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000
FXUS63 KLSX 251122
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
622 AM CDT Thu Apr 25 2024

.KEY MESSAGES...

- A multi-day severe weather period is just beginning across the
  central United States. While we are confident that severe
  weather will occur regionally every day from today through
  Sunday, when and whether this threat will exist in our forecast
  area is more uncertain. Most days we will be on the periphery of
  the greater threat area, with the greater threats to the west.

- Sunday evening represents the greatest severe thunderstorm
  threat for the entirety of our forecast area as a cold front
  shifts the threat area eastward.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 316 AM CDT Thu Apr 25 2024

While high pressure moves east through southern Canada today turning
our winds to the southeast, our eyes begin to look to the west as
Gulf of Mexico moisture begins to move north through the Plains.
This is all in response to a broader trough developing across the
southwestern US leading to troughing in the lee of the Rockies and a
persistent southerly flow east of it pulling in the low level
moisture. Thunderstorms today are expected to form along the leading
edge of this moisture across Kansas and into southwest Missouri
where a broader convective complex is likely to develop during the
afternoon and track southeast toward Arkansas. While this does
represent the opening round of a multi-day period of severe
thunderstorms, we have strong confidence that this threat remains to
our southwest today. Low level moisture, and thus the available
instability, does not make much progress eastward today, but we are
likely to see increased cloud cover blowing off from this activity
in a general WNW flow aloft. This will lead to slightly cooler
temperatures today especially in central Missouri where clouds will
be thickest. It`s also in central Missouri where we have the best
chance of seeing some rain along the periphery of the broader
convective complex. Although the stratiform rain region may clip our
area, we expect thunder to be pretty limited as instability
decreases rapidly to the east.

While the convective complex drops southeast into Arkansas this
evening, the first prominent shortwave trough moves across the
central Rockies and into the Plains tonight. In response to this, a
southwesterly low level jet will begin to push the moisture
northeastward into our area. The core of this moisture surge will be
at about 850MB where a warm front serves as a focus for elevated
showers and thunderstorms to form and push northeast through much of
our area. Guidance suggests this doesn`t begin until early Friday
morning and only exits our forecast area around midday. Given this
scenario the greatest threat from any thunderstorms would be hail
since they are being forced by elevated moisture advection. Forecast
soundings remain rather modest when it comes to the instability,
with around 500-700 MUCAPE to work with. While not a lot, this could
also be an underestimation by the courser resolution long range
guidance. Even so, the high end potential here should be marginally
severe hail.

As the shortwave trough continues to push off to the northeast and
the warm front does so as well, we`ll open ourselves up fully into
the warm sector on Friday. Dewpoints will rise into the 60s setting
a base for a moist, unstable air mass that lasts until Sunday. In
the wake of the Friday morning trough, we`re likely to see this air
mass capped by a temperature inversion aloft. This combined with the
lack of a clear focusing boundary to help initiate additional
thunderstorms will make it difficult to see another round of
thunderstorms forming in our area Friday afternoon. Further to the
west along the dryline there is a comparatively greater chance for
thunderstorm initiation Friday afternoon especially further north
closer to the front and surface low. The best chance for severe
weather in our area Friday evening will likely be a result of the
propagation of dryline storms eastward. Even that will be difficult
with storm motions likely more northerly than easterly. That said,
if we do get storms in our area they will be in an environment
favorable for supercells which would have a risk for large hail,
tornadoes, and damaging winds. Confidence in this occurring this far
east has decreased relative to prior forecasts and this is reflected
in the new Day2 SPC outlook which has shifted the slight risk area
westward.

Kimble

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 316 AM CDT Thu Apr 25 2024

Shortwave ridging in the wake of Friday`s trough will reinforce the
capping inversion across our area. While we will likely see our most
extreme levels of surface based CAPE developing as a result of the
continued moist air mass and strong heating (warmest day of the
forecast), it is less likely that we will be able to realize that
potential energy during the day on Saturday. Further west, though,
the next trough will be moving across the Rockies and into the
Plains setting up much more likely initiation of severe
thunderstorms along the dryline in Kansas and Oklahoma and
northeastward along the remnant front into Iowa. So once again the
focus area for severe thunderstorms will be to our west and
northwest on Saturday. That said, the instability and shear
environment will be quite strong on Saturday, so any storms which
are able to make their way into our area will have a good
environment to work with. This is more likely late in the evening
into the overnight when storms will have likely formed into clusters
and pose a greater risk for damaging winds or QLCS tornadoes.
There`s a lot more uncertainty on this happening as it will depend
on the evolution of convection forming to our west. If our area does
see severe weather Saturday night it will likely be in central or
northeast Missouri.

As the trough moves slowly NNE on Sunday it will have the effect of
pushing the dryline and cold front further to the east setting the
stage for the next round of thunderstorm initiation in the warm
sector. This is when the main severe weather threat area shifts into
our area representing our greatest overall threat of this entire
period. Confidence has grown relative to prior forecasts in the area
of thunderstorm initiation being in or near our forecast area likely
along a prefrontal trough or remnant convective outflow from
Saturday night`s storms. With the upper trough axis approaching
we`ll also have added support aloft and strong shear. The greatest
uncertainty will be on the degree of destabilization in the wake of
Saturday night`s convection, but even that uncertainty is more on
whether we will see the more extreme levels available on Saturday.
We are confident we will see at least enough instability for
thunderstorms to form once again initially in the form of supercells
merging into a broader convective complex with time. All severe
weather hazards will be possible including tornadoes, large hail,
and damaging winds. In addition there is at least some potential for
training storms to produce locally heavy rainfall.

As the trough makes progress eastward it will push a cold front
through our area Sunday night into Monday, pushing away the moist
and unstable air mass. The air behind the front will not be
particularly cold, though, as ridging aloft quickly builds back in
behind it. Temperatures next week remain warm, although greater
uncertainty exists in the overall upper air flow pattern and when
rain chances might return.

Kimble

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 618 AM CDT Thu Apr 25 2024

Mostly VFR expected today. Some showers and storms over western
Missouri may sneak toward the central Missouri TAF locations this
afternoon. Thunder is less likely, though, as instability is weak
this far east. Ceilings will begin to lower toward the MVFR range
in central Missouri later this evening and then further east by
Friday morning. Another round of showers and thunderstorms is
expected to develop and track northeast through the region late
tonight into Friday morning. This potential is only at the tail
end of the 12Z TAF in central Missouri and at St Louis (after
12Z).

Kimble

&&

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

$$

WFO LSX