810
FXUS63 KLSX 181753
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1253 PM CDT Mon Aug 18 2025

.KEY MESSAGES...

- Hot and humid conditions continue today. Heat relief begins
  tomorrow, with all areas seeing relief by Wednesday.

- Isolated thunderstorms are expected this afternoon, but the more
  widespread rain chances are on Tuesday.


&&

.SHORT TERM... (Through Tuesday night)
Issued at 209 AM CDT Mon Aug 18 2025

The upper level ridge which has brought our current spate of hot
weather continues to weaken as it remains over Missouri today.
Expect similar conditions today as were seen yesterday, except
perhaps a degree or so cooler. Even so, with similar dewpoints in
the low to mid 70s, expect heat index values rising above 100
degrees again area wide, with many areas seeing 105 degrees again.
We did give some thought to whether to upgrade to an Extreme Heat
Warning for metro St Louis as we have seen sporadic heat index
values crack the 110 degree mark the past two days. This looks a bit
less likely today. However, today will likely be day 3 of widespread
heat index values over 105, just one day short of criteria for a
long duration warning. With continued uncertainty on the heat
tomorrow we just do not have the confidence we`ll meet the criteria
for an Excessive Heat Warning. This being a late season heat wave
after a full summer getting accustomed to heat also played a role in
the decision.

Just as we`ve seen the past few days, we expect isolated pulse
thunderstorms this afternoon, mainly from 2-8PM. The heat and
humidity is generating a lot of instability in an environment with
nearly zero shear, so once again look for locally heavy downpours
and the potential for gusty winds with the stronger storms. However,
they won`t be moving much and individually will not last very long.
However, they may provide some late day heat relief to some areas.

A weak shortwave trough moving east into the Great Lakes will set up
a cold front across Iowa this evening. This will serve as a more
obvious trigger for thunderstorms late this afternoon which are
expected to develop into a broader mesoscale convective system.
While the primary steering flow for these storms is west to east,
additional development is likely as outflow heads south into
northern Missouri and central Illinois overnight. The better threat
for damaging winds from this MCS will remain to our north, reflected
well in the Day1 SPC outlook, but our area may be peripherally
affected by non-severe storms drifting southward overnight. This
activity likely dissipates during the early to mid morning hours on
Tuesday, however it will play a role in the forecast for our final
day of heat.

While the cold front itself lags behind, the outflow from Tuesday
morning`s storms will effectively bring at least some minor heat
relief to northern portions of our forecast area. More extensive
cloud cover takes the edge off the heat despite continued humid
conditions. South of the outflow, though, little if any heat relief
is expected, and another day of 105 degree heat index values is a
real possibility. Unfortunately it`s not clear how far south that
outflow will push and thus how far north the high heat will be. For
that reason, we`ve maintained the current Heat Advisory through this
evening. However, it is likely that it will need to be extended for
a portion of our area into Tuesday. Tuesday`s outflow boundary will
serve as a trigger for more widespread thunderstorms Tuesday
afternoon with the surface cold front making progress south through
our area Tuesday night, finally ending this round of heat.

Kimble

&&

.LONG TERM... (Wednesday through Sunday)
Issued at 209 AM CDT Mon Aug 18 2025

The upper ridge reorganizes and strengthens over the Rockies and
Four Corners region Tuesday into Wednesday with the downstream
troughing over the Great Lakes and Northeast helping to push a
cooler air mass southward. We may still see the cold front in the
southern portion of our forecast area on Wednesday, with
temperatures along and ahead of the front still reaching near 90
degrees with afternoon thunderstorms developing along it. But
further north that cold frontal passage will become more noticeable
as cooler air mixes into the region from the north. Temperatures
only reach the mid 80s on Wednesday north of the front, with
dewpoints dropping into the 60s in the afternoon. This air mass will
be with us for a few days as surface high pressure builds south into
Missouri and Illinois by Friday. These temperatures are near or just
a bit below normal for this time of year.

Guidance continues to show strong agreement on a strong trough
developing into a deep upper low tracking east through the Canadian
prairies on Friday. As this heads east, it will send another cold
front southward through the Midwest, arriving in our area Friday
night or Saturday. With no real moisture return ahead of the front,
any rain with it looks spotty. The air mass behind this front is
much cooler, and although the core of the cool air will be pushing
through the Great Lakes, we`ll get a piece of it too! There`s still
uncertainty on just how much of the cool air will reach us, but the
trend has been toward cooler temperatures. Among the long range
ensemble guidance, the lower quartile (25th percentile) of 850MB
temperatures this weekend is at about 10C which would translate to
70s for highs in full mixing. Dewpoints falling through the 50s will
also help it feel nice and allow for some cool nights as well.

Kimble

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1247 PM CDT Mon Aug 18 2025

While most of the TAFs for the local terminals show dry and VFR
flight conditions through the period, there will be a low chance
for thunderstorms at times at all local terminals. The first
chance is this afternoon and evening when airmass thunderstorms
are forecast across the region. Given these storms do not have a
focused forcing mechanism, where and when they form relative to
the TAF sites is highly uncertain. Any storms forming on or near
local terminals will be capable of heavy rainfall and gusty
outflow winds. The next chance will be tonight as outflow from a
thunderstorm complex north of the region moves southward. The
greatest chance for impacts is at KUIN, though there is a low
chance that the outflow and associated impacts can make it further
south to the other local terminals. Depending on what happens
tonight, an additional rounds of storms is forecast for portions
of the area tomorrow afternoon, though they might form well south
of the local terminals.

Elmore

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT this evening for Audrain MO-Boone
     MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
     Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
     MO-Ralls MO-Reynolds MO-Saint Charles MO-Saint Francois MO-
     Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-
     Shelby MO-Warren MO-Washington MO.

IL...Heat Advisory until 8 PM CDT this evening for Adams IL-Bond IL-
     Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey
     IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Pike IL-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX