Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

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FXUS63 KLSX 202323
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
623 PM CDT Mon Apr 20 2026

.KEY MESSAGES...

- A notable warmup begins on Tuesday lasting through Thursday,
  with temperatures 5-10 degrees above average.

- Widespread showers and thunderstorms are expected Thursday night
  into early Friday with potential for additional development
  Friday afternoon/evening.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 251 PM CDT Mon Apr 20 2026

A surface high, that was centered across the Great Lakes this
morning, has slid southeast throughout the day and is now positioned
near the Ohio River Valley. The progression of this surface feature
has allowed area surface flow to veer from easterly to southerly
with return flow commencing earlier across central MO. This, coupled
with abundant solar insolation under a sunny sky, has led to
afternoon temperatures reaching the low/mid 70s across MO, with
mid/upper 60s across IL. Even with a clear sky persisting into
tonight, nocturnal cooling will be limited, thanks to continued low-
level southerly winds bringing WAA along with PBL mixing, keeping
temperatures largely in the lower 50s before sunrise on Tuesday.

Recent GOES-19 mid-level water vapor imagery reveals broad ridging
across the Intermountain West with amplified troughing on either
side, just off the West Coast and across the eastern United States.
This large-scale pattern places our region under mid/upper-level
northwesterly flow, which limits chances for precipitation across
the area over the next few days. A mid-level shortwave and surface
reflected low far to the north across Ontario will send a weak cold
front southward, approaching the area on Tuesday. The approaching
boundary will tighten the surface pressure gradient, leading to
increased low-level southwesterly flow, gusting to 20-30mph by the
afternoon on Tuesday. A setup like this favors downsloping off the
Ozark Plateau, which coupled with steady WAA, will work to boost
temperatures. As a result, Tuesday should be near 10 degrees warmer
than today, with afternoon highs expected to reach the upper
70s/lower 80s areawide.

High-resolution guidance is revealing the weak boundary stalling
just to the north across southern IA/northern IL Tuesday evening.
Nearly all of deterministic guidance have a very subtle mid-level
shortwave propagating across the area Tuesday evening within the
mid/upper-level northwesterly flow. As this moves overhead, height
falls aloft may be just enough to produce sufficient lift to help
initiate showers and isolated thunderstorms along the stalled
boundary. Low-level southerly flow colliding into the boundary
results in abundant convergence leading to moisture pooling across
this area. As a result, REFS/HREF mean CAPE values of 1,000-1,500
J/kg exist along with mean 0-6km Bulk Shear around 30kts, indicating
the environment should be supportive for thunderstorms. However,
forecast soundings across that area reveal a 750mb cap/inversion
that may limit shower/thunderstorm coverage. The most recent CAMs
indicate scattered shower and thunderstorm development around sunset
on Tuesday, across far northern MO and west-central IL, with
decreasing coverage as precipitation slides southeastward into a
less favorable environment. As a result, a low chance (10-20%) for
scattered showers and thunderstorms exists across northern MO/west-
central IL Tuesday evening into Tuesday night.

Peine

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 251 PM CDT Mon Apr 20 2026

By Wednesday, the mid-level ridge out west will be slowly sliding
eastward with the ridge axis nearing the Great Plains. The
continuation of mid/upper-level northwesterly flow will help keep
the area dry Wednesday into Thursday, even though low-level
southerly warm/moist advection will be persisting as well. Guidance
reveals the mid-level ridge axis advancing across the area Wednesday
night, leading to the beginning of deep southwesterly flow on
Thursday. It appears that Thursday will be the warmest day of the
week, with areawide highs in the low 80s, as mean 850mb temperatures
progged by the LREF may reach the mid teens (C), which is near the
90th climatological percentile.

An active weather pattern returns near the end of the week as
longwave troughing settles in across the Intermountain West behind
the departing ridge to the east. Long-range guidance reveals a
closed low within the longwave trough ejecting in to the High Plains
Thursday. Guidance is in good consensus that this closed low rapidly
lifts north into the Upper Great Plains as a Fujiwhara effect occurs
between it and other lobes of energy swinging around the base of the
nearly stationary longwave trough. With the ejection of the robust
closed mid/upper-level low, surface cyclogenesis ensues across the
Upper Great Plains Thursday afternoon that sends a cold front
eastward across the Central Great Plains. Ahead of this front, on
Thursday, the highest LREF mean SBCAPE approaches 1,000 J/kg across
central MO with 0-500mb Wind Shear near 30kts. As it stands now, the
greatest threat for strong to severe thunderstorms would be across
western into central MO, with gradual weakening thunderstorms with
eastward extent, a result of waning instability into the night.

It is important to note that the exact timing/orientation of this
frontal passage remains uncertain given the plethora of influencing
variables at this range. What`s more certain, is that shower and
thunderstorm chances, along with the potential for severe weather,
increase as the front approaches, which currently appears to be late
Thursday evening into Friday morning. Some long-range guidance
reveals this cold front pivoting clockwise and settling in a
southwest-northeast orientation near the area on Friday. The
location of this boundary on Friday afternoon/evening will greatly
influence where the best chances for strong to severe thunderstorms
is, which varies significantly in guidance. With the potential for a
stalled boundary around the region Friday and into the weekend,
along with multiple shortwaves swinging around the base of the
longwave into the Plains, a wet and active pattern may setup from
Friday into early next week. The latest outlook from the Climate
Prediction Center highlights this well, where the 6-10 day
precipitation outlook places our area in a 50% chance for above
normal rainfall.

Peine

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 623 PM CDT Mon Apr 20 2026

Dry and VFR flight conditions will persist into Tuesday evening
across the region with winds gradually veering from
southeasterly/southerly overnight and then strengthening Tuesday
morning with occasional 23 to 27 kt gusts through the day. There
will also be a low-level jet in place tonight, but its modest
strength and marginal low-level directional and speed shear
precludes impactful LLWS.

Pfahler

&&

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

$$

WFO LSX