Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

885
FXUS63 KLSX 192351
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
651 PM CDT Sun Jul 19 2026

.KEY MESSAGES...

- Heat and humidity spread area wide on Monday, but a cold front
  on Tuesday finally brings this heat spell to an end.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 200 PM CDT Sun Jul 19 2026

A poorly defined front is draped across our region this
afternoon, and while it has at times been the focus for enhanced
cumulus and subsequent thunderstorm development, other areas of
enhanced moisture and lift have also spawned convection on both
sides of the boundary. The focus, though, has been largely along
and south of the I-70 corridor. Storms today are capable of
heavy downpours and locally gusty winds, but organized severe
weather is not expected. These should dissipate with the loss of
daytime heating this evening, giving way to a quiet night. Low
temperatures will be cooler to the east where a little drier air
has infiltrated behind the front.

Winds turn southwesterly on Monday and the heat and humidity
return to the entire region. The intensity of the heat will be
even a bit greater than it has been in recent days, with high
temperatures topping out in the low to mid 90s. But combine this
with dewpoints in the mid 70s and we`ll see heat index values
rising above 100 for just about the entire area. Only our far
eastern zones in central Illinois may miss out on 100+ heat
index values. The Heat Advisory has been expanded northward to
include the rest of Missouri as well as Adams County, IL in
coordination with neighboring forecast offices. The Advisory
here is only for Monday, though, as heat relief begins from
north to south on Tuesday.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Issued at 200 PM CDT Sun Jul 19 2026

Tuesday features our next air mass change. A trough currently
dipping south out of the Canadian prairies will move through the
Great Lakes Monday and carve out a larger trough over the
eastern US on Tuesday. This sends a cold front southward through
the Midwest behind it, likely arriving near the Iowa/Missouri
border area by dawn on Tuesday. The front continues pushing
south through our area during the day. Although a cooler and
considerably less humid air mass is pushing in behind the
front, it will take some time to arrive. So even northern areas
where the front moves through early will still make a run at 90
degrees. But further south where the front is later to arrive
this is likely to be the worst day of this air mass. Heat index
values peak locally near 110 degrees as moisture pooling ahead
of the front combines with temperatures peaking in the mid 90s.
By Tuesday night into Wednesday, though, the whole area will
notice the air mass change.

The surface high with the incoming air mass comes charging south
out of Canada before stalling over Illinois and the southern
Great Lakes Wednesday into Thursday. These will be our coolest
days of this air mass despite some sunshine initially. Daytime
highs top out in the low to mid 80s, but with much lower
dewpoints we`ll see nighttime lows in the low 60s or even upper
50s. As we head toward the end of the week, we start to see some
rain chances return to the forecast. This is related to
shortwave troughs rotating around a ridge over the Rocky
Mountains, with the northwesterly flow sending these waves
southeastward. The greatest chance for impacts locally are on
Friday. While there`s still uncertainty on who may get some
rain out of this, there will at least be a more cloud cover
around, helping to delay the modification of the air mass. Thus
high temperatures Friday are still forecast in the low 80s. With
more sun we`d see a warming trend into the mid to upper 80s,
but if the rain is more widespread locally we could even stay in
the 70s. Eventually, though, the ridge is expected to expand
its influence to the east leading to gradually warming
temperatures this weekend. How quickly the humidity returns and
how hot it gets are still uncertain.

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 633 PM CDT Sun Jul 19 2026

Thunderstorms that percolated at most of the terminals (save for
KUIN) along a weak front this afternoon are finally
dissipating, with the only activity of note focused across the
western St. Louis metropolitan area at this hour. By 01Z at the
latest, all convection will cease and dry conditions will
prevail.

There is a persistent signal for visibility reductions tonight
across parts of east-central Missouri into southwest Illinois,
including the St. Louis terminals. This area saw several heavy
downpours, which is keeping dewpoint depressions fairly low.
MVFR fog was added to KSUS, KCPS, and KJEF, but we will need to
keep an eye on KSTL for any development there as well. After fog
erodes tomorrow morning, dry and VFR conditions resume.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 7 PM CDT Tuesday for MOZ041-047>051-
     059>065-072>075-084-085-099.
     Heat Advisory from noon to 8 PM CDT Monday for MOZ018-019-026-
     027-034>036-042-052.
IL...Air Quality Alert until midnight CDT Monday night for
     ILZ058>060-065-095>097.
     Heat Advisory until 7 PM CDT Tuesday for ILZ069-074-079-
     100>102.
     Heat Advisory from noon to 8 PM CDT Monday for ILZ095.

&&

$$

SHORT TERM...Kimble
LONG TERM...Kimble
AVIATION...MRB