Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

918
FXUS63 KLSX 030611
AFDLSX

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

.KEY MESSAGES...

- Heat and humidity will continue through Independence Day with
  afternoon/evening heat index values of 100 to 108 F.

- Daily chances of showers and thunderstorms exist through at
  least Sunday, but coverage is uncertain. Thunderstorms could
  contain gusty winds, but the overall severe weather threat is
  low at this time.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 238 PM CDT Thu Jul 2 2026

An upper-level ridge centered over the Ohio Valley will continue to
gradually weaken through Friday, allowing a slow cooling of a
persistent capping inversion that has kept the CWA in previous days.
This weakening capping inversion and corridors of weak confluent
surface flow are expected to result in isolated showers and
thunderstorms across portions of the area through the evening. The
combination of temperatures around 90 to the mid-90s F and dewpoints
in the 70s F has not only led to heat index values of 100 to 108 F
this afternoon in many areas but also 2000 to 4000 J/kg of MLCAPE
per the SPC Mesoanalysis. Therefore, localized microbursts with
gusty winds cannot be ruled out with these thunderstorms but
nebulous large-scale forcing may limit overall thunderstorm
intensity.

On Friday, upper-level flow over the Mid-Mississippi River Valley
will become more zonal as the downstream ridge weakens further,
opening the region to glancing passages of shortwave troughs/MCVs
just to the north. Temperatures, moisture, and instability will
overall be similar to today, but these features could be just enough
to result in greater coverage and support of showers and
thunderstorms along/north of I-70 especially with the potential for
an incoming outflow boundary associated with early day weakening
thunderstorms to the northwest across IA/northern MO. However, the
conditionality of this greater coverage and timing uncertainties
preclude higher PoPs at this time. Deep-layer wind shear of 15 to 20
kt does also suggest that these thunderstorms will pulse in
intensity but could grow upscale into disorganized multicell
clusters with time. The environment will be favorable for
microbursts containing gusty to localized damaging winds, but the
threat is anticipated to be isolated/sporadic which precludes
messaging a severe threat at this time.

Pfahler

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 238 PM CDT Thu Jul 2 2026

On Saturday (Independence Day), there is a stronger signal in CAMs
for weakening showers and thunderstorms, perhaps as an MCS, entering
the CWA during the morning with a lingering MCV and/or outflow
boundary serving as a focus for showers and thunderstorms during the
afternoon and evening in addition to a cold front arriving from the
north later in the day. Therefore, there is a higher ceiling for the
coverage of showers and thunderstorms, but the exact coverage varies
significantly in model guidance due to the subtleties of the
pattern. The environment will be similar to Friday but with slightly
stronger deep-layer wind shear of 20 to 25 kt, which leads to a
repeated concern of isolated gusty to damaging winds with
microbursts and upscale growth into multicell clusters and/or an
MCS. That being said, the severe threat is nuanced and low enough at
this point not to message. Otherwise, greater coverage of
showers/thunderstorms and clouds will lead to slightly cooler high
temperatures, but dewpoints will once again allow for another day of
100+ F heat index values in most areas. Accordingly, the Extreme
Heat Warning has been extended through Saturday evening along the
Missouri River through the St. Louis metro into southwestern IL
where the highest heat index values (105+ F) are forecast with a
Heat Advisory on Saturday elsewhere.

Coverage of showers and thunderstorms may actually increase and peak
Saturday night into Sunday morning as the cold front continues to
progress southward through the CWA and an upper-level trough passes,
although it will not be favorably in phase with maximum diurnal
heating/instability and there is little semblance of a LLJ. These
factors could limit organization and intensity of thunderstorms
overnight along with how widespread they become.

Global model guidance suggests that northwesterly flow and rising
mid-level heights at the forward flank of an upper-level ridge will
promote development of a high pressure center across the Upper
Midwest and Great Lakes. This high pressure center will influence at
least the northern half of the CWA with gradual filtering of a
cooler and slightly drier airmass into the CWA, but there are
differences in how much cooler and drier owing to varied depictions
on the strength/persistence of northerly low-level advection and if
the front stalls just south of the CWA. This uncertainty is
reflected in the approximately 5 F interquartile range in NBM high
temperatures Sunday, onward, spread from below average to near
average. Aside from southeastern MO/southwestern IL on Sunday
(potentially still ahead of the front), there is strong support for
peak heat index values to fall below 100 F, ending impactful heat
concerns. There will also be opportunities for showers and
thunderstorms, mainly diurnally and closer to the front, with
highest probabilities south of I-70 through Tuesday. However, model
guidance does suggest the front might begin to lift back northward
during the second half of the week.

Pfahler

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 106 AM CDT Fri Jul 3 2026

VFR flight conditions are expected to prevail through the period.
Showers and thunderstorms across Iowa may extend into parts of
northeast Missouri and west central Illinois early this morning,
however coverage looks to be limited (if there is any convection
at all), so think prob30 at UIN should cover it for that TAF site.
Isolated to widely scattered thunderstorms look likely again
across much of the area this afternoon. However, coverage again
looks sparse, so confidence is low that any thunderstorms that do
form will affect our terminals. Any storms that develop should
dissipate by late afternoon/early evening.

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