853
FXUS63 KLSX 052347
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
647 PM CDT Sun May 5 2024

.KEY MESSAGES...

- A few more opportunities for thunderstorms and severe weather
  are coming up. The two main threat periods locally are late
  Monday night and Wednesday.

- A stretch of cooler and drier weather is expected later this
  week.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 205 PM CDT Sun May 5 2024

High pressure over the Great Lakes has brought briefly cooler and
drier weather into our area today. While larger scale ridging is
moving through the Northern Plains, a southern stream shortwave
trough is exiting slowly out of the Southern Plains and toward the
lower Mississippi Valley today. Showers and thunderstorms have
been widespread across Oklahoma and Arkansas, moving into
southwest Missouri. Thunderstorms are generally on the leading
edge of the broader precipitation area as the low level moisture
rotating into the system undercuts some drier air (steeper lapse
rates) aloft, but overall instability remains weak so strong or
severe storms are not expected. We do expect at least some of this
activity to cross through the southern portion of our forecast
area this evening and into the overnight, although it will be
weakening as it very slowly moves through. It`s not clear it will
make it all the way to St Louis for example. But at least some
remnant clouds will. These clouds and any remnant showers will
finally dry up and disperse Monday morning. Expect a significantly
warmer day on Monday especially if more sun develops.

A trough moves through the northern Rockies on Monday, nudging the
ridge eastward toward the Great Lakes. As it does so, a renewed
surge of moisture will make its way north through the Great Plains
on Monday just as the trough begins to move out as well. This will
set up a renewed round of severe thunderstorms over the central
and southern Plains on Monday with the focus for this activity
remaining to our west during the day. It is expected that the
storms that form in the Plains will develop into a broader
convective line and march east during the overnight hours Monday
night into early Tuesday morning. This is when we`ll see the
impacts locally, mainly after midnight Monday night through about
dawn on Tuesday. Given the expected evolution, the primary severe
weather threat will be damaging winds. However, strong low level
shear does exist during this time period so there will be at least
some threat for embedded tornadoes in portions of the line which
can become oriented perpendicular to the low level shear vectors.
The overall strength of the convective complex is expected to be
weakening as it tracks out of western Missouri and through our
area, so we`re not sure just how strong or widespread the severe
weather threats in our area are. This is reflected in the SPC
outlook for Monday which decreases the threat level with eastward
extent.

Although overall rain amounts Monday night are not expected to be
particularly excessive, river levels remain high across the region
and the additional rainfall will lead to renewed rises. Additional
river flooding may result.

Kimble

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 205 PM CDT Sun May 5 2024

Monday night`s thunderstorm complex will move east Tuesday morning
and lay down an effective cold front across southeast Missouri.
Some models indicate renewed convection may form along this front
Tuesday afternoon, but this will most likely be outside of our
forecast area, focused more in the Ohio River Valley. Winds aloft
remain quite strong out of the WSW so storms that form along and
ahead of the front will be able to take advantage of strong shear
to become organized and pose a threat for mainly large hail and
damaging winds. But again, this threat will likely be southeast of
our forecast area. Subsidence in the wake of the departed trough
should lead to dry conditions after the morning rain. Temperatures
are forecast to be even warmer, mostly in the 80s, but it could be
cooler if clouds linger through the day.

The next trough organizes over the northern Rockies on Tuesday and
moves east on Wednesday. This will result in another surge of
moisture northward ahead of it and another round of potential
severe thunderstorms. The threat area for this round looks more
centered on our forecast area on Wednesday as the timing of the
trough aligns well with peak heating. What`s unclear at this point
is where the remnant frontal boundary will be thus setting the
bounds of the warm sector and primary severe threat area. Most
likely the warm front will shift north into at least northern
Missouri putting the bulk of our area squarely within the warm
sector where strong instability could develop in an environment
with strong shear as well. Thus thunderstorms that form during the
afternoon Wednesday would likely take the form of supercells with
large hail, damaging winds, and tornadoes all possible. Some
models indicate that more widespread convection will develop late
Tuesday night into Wednesday morning as the moisture pushes back
to the north. This represents another source of uncertainty for
the severe weather threat Wednesday as morning convection (with a
hail threat) could disturb the destabilization in the warm sector
and reduce the threat later in the afternoon.

As the trough moves east, a cold front will push south behind it,
ending the thunderstorm threat Wednesday night. Another secondary
front drops down from the north on Thursday reinforcing a drier
and cooler air mass for the rest of the week. Models begin to
diverge more in the handling of the overall upper air pattern this
weekend. The primary differences revolve around another trough
dropping south through the western Great Lakes (how strong will it
be?) and the remaining piece of the western trough over the
southern Rockies. The result is more uncertainty in the
temperature forecast this weekend as a deeper trough would result
in cooler temperatures while a weaker or more progressive one
would open us back up to ridging and warmer temperatures. Either
way, though, Gulf of Mexico moisture remains suppressed either to
the Gulf Coast or even further south to the Yucatan. So we expect
we will have a relatively prolonged break from the severe weather
episodes we have been seeing lately.

Kimble

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 637 PM CDT Sun May 5 2024

Light showers will affect COU and JEF with unrestricted
visibilities, and will affect the St. Louis terminals between
03-09Z. MVFR/possible IFR ceilings are expected to move into
JEF/COU/STL/SUS/CPS tonight and move out between 22-00Z on Monday
afternoon. MVFR ceilings will move into UIN on Monday morning and
move out late Monday. Winds will remain light through the period.

Britt

&&

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

$$

WFO LSX