685
FXUS63 KLSX 031854
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
154 PM CDT Sun May 3 2026

.KEY MESSAGES...

- Scattered showers and thunderstorms are expected from the late
  evening into early tonight, mainly between U.S. Route 36 and
  the I- 70 corridor.

- Additional chances for more widespread showers and thunderstorms
  are forecast Monday night, with periods of rain continuing into
  Tuesday night.


&&

.SHORT TERM... (Through Late Tuesday Night)
Issued at 151 PM CDT Sun May 3 2026

A weak cold front is approaching from the northwest this afternoon,
with recent surface analysis unveiling this front across
southeastern IA, stretching southwestward into northwestern MO.
Water vapor imagery reveals that broad cyclonic flow envelops
southern Canada with shortwave ridging out west ahead of a cutoff
low across the Baja Coast. This pattern places our area under
northwesterly flow aloft with a low/mid-level shortwave propagating
over the region today. This feature has worked to produce scattered
showers across northeast MO/west-central IL today. Overall, this
activity has been light in nature, with virga being a common
observation, thanks to dry air in the low-levels and meager forcing
for ascent aloft. The most notable impact on the sensible weather
has been an increase in mid-level clouds from the northwest, leading
to slightly cooler afternoon temperatures where clouds have been
more prevalent. Otherwise, increased southwesterly low-level flow
today ahead of the approaching cold front, has boosted temperatures
into the low 70s for most locations this afternoon.

The previously mentioned weak cold front will work its way southward
into the area, eventually washing out as it nears the I-70 corridor
early tonight. Recent CAMs show convective initiation between U.S.
Route 36 and the I-70 corridor late this evening around sunset with
southeastward progression of showers and thunderstorms overnight as
the 850mb low-level jet strengthens to 40-50kts. The 850mb jet will
be parallel to the remnants of the cold front, potentially resulting
in training showers and thunderstorms. Fortunately, mean PWATs
progged by high-resoltion guidance stay below 1", which lowers the
threat for heavy rainfall. Regardless, localized amounts of up to 1"
of rainfall are possible for locations where multiple rounds of
showers/thunderstorms move over, which may cause typical ponding on
roadways where the heavier rates occur. A marginal threat for severe
weather exists with both the HREF and REFS revealing pockets of
1,000 J/kg SBCAPE by mid-evening, with quickly waning instability
after sunset. Forecast model soundings reveal a cap inversion around
the 800-700mb layer, with potential erosion of this cap into the
late evening. Effective layer shear near 30kts may be sufficient for
organization of sustained updrafts, resulting in the potential for
an isolated strong to severe thunderstorm. As a result, this threat
remains conditional on a narrow window of favorable conditions when
thunderstorms would be able to tap into better instability. Storms
that are able to become strong/severe late this evening have the
potential to produce hail up to 1 inch, with a lesser threat for
damaging winds.

A secondary and stronger cold front will be making its way southward
across the Upper Midwest on Monday, leading to yet another day of
increased low-level southerly flow resulting in warm/moist
advection. In fact, temperatures are forecast to be even warmer on
Monday with widespread highs in the upper 70s along with lower 80s
possible across southern MO and the St. Louis Metro heat island. By
Monday evening, the HREF and REFS both reveal near 2,000 J/kg of
SBCAPE. However, forecast model soundings show that the open warm
sector Monday afternoon/evening across the area remains well capped,
with most CAMs not initiating anything until the low-level jet ramps
up around sunset. At that time, the most likely position of the cold
front will be near northern MO, where additional showers and storms
may be ongoing. Once again, with this conditional setup, a marginal
threat for severe weather is in place with hail and damaging winds
the primary threats. As the front sinks southward into the area
interacting with the nearly parallel 850mb low-level jet, widespread
showers and thunderstorms are expected across the area. Model
guidance still varies on the southward evolution of this cold front,
with slower solutions not exiting the front out of the area until
Tuesday night. As a result, confidence remains low with the forecast
for temperatures and precipitation chances on Tuesday. Locations
under the post-frontal airmass on Tuesday may experience a falling
of temperatures throughout the day, or a flattening of the typical
diurnal temperature curve, with highs in the 50s expected north of
the I-70 corridor.

Peine

&&

.LONG TERM... (Wednesday through Next Sunday)
Issued at 151 PM CDT Sun May 3 2026

At the start of the extended, continued broad cyclonic flow across
southern Canada will be phasing with a cutoff low near the Great
Plains. As these features move over the region, mostly cloudy
conditions along with scattered chances for showers will exist on
Wednesday. At the surface, the aforementioned cold front will be
well off to the southeast, placing the entire region under the much
cooler post-frontal airmass. With modest low-level CAA and a mostly
cloudy sky forecast, high temperatures on Wednesday will be at least
20 degrees below average with afternoon temperatures maxing out in
the 50s across the entire area.

By the end of next, model guidance varies with the placement of many
important large scale features, resulting in more uncertainty with
temperatures and precipitation chances for the area. The general
consensus is that a stretch of dry weather will prevail Thursday
through the end of the week with a gradual warming trend from the
mid-week chill. Recent LREF temperature IQRs indicate this well with
spreads of 7-10 degrees centered near values that are below average
on Thursday, but near average Friday into Saturday.

Peine

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1242 PM CDT Sun May 3 2026

An area of mid-level clouds is moving southeast across northeast
MO/west-central IL with an isolated shower as well. Most of what is
seen on radar imagery is not reaching the ground as it falls into
dry low-level air and evaporates. The latest guidance has trended
further north with the convection overnight and now is focused on an
area between U.S. Route 36 and the I-70 corridor. Now, probabilities
are highest, near 30-40%, for a thunderstorm at the St. Louis metro
terminals and KUIN late this evening into the overnight. Confidence
is lower in the occurrence of thunderstorms at KCOU/KJEF, so held
off on any mention for now, as trends will continue to be monitored.
The main threat with thunderstorms will be MVFR visibilities, with a
lower potential for small hail and gusty winds. Gusty southwesterly
winds this afternoon fade after sunset with gusty southerly winds
returning after sunrise on Monday.

Peine

&&

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

$$

WFO LSX