold frontal passage on Tuesday introduces more uncertainty
in the temperature and heat index forecast, but there is a growing
consensus in ensemble membership for one more day of 100+ F heat
index values along/south of I-70. Aside from isolated diurnal
showers and thunderstorms on Monday, global ensemble membership
indicates greater support (20 to 50 percent) for showers and
thunderstorms on Tuesday with remnant activity along the LLJ
skirting northern parts of the CWA during the morning and then
development during the afternoon/evening near the front. With
stronger deep-layer wind shear (20 to 35 kt), the low threat of a
few severe thunderstorms will need to be monitored for Tuesday
afternoon/evening; however, uncertainties in the strength of a
capping inversion decreases confidence in how much development,
especially surface based, occurs decreases confidence.

Behind Tuesday`s front, a much cooler and drier airmass will be
filtered into the Mid-Mississippi River Valley with below average
high temperatures in the 80s F. There is strong support for dry
conditions on Wednesday as well with a surface high pressure center
building into the area, but a series of upper-level shortwave
troughs in the northwesterly flow will provide periods of showers
and thunderstorms regionally through the end of the week. That being
said, the favored corridor of showers and thunderstorms will be
closer to the front west/southwest of the CWA through Thursday, but
the front will attempt to lift northeastward with time, eventually
shifting this corridor closer to the CWA. Until then, the main
impact of the shortwave troughs will be increased cloud cover, which
may further reinforce the seasonably cool temperatures ahead of the
warm front.

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 637 AM CDT Sat Jul 18 2026

Largely dry and VFR flight conditions are expected through tonight
across the region but there are low chances of showers and
thunderstorms, late this morning through afternoon along/south of I-
70 and this evening north of I-70. Confidence is relatively higher
(although not high) for thunderstorms to impact KUIN this evening,
warranting a PROB30 group. Thunderstorms will be capable of brief
MVFR to IFR flight conditions and gusty winds. Very patchy lower
ceilings (sub-VFR) cannot be ruled out late tonight into Sunday
morning, but the probability of impacts is too low to include in
the TAF at this time.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon today to 7 PM CDT Tuesday for MOZ041-
     047>051-059>065-072>075-084-085-099.
IL...Heat Advisory from noon today to 7 PM CDT Tuesday for ILZ069-
     074-079-100>102.

&&

$$

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