Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

167
FXUS63 KLSX 161154
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
654 AM CDT Wed Sep 16 2026

.KEY MESSAGES...

- A late season heat wave continues through at least Friday with
  peak heat index values of 100-105F+ along/south of I-70.

- Widespread showers and thunderstorms will be limited to
  northeast MO/west-central IL through this morning with
  scattered showers and sub-severe thunderstorms possible
  (20-30%) across the area this afternoon/evening.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 340 AM CDT Wed Sep 16 2026

Regional radar imagery reveals a region of widespread
shower/thunderstorm activity enveloping the northern third of MO
into west/central IL. Nearly all of this activity is being driven
and sustained by a southwesterly 20-25kt nocturnal LLJ, 30-40kts of
effective bulk shear, and around 1000 J/kg of MUCAPE, per the latest
SPC mesoanalysis. The area of focus for this activity has and will
continue to be determined by the location of a slow moving low-level
cold front. This front is interacting with the LLJ overrunning it,
leading to a swath of enhanced moisture convergence and broad
forcing for ascent along/poleward that will continue through the
early morning hours. The main threat with this activity is heavy
rainfall as the environment is supportive of warm rain processes
leading to efficient rainfall. Convection has been fairly transient,
which has limited the duration of heavy rainfall at any given
location for most. However, for a few localized areas there have
been instances of training leading to multiple rounds of convection.
Therefore, the low threat for localized flooding will continue
through the morning for portions of northeast MO/west-central IL.
The effective cold front is tough to locate, but appears to be (as
of 09z) located from Warsaw MO, northeastward through the St. Louis
metro, to south of Springfield IL. This front should continue to
slowly slide southeast, potentially stalling just to the south of
the St. Louis metro this morning. As a result, showers and
thunderstorms will be limited to locations along/north of I-70 with
more widespread convection limited to northeast MO and west-central
IL. As the nocturnal LLJ weakens after sunrise, coverage of showers
and thunderstorms should wane becoming more scattered across
northern portions of the area by the late morning, ending the heavy
rainfall/flooding threat as well.

The record strong mid-level ridge that is currently centered over
the Tennessee Valley expands to the north and west today, aiding in
lifting the boundary poleward as a warm front. Confidence remains
low regarding high temperatures across the northern half of the area
today, as there is still variance among guidance regarding how fast
the front lifts northward. A rather sharp temperature gradient will
exist on each side of this boundary, with scattered showers/storms
and low stratus to the north leading to temperatures that struggle
to reach the low 80s this afternoon. While unseasonably hot
conditions will continue on the equatorward side of the boundary
thanks to 850mb temperatures around 20C (95th percentile) and
continued warm air advection. Remnant cloud cover from convection
this morning could slow the warmup for some along/south of I-70, but
is forecast to be temporary with clearing by the afternoon with
highs in the low/mid 90s for most. Near record high humidity will
combine with the unseasonable heat to create heat index values of
100-105F where there is a Heat Advisory in effect. With sufficient
heating, forecast soundings indicate some locations in the open warm
sector become uncapped allowing for isolated sub-severe thunderstorm
chances (20-30%) this afternoon/evening. Mid-level height rises
aloft reinforce subsidence, which should limit the coverage and
intensity of convection across the area.

Confidence is high that the front continues its trek northward into
Thursday with nearly the entire area expected to be free of
rain/clouds, aside from portions of northeast MO/west-central IL,
where low rain chances linger. That being said, near record level
heat and humidity will continue to impact the region with highs in
the upper 90s, potentially nearing 100F for some locations
along/south of I-70 on Thursday.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 340 AM CDT Wed Sep 16 2026

By Friday, a longwave trough will be traversing the Intermountain
West working to initiate leeside cyclogenesis that will enhance
southerly flow to the west of the region. At the same time, a
surface high will be sliding southward into the Upper Great Lakes
creating northeasterly flow into the boundary across the Ohio river
valley. The evolution of these features should favor a clockwise
pivot of this boundary with equatorward movement possible across
portions of IL and points to the east. As a result, the boundary may
creep equatorward, back into the area, bringing chances of showers
and thunderstorms with it. This may also keep the heat from
expanding to far to the north, potentially saving northeast MO/west-
central IL from highs in the 90s Friday afternoon. Into the weekend,
the anomalous mid-level ridge centered over the region weakens as
the slow moving longwave trough out west approaches. Slow moving is
the key word here as an mid-level cut off low may halt the
progression of the feature. Guidance has continued to slow the
evolution of the trough, which has led to increased confidence that
heat continues Saturday and potentially into Sunday. The only bright
side is that humidity levels appear to gradually lower into the
weekend, which may lead to heat index values that are the same as
the ambient air temperature. Although, with potential for ambient
temperatures in the mid/upper 90s for some on Saturday, the lower
moisture won`t provide much relief, if any.

The longwave trough out west is progged by long-range guidance to
eject into the Northern Plains Sunday night into Monday. The LREF
IQR temperature spreads on Sunday and Monday are now both 10-15
degrees, indicating this is when guidance expects a cold front to
move through the region. A slower front would lead to another
unseasonably hot day for most while a faster front would result in a
sooner arrival of real heat relief. Temperature IQR spreads decrease
to 5-10 degrees Tuesday and beyond next week, but this time,
centered on values that would be near or slightly above
climatological normals. Values like this reveal the signal is there
for an end to the heat by the start of next week.

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 654 AM CDT Wed Sep 16 2026

A weak cold front has slowly moved south of all local terminals and
is now positioned just to the south of the St. Louis metro. Along
and to the north of this front there is widespread low/mid-level
cloud cover, including areas of IFR/low MVFR stratus. These
impactful ceilings are more widespread further north (at KUIN) and
west along with showers and thunderstorms across northeast/west-
central IL. These IFR ceilings along with intermittent showers and
isolated convection will continue to impact KUIN through the
morning. More IFR ceilings are just to the west of KCOU/KJEF with
about a 50% chance they impact KCOU/KJEF from 12-16z. The St. Louis
metro terminals will be on the southern periphery of these IFR
ceilings for a few hours this morning, likely altering back and
forth between BKN and SCT ceilings around 1kft. Gradual improvements
to ceilings are expected first at the St. Louis metro terminals,
followed by KCOU/KJEF, and lastly KUIN by the early afternoon. There
is a low 20-30% chance for a few scattered showers and sub-severe
thunderstorms this afternoon and evening near the St. Louis metro so
a PROB30 was included to indicate that low threat. Surface winds
light, generally less than 10kts, but will be quite variable across
the region over the next 30 hours with the front nearby the
area.

&&

.CLIMATE...
Issued at 231 PM CDT Mon Sep 14 2026

Daily record high temperatures may be threatened for both St. Louis
(KSTL) and Columbia (KCOU) this week. NOTE: Records at Quincy (KUIN)
are not shown below due to cooler temperatures forecast:

          9/15        9/16        9/17        9/18        9/19
KSTL   101/1939     98/1954     97/1931     97/1925     96/1955
KCOU   101/1893     99/1954     98/1895     98/1893     98/2022

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Friday for MOZ047>049-059>065-
     072>075-084-085-099.
IL...Heat Advisory until 8 PM CDT Friday for ILZ064-069-070-074-079-
     100>102.

&&

$$

SHORT TERM...Peine
LONG TERM...Peine
AVIATION...Peine
CLIMATE...Gosselin