Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

230
FXUS63 KLSX 170838
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
338 AM CDT Fri Jul 17 2026

.KEY MESSAGES...

- Seasonably warm and humid conditions will persist across much
  of the area through Tuesday with afternoon heat index values
  around 100 F in the warmest locations.

- Opportunities for showers and thunderstorms exist through
  Tuesday, with the strongest thunderstorms capable of gusty
  winds and locally heavy rainfall today and Saturday.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 337 AM CDT Fri Jul 17 2026

Patchy fog in river valleys and locations that received substantial
rainfall on Thursday will dissipate through sunrise. Thereafter,
another round of predominantly diurnal, scattered showers and
thunderstorms is expected today, mainly along/east of the
Mississippi River from late morning through the afternoon when/where
large-scale ascent is relatively greatest from an upper-level vort
max passing slowly to the north of the CWA. However, with a weak
capping inversion, showers and thunderstorms cannot really be ruled
out anywhere across the CWA today. Depictions of 1500 to 2500 J/kg
MLCAPE and modest "inverted-V" sounding signatures suggest that
localized gusty winds are possible with the strongest thunderstorms;
however deep-layer wind shear will be exceptionally weak (<10 kt)
and limit the severe threat. Ingredients are not expected to be
quite as optimal for efficient, heavy rainfall today with the
departure of deeper moisture, but very slow storm motions still
means that locally heavy rainfall cannot be confidently ruled out.
High temperatures today are forecast to be similar or even slightly
warmer in locations that saw thunderstorms on Thursday, with
afternoon heat index values flirting with 100 F.

On Saturday, upper-level longwave troughing will become dominant
across the Great Lakes, allowing a weakening cold front to enter the
Mid-Mississippi River Valley during the evening into overnight.
Additional, at least diurnal, isolated showers and thunderstorms
will develop once again, but the HREF and global ensemble membership
support much greater coverage of showers and thunderstorms along the
approaching cold front from the north. The environment on Saturday
will feature similar instability and slightly higher deep-layer wind
shear (10 to 20 kt), especially in west-central/south-central IL
closer to the front which may be supportive of loosely organized
multicell thunderstorms clusters. Although an isolated microburst
with damaging winds is not out of the question, the relatively
greater severe threat would likely be near the front during peak
heating, which model guidance generally agrees will be
north/northeast of the CWA at this point. Less clouds and dry
conditions most of the day will support high temperatures from
around 90 to the mid-90s F accompanied by peak afternoon heat index
values of 100 to 105 F.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 337 AM CDT Fri Jul 17 2026

The aforementioned front will stall somewhere across the CWA on
Sunday before lifting back to the north on Monday ahead of an upper-
level trough navigating northwesterly flow extending into the Mid-
Mississippi River Valley. Finally, this trough is anticipated to
force a cold front passage sometime Tuesday. The details of frontal
timing and position through this period are uncertain in addition to
the potential impact of showers/thunderstorms and clouds. Both
fronts will possess pronounced differences in temperatures ranging
from high temperatures in the 90s F and 100+ F peak heat index
values to the south in the warm sector, to near or below average
temperatures post-frontal/north of the fronts. These factors
diminish confidence at this time that Heat Advisory duration
criteria (100+ F heat index for four days) will be reached going
forward. However, conceptually, the hottest days of the period would
be Monday or, with more uncertainty, Tuesday with the arrival of a
low-level thermal ridge. Global ensemble model probabilities of heat
index values 105 F or higher are between 20 and 50 percent both
days, indicating that there is a potential for reaching single-day
Heat Advisory criteria those days. Behind Tuesday`s cold front, a
seasonably cool airmass will settle across the Mid-Mississippi River
Valley.

In terms of precipitation, global ensemble model membership includes
showers and thunderstorms at times through Tuesday, mainly during
afternoon and evenings. That being said, membership is greatest on
Sunday near the stalling front with less forcing on Monday and then
a prominent capping inversion on Tuesday that could limit
coverage of showers and thunderstorms with cold frontal passage.
Dry conditions are then most likely behind the front Wednesday
and Thursday.

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1051 PM CDT Thu Jul 16 2026

Thanks to rainfall Thursday afternoon and evening, portions of
the area have enough low-level moisture to lead to patchy fog.
There is an increasing signal for fog development at KCOU, KJEF,
and KSUS through the early morning hours, but winds may prevail
just enough to mitigate this threat. Outside of this fog, VFR
flight conditions are expected through the period. Isolated to
scattered showers and thunderstorms are forecast across portions
of the area during the afternoon, but confidence is currently
high that they will occur well east of the local terminals.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.

&&

$$

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