Area Forecast Discussion

694
FXUS63 KLSX 301128
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
628 AM CDT Wed Apr 30 2025

.KEY MESSAGES...

- Showers and thunderstorms remain in the forecast through
  Thursday. Although a few strong to severe storms cannot be
  ruled out through the next 36 hours, the greatest chance is this
  evening.

- Dry and seasonably cool weather sets up a beautiful weekend.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 240 AM CDT Wed Apr 30 2025

At the big picture, a trough is currently situated near the Four
Corners region of the desert Southwest while a broad, low amplitude
ridge exists across the southeast US having been recently pushed
back by a fast moving northern stream trough now in New England.
At the surface, a cold front has pushed southward across the
central US including our entire forecast area in the wake of the
northern stream trough. However, moisture continues to advect
northward through the Southern Plains ahead of the Southwest
trough. This has led to the development of more widespread
thunderstorms across Oklahoma overnight near where this moist
advection intersects the frontal boundary. These storms have
already begun to organize into an MCS which will move off to the
northeast through Missouri today. This MCS is expected to outrun
its source of low level moisture and instability as it tracks
north of the front, gradually weakening as it moves through our
forecast area during the morning hours. Flow within and around the
MCS itself will likely pull in greater instability just ahead of
it, but this is expected to be elevated in nature as the surface
front itself takes longer to react and buckle back northward
today. So current thinking is that this MCS will weaken and pose a
limited severe weather threat for our area. However, this could
change if better surface based instability is able to develop
ahead of it this morning.

As the southwestern trough moves out into the Plains today it will
strengthen the low level moist southerly flow ahead of it,
triggering another round of convection over Oklahoma. This again is
likely to organize into a broader MCS and push northeastward into
our forecast area this evening into the early overnight hours. By
this time, the surface front will have pushed further northward
opening more of the forecast area up to some surface based
instability enabling this MCS to maintain its strength longer. Wind
shear will be increasing as well as the upper trough approaches, so
it is likely that this represents our greatest threat for severe
weather. Damaging winds will be the primary threat, but there may be
enough low level wind shear for a few QLCS tornadoes as well. The
biggest limiting factor will be surface based instability which will
be on a diurnal downward trend.

While this next MCS moves northeast and out of our forecast area
early Thursday morning, surface low pressure will be tracking
through northeast Missouri in response to the arrival of the upper
trough. This will have the effect of halting the northward
advance of the warm front and sending it back southeast as a cold
front during the day on Thursday. Additional shower and
thunderstorm activity is expected as the trough moves through,
primarily ahead of the surface cold front. Wind shear will be
weaker near the core of the upper trough, and it`s unclear how
much instability will be able to redevelop ahead of the front in
the wake of the early morning MCS. These factors lead to a much
more limited severe weather threat with this round. In fact,
latest CAMs have any shower and thunderstorm activity on Thursday
much more isolated in nature and the SPC outlook has shifted the
core of the severe threat to the east of our forecast area.

One final note on total rainfall amounts. There are likely to be
swaths of 1 to 2 or perhaps even 3 inches of rain with each of the
broader convective complexes that move through the region this
morning through tonight. Each of these swaths will be relatively
narrow in width, with a broader area of more moderate rainfall of
0.5 to 1 inches. While 1 to 2 inches over the course of a few hours
is not likely to cause flash flooding in general, this assessment
could change if these swaths move across areas already significantly
moistened by earlier rainfall. If any areas are affected by the more
significant swaths of rainfall from both rounds, then a greater
flash flood threat will result. As far as river flooding, the
combined effect of several days of rainfall will lead to rises on
nearly all creeks and rivers across the region. The degree of the
rises on each river will be determined by where those swaths of
heavy rain fall. There is a comparably greater chance that a few
rivers will rise to flood stage in the coming days as all of this
water has to go somewhere and will gradually collect into the larger
rivers over the course of hours to days. At this time, confidence in
flash flooding or river flooding in any particular area is too low
for a Flood Watch.

Kimble

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 240 AM CDT Wed Apr 30 2025

Thursday`s cold front sends a cooler and drier air mass into the
area. Dewpoints fall into the 40s on Friday while temperatures reach
the 60s to low 70s, near or slightly below normal for this time of
year. A secondary trough dropping south from the northern stream
brings renewed cold advection and one more chance of showers. These
showers on Friday will be driven by cold advection aloft rather than
by moist advection in the low levels. As a result, instability will
be meager and moisture quite limited. Rainfall amounts from this
will be minor, with ensemble probability of 0.1 inch of rainfall
peaking at only about 20 percent in our area on Friday. While we
cannot rule out a few strikes of lightning, we do not anticipate
severe weather.

Most guidance has Friday`s trough exiting quickly to the east,
replaced by ridging downstream of a developing western US trough.
However, cluster analysis shows that some guidance, led by the ECMWF
and CMC, actually pull Friday`s trough so far south that it gets cut
off beneath the developing ridge and lingers over the Ohio Valley
this weekend and right into next week. So while the general trend
favors dry weather, ridging, and an eventual warm up, a lingering
cut off low could spoil that somewhat. The practical difference
would be lingering 60s and 70s vs a return to the 80s. There will
also be some chances for light showers in the vicinity of the cut
off low while ridging would favor more sunny conditions.

Despite the uncertainty in the upper air flow pattern mentioned
above, all scenarios point to Gulf moisture remaining cut off and
moisture return focused over the western High Plains. So we`re
not expecting any significant precipitation for several days, at
least until that western trough begins to move east late in the
week.

Kimble

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 624 AM CDT Wed Apr 30 2025

It`s a rather complex forecast today and tonight. A dying
thunderstorm complex is moving across the region this morning.
It`s not certain whether thunder will occur at any TAF site, but
this seems most likely in the southeast part of the St Louis
metro in the next few hours. We do expect a break in the rain
after this moves through, but some lower ceilings are lurking
nearby and are likely to keep conditions MVFR or IFR for much of
the morning into the afternoon at most sites. A warm front lifting
north today may bring some clearing to VFR at central MO as well
as the St Louis metro around midday or the early afternoon. But
another round of thunderstorms is expected tonight, with a greater
chance of impacting both central MO as well as St Louis. A return
to lower ceilings is likely after this rain ends. Further north at
Quincy, lower ceilings have yet to materialize and may not do so
for some time. Quincy is also the least likely to see
thunderstorms of any TAF site.

Kimble

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion