032
FXUS63 KLSX 010327
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1027 PM CDT Sat May 31 2025

.KEY MESSAGES...

- The warmest temperatures the area has seen in a couple weeks are
  forecast to start the week, with highs on Monday and Tuesday
  pushing 90 degrees for some locations.

- While there are low rain chances (30% or less) for portions of
  the area tonight and tomorrow, the next chance (60-80%) for a
  widespread soaking rain is late Tuesday into Wednesday, with
  active weather continuing into next weekend.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 214 PM CDT Sat May 31 2025

An upper-level trough is digging across the eastern CONUS, with the
Middle Mississippi Valley beneath northwesterly flow on its western
periphery per recent water vapor imagery. This northwesterly flow
has kept our weather relatively calm, our humidity low, and our
skies smokey today. Thanks to the mostly clear skies and current
mesoanalysis showing 850 mb temps around 13C, surface temperatures
have warmed into the low to mid 80s for most locations - the warmest
we`ve seen since mid-May.

This evening and tonight, two features will be responsible for low
chances of isolated to scattered showers and thunderstorms. The
first is a backdoor cold front this evening that will move into our
west-central and south-central Illinois counties. Forcing along this
front will be weak and vertical thermal profiles poor for convective
development. However, some deterministic soundings show weak
instability that may yield a brief isolated shower, possibly a
thunderstorm, in this part of the area early to mid evening. The
second feature is a tightly-wound shortwave currently moving south-
southeastward through the Central Plains. This shortwave`s
trajectory will keep it just west of the CWA tonight, though some of
its associated convection may clip portions of central and
southeastern Missouri overnight. Meager MUCAPE (a couple hundred
J/kg) will allow for some convection to carry a lightning threat,
but the low instability and weak deep layer shear (20-25 kts) will
mitigate a threat of strong to severe storms.

Sunday will be similar to today as northwesterly flow persists over
the region. Recent HRRR runs indicate that the wildfire smoke aloft
will be more dense, but that is unlikely to have much impact on
temperatures, which are expected to be a degree or two warmer as
ridging slowly pushes into the region from the west. The front, or
rather, it`s remnants, may serve as the focus for isolated to
scattered convection across far southeastern Missouri during
afternoon peak heating.

Elmore

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 214 PM CDT Sat May 31 2025

At the start of the extended period on Monday, guidance consensus is
that the axis of the upper-level ridge will be almost directly over
the Middle Mississippi Valley, with the mid-level ridge being
relatively displaced eastward. This will place the CWA squarely
beneath deep southerly to southwesterly flow that will ramp up warm
air advection and push 850 mb temperatures to around +16C. As a
result, surface temperatures will warm to the mid to upper 80s -
about 5-10 degrees above climatological normals. For Tuesday,
temperatures will be similar to those on Monday, with perhaps more
widespread upper 80s as warm air advection ramps up ahead of an
approaching trough. This approaching trough will increase cloud
cover over the region, though by how much and its impact on
temperatures remains uncertain at this time.

The bigger story with the approaching trough is that it marks the
start of what will be an active period for the CWA from the middle
to at least late next week. An initial shortwave within the trough
will eject out of the Plains and into the Midwest Tuesday into
Wednesday per deterministic guidance, with a low traveling along a
cold front draped from the Central Plains into the Great Lakes. As
it and its parent shortwave approach the CWA late on Tuesday, lift
will increase via these features and a low-level jet, leading to
widespread showers and thunderstorms developing along the front into
Wednesday. Both deterministic and ensemble guidance have SBCAPE
reaching 1,000-1,500 J/kg ahead of the front among moderate deep-
layer shear of generally 30 kts, leading to a low concern for strong
to severe storms. However, the deep layer shear vector and low-level
jet nearly parallel to the front is expected to lead to destructive
interference among updrafts and limit updraft strength. If
anything, this looks like a setup favorable for locally heavy
rainfall, though guidance is currently not too excited on QPF
amounts. Confidence in impacts from heavy rainfall remains low at
this time.

Guidance consensus breaks down late Wednesday into the remainder of
the period on the phasing of the trough and subsequent shortwaves.
Generally, southwesterly flow aloft is expected to persist through
the end of the workweek, with a shortwave or two within this flow
bringing subsequent rounds of rain to portions of the area. For the
weekend, guidance varies on whether the trough shifts eastward, or
remains just west of or over the Middle Mississippi Valley.
Regardless, southwesterly to quasi-zonal flow over the CWA is seen
in the majority of deterministic guidance and global ensembles,
keeping chances going for shortwaves within the flow to bring rounds
of rain to the area through the weekend. At the very least, the
constant chances of rain and associated cloud cover will keep
temperatures around seasonal normals (low to mid 80s) through this
stretch.

Elmore

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1020 PM CDT Sat May 31 2025

Dry, VFR conditions are forecast at the terminals through the
valid TAF period. The mid-level shortwave that is forcing
convection across eastern Kansas and western Missouri is staying
west enough of the central Missouri terminals to keep any threat
of thunder at bay. A few storms approached KCOU and KJEF but
collapsed before getting near enough for impacts. With the main
thrust of the upper-level wave moving south, the chances for any
thunder overnight in central Missouri are very low (less than
10%).

The backdoor cold front continues to push east out of central and
northern Illinois, though is slower than previously forecast.
With such weak winds overall and the threat for any convection
along this front so low, operational impacts are very unlikely
(less than 10%). Guidance continues to hint at fog at KJEF just
before sunrise, but there`s little to support that in the current
observations. On the very off-chance that fog develops, it will
be short- lived and unlikely to reach IFR flight category.

MRB

&&

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

$$

WFO LSX