086
FXUS63 KLSX 090251
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
951 PM CDT Fri Aug 8 2025

.KEY MESSAGES...

- Hot and humid conditions are expected on Saturday with peak heat
  index values around the 100 degree mark.

- Chances (40-60%) of showers and thunderstorms increase Saturday
  night through Sunday night, mainly across portions of northeast
  Missouri and west-central Illinois. Locally heavy rainfall and
  gusty winds are the main threats particularly, Sunday
  afternoon/night.


&&

.SHORT TERM... (Through Sunday Morning)
Issued at 235 PM CDT Fri Aug 8 2025

A mostly clear sky with persistent southerly winds is forecast
overnight tonight. The aforementioned southerly flow will help
keep temperatures on the mild side, with lows only dropping back
into the low to mid 70s. The warmest conditions are expected to be
across portions of central Missouri northeastward into west-
central Illinois where winds will be closer to 10 mph. Meanwhile,
the coolest locations are expected to be within favored river
valleys where winds are more susceptible of decoupling overnight.

Conditions on Saturday look very similar to this afternoon, though
there may be a bit more diurnal cumulus development during the day.
In addition, some mid/high level cloudiness is expected to spill
into portions of northeast Missouri and west-central Illinois. Highs
are forecast to range from the low to mid 90s on Saturday. Humidity
levels also should be similar to today, so peak heat index values
once again around the 100 degree mark are expected for most
locations. Some spots may get close to 105 degrees, but should not
be widespread enough to warrant a heat advisory for magnitude
criteria (105 degrees).

In addition to the heat, the other potential concern is the
possibility of showers and thunderstorms Saturday afternoon. Model
soundings show a pretty strong cap, aided by 700-hPa temperatures of
+12 to +13C. The stronger mid/upper level forcing for ascent is also
well to our north ahead of an area of vorticity moving across the
Upper Midwest.  The front itself is expected to get closer to the
Missouri-Iowa border by early evening, so convergence does increase
near the front itself. This very well may be enough to generate
convective initiation, but chances are higher to our northwest based
on the expected location of the front. If any storms do occur in our
area during the afternoon/early evening, the threat for any
organized severe weather looks low. While instability will be high,
effective shear is likely to be around 15 knots, which suggests
pulse convection.

Low-level convergence increases on the nose of a 35-40 knot jet
across the mid-Missouri Valley overnight Saturday night.
Thunderstorms are expected to organize into an MCS across Iowa. Some
of this activity may try and skirt into parts of northeast Missouri
and west-central Illinois late Saturday night into Sunday morning as
the low-level jet veers more to the southwest. However, forward-
propagating Corfidi vectors suggest more of a southwest-northeast
movement. Given that deterministic models show a pretty significant
midlevel shortwave trough moving across the Upper Midwest during
this time frame, the forward propagating vectors should be pretty
representative of the motion of this MCS.


Gosselin

&&

.LONG TERM...  (Sunday Afternoon through Next Friday)
Issued at 235 PM CDT Fri Aug 8 2025

(Sunday Afternoon - Wednesday Night)

There is uncertainty as to how far south the front gets by Sunday
afternoon, and of course this will depend on the antecedent round
late Saturday night into Sunday morning. Conceptually speaking
however, the front should be at least slightly further to the south.
This boundary may serve as the focus for renewed development by
Sunday afternoon as another midlevel shortwave trough ejects out of
the south-central Plains. This feature also looks to be slightly
south of its predecessor. A moist and unstable air mass will remain
in place along/south of the cold front Sunday afternoon, and with
higher deep-layer shear, this increases the threat for strong to
severe thunderstorms through early evening across northeast Missouri
and west-central Illinois. Multicellular clusters along with
potentially a few bowing segments or even supercells may be
possible. Damaging wind gusts would be the main severe threat.

Attention Sunday night into early Monday morning will quickly shift
to the potential for heavy rainfall/flash flooding in our far north.
While there remains some uncertainty on the location of the front
and where the heavy rain axis will ultimately set up, Sunday night`s
activity should be at least slightly further southeast given further
weakening of the mid/upper level ridge and the expectation that
previous rounds of storms should at least help augment the effective
boundary more equatorward. Strong moisture convergence is forecast
once again Sunday night on the nose of a 30-40 knot low-level jet,
helping to lead to widespread convection in the form of a possible
MCS. The environment in northeast Missouri and west-central Illinois
is likely to lead to efficient rainfall given precipitable water
values above 1.75" (>90th percentile of climatology) and deep warm
cloud depths. Convection may also tend to train along/near the quasi-
stationary front, which would also increase the flash flood threat.
The threat for river flooding will also increase into early next
week. Widespread minor river flooding on upstream portions of the
Mississippi River is likely given the elevated flows currently
observed. The latest HEFS shows ~20% probabilities for moderate
flooding as well on a few points including at Clarksville and
Winfield. However, those probabilities likely are too low given the
setup and should climb higher in the next 24-48 hours. Other
tributaries may also see minor flooding in northeast Missouri and
west-central Illinois, but the bigger concern would be on the
Mississippi.

Additional rounds of showers and thunderstorms are possible at least
through midweek as the front remains stalled somewhere across the
region. There are some differences though that do decrease the
threat for any organized convection capable of producing widespread
heavy rainfall and/or severe weather. First, the quasi-stationary
front itself is expected to become more diffuse with time, with
weakening convergence along it. Secondly, the mid/upper level flow
aloft dampens, decreasing effective shear. Finally, deterministic
guidance, at least at this point, are not showing nearly as strong
of a low-level jet any night as those anticipated Saturday and
Sunday nights.

Continued chances of showers and thunderstorms and associated cloud
cover spreading a bit more to the south should mean high
temperatures closer to normal (mid 80s to near 90 degrees). There
may more a bit more room to go cooler if there is more widespread
shower and thunderstorm activity any given afternoon. Meanwhile, for
nighttime lows, they should remain on the mild side (staying above
normal) given the anticipated cloud cover. Even the 25th percentile
of the NBM shows lows a few degrees above normal areawide, which
illustrates the high confidence in warmer than normal lows
continuing well into next week.


(Thursday - Next Friday)

Clusters of the 500-hPa height pattern show a subtle return to more
mid/upper level riding across the Mississippi Valley by late next
work week. This ridge is not forecast to be as strong as the current
version, but may be strong enough to lead to mostly dry weather
beneath it along with daytime temperatures returning to above normal
levels for mid August.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 948 PM CDT Fri Aug 8 2025

Scattered light showers have developed in central Missouri and
will continue to percolate through the early morning hours. No
impacts are expected to terminals from these showers. Dry
conditions are forecast from early morning until the last 6 hours
of the TAF period when showers and thunderstorms enter northeast
MO/west-central IL from the northwest. Otherwise, winds will
become gusty for northeast MO, central MO, and west-central IL
this afternoon with gusts up to 20 kts forecast.

Jaja

&&

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

$$

WFO LSX