880
FXUS63 KLSX 200730
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
230 AM CDT Wed Aug 20 2025

.KEY MESSAGES...

- Mild temperatures are expected the next few days, but a cold
  front this weekend will bring the potential for the coolest air
  we`ve seen in months.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 229 AM CDT Wed Aug 20 2025

Our heat relieving cold front has finally pushed fully south of our
forecast area, and northerly winds are bringing some slightly cooler
and slightly less humid air into the region. As temperatures cool
aloft we`ll actually set up some low level instability between the
still warm and humid surface and the incoming cold air above it.
This should lead to a fair amount of afternoon cumulus and even the
potential for some showers mainly in southern Iowa and northern
Missouri where that low level cold advection is strongest. Most
model soundings show at least some modest capping at about 600MB or
so, but as the day goes on cold advection may even erode this cap
and open up deeper instability for a few thunderstorms. Wind shear
remains pretty weak, so we`re not looking at a significant severe
weather threat with this, but it will be one final chance at rain
for a while. About 30 to 50 percent of 00Z HREF members produce
measurable precipitation across the northern portion of our forecast
area, while the lower resolution LREF guidance is even higher
especially over western Illinois. This all supports the idea of
adding at least some minor PoP up there this afternoon.

Any afternoon showers dissipate quickly with the loss of heating
this evening leading to a mostly clear night tonight. Low level
northeast flow continues and dewpoints finally begin to drop into
the 60s setting up an even cooler and drier day on Thursday. The
surface high will be nudging closer to the area and cool/dry
advection continues leading to highs only in the low to mid 80s
despite plenty of sun.

Kimble

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 229 AM CDT Wed Aug 20 2025

A potent trough moving through southern Canada spreads its influence
across the northern tier of the US late this week into this weekend.
So just as our surface high finally settles in and temperatures
begin to rebound, another cold front takes aim at us. This one
arrives in our area Saturday morning and pushes southward through
the day. With a blocking high ahead of it, there won`t be much
moisture to work with which is why NBM PoP remains mostly below 20
percent right now. However, I wouldn`t be surprised to see that tick
upward in the coming days as the timing comes into better focus.
Approximately 30 to 50 percent of long range ensemble members
produce measurable rain across northern MO and IL sometime Friday
night into Saturday. Even if this occurs, it will be fairly light
and inconsequential as the forcing aloft is weak and there is a
distinct lack of a robust moisture connection.

Temperatures behind the front continue to look mighty pleasant for
this time of year. NBM temperatures keep ticking downward with each
new forecast, now with widespread 70s for highs early next week.
High temperatures may even be a bit cooler especially if some cloud
cover comes into play, but for now even full sun and deep mixing
only yields temperatures in the mid to upper 70s Monday into Tuesday
as the core of the surface high pushes into our area from the
northwest. Another measure of the potential of this air mass is the
dewpoint. Again, NBM trends over the past several cycles have been
to pull dewpoints down even further. Now the probabilistic NBM
indicates 50 to 80 percent chance of dewpoints falling into the 40s
by Tuesday morning. That suggests the potential for some very cool
mornings especially if we can get a clear, calm night or two. That
potential looks greatest Tuesday night into Wednesday morning. For
reference, St Louis has not dropped into the 50s since June 10 (59).
Columbia and Quincy both dropped into the 50s in the first few days
of this month, but even there we have the potential to drop even
further to levels not seen since May.

Kimble

&&

.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 622 PM CDT Tue Aug 19 2025

Dry and VFR flight conditions will continue through the end of the
period under northerly winds that will diminish in speed
overnight. Winds will pick up again tomorrow but remain from the
north, with speeds roughly around 10 kts.

Delia

&&

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

$$

WFO LSX