Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

303
FXUS63 KLSX 151729
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1229 PM CDT Sat Aug 15 2026

.KEY MESSAGES...

- Localized flash flooding remains a threat through mid-morning
  across northeast MO and west-central Illinois, due to ongoing
  thunderstorms producing heavy rain rates (1-2+ inches per
  hour). Another round of locally heavy rain is possible
  (40-60%) in these same areas overnight tonight, although
  confidence is relatively low.

- Dangerous heat is expected again today and tomorrow,
  particularly along and south of I-70. More widespread heat
  relief is expected Monday and Tuesday.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 236 AM CDT Sat Aug 15 2026

The most pressing item of concern is the ongoing development of
thunderstorms across northern Missouri and central Illinois, and the
threat of flash flooding that they pose overnight through tomorrow
morning. In these areas, numerous thunderstorms have developed and
expanded over the past few hours, producing exceptionally high rain
rates (2+ inches per hour) within their convective cores. These
storms are being forced by a moderate low level jet (25-30kt)
interacting with a stalled front, within  a regime of extremely high
moisture (precipitable water of 2.2-2.5 inches per mesoanalysis).
This is very close to observed all-time maximums from the historical
sounding database at Lincoln, IL, and is no doubt contributing to
very efficient precipitation rates. So far though, the most
persistent and strongest showers have remained just north of our
area, with only a few showers grazing our portion of northeast
Missouri, and we have not seen any substantial "training" as has
been observed just to our north and northeast. This activity is
expected to continue to develop in these areas, and while we expect
the most significant impacts to remain just to our north, we will
need to keep a close eye out for a southward outflow surge that
could carry heavier rain into our area over the next few hours.

These storms are expected to continue to develop for several hours
in the same areas, and gradually wind down from mid to late morning
as the nocturnal low level jet wanes. For the remainder of the day,
the focus shifts to the continuation of extreme heat in many areas,
particularly along and south of the I-70 corridor where thunderstorm
outflow winds are not expected to provide any meaningful relief. In
these areas, yet another day of 105-115 degree heat index values can
be expected. However, we did opt to shave a row of counties out of
the Heat Advisory in northeast MO and west central IL
(Shelby/Marion, MO and Pike, IL), as these areas are not likely to
see heat index values above 100 degrees today thanks to outflow
winds and cloud cover. This was the case yesterday, and it is more
likely than not that we will see something similar today, followed
by a surging cold front tomorrow. As such, we felt it was time to
pull some of those northern areas from the ongoing Advisory.
Otherwise, while we may see a few showers this afternoon, most areas
will be dry once this morning`s activity winds down.

Overnight tonight, a very similar pattern is expected to unfold
across northern Missouri and central Illinois, with a strengthening
nocturnal low level jet overriding a stalled surface front, and
pooled moisture once again reaching nearly 2.2 inches of
precipitable water. Current high resolution model guidance suggests
that tonight`s LLJ may be slightly weaker and veered compared to
this morning`s iteration, and we suspect that this is the main
reason that modeled precipitation amounts are slightly lower. Still,
considering the persistence of extremely high moisture and locally
saturated soils from recent rain, we will need to monitor this
closely for renewed Flash Flooding concerns tonight and tomorrow
morning.

During the day Sunday, at long last a passing Great Lakes shortwave
is expected to both suppress the upper ridge and nudge the stalled
frontal boundary southward, although the timing of this feature
remains somewhat uncertain. While we do have at least some
confidence that this will provide a bit of extra heat relief along
the northern fringes of our current heat headlines, this front is
likely to be weakening and slowing down as it arrives, and the
Ozarks and southwest Illinois are still in line for one more day of
similar heat and humidity Sunday. We will also need to keep an eye
on the potential for scattered showers and thunderstorms along the
front throughout the day Sunday, but especially late afternoon and
evening as the low level jet starts to ramp up. Forcing along this
boundary is not expected to be very strong, but with such ample
moisture in place, it likely won`t take much to force a few showers
and thunderstorms with a little added boost from the LLJ. Bursts of
heavy rain and occasional strong wind gusts would be the main
hazards with this activity, although weak wind shear should limit
the overall severe weather threat.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 236 AM CDT Sat Aug 15 2026

By Monday, the cold front is expected to stall somewhere across
southern Missouri, ushering in a relatively mild and dry airmass
into most of the area. Confidence is still relatively high that this
should provide enough cooling to allow for all of our current heat
headlines to expire Sunday evening, although this may not be a slam-
dunk in our southernmost Ozark counties depending on where exactly
the front stalls. There is still a reasonable potential for a
weaker/slower front to stall before it completely exits the area,
and if this materializes, places like Reynolds/Iron/Madison counties
in southeast Missouri may hover near Advisory levels of heat (100-
105 degree heat index) Monday and Tuesday, before heat returns back
north slightly ahead of another cold front Wednesday. In any case,
Monday and Tuesday should, for the most part, feature more
comfortable temperatures/humidity, and low chances (~20%) for
additional showers in the vicinity of the stalled front.

On Wednesday, confidence continues to increase that a deeper,
stronger shortwave will dig into the Great Lakes, further suppress
the central CONUS upper ridge, and send a reinforcing cold front
into the area sometime during the day. There remains a high degree
of timing variability among members, with LREF cluster analysis
unsurprisingly suggesting that a weaker shortwave would lead to a
slower/weaker front and warmer Wednesday temperatures. This is also
evident in much wider temperature spreads Wednesday, as some areas
may actually see another day of Advisory-level heat and humidity
Wednesday, while other areas stay relatively mild. The distribution
of such will all depend on how quickly this front passes through our
area. Meanwhile, confidence is also increasing that this front will
bring another round of showers and thunderstorms between late
Tuesday and late Wednesday, with at least a modest potential for
strong or severe storms. Confidence remains low regarding this
potential though, due to both the aforementioned timing uncertainty
and limited wind shear. Behind the cold front though, trends
continue to drift towards further temperature moderation, although
we do note that ensemble spreads remain relatively high, and
likewise forecast confidence in the current deterministic NBM
temperatures is relatively low.

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1224 PM CDT Sat Aug 15 2026

Dry and VFR flight conditions will persist at all local
terminals through this evening. Then a cold front will slowly
enter the region from the north, bringing a threat for scattered
showers and thunderstorms to KUIN. There is a very low chance
that the remaining local terminals could see a brief shower
and/or thunderstorm tonight, but the better chances look to be
tomorrow afternoon after 18z.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 9 PM CDT Sunday for MOZ035-036-042-052.
     Extreme Heat Warning until 9 PM CDT Sunday for MOZ041-047>051-
     059>065-072>075-084-085-099.
     Heat Advisory until 9 PM CDT this evening for MOZ034.
IL...Heat Advisory until 9 PM CDT Sunday for ILZ058>060-064-065-098-
     099.
     Extreme Heat Warning until 9 PM CDT Sunday for ILZ069-070-074-
     079-100>102.

&&

$$

SHORT TERM...19
LONG TERM...19
AVIATION...Elmore