Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

431
FXUS63 KLSX 031921
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
221 PM CDT Fri Jul 3 2026

.KEY MESSAGES...

- Heat and humidity will persist through Independence Day with
  peak heat index values reaching 100 to 108 F in many locations.

- Opportunities for showers and thunderstorms exist through
  Sunday, with a few strong to severe thunderstorms capable of
  isolated damaging winds and marginally severe hail.


&&

.SHORT TERM...  (Through Late Saturday)
Issued at 219 PM CDT Fri Jul 3 2026

Continued weakening of the upper-level ridge across the Ohio Valley
and associated cooling capping inversion has allowed greater
coverage of showers and thunderstorms developing this afternoon
compared to Thursday. Most of these showers and thunderstorms have
formed near an axis of confluent surface flow along the I-44 (MO)
and I-70 (IL) and remnant morning MCS-generated gravity wave across
northeastern MO/west-central IL all within a broad, weak upper-level
jet streak entrance region. MLCAPE of 2500 to 4000 J/kg, 1000+ J/kg
DCAPE, and 10 to 20 kt of deep-layer wind shear has resulted in an
environment favorable for pulse thunderstorms with a few microbursts
possible with localized damaging winds. Otherwise, yet another day
of heat and humidity is underway with heat index values climbing to
above 100 F in many locations this afternoon/early evening (where
thunderstorms have not occurred).

Most of the ongoing showers and thunderstorms are diurnally
driven/aided and are expected to decrease in coverage through
sunset. However, one point of complication is a multicell cluster of
thunderstorms crossing into western/northwestern MO that will track
eastward toward the CWA (central/northeastern MO). CAMs have not
handled these thunderstorms well, which decreases confidence in
their evolution; however, the general idea is that they will become
increasingly outflow dominant and weaken before reaching the CWA as
they track further eastward away from stronger, more supportive wind
shear. Exactly how quickly this process occurs will need to be
monitored considering there is still plentiful downstream
instability. The majority of additional showers and thunderstorms
tonight will be focused near the nose of LLJ along a slow-moving
cold front across IA/northern MO. That being said, the development
of a cold pool may cause some of these showers and thunderstorms to
drift southward into northeastern MO/west-central IL overnight,
although this is unlikely to occur before Saturday morning.

During the morning on Saturday (Independence Day), there has been a
relatively coherent signal in CAMs for a decaying MCS to track
east/southeastward into central/northeastern MO, with its associated
MCV and outflow boundary being catalysts for additional showers and
thunderstorms developing late morning through the afternoon. An
additional catalyst will be the weak cold front will be slowly
making its way southward into the CWA during the evening.
Deterministic model guidance depicts 2000 to 3500 J/kg MLCAPE during
the afternoon along with slightly stronger deep-layer wind shear of
15 to 25 kt, which could allow some loose organization of
thunderstorms into multicell clusters or an MCS. With other
parameters similar to today, the threat of microbursts with damaging
winds exists again. Around 750 to 1000 J/kg of hail CAPE also
suggests a brief instance of marginally severe hail is possible with
thunderstorms at peak intensity. Heat index values are forecast to
be slightly lower than today with more cloud cover impacting
temperatures, but yet another day of 100+ F afternoon heat index
values are likely, especially in Saturday`s Extreme Heat Warning
area.

Pfahler

&&

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 219 PM CDT Fri Jul 3 2026

Showers and thunderstorms will continue at times Saturday night into
Sunday as the cold front slowly works its way southward and an upper-
level trough passes, but there will undoubtedly be significant
impacts from antecedent activity on Saturday, which leads to
increasing uncertainty on the coverage, location, and amount of
instability available through this subsequent time period. It does
appear that showers and thunderstorms will at least be incrementally
focused southward. High temperatures will also generally be cooler
on Sunday, but heat index values could still flirt with 100 F in the
St. Louis metro, southeastern MO, and southwestern IL depending on
how prevalent breaks in cloud cover are.

An upper-level ridge will amplify into the Mid-Mississippi River
Valley Monday and Tuesday, fostering the building of surface high
pressure center across the Midwest and Great Lakes. As a result,
weak northerly flow will become established with gradual filtering
of cooler and slightly drier air into the CWA in the wake of the
cold front (or what is left of it). Therefore, temperatures will
cool to more seasonable values and impactful heat will end. With
moisture and modest instability lagging behind the front, diurnal
isolated to widely scattered showers and a few thunderstorms are
also depicted by some ensemble model membership, mainly across
southeastern MO/southwestern IL.

Global model guidance is in general agreement that upper-level flow
will become more quasi-zonal Wednesday through the end of the week
as the ridge de-amplifies. This pattern would provide warming
temperatures and opportunities for showers and thunderstorms as a
front lifts northward and wavers around the Mid-Mississippi River
Valley beneath multiple passing shortwave troughs.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1208 PM CDT Fri Jul 3 2026

The main concern this TAF period will continue to be the potential
for showers and thunderstorms. Isolated to scattered showers and
thunderstorms are expected to persist this afternoon and early
evening with the highest chances at KUIN and St. Louis metro
terminals, although there is a greater near-term threat at KCPS. The
timing, coverage, and placement of thunderstorms beyond this evening
is uncertain, but there is a window of relatively higher chances
again late Saturday morning through afternoon. However, no explicit
mention has been included due the uncertainty and long lead time.
Thunderstorms will be capable of brief MVFR to IFR visibilities and
gusty winds. Winds, otherwise, will be light throughout the
period.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Extreme Heat Warning until 10 PM CDT this evening for Audrain MO-
     Crawford MO-Iron MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
     Marion MO-Monroe MO-Pike MO-Ralls MO-Reynolds MO-Saint
     Francois MO-Shelby MO-Washington MO.

     Heat Advisory from 10 PM this evening to 8 PM CDT Saturday for
     Audrain MO-Crawford MO-Iron MO-Knox MO-Lewis MO-Lincoln MO-
     Madison MO-Marion MO-Monroe MO-Pike MO-Ralls MO-Reynolds MO-
     Saint Francois MO-Shelby MO-Washington MO.

     Extreme Heat Warning until 8 PM CDT Saturday for Boone MO-
     Callaway MO-Cole MO-Franklin MO-Gasconade MO-Jefferson MO-
     Moniteau MO-Montgomery MO-Osage MO-Saint Charles MO-Saint
     Louis City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO.

IL...Extreme Heat Warning until 10 PM CDT this evening for Adams IL-
     Bond IL-Brown IL-Calhoun IL-Fayette IL-Greene IL-Jersey IL-
     Macoupin IL-Marion IL-Montgomery IL-Pike IL.

     Heat Advisory from 10 PM this evening to 8 PM CDT Saturday for
     Adams IL-Bond IL-Brown IL-Calhoun IL-Fayette IL-Greene IL-
     Jersey IL-Macoupin IL-Marion IL-Montgomery IL-Pike IL.

     Extreme Heat Warning until 8 PM CDT Saturday for Clinton IL-
     Madison IL-Monroe IL-Randolph IL-Saint Clair IL-Washington
     IL.

&&

$$

WFO LSX