Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

252
FXUS63 KLSX 130830
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
330 AM CDT Wed May 13 2026

.KEY MESSAGES...

- After a couple of dry and mild days, warmer and more humid
  weather moves in for the weekend.

- Although there will be occasional showers and thunderstorms
  Friday through Sunday, there will be more dry time than wet. The
  better thunderstorm chances come early next week.

&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 329 AM CDT Wed May 13 2026

A cold front is in the process of moving through the forecast area
this morning as part of a broader low pressure system over Lake
Huron. The core of the cooler air behind this front is pushing east
through the Great Lakes, so the effect on our temperatures locally
is fairly minor. With full sun today we top out in the mid 70s,
about 5 to 10 degrees cooler than yesterday but right around normal
for this time of year. Surface high pressure currently in Nebraska
will gradually move east today through Thursday. Northerly winds
today go calm tonight leading to good cooling conditions. With dry
air in place we`ll see a cool night, in the 40s for most, but some
upper 30s are possible in valley locations, as is some patch fog.
Southerly flow returns by Thursday yielding temperatures starting to
inch up again.

Kimble

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 329 AM CDT Wed May 13 2026

High amplitude ridging currently over the Rockies nudges east and
flattens out over the coming days. As it does so, we`ll gain access
to some warmer temperatures. Southerly flow over the Plains begins
to recharge the moisture over Texas as early as Thursday, with this
moisture expanding north and east Friday into the weekend. The
combination will lead to warmer and more humid conditions across our
area, with plenty of instability.

On the topic of temperatures late this week and this weekend,
ensemble forecasts continue to point to above normal 925mb and 850mb
temperatures, but not excessively so (near the 90th percentile of
climatology). This is consistent with the expected flattening of the
ridge which should dampen the potential for more extreme heat.
Unfortunately the NBM continues to suffer from known bias correction
issues that magnify during the transition season, and it`s forecast
near daily record highs is inconsistent with the rest of the
available guidance. This was evidenced yesterday when temperatures
locally peaked in the warm sector ahead of the front with plenty of
sunshine, yet still afternoon highs were about 3 degrees below NBM
forecasts. This issue seems more pronounced in the longer range
portion of the forecast when the total number of guidance sources
within NBM is smaller. We`ve continued to coordinate with WPC to try
to tone down some of these extreme temperatures in the forecast this
weekend, though it is likely that current forecast values are still
too high. Ensemble mean forecast 925MB temperatures have remained
consistent over the last several days, while the spread has
decreased indicating greater confidence. A 925MB temperature of +24C
represents a high temperature of 86F at ground level assuming good
mixing. While some variability remains possible if conditions end up
warmer than the current ensembles suggest, the odds of seeing
widespread 90 degree temperatures this weekend are estimated at less
than 10 percent. With dewpoints climbing into the 60s to even near
70 degrees, humidity will be noticeable and add a degree or two to
the heat index.

As far as rain chances go, current forecast continues to paint at
least low end chances for showers and storms every period from
Thursday night through Tuesday due to the presence of that warm,
humid air mass and the availability of instability every day.
However, it`s worth noting that shower and thunderstorm activity
will likely be more focused in time and space than what is currently
depicted, and most of this weekend will be dry. The initial round of
showers and storms Thursday night is associated with the initial
arrival of low level moisture being advected northward in
association with a shortwave trough moving through the Upper
Midwest. Northern sections of our forecast area are favored with
this round through Friday morning. There are indications that a weak
and nearly stationary front develops and wobbles north and south
over the coming days as additional subtle shortwave troughs traverse
the area. This provides the focus for additional rounds of showers
and thunderstorms through Saturday and potentially Sunday.
Confidence remains low on the timing and track of these features
which is why POP remains in a modest range through most of this
period. Subsequent forecasts will likely refine this over the coming
days into more focused times and geographic areas.

A trough moving onshore into the Pacific Northwest on Saturday digs
a larger trough over the western US this weekend with a downstream
ridge amplifying over the East Coast. Our area will be in the
developing southwest flow between these features. As the trough
moves east it will bring a more focused threat for thunderstorms as
it moves into our area Monday into Tuesday. The warm and humid air
mass will maximize the instability just as greater shear arrives
with the approach of the upper trough. So this will represent our
next best chance for strong to severe thunderstorms regionally.
Whether that occurs locally will depend on the timing of the wave
and whether it moves through our area during peak heating.

Kimble

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1041 PM CDT Tue May 12 2026

A narrow swath that contains a few scattered pockets of rain
showers continues to slide southeastward along and ahead of an
approaching cold front. Currently, this activity stretches from
central IL southwestward into east-central MO with the greatest
coverage of showers off to the east across IL. A low chance
(10-20%) for a brief shower remains at the St. Louis metro
terminals until around 06z. The cold front has passed KUIN within
the last hour and will be nearing KCOU/KJEF and the St. Louis
Metro terminals within the next few hours. Light southwesterly
winds ahead of the front remain light while veering
northwesterly behind the front for the remainder of the night.
Northwesterly winds strengthen by late Wednesday morning, gusting
to 20-25kts (strongest across northeast MO/west- central IL). A
surface high slides south into the area tomorrow evening, leading
to light and variable winds Wednesday night.

Peine

&&

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

$$

WFO LSX