Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

105
FXUS63 KLSX 090526
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1226 AM CDT Wed Sep 9 2026

.KEY MESSAGES...

- It will be warm and humid again on Wednesday ahead of a cold
  front. However, current thinking is that clouds and afternoon
  showers and thunderstorms will keep heat index values below
  Heat Advisory criteria

- Thunderstorms on Wednesday afternoon and evening may produce
  gusty winds and hail. Continuing storms overnight night may
  produce locally heavy rainfall.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 257 PM CDT Tue Sep 8 2026

High pressure remains in control of our weather this afternoon.
The high is drifting to the east and a cold front is moving
through the northern Plains into the Upper Midwest. This cold
front will continue to move southeast tonight and most
deterministic guidance shows it in central or southern Iowa by
12Z Wednesday. Short range guidance shows a 25-30kt low level
jet intersecting the 850mb front over Iowa tonight to produce
some modest to strong moisture convergence resulting in
overnight convection. The convection slides south ahead of the
front Wednesday, resulting in at least a chance (30-50%) of
showers and thunderstorms for our area by mid- afternoon.
Current thinking is that scattered convection and clouds will
suppress temperatures enough to keep heat index values below 105
across the area, despite west-southwest flow ahead of the
front. Therefore we are not extending the current Heat Advisory
for central Missouri beyond its expiration time of 8 PM this
evening, nor are we issuing any new advisories at this time.
However, a new Heat Advisory may be needed for parts of the area
on Wednesday if there is less convection and fewer clouds than
expected. The RAP is showing as much as 3000 J/Kg of MLCAPE
Wednesday afternoon generally along and south of I-70. Deep-
layer wind shear is lacking at less than 25kts for most of the
area, though there is a small area where 30-35kts of shear
overlaps with some of the higher instability across parts of
central into east central Missouri as well as south central
Illinois. This jives well with SPC`s day 2 marginal outlook
highlighting the possibility of damaging wind gusts. Don`t think
the severe threat will last too long into the evening if it
materializes at all.

Attention turns to the possibility of heavy rain Wednesday night as
the front sags slowly southward.  Mean precipitable water values are
forecast to be between 2-2.25 inches along and south of I-70
Wednesday night by both the HREF and LREF which is right around the
99th percentile of climatology.  Forecast soundings show warm-cloud
depths in excess of 14kft with tall/skinny CAPE profiles.  However,
the soundings do have some drier air at times, particularly in the
low levels.  There is a low level jet to force convection Wednesday
night, although current guidance indicates it won`t be
particularly strong at 25-30kts.  The HREF 6hr LPMM QPF shows some
bullseyes of 4+ inches mainly south of I-70 by 12Z Thursday morning,
and this along with previously mentioned parameters does increase
confidence in locally heavy rain.  However, the threat for
widespread flooding is balanced by dry ground conditions, so we will
not issue a Flood Watch at this time.  A watch may be necessary
though if subsequent model runs show an increasing likelihood for
heavier rain.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 257 PM CDT Tue Sep 8 2026

The front continues southward on Thursday, and eventually drier air
moves in, ending the precip across southeast Missouri by 00Z Friday.
 Weak high pressure builds into the Upper Midwest and Great Lakes
Thursday night into Friday which will produce dry and relatively
cooler weather for Friday.  However another short wave trough moves
across the Midwest in quasi-zonal flow aloft Friday night into
Saturday.  This pushes another cold front with another round of
convection into Iowa Friday night, and the front stalls over
northern Missouri on Saturday.  The front and any outflow boundaries
that overnight convection produces will provide a focus for
scattered convection Saturday afternoon, though drier low level air
lingering ahead of the front looks like it will keep the atmosphere
from becoming too unstable.  A chance for showers and thunderstorms
continues Sunday into next week as a strong short wave trough moves
from the Intermountain West across the Northern Plains.  This forces
low level cyclogenesis over the Plains and turns the low level flow
over the Mid Mississippi Valley back to the southwest. Temperatures
remain 5-10 degrees above normal in the mid 80s to low 90s through
the end of the forecast period due to the persistent southwest flow.

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1226 AM CDT Wed Sep 9 2026

An outflow boundary will pass through KUIN at the start of the
period, quickly veering winds to the north. Otherwise, the main
concern overnight is LLWS at KCOU, KJEF, and KUIN as a low-level jet
overspreads the area. A cold front will slowly advance southward
today, initially threatening showers and weakening thunderstorms at
KUIN that could linger well into the day. Winds will also veer
toward northwesterly with its passage, reaching KCOU, KJEF, and St.
Louis metro terminals around late morning or midday.

Additional showers and thunderstorms are expected to develop along
and behind this front sometime this evening, increasing in coverage
and continuing at times overnight. KCOU, KJEF, and St. Louis metro
terminals have the highest probabilities for impacts which include
MVFR flight conditions, possibly IFR flight conditions and gusty
winds with the strongest thunderstorms. Although the threat for
thunderstorms will continue overnight, confidence at this time is
relatively higher in the threat this evening. Winds outside of
thunderstorms will generally be light.

&&

.CLIMATE...
Issued at 258 AM CDT Sun Sep 6 2026

Record Highs...Record Max Lows (value/year):

September 8:
KSTL: 101/1947...77/1939
KCOU: 103/1947...75/1922
KUIN: 102/1947...79/1922

September 9:
KSTL: 101/1925...79/1925
KCOU:  99/1925...76/1947
KUIN: 100/2013...75/1912

&&

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

&&

$$

SHORT TERM...Carney
LONG TERM...Carney
AVIATION...Pfahler
CLIMATE...Pfahler