Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

962
FXUS63 KLSX 091721
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1221 PM CDT Sun Aug 9 2026

.KEY MESSAGES...

- Dangerous heat and humidity arrive today and persist through
  Wednesday. The Extreme Heat Watch was upgraded to an Extreme
  Heat Warning, with a Heat Advisory issued across the rest of
  the region.

- A cold front brings an end to this heat by Thursday and will
  serve as a catalyst for showers and thunderstorms through the
  weekend. There is an increased concern for flash flooding,
  but exactly where the heavy rain axis will develop is far from
  certain.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 422 AM CDT Sun Aug 9 2026

Radar indicates showers and thunderstorms in northwestern MO and
southern IA, forced by a low-level jet across the central Plains
and 850mb-700mb warm air advection. This convection should
remain north of our forecast area, but the chance of some
scatter showers in our northern counties is not out of the
question as the rain moves to the east. An upper level ridge
continues over much of the western CONUS, with our forecast area
on the eastern edge of the ridge. The ridge will be the
controlling factor for our weather over the coming days. The
building ridge will allow the heat and humidity to continue and
worsen through the day. Guidance indicates that 850mb
temperatures are ranging from 20-23 C resulting in surface
temperatures in the low 90s. These high temperatures combined
with dewpoints in the low to mid 70s will cause heat index
values to range from 100-108F this afternoon. The Extreme Heat
Watch was upgraded to an Extreme Heat Warning with a Heat
Advisory Issued for the remaining counties in our forecast area
due to the increased confidence of dangerously-high heat index
values over the next few days.

Northeastern MO does have a low (10-20%) chance of experiencing
some convection overnight tonight into Monday morning. Model
soundings show a strong capping inversion and subsidence from
the ridge building despite having 3000+ J/kg of surface-based
CAPE and being located at the nose of the low level jet
providing lift. As a result we can not rule out some isolated,
elevated storms to occur in the northeastern MO and west central
IL. The worst-case threat for these storms is an extremely low
risk of hail, and as a result we have decided not to message the
Marginal Risk from SPC.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 422 AM CDT Sun Aug 9 2026

The upper level ridge will continue to dominate our weather for
the first half of the week. Ensemble models have increased
confidence that air temperatures will remain in the low to mid
90s F through Wednesday. Temperatures at 850mb increase to
22-24C (above 95th climatological percentile) during this time,
further supporting that warm surface temperatures will continue.
An approaching cold front, which will be discussed further
later, may bring some cloud cover into northeastern MO and west
central IL that may mitigate the heat some on Wednesday. High
dewpoints will also persist in the region continuing to cause
heat index values to be 100-108F. Some relief from the heat is
projected after Wednesday, but to what magnitude remains
uncertain. A longwave trough deepens over the Great Lakes region
bringing a cold front that will move through the region, but the
exact positioning and magnitude of this front is currently
uncertain. The cold front will bring relief from the dangerous
heat, but the exact magnitude of the cool down is uncertain.
Model guidance has IQRs of 5-10 degrees for Thursday through
Friday and 10-15 degrees for next weekend.

The cold front will also be the catalyst for multiple rounds of
rain Wednesday through Friday across the region. Ascent from
upper level divergence coupled with the forcing from the cold
front will interact with highly anomalous moisture values and
will introduce a threat for heavy rain and flash flooding. The
confidence of the totals and location of the heaviest rainfall
is very uncertain due to the fronts varying position in
ensemble guidance. Cluster analysis of ensemble members shows
that the swath of heavy rain could occur anywhere between
northern Arkansas and southern Iowa. Flash flooding will be a
concern where this swath of heavy rain materializes. This amount
of rain would also threaten river flooding, particularly on
smaller creeks and streams, but the exact placement of the heavy
rain will be crucial in determining where this occurs.

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1213 PM CDT Sun Aug 9 2026

VFR flight conditions are expected to prevail through the
period. Southerly winds with occasional gusts will continue this
afternoon. Gusts will diminish this evening. There`s a
possibility that low level wind shear will develop over central
and northeast Missouri into west central Illinois this evening
and persist overnight. However, confidence in LLWS is low at
this time so I did not include it in the terminal forecasts.
Some models also show showers and thunderstorms developing
overnight across parts of the area, and continuing into Monday
morning. However, the majority of guidance is dry through the
period so have kept TAFs dry for this update.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Wednesday for MOZ018-019-026-027-
     034>036-042-052-072>075-084-085-099.
     Extreme Heat Warning until 8 PM CDT Wednesday for MOZ041-
     047>051-059>065.
IL...Heat Advisory until 8 PM CDT Wednesday for ILZ058>060-064-065-
     070-095>099.
     Extreme Heat Warning until 8 PM CDT Wednesday for ILZ069-074-
     079-100>102.

&&

$$

SHORT TERM...Asel/MRB
LONG TERM...Asel/MRB
AVIATION...Carney