Area Forecast Discussion

800
FXUS63 KLSX 281938
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
238 PM CDT Sat Jun 28 2025

.KEY MESSAGES...

- Seasonably hot and humid conditions continue with periods of
  showers and thunderstorms through Monday.

- A cold front on Monday brings a shift toward slightly cooler and
  much less humid conditions for midweek.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 237 PM CDT Sat Jun 28 2025

Summer-like heat and humidity continues as we remain along the
northern periphery of a broad, flat-topped ridge. Yesterday`s front
has stalled and gotten a bit washed out in the low levels as the
parent shortwave trough that drove it is now well east in New
England. With the heat and humidity we`re seeing plenty of
instability developing with convection mainly focused in a few
clusters from southern Missouri eastward into KY and TN. The primary
cluster affecting us today is associated with a remnant MCV just now
entering southwestern Missouri. Most of the convection has been
ahead of the MCV where subtle low level warm/moist advection toward
the remnant front has encouraged more lift. Plenty of sun this
morning also has led to strong instability this afternoon leading to
broader convective coverage than what some high resolution guidance
had been suggesting. We expect this activity to move VERY slowly to
the east this afternoon as the overall wind flow is quite weak.
Convection may propagate a little faster once larger outflow gets
generated with the potential for new convection on the outflow
mainly oriented to the east. Eventually, though, we`ll lose access
to daytime heating and instability will wane this evening, leading
to a decrease in convection as it inches to the east.

While convection is expected to dissipate this evening, the MCV core
itself will gradually move east as well. This is expected to
generate new thunderstorm activity during the overnight hours over
southwest Missouri moving into southeast Missouri Sunday morning. It
will still be moving pretty slowly and we expect a gradual decrease
in convection as the morning moves on. Lingering clouds may hold
temperatures back a few degrees, but the humidity remains with us
keeping those heat index values in the mid to upper 90s.

Late Sunday afternoon into early Sunday evening, convection is
expected to fire to our north along or just ahead of a developing
cold front from Nebraska to Iowa. Guidance varies considerably on
when and where along this boundary convection will develop, but the
general consensus on a compact MCS developing and diving southward
through the overnight is fairly well agreed upon. If this forms
further west into Nebraska, then the area impacted will be across
western Missouri. If it forms further east into central Iowa, then
it stands a much better chance of dropping south through parts of
northeast and central Missouri. This MCS does look capable of
producing damaging winds across the region, but this threat
decreases as storms head southward from their origin and as the
available instability decreases overnight. The new Day2 outlook from
SPC highlights this threat well. While a small portion of our area
is included in this outlook, we`re holding off on messaging this
more broadly just yet due to the uncertainty on the east/west
location of where this MCS will track and whether it will maintain
its severe weather potency as it moves south. Either way, though,
the chances of rain associated with the nearby MCS increase
overnight for most of the area.

Kimble

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 237 PM CDT Sat Jun 28 2025

Sunday night`s MCS will likely still have remnant showers associated
with it Monday morning, but the next potential for thunderstorms
will be associated with the cold front itself which will be dropping
south through the region during the day on Monday. Considerable
uncertainty exists on if this boundary will trigger additional
convection Monday afternoon, with a lot of that uncertainty hinging
on how much cloud cover from the morning MCS hinders destabilization
ahead the front. If storms do form, though, they`ll have a little
better shear to work with and stand a better chance of becoming
severe. The Day3 Outlook now includes most of our area in a Marginal
Risk due to this uncertain threat.

Zooming out we see that this front is being driven by a much deeper
trough dropping southeast out of the Pacific Northwest and tracking
through the center of the country. This is what is enabling a full
pattern change and a change in air mass for our region. While
humidity lingers ahead of the front Monday, with rain chances
continuing until the trough axis has fully passed through the area,
the effects of the front`s passage will be fully noticeable by
Tuesday. Dewpoints fall a good 10+ degrees back into the low to mid
60s. Plenty of sunshine will enable strong surface heating, so
afternoon high temperatures will remain somewhat similar to what we
saw the preceding few days, in the 80s, but it will feel much
better. Low temperatures also drop into the 60s for the first time
in a while. This air mass lasts through about Wednesday when the
surface high pushes off to the east enabling a return southerly flow
across the region.

Ridging over the western US nudges eastward later in the week with a
gradual warm up expected. Southerly flow also increases humidity
across the area, with dewpoints rising back to 70 or higher by
Friday, July 4. This combined with temperatures climbing back into
the 90s will bring the heat index values back into the upper 90s to
near 100 degrees for Independence Day. Although rain chances do
start to creep back into the forecast later in the week with the
arrival of the more humid, unstable air mass, these chances have
come down a bit from previous forecasts as it is now more likely
that we stay away from the influence of an eastern US trough and its
associated frontal waves. Better rain chances may actually hold off
until this weekend when a more pronounced trough rounds the top of
the ridge sending a front toward us from the northwest.

Kimble

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1218 PM CDT Sat Jun 28 2025

Mostly VFR conditions with light winds are expected today and
tonight outside of thunderstorms. The primary threat area for
thunderstorms this afternoon is across southern Missouri. This has
the best chance of reaching toward JEF in the next few hours,
though there`s an outside chance of a storm reaching up into the
St Louis metro as well. This activity is expected to die down
around sunset. Additional storms may develop overnight associated
with a weak convective vortex drifting across southern Missouri.
There`s a bit more confidence in this round of storms owing to the
more clear forcing associated with it, however there remains
uncertainty on the location and timing of its impact. These have a
slightly better chance of reaching into the St Louis metro Sunday
morning and this has been included via the PROB30 designation for
now.

Kimble

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion