Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

963
FXUS63 KLSX 191054
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
554 AM CDT Sun Apr 19 2026

.KEY MESSAGES...

- Locations in northeast Missouri and west-central Illinois will
  cool to the mid 30s tonight and Monday morning. Some patchy
  frost (<50% chance) is possible on elevated, exposed surfaces.

- Widespread 70s are expected Tuesday and Wednesday, warming into
  the 80s Thursday (10 degrees above normal).

- An approaching cold front increases chances (70-80%) for
  widespread showers and thunderstorms late Thursday into Friday.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 230 AM CDT Sun Apr 19 2026

A broad view of MSLP plots show a tandem of surface highs that will
influence local conditions through Monday. The first of these is
centered near the TX/OK border to our southwest. The second is
positioned over the south-central Canadian Provinces.

The surface high near the TX/OK border is already influencing
surface flow, as winds have gradually shifted from northwest to
westerly. The surface high is vertically stacked below mid-level
ridging with subsidence at the anticyclonic side of the upper
level jet maintaining mostly clear skies early this morning.
Radiational cooling will bring temperatures into the 30s to
low-40s by sunrise. Patchy frost is possible in the coolest
locations, but not without a few caveats. The surface high is
departed far enough to the southwest that we`ll hold onto the
light surface winds north of I-70 and light/variable conditions
south of I-70. Mid/upper 30s are more broadly spread across
northeast Missouri and west-central Illinois, where winds remain
5-7 mph. Outside of that, temperatures below 40 degrees will
become more localized to typically cooler microclimates of the
low-lying valleys. Surface dewpoints are also pretty low, ranging
from the upper 20s to mid-30s, resulting in dewpoint depressions
of 5+ degrees in the cooler locations. HREF shows the highest mean
RH values peaking at 70-75% roughly between Moberly/Columbia, MO
extending east through Hannibal/Troy, MO. HREF 90th percentile
values barely reach 90% in these locations with lower RH outside
of this corridor. Lastly, recent soaking rainfall has moistened
the surface enough that latent heat release will combine with warm
soils (60s) to help mitigate frost potential. This is not likely
to be a harmful frost with primary concerns being elevated,
exposed vegetation that is more susceptible to frost.

Ahead of the second surface high, westerly flow draws warmer air
into the region from the central Plains. A reinforcing cold front
stretches through the Great Lakes Region into the Mississippi Valley
and the surface high will serve as the impetus for this front. The
boundary pushes into northern sections of the CWA just after noon
today, progressing southward through early this evening. While the
front isn`t strong, it shifts winds back out of the northwest in the
later half of the day, and out of the north tonight before winds go
calm to light/variable tonight. This simply maintains the west-east
temperature gradient through Monday with Warmer temperatures staged
west (MO) and cooler air to the east (IL).

Another chilly morning is expected Monday. The only difference is
that winds lighten a bit further than Sunday morning. Patchy
frost is once again on the table, but the potential is only
slightly better than this morning with the lighter wind component.
Conditions remain comfortably mild and dry during the day with
highs ranging from the low/mid-60s on the Illinois side of the
river and mid- 60/low-70s in Missouri. Morning lows will bottom
out in the mid-30s to low-40s.

Note: No headlines were issued for the frost potential given that
frost remains patchy. Certainty remains limited with multiple
caveats working against conditions that would otherwise lead to
vegetation damage. Those with susceptible vegetation can water
and/or cover plants to reduce the potential that frost would
develop on elevated, exposed plants.


Maples

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 230 AM CDT Sun Apr 19 2026

Monday night into Tuesday marks the start of a significant warm-up
that will extend through at least Thursday. The northern surface
high, mentioned in the short term section, quickly shifts eastward
into the mid-Atlantic Region. The two surface ridges consolidate
into a broader ridge that encompasses the eastern seaboard heading
into Tuesday. Surface flow quickly shifts out of the south Monday
night, which will bump Monday night`s lows by 10-12 degrees. In
previous forecasts, yet another reinforcing cold front looked like
it might push into the region late Tuesday. Latest trends in the
deterministic NAM/GFS/ECM show the front stalling before it reaches
the CWA. Additionally, NBM temperature data is impressively tight
with the greatest spreads in the 2D output remaining north from Iowa
into northern Illinois. This provides high confidence that we`ll
remain warm and precipitation-free. West-southesterly mid-level flow
will bring 10-15C air into the area, resulting in highs well into
the 70s with a few 80s in urban areas.

NBM IQRs remain tightly clustered Tuesday through Thursday with the
main theme being the return to warmer-than-normal temperatures. The
drivers to the warmth will be the eastward shift in the mid-level
ridge with strengthening southwesterly flow setting up between it
and a developing trough over the Plains. Mid-level temperatures are
even more anomalous to the west (90th-99th percentile). Overhead,
mid-level warmth reaches the 80-90th percentile Wednesday and
Thursday with highs warming slightly each day with 80s more broadly
represented Thursday.

Spread increases Friday, though deterministic guidance seems to be
coming into slightly better agreement with a late week system that
could bring another round of soaking rain to the area. Global
guidance shows a cutoff upper low meandering into the northern
Plains as an amplified upper trough crosses western Canada and
consumes the lead upper low. This keeps the broad cyclonic flow
further north and leaves the region within the warm sector ahead of
a cold front that stretches southward of the surface low rounding
the upper trough. While this builds confidence in the northern
trends, it leads to some question with the front and precipitation
chances late Thursday into Friday. In these scenarios, the southern
end of the cold front typically take on more of a NE-SW orientation,
bending more W-E with southward progress. This reduces surface
convergence as the surface boundary parallels flow aloft. On top of
that, timing (late Thursday/early Friday) will be key in whether we
are able to tap into peak diurnal instability. At this juncture, it
looks like the front moves in overnight. Thunderstorms outrun better
instability to the west/northwest, much the same way it has in
recent events. This shows up well in the LREF 24-hour QPF spreads,
which increase and expand across the Midwest through late Friday.

All that being said, the pattern favors weakening convection that
could bring widespread soaking rain Thursday night into Friday. LREF
25th-75th percentile ranges show approximately 0.25-0.75" covering
much of the CWA. Keep in mind that convective potential could lead
to localized rainfall that is much higher, pending the exact
outcome. The main message here is for another chance at soaking
rainfall with details becoming more clear over the next few days.


Maples

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 554 AM CDT Sun Apr 19 2026

High pressure will support VFR conditions through the TAF period.
Prevailing groups mainly account for wind gusts through the
daylight hours, along with wind shifts resulting from a
reinforcing cold front. This front is expected to through the
region from north (KUIN) to south (Central MO/metro terminals)
between 18z and 22z, respectively.

Maples

&&

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

$$

WFO LSX