Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

836
FXUS63 KLSX 031127
AFDLSX

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

.KEY MESSAGES...

- Dangerous heat and humidity will continue through Independence
  Day. Heat index values are expected to continue in the 100-108
  range each day.

- There is a chance for thunderstorms today through the weekend. A
  few storms could be strong to severe, capable of producing gusty
  winds and isolated wind damage today and Independence Day.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 322 AM CDT Fri Jul 3 2026

The primary weather story continues to be the heat and humidity,
which will continue at least through Saturday. The upper level
ridge is continuing to slowly weaken over the Mid Mississippi
Valley which is helping to lower temperatures a bit from earlier
in the week. However, this hot and humid airmass is locked in
place, and without something to flush it out, heat index values
will continue in the 100-108 range (or near enough that it won`t
matter). Today will be the 6th straight day with highs in the 90s
across most of the area and correspondingly high heat index values
above 100. Emergency Management and Department of Health Agencies
in the St. Louis Metro area have reported a distinct uptick in
heat-related illness calls over the past several days, and this
trend is likely occurring across most of the area as this heat
wave drags on. Therefore there will be no change in heat headlines
for today and Independence Day at this time.

The one potential relief from the heat toady and tomorrow will come
in the form of isolated to widely scattered thunderstorms.  Models
are showing sparse and largely disorganized convection at various
locations across our area today, and with no surface features to
focus convective initiation this afternoon have opted to handle the
situation with widespread 20 PoPs.  Deterministic guidance is
showing a weak short wave moving into northern Missouri late this
afternoon/evening, likely the remnants of convection currently
ongoing over the Great Plains.  This weak forcing combined with 2000-
4000 J/Kg CAPE could allow some of the storms this afternoon to grow
into loosely organized clusters with some 50-60 mph wind potential.
Expect any storms that form will dissipate quickly as daytime
heating diminishes.  There are some indications that a diminishing
MCS will dive into northern Missouri late tonight with the remnants
reaching central and northeast Missouri before sunrise Saturday.  The
remnant outflow could provide more of a focus for storms Saturday
afternoon, however guidance is mixed on this, and most CAMs show
less convection than they do for this afternoon.  Am therefore
continuing with widespread 20 PoPs due to the uncertainty.
Thermodynamics/kinematics Saturday afternoon are similar to today
so an isolated damaging wind threat continues.

Carney

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 322 AM CDT Fri Jul 3 2026

Deterministic GFS and ECMWF are showing a solid synoptic-scale
shortwave moving into the Mid Mississippi Valley on Sunday.  This is
backed up by the LREF mean and all 4 of its clusters.  Most short-
range deterministic guidance including several CAMs show another MCS
ahead of the wave late Saturday night into Sunday morning. The
falling heights and decaying MCS should finally break the heat wave
on Sunday for most of the area.  There is a chance that some areas
along and south of I-70 could reach advisory criteria again Sunday
afternoon, particularly if there is some rain Sunday morning which
would raise dew point temperatures and therefore afternoon heat
index values.  Regardless, afternoon convection will likely be more
widespread on Sunday with the short wave digging in overhead.
Therefore confidence in reaching advisory/warning criteria is low.

The remainder of the forecast period through next Thursday looks
cooler than what we`ve experienced over the past week, with weak mid-
upper level northwest flow dominating the pattern.  However, nobody
is likely to call the weather "chilly" next week.  A weak surface
ridge tries to nose into the Mississippi Valley with lower dew point
temperatures Monday and Tuesday.  However, long days and high sun
angle will continue to push temperatures into the mid to upper 80s
in most locations.  Low level flow turns back to the south and
southwest Wednesday and Thursday as the ridge moves east which
returns us to a warm/moist advection regime.  Current thinking is we
won`t get back into heat headline territory as the upper level
pattern will remain troughy with a couple of weak short waves moving
across Missouri and Illinois.  However, there is a distinct warming
trend in the ensemble guidance with the LREF interquartile range
rising from 83-86 at St. Louis on Tuesday to 87-91 by Thursday.

Carney

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 613 AM CDT Fri Jul 3 2026

VFR flight conditions and light south to southwest flow is
expected to prevail through the period. Isolated to widely
scattered showers and thunderstorms are possible again this
afternoon into early this evening. The coverage continues to look
very sparse, and the chance that any thunderstorm will directly
impact one of the terminals is too low (~20 percent) to mention
in the TAFs at this time. However, any storms that form will be
capable of producing wind gusts in excess of 40kts and IFR flight
conditions in brief heavy downpours. There is another chance for
thunderstorms before 12Z Saturday morning across parts of central
and northeast Missouri into west central Illinois.

Carney

&&

.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