Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

421
FXUS63 KLSX 202339
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
639 PM CDT Sat Jun 20 2026

.KEY MESSAGES...

- Thunderstorms are expected across the area late tonight through Sunday
  night. Some of these storms may be strong to severe Sunday
  afternoon/evening with damaging winds and tornadoes.

- Locally heavy rainfall is also possible through Sunday night,
  mainly across northeast Missouri and west-central Illinois.

- Seasonably cool temperatures are forecast next work week.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 252 PM CDT Sat Jun 20 2026

Dry weather is expected for much of the night, but all eyes were
to our west and an MCS that is expected to develop overnight.
Forward- propagating Corfidi vectors are due west to east, though
the vectors do veer more toward the northwest as you head into
eastern Kansas late tonight into early Sunday morning. Given that
convection is expected that far west, at least some southward
movement is expected. This is also buttressed by a veering low-
level jet with time as well. Storms should continue to develop on
the western flank of the MCS, and travel southeast with time.
Therefore, while there is still some uncertainty with respect to
the track of the heart of the MCS, it does appear likely that at
least some thunderstorms will move through central and east
central Missouri by mid to late morning. Most CAMs generally
weaken this activity with time/eastward extent, but this is a
common bias. Given the low-level jet and associated moisture
convergence does not abate at all Sunday morning, this weakening
trend is likely overdone in model guidance.

There is still concern for heavy rainfall very late tonight (after
0900 UTC) through Sunday morning across northeast Missouri and west-
central Illinois. This is where the heart of the MCS is most likely
to travel through. Some training appears likely as well in an
environment that should be conducive to efficient warm rain
processes (deep warm cloud depths, anomalously high precipitable
water values). The duration of thunderstorms in this area is also
notable (~6 hours). Amounts of 1-3" are likely, with some spots of
up to 4" possible (similar to HREF`s LPMM). In terms of a severe
threat, some small hail and gusty winds are possible, but the
overall severe threat is low with the MCS itself. There is a low
possibility that storms may strengthen with time toward midday into
the early afternoon as instability climbs. If this occurs, parts of
western Illinois may see an increasing risk for damaging winds.

The morning MCS should put out a composite outflow boundary to its
south/southwest. Exactly where this boundary will be is uncertain
and very crucial for the afternoon/evening severe threat. Past
experience suggests it will be farther south/southwest than most
model guidance. Given the positive factors detailed above re: storms
continuing through the morning hours at least with a southerly
component, the most likely location still appears to be from central
to southeast Missouri around midday. This boundary may try and move
northward into the early afternoon if convection completely dies
off. Additional convective initiation is expected by mid afternoon,
with the aforementioned remnant outflow boundary likely the focus.
The convective mode is likely to be mixed, with multicell clusters,
small bowing segments, and transient supercells all possible given
effective deep-layer shear of ~35 knots. Large hail is possible in
supercells along with tornadoes. The tornadic threat may be even
more heightened along/near a weak surface low which could transverse
the boundary. Similar to yesterday, whether this actually happens or
not is unclear. Regardless, there does appear to be enhanced low-
level shear/helicity for tornadogenesis near the composite outflow.
The threat for damaging winds also will exist, mostly within any
bowing segments. As alluded to above, the severe risk remains highly
uncertain for Sunday afternoon/evening and largely will be
dependent on exactly how the morning MCS evolves. Currently, the
best chances for severe thunderstorms appears to be from central-
southeast Missouri and points southwest, but this area may expand
northeastward if the morning MCS ends up further north and/or weaker
than currently anticipated.


Gosselin

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 252 PM CDT Sat Jun 20 2026

(Monday - Monday Night)

There are signs of another midlevel shortwave trough moving across
the mid south early Monday. This feature may help continue the
threat for showers and a few thunderstorms across southeast Missouri
and southwest Illinois. Farther north, drier and cooler weather will
continue to advect southward behind Sunday night`s cold front. High
temperatures in the 70s are forecast on Monday, with lows Monday
night in the upper 50s to low 60s.


(Tuesday - Next Saturday)

Broad northwest flow aloft is expected through the remainder of the
week per ensemble guidance. There are signs of a cold frontal
passage sometime between Wednesday and Thursday associated with a
midlevel shortwave crossing the Great Lakes. There is a fair amount
of timing uncertainty with this feature, but this likely will be the
only rain chance for the work week.

There remains high confidence in at least slightly below normal
temperatures through the end of the work week, owing in large part
to 850-hPa temperatures running 2-4C below normal. Even the 75th
percentile of the NBM for highs is at least a couple of degrees
below seasonal normals, with the 25th percentile closer to 6-8
degrees below average.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 636 PM CDT Sat Jun 20 2026

VFR conditions predominate for the next 12 to 24 hours, with the
exception of some periods of thunderstorms. The first round of
thunderstorms begins tomorrow morning potentially affecting all
TAF sites. Confidence is relatively high in storms occurring,
though timing may shift a few hours from current forecasts. After
the initial round of storms, another round develops along a cold
front from central Missouri to the St Louis metro tomorrow
evening. This round has less confidence with it as the placement
of that front and the instability remaining along it are still in
question. There`s also the potential for some MVFR or IFR ceilings
to develop mainly to the north in the vicinity of UIN after
several hours of rain tomorrow morning. These lower ceilings could
push down toward the other TAF sites behind the front tomorrow
evening, though confidence is lower in this.

Kimble

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch from Sunday morning through late Sunday night for
     Franklin MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO.

     Flood Watch from Sunday morning through Monday morning for
     Crawford MO-Iron MO-Jefferson MO-Madison MO-Reynolds MO-
     Saint Francois MO-Sainte Genevieve MO-Washington MO.

     Flood Watch from 1 AM CDT Sunday through Sunday evening for
     Audrain MO-Boone MO-Callaway MO-Cole MO-Gasconade MO-Knox
     MO-Lewis MO-Lincoln MO-Marion MO-Moniteau MO-Monroe MO-
     Montgomery MO-Osage MO-Pike MO-Ralls MO-Shelby MO-Warren MO.

IL...Flood Watch from Sunday morning through late Sunday night for
     Bond IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison
     IL-Montgomery IL.

     Flood Watch from Sunday morning through Monday morning for
     Clinton IL-Marion IL-Monroe IL-Randolph IL-Saint Clair IL-
     Washington IL.

     Flood Watch from 1 AM CDT Sunday through Sunday evening for
     Adams IL-Brown IL-Calhoun IL-Pike IL.

&&

$$

WFO LSX