Area Forecast Discussion

386
FXUS63 KLSX 262324
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
624 PM CDT Fri Jul 26 2024

.KEY MESSAGES...

- Showers and thunderstorms become more numerous Saturday into
  Sunday. Just about everyone will see at least some rain at some
  point.

- Hot weather returns next week with heat index values over 100
  expected most of the week. Occasional rounds of thunderstorms
  may bring local heat relief.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 241 PM CDT Fri Jul 26 2024

The surface front is located just to our south this afternoon across
far southern Missouri although it has become increasingly less
defined. Slightly cooler air behind the front has led to more
pleasant temperatures today, although the drier air has been lagging
behind. Regional satellite imagery today has shown high level clouds
streaming southwest to northeast from Texas to Indiana while just to
our north the high level blow off from earlier thunderstorms in
southern Iowa was tracking northeast to southwest. This is
indicative of the overall upper level flow regime which implies a
weak trough axis over our area. More effectively, though, it`s a
weakness between two ridges, one to the southeast near the Florida
Panhandle and the other nudging into Minnesota. The result is little
to no mid and upper level forcing across our area today. Showers and
thunderstorms have already begun to develop in the unstable air mass
near the frontal zone. With little to no flow these will take the
form of pulse thunderstorms with concentric outflows potentially
driving new development into the evening when the activity should
cease. The severe weather risk is very low, with lightning and local
downpours the primary threats.

The ridge advancing eastward across the northern US fully cuts off
the upper trough in Texas tonight. As the ridge moves east on
Saturday it will allow the trough to escape northward behind it.
This is going to bring our best rain chances this weekend. This
trough has been hanging out in coastal Texas for several days, so it
will bring some tropical moisture with it. PWATs in parts of the
area rise to near or over 2 inches, a sign that when it rains it
will pour. Much of the rain will be generated by the synoptic
forcing from the upper low, but convective elements will take full
advantage of the depth of moisture. Although storm training doesn`t
look like a strong factor, just these heavy rainfall rates alone
could lead to some minor flooding issues especially if they fall in
vulnerable locations. For Saturday, the best synoptic forcing moves
through eastern Missouri, but due to extensive cloud cover overall
instability is limited. Further west some breaks in the clouds are
more likely over central Missouri where afternoon pop up
thunderstorms are a bit more likely. Overall moisture content (PWAT)
is lower to the west, though, so the best moisture and best
convective potential are not aligned well enough for us to consider
a Flash Flood Watch at this point. If flash flooding occurs it is
likely to be much more localized than what occurred last week.

Kimble

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 241 PM CDT Fri Jul 26 2024

The upper low moves out of the area on Sunday, with the rain exiting
to the east as well. While POPs for any one period of the forecast
are still somewhat modest due to timing uncertainties, when
integrated over the entire weekend it is clear that just about
everyone will see rain at some point. Ensemble probability of
measurable rainfall over the combined Saturday to Sunday period is
above 90 percent giving us the strong confidence that everyone will
get wet at some point. The heavy rain threat will remain much more
localized.

As the trough moves east, ridging over the western US expands east
behind it opening us up to some hot weather. One more trough
tracking east around the top of the ridge brings another chance of
thunderstorms on Monday, but this should be more progressive as we
head towards a hotter weather pattern. Daytime highs in the mid 90s
are expected most days this week, with some daily variation.
Humidity remains moderate to high, leading to heat index values
routinely rising to 100 or higher. The greatest uncertainty is with
rain chances as being on the eastern side of the ridge we`ll be in
the line of fire for frequent "ridge runners," or convective
complexes that generally form and track around the periphery of a
ridge. While these would carry the potential for damaging winds when
they occur, they also would bring heat relief especially if
extensive clouds impact daytime heating. Confidence on which day
this might occur is low, but it seems likely we`ll be affected at
some point. On days where storms stay away we could be even hotter,
with heat index values exceeding 105.

The trend later in the week is for the ridge to retrograde to the
west pulling the core of the heat westward with it. So there does
seem to be an end point to this heat wave centered around the end of
the coming week.

Kimble

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 623 PM CDT Fri Jul 26 2024

With most isolated showers dissipating through sunset aside from
southeastern MO, dry and VFR flight conditions are expected to
persist into Saturday. The only exception is another night of
potential fog development in river valleys, but conditions should be
less favorable than previous nights with fog not included in the
current TAF. On Saturday, MVFR flight conditions are anticipated at
St. Louis metro terminals, KCOU, and KJEF with lower cumulus and
stratocumulus developing in the afternoon and scattered to numerous
showers and possibly a thunderstorm during afternoon and evening.
Precip and lower ceilings will most likely remain south of KUIN.

Pfahler

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion