624
FXUS63 KLSX 151052
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
552 AM CDT Sun Jun 15 2025

.KEY MESSAGES...

- There is a Slight Risk (15 percent) of a few strong to severe
  thunderstorms Tuesday night and Wednesday; however, the risk is
  more conditional and uncertain Tuesday night. There is also a
  conditional threat of locally heavy rainfall.

- A warming trend early this week will support some high
  temperatures reaching 90 F, threatening the long run of sub-90 F
  temperatures.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 320 AM CDT Sun Jun 15 2025

Height rises/subsidence in the wake of a departing upper-level
closed low will dominate the Mid-Mississippi River Valley with dry
conditions through the morning and daytime today. With mostly clear
skies and lingering BL moisture, GOES-19 Nighttime Microphysics and
surface observations have detected increasing fog across the CWA
early this morning with the greatest coverage and lowest
visibilities in river valleys. Some locally dense fog is possible in
river valleys, but conditions are not as favorable for widespread
dense fog as they were Friday night with greater BL mixing and less
precipitation on Saturday. Following dissipation of fog shortly
after sunrise, mainly upper-level clouds and diurnal cumulus will be
overhead, allowing slightly warmer high temperatures than Saturday
and more firmly in the 80s F.

An upper-level shortwave trough will very slowly propagate across
the Mid-Mississippi River Valley tonight into Monday driving the
next chance showers and thunderstorms. HREF membership is in
agreement that showers and thunderstorms will be focused on a
potential remnant MCV tracking into southeastern MO and southwestern
IL. However, additional, more isolated showers and thunderstorms
could develop elsewhere across the CWA during the afternoon as weak,
broad ascent combines with weakly "capped" modest diurnal
instability of 500 to 1000 J/kg. Weak deep-layer wind shear of 10
to 20 kt will preclude sufficient organization of thunderstorms
for severe weather. Coverage of showers and thunderstorms will
wane through the evening as the shortwave trough axis shifts to
the east. High temperatures on Monday will generally be similar to
today, perhaps slightly cooler in the presence of more prevalent
clouds in southeastern MO and southwestern IL.

Pfahler

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 320 AM CDT Sun Jun 15 2025

After the departure of Monday`s upper-level shortwave trough, quasi-
zonal flow on Tuesday will gradually transition to southwesterly
ahead of an approaching trough and be navigated by a series of
perturbations/shortwave troughs. This pattern will provide multiple
rounds of showers and thunderstorms across the Mid-Mississippi River
Valley including a potential MCS Tuesday night/Wednesday morning in
northeastern MO/west-central IL and additional development across a
larger portion of the CWA with a cold front on Wednesday. With an
upper-level jet streak glancing the CWA, increasing deep-layer wind
shear of 25 to 35 kt and seasonably high instability, there is a
risk of a few strong to severe thunderstorms Tuesday night and
Wednesday. Confidence is lower in the MCS Tuesday night being severe
since its presence still appears conditional (e.g., dependent on its
upstream development and subsequent track, timing) and it may be
arriving as low levels are stabilizing. The risk is less conditional
on Wednesday, but there are still plenty of factors to consider
including the impact of the potential morning MCS and timing of the
cold front/trough, likely determining the most favorable locations
and time. If thunderstorms become severe Tuesday night or Wednesday,
the main hazards will be damaging winds and large hail, but a
tornado or two cannot be ruled out. With PW approaching 2" there
will also be a conditional threat of locally heavy rainfall if any
thunderstorm training or backbuilding can occur on the flank of an
MCS. Showers and thunderstorms could linger into Thursday until the
front clears the CWA.

In this pattern Tuesday and Wednesday, low-level southwesterly WAA
will attempt to further warm temperatures with the NBM indicating
some high temperatures reaching the low-90s F near the Missouri
River (90+ F probabilities 50 to 80 percent), aided by downsloping
off the Ozark Plateau. With higher chances of showers and
thunderstorms on Wednesday, confidence is lower in high temperatures
that day and much of the NBM distribution is slightly cooler than
Tuesday. The climatological implication of these temperatures is
that they could threaten to end the going long streak of sub-90 F
temperatures at KCOU and KSTL.

Late Thursday through the end of the week, upper-level flow will
become northwesterly as an upstream ridge migrates eastward across
the central CONUS. More limited opportunities for showers and
thunderstorms are expected with 20 percent or less of ensemble model
guidance having any measurable rain in the CWA. Instead, the main
impact of this pattern will be another period of low-level
southwesterly WAA that introduces another warming trend into next
weekend with even higher NBM probabilities of 90+ F high
temperatures (60 to 90 percent by Saturday) across most of the
CWA.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 552 AM CDT Sun Jun 15 2025

Fog, especially in river valleys, and sporadic stratus will
dissipate/lift shortly after sunrise this morning with MVFR to IFR
flight conditions improving to VFR. More widespread MVFR/IFR
ceilings in southwestern IL will likely remain to the east of all
terminals. Otherwise, dry and VFR flight conditions will persist
through tonight with any showers and thunderstorms expected to
remain to the south of all terminals and across southeastern
MO/southwestern IL through Monday morning.

Pfahler

&&

.CLIMATE...
Issued at 320 AM CDT Sun Jun 15 2025

St Louis still hasn`t reached 90 yet in 2025. This puts 2025 in
the Top 20 all time latest dates of the first 90 degree
temperature, and the latest in at least 30 years (Jun 19, 1995).
The all time latest first 90 degrees was July 4 in both 1961 and
1912. Records began in St Louis in 1874.

Columbia has also not yet seen a 90 degree reading in 2025. The
latest date of first 90 degrees on record was July 14, 1904.
Records began in Columbia in 1890.

Quincy reached 90 degrees on May 15 of this year.

Based on the current forecast, our next best chance to see 90
degrees is Tuesday.

Kimble/Pfahler


&&

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

$$

WFO LSX