Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

637
FXUS63 KLSX 270750
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
250 AM CDT Mon Jul 27 2026

.KEY MESSAGES...

- There is a low chance (20-30%) for showers and thunderstorms
  this afternoon and evening. If they do form, thunderstorms
  will be capable of damaging wind gusts.

- Another day of dangerous heat is expected today, with
  afternoon heat index values reaching 105-115 degrees.

- Relief from the heat will begin Tuesday, with seasonable
  temperatures expected Wednesday into the weekend.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 247 AM CDT Mon Jul 27 2026

A highly amplified ridge remains positioned across the western U.S.;
its axis centered over the Rockies and Great Plains per water vapor
imagery. This has the weather in the CWA driven by the northwesterly
flow on the eastern periphery of the ridge. This flow will
potentially bring yet another MCS into or in the general vicinity of
the Middle Mississippi Valley during the afternoon and evening.

The convection responsible for this potential MCS has just recently
developed across eastern Minnesota and northern Wisconsin, and it`s
going to take at least through the early morning hours for the CAMs
to be able to appropriately latch on to it. Currently, either the
MCS itself or its outflow has a low chance (20-30%) of impacting the
CWA during the afternoon and evening. This will be crucial for
convective initiation, as a majority of soundings show a stronger
cap in place today compared to yesterday. If convection is able to
impact the CWA, it will do so among 3,000-3,500 J/kg of SBCAPE and
25-30 kts of deep layer shear per the 00z HREF and REFS ensemble
means. This supports a threat for strong to severe thunderstorms for
any updrafts that form or enter the CWA. High LCLs and inverted V
soundings will lead to a damaging wind threat, particularly if
convection is organized as an MCS.

The temperatures and dewpoints responsible for this degree of
instability will also yield another day of dangerous heat. Dew
points today will be a little lower than the last two days,
particularly across central and southeastern Missouri thanks to
southwesterly surface flow moving down off the Ozarks. However, with
850 mb temperatures at their warmest for this stretch of heat over
our CWA and the slightly lower dew points allowing for surface
temperatures to warm more readily, air temperatures will top out
near or at 100 degrees this afternoon for a majority of the area.
This will lead to another round of heat index values of 110-115
degrees. If the CWA is impacted by an MCS or other convection,
current thinking is that it would happen either right at or just
after peak heating, with minimal impact to the peak heat potential.

A weak backdoor cold front will sink through the CWA tomorrow, with
north-northeasterly low-level flow in its wake ushering in cooler
and drier air. Given the weakness of the front and post-frontal
flow, the relief from the heat will be gradual across the CWA, with
confidence high that only areas north of I-70 will see the most
relief and fail to warm above 90 degrees. South of I-70, afternoon
temperatures in the low to mid-90s are expected, with dew points in
the low to mid-70s still enough to produce heat index values of 100-
105 degrees. There is still enough uncertainty in convective trends
to our west tonight and the speed of the post-frontal air mass that
we are holding off on tomorrow`s heat headline decisions for
now.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 247 AM CDT Mon Jul 27 2026

Guidance consensus remains consistent in northwesterly flow
prevailing over the Middle Mississippi Valley as the upper-level
ridge holds over the western CONUS through the extended forecast
range. With flow lower in the atmosphere persisting generally out of
the northeast, the thermal ridge responsible for our early week heat
will remain focused over the Great Plains. This pattern will keep
local temperatures seasonable through the weekend.

A strong shortwave is still seen in the majority of guidance diving
south-southeastward across the Midwest late Thursday through
Saturday, settling into the Ohio Valley on Sunday. This wave and
it`s associated surface system will bring rain chances to the region
to end the week. Phasing of the wave varies enough among guidance to
produce widely different evolutions of the surface system, leading
to low confidence in specifics on timing and placement of rainfall.
There at least has been some consistency in the CWA being in the
warm sector of the surface system Thursday nigh into Friday,
favoring the greatest chances for rainfall through that stretch.
Some guidance has a slower shortwave, which would then linger rain
chances through Saturday and even possibly Sunday, whereas a faster
wave would confine rain chances to late Thursday into Friday. The
current forecast splits the difference, with a bulk of the rain
chances (60%) Thursday night into Friday, with low chances (20-30%)
lingering into Saturday.

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1050 PM CDT Sun Jul 26 2026

A field of mid/high-level clouds, that were previously supporting
showers and isolated thunderstorms across eastern Iowa, has
continued to slide southeastward into west-central Illinois. Models
have trended toward less shower and thunderstorm development with
this area, which has led to a continuation of dry and VFR TAFs for
this package. However, as this cloud field slides southward across
western Illinois over the next few hours, it will be an area to
monitor for isolated convection, as there remains a 10-20% chance
for thunderstorms along and to the east of the Mississippi River.
The chance for fog is low tonight, thanks to intermittent mid/upper-
level clouds sliding across the area, although, there remains a 15%
chance that KSUS/KUIN/KCPS may see brief patches of fog before
sunrise. Confidence in the coverage/timing of any potential fog was
too low to include mention. Calm and mostly variable winds continue
tonight with southwest winds prevailing tomorrow.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Extreme Heat Warning until 9 PM CDT this evening for MOZ018-
     019-026-027-034>036-041-042-047>052-059>065-072>075-084-
     085-099.
IL...Extreme Heat Warning until 9 PM CDT this evening for
     ILZ058>060-064-065-069-070-074-079-095>102.

&&

$$

SHORT TERM...Elmore
LONG TERM...Elmore
AVIATION...Peine