945
FXUS63 KLSX 231049
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
549 AM CDT Sun Jun 23 2024

.UPDATE...
Issued at 546 AM CDT Sun Jun 23 2024

The last of the pre-frontal showers and storms have exited the
forecast area to the southeast. We`ve removed mention of these
from the forecast.

Kimble

&&

.KEY MESSAGES...

- A cold front this morning offers some modest heat relief today, but
  the heat returns in force, with humidity accompanying it,
  Monday into Tuesday. Heat Index values of 105 are expected at
  times, especially on Tuesday.

- The next cold front Tuesday night provides the next opportunity
  for rain and thunderstorms as well as another brief break from
  the heat. But the heat and humidity look to return for the
  weekend.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 247 AM CDT Sun Jun 23 2024

A cold front is moving through the region this morning, accompanied
by some scattered showers and thunderstorms. These thunderstorms
will continue to weaken and dissipate through the morning as the
better forcing with the trough pushes off to the east and low level
convergence along the front wanes. We will see a slightly less hot
air mass moving in behind the front today as well as some drier air.
This will give us a day`s break from the more extreme heat, with
mostly upper 80s to low 90s expected. That`s right around normal for
this time of year.

The center of surface high pressure passes through the region
tonight allowing the air conditioning units to get a bit of a break
with lows falling into the 60s outside of the strongest urban
influences. But as the high moves east on Monday it will open us
back up to the return of the heat, this time accompanied by
humidity. Ridging builds back in from the west as low level flow
turns southerly. The core of the heat and humidity on Monday is
focused to our west across the Plains and into western Missouri.
But central Missouri will likely get in on this heat by the
afternoon and heat index values will approach or exceed 105
degrees for a time. In coordination with neighboring offices to
the west, we have issued a Heat Advisory for the areas most likely
to see these heat index values on Monday. Further to the east, it
will still be hot on Monday, but more normally so. East of the
Mississippi River in Illinois it mainly tops out near 90 with much
lower humidity.

Kimble

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 247 AM CDT Sun Jun 23 2024

After a balmy night Monday night, the heat and humidity spread
across the entire forecast area on Tuesday. There`s still some
uncertainty on the temperature forecast as NBM continues to run at
the top end of the guidance spread and the trend the last several
days has been for temperatures to run a little cooler than
forecast. That said, it`s still going to be hot, whether we get
to 101 or merely 98. And with dewpoints in the 70s, the humidity
is going to be palpable. The entire area is likely to see heat
index values above 100, with many areas reaching up to 105 or so.
It is likely that another more widespread Heat Advisory will be
needed for Tuesday.

The next cold front will be driven through the area by a shortwave
trough pushing east through the Great Lakes along the southern
periphery of a larger scale trough in Hudson Bay. In the hot and
humid air ahead of the front, thunderstorms are expected to form.
This threat is greatest to our north on Tuesday. While there remains
a slight chance (20%) of thunderstorms during the day on Tuesday for
most of our area, models have trended drier which makes a lot more
sense considering the emerging consensus that the front and better
forcing don`t move in until Tuesday night. It`s this time frame,
Tuesday night into early Wednesday, that we`ll see our best rain
chances. While we can`t rule out severe thunderstorms with the very
unstable air ahead of the front, the better focus for that threat is
more to our north and east closer to the shortwave trough, better
forcing, and stronger shear. The main story here is the chance for
rain which has been harder to come by locally in recent weeks. Even
that isn`t a slam dunk, though, as there are some indications that
the shortwave comes through in two parts: the stronger wave to the
north and a weaker one to the southwest. That may put us in between
the two better-focused areas with locally less widespread
thunderstorms Tuesday night.

The front does provide at least modest heat relief as cool high
pressure builds into the Upper Midwest Wednesday. The core of the
push of modified Arctic air remains to the north, though, closer
to and north of the surface high. Our temperatures are more likely
to end up near normal, upper 80s to low 90s on Wednesday and
Thursday. The cooler side of that range is more likely if we hang
on to clouds and showers a bit longer on Wednesday. Dewpoints,
though, will fall back significantly in the drier air. Even NBM is
forecasting some 50s dewpoints which for this time of year is
notable.

Ridging builds back in from the west rather quickly later this week.
As surface high pressure moves east we`ll see southerly return flow
and heat back up again for the weekend. By Friday we`ll be solidly
in the hot air again, with moisture return bringing dewpoints back
to near 70. The ridge isn`t as strong this time, though, so the heat
may be a little less intense than our last few rounds. Troughing
over the northern Rockies and into the northern plains actually
keeps the storm track fairly close to us, with waves on Friday and
especially Saturday bringing thunderstorm chances. As the main
trough axis passes by around Saturday we see another cold front
moving in, knocking back the heat again. NBM interquartile range on
max temperatures Friday to Sunday is rather large for this time of
year, in the 5 to 10 degree range. This is a reflection of the
uncertainty on timing and positioning of these waves and the
resulting potential impacts on daytime temperatures. The
deterministic NBM, and our going forecast, represents the upper end
of the spectrum with highs getting back into the mid to upper 90s.

Kimble

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 546 AM CDT Sun Jun 23 2024

Winds out of the northwest are expected today. With this drying
flow the trend will be toward a clearing sky and VFR conditions. A
brief period of MVFR ceiling is possible near St Louis early this
morning but if it occurs it will not last very long with the
clearing trend continuing through the morning. Winds go light or
calm this evening under a clear sky.

Kimble

&&

.CLIMATE...
Issued at 247 AM CDT Sun Jun 23 2024

With hot temperatures forecast, here are some of the daily high
temperature records:

            MON JUN 24     TUE JUN 25
ST LOUIS    102 in 1988    102 in 1954
COLUMBIA    103 in 1988    102 in 1988
QUINCY       99 in 1988    102 in 1931


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon to 8 PM CDT Monday for Boone MO-Cole MO-
     Moniteau MO.

IL...None.
&&

$$

WFO LSX