307
FXUS63 KLSX 171929
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
229 PM CDT Tue Jun 17 2025

.KEY MESSAGES...

-There is a conditional chance for severe thunderstorms and damaging
winds overnight tonight into early Wednesday morning.

-There is another conditional threat for severe thunderstorms
Wednesday afternoon and evening across a portion of the forecast
area, though it depends on how the overnight convection plays out.

-Confidence is high that our first long duration heat event will
 start Friday and last into next week as high temperatures climb
 well into the 90s.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 227 PM CDT Tue Jun 17 2025

Scattered showers and thunderstorms have formed across north-central
Missouri in response to an approaching mid-level shortwave and 2500-
3000 J/kg of SBCAPE across the area. These storms will expand in
coverage and move east into southern Iowa, northeast Missouri, and
west-central Illinois with the shortwave this afternoon and evening.
What remains uncertain is how far south these showers and
thunderstorms extend into our forecast area. Some guidance sources
show showers and thunderstorms largely north of I-72 in west-central
Illinois and areas west, while other sources show convection
extending as far south as the St. Louis metro. What I am confident
of is that if any thunderstorms form in our area, they will only
have access to 15-25 kts of 0-6 km shear, which will limit their
strength and duration. Severe thunderstorms are not expected this
afternoon.

An MCS has formed over Kansas this afternoon and will track east
into southwest Missouri overnight with another mid-level shortwave.
This complex is expected to move through an area characterized by
1500-2000 J/kg of MUCAPE, rather than riding along the MUCAPE
gradient, which would keep it tethered more to the mid-level
forcing. Hi-res guidance has come into better consensus over the
location and somewhat of the timing of this system, and the MCS is
expected to swing from southwest Missouri/northern Arkansas into
southeast Missouri and southern Illinois overnight, exiting during
the early morning. Whatever happens to the MCS tonight and tomorrow
morning will dictate our severe weather chances Wednesday afternoon
and evening along a cold front and surface trough that is dropping
south through the region. The two extremes are as follows:


1. MCS remains robust as it enters the CWA (best case scenario):

In this scenario the MCS remains strong, with some severe components
as it enters the western forecast area. Damaging winds are the main
threat with this line of storms as it moves through.  This scenario
would result in a stronger outflow boundary pushing through the
forecast area tomorrow morning. Given the expected trajectory of the
MCS, the outflow would stabilize the entire forecast area, limiting
our severe weather potential to nearly nothing. HREF ensemble
guidance highlights this possibility as the ensemble minimum SBCAPE
values are less than 100 J/kg areawide.

2. MCS weakens before entering CWA (worst case scenario):

In this scenario the MCS structure and convection weakens
considerably before entering the CWA, though convection may still be
ongoing early Wednesday morning. This would result in a weaker
outflow boundary and stratus shield impacting the forecast area,
allowing destabilization Wednesday afternoon and a more robust and
widespread severe weather event. Between 1000-2500 J/kg is
reasonable to develop across the area in this scenario, which would
be enough for severe thunderstorm development. The best chances for
severe thunderstorms would be across southeast Missouri and southern
Illinois where destabilization is most likely.

If scenario 2 were to occur, thunderstorms would generate along the
southward moving surface trough ahead of the cold front, organizing
in the 35-45 kts of 0-6km shear. They would initially be isolated,
and 7-7.5 degree lapse rates, a reasonable amount of CAPE in the
hail growth zone, as well as inverted V soundings all point to
damaging wind and hail the main concerns. There is a lesser (but non-
zero) threat for tornadoes at this point, but limited helicity (~100
m2/s2) and high LCLs will limit the potential. The 0-6km shear is
nearly parallel to the boundary, so over time these storms are
expected to develop into linear segments and the threats will
transition to damaging wind and tornado threat.

The first wave of convection will exit the forecast area during the
early evening which additional showers and thunderstorms will fire
up along the cold front that will follow quickly behind. If any
instability lingers across the area, I cannot rule out a severe
thunderstorm or two developing along the front, but confidence in
this occurring is much lower than the severe threat along the
surface trough should scenario 2 occur.

There is also the potential for a solution midway between the two
extremes described. In this situation, the MCS would roll through
and stunt instability, but not completely wipe it out. This would
result in higher moisture near the surface and some instability
present. Coupled with the forcing along the front and strong
dynamics, the tornado threat would be highest in this solution,
while wind and hail would have lower threats.

Delia

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 227 PM CDT Tue Jun 17 2025

The mid-level trough responsible for Wednesday`s cold front and
associated convection will push through the mid-Mississippi Valley
on Thursday, while in the lower-levels southwesterly flow will
already be established. Warm air advection along this flow will
start the warming process, offsetting the cooler air we might have
had from the passage of the cold front. Behind this trough, a mid-
level ridge will barge into the region starting Friday, finally
settling over the Ohio River Valley Sunday, and leaving the region
in southwesterly flow. 850 mb temperatures will jump into the upper
teens to low 20s C, and with a surface high stationed over the
Southeast US, we are looking at a favorable synoptic set up for at
least a Heat Advisory event over the weekend. Temperatures are
forecast to peak in the 90s areawide, with areas around the St.
Louis metro expecting heat index values above 100 for multiple days.

This will be the first long duration heat we have experienced this
year, and actually the first 90 degree days for much of the forecast
area, including the St. Louis metro. The combination of the high
heat and humidity and warm overnight lows will be dangerous for
certain groups including those without adequate shelter or cooling
systems. The St. Louis metro area in particular will be at risk as
the Urban Heat Island will exasperate these conditions even further,
resulting in locally hotter high and overnight temperatures.

Delia

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1150 AM CDT Tue Jun 17 2025

Multiple rounds of showers and thunderstorms are expected over the
next 24-36 hours. The first is this afternoon mainly across
central and northeast Missouri and west-central Illinois.
Confidence is low on the development of showers and thunderstorms,
as well as where they will be, so have held off mention in the
TAFs for now.

The next chance will be tonight, as a complex of thunderstorms
moves west to east into the region, mainly impacting the central
Missouri (KCOU, KJEF) and west-central Illinois (KUIN) terminals.
Uncertainty remains in how widespread rain will be when it moves
into the area, but confidence is high enough to include a mention
in the TAFs. Ceilings are expected to drop to MVFR conditions with these
thunderstorms that move through.

The last chance will be during the afternoon Wednesday into
Wednesday evening when a cold front will drop through the region
bringing the last chance for showers and thunderstorms. Again,
there is uncertainty in how widespread these storms will be, and
that will depend on the behavior of thunderstorms earlier in the
day. Have used a PROB30 group to block this time in the KSTL TAF.
Ceilings are expected to drop to MVFR conditions with any
thunderstorms that develop.


Winds will prevail from the south to southwest today through much
of the period.

Delia

&&

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

$$

WFO LSX