704
FXUS63 KLSX 230753
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
253 AM CDT Thu May 23 2024

.KEY MESSAGES...

- Although there`s at least some chance for thunderstorms nearly
  every day, there are two primary time frames we`re watching for
  the best potential. The first is on Friday when a cold front
  moves through with the potential for damaging winds and possibly
  some hail. The uncertainty here revolves around whether there
  will be a round of storms in the morning or if the stronger
  activity waits until the afternoon.

- After a dry Saturday, the best day of the weekend, the next
  trough brings the potential for two rounds of storms Saturday
  night and Sunday. There`s a potential for a greater severe
  weather threat on Sunday but there`s uncertainty on the timing
  of this trough and whether the strong instability will exist in
  our area or not.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 246 AM CDT Thu May 23 2024

As surface high pressure is moving east this morning, low level
moisture return has begun with some showers and thunderstorms along
the leading edge of the better moisture. This activity will
primarily be confined to the morning hours when the moist advection
on the low level jet is most pronounced. As we get further into the
day mid level shortwave ridging will promote subsidence and suppress
convection despite increasing instability in the gradually
moistening air mass. So after the morning showers it should actually
be a pretty nice, dry day for most with temperatures near 80.

The next shortwave trough is actually moving out into the Northern
Plains today, sparking off a new round of thunderstorms over
Nebraska this evening. As the trough moves northeast it will send a
cold front eastward tonight into Friday. Similar to our last episode
on Tuesday, the best forcing with the trough will be moving well to
the north so the coverage of storms further south in our area is a
little more in question. Instability will tend to be maximizing
ahead of the front as moisture pools and temperatures aim for their
warmest levels of this air mass, but convergence along the front
will be weakening and lift from the shortwave trough stays well
to the north. In addition, deep layer shear is only weak to
moderate so it will be more difficult to get supercell structures
from thunderstorms, with a tendency more toward multicell clusters
with the primary severe weather hazard being damaging winds.

While that is the general synoptic setup, there are more
complications involved. The thunderstorms that get going over
Nebraska tonight are forecast by most of the CAMs to form into a
broader convective complex which pushes southeast well ahead of the
front and moves clear across our area during the morning to early
afternoon. If this is how it plays out, then our primary severe
weather threat would be with this line, with the intensity perhaps
decreasing as it moves in but reinvigorating as daytime heating adds
instability ahead of it. This would also mean that during the
afternoon most of the area will be behind the MCS affected by the
cold pool which limits the potential for redevelopment as the actual
front moves in later in the day. It`s worth noting that while many
CAMs show this scenario, most of the global scale models don`t play
out the morning MCS concept, instead focusing the threat along the
cold front in the afternoon. While the CAMs have a general bias
toward over-convecting and this could be causing the projected MCS
to blow through too early, the global scale models have a tendency
to underplay these mesoscale events and focus more on the synoptic
scale forcing of the cold front. It`s hard to have much confidence
in which way it will play out. The message we have the most
confidence in is that there is a threat for strong to severe
thunderstorms with primarily a threat for damaging winds on Friday,
but the timing could be anywhere from mid morning to late afternoon.
This also has an effect on the temperature forecast as a morning MCS
would prevent strong heating.

Kimble

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 246 AM CDT Thu May 23 2024

The cold front Friday night is strong enough to clear the area and
push the moist, unstable air mass out of the way giving way to the
driest day of the forecast on Saturday. It won`t last long, though,
as surface high pressure already begins to push east of the forecast
area Saturday afternoon and moisture begins surging back north in
the plains ahead of the next trough.

This next trough will be further south than the last several,
placing us well within the better synoptic forcing. We`re expecting
the development of a surface low with a warm front lifting north and
a cold front swooping southeast in quick succession. The most
widespread rainfall will be with the warm front Saturday night into
Sunday morning. In this scenario the severe weather hazard would
primarily be large hail with elevated thunderstorms if enough
instability can develop. Whether another round develops in the
warm sector ahead of the cold front on Sunday is much more in
question. It`s this round that would have the greater overall
severe weather threat if all the variables line up. The timing of
the wave is the biggest factor here, as if the low moves by in
the morning and swings the cold front through early, we may not
develop enough surface based instability ahead of the front in our
area. While there is still timing and track uncertainty on this
wave, the trend continues to be for a rather quick passage of this
trough which would have a tendency to place the better severe
weather threat to the southeast of our area.

Sunday`s trough initiates a pattern change across the continent. As
it moves east it will deepen and establish a broader trough over
the eastern US with a ridging building over the west. The
resulting northwest flow over our region will promote drying and
the potential for cooler temperatures as well. While dry and cool
is the rule, it`s not absolute. Shortwave troughs dropping through
the flow could bring brief periods of showers, but we won`t have
the Gulf of Mexico moisture to work with to create a greater
severe weather threat. Whether temperatures will be a few degrees
below normal or a few degrees above will depend on whether we`re
closer to the broader trough or the broader ridge. The general
trend will be for initially cooler temperatures followed by a
gradual warming. The greatest uncertainty is on Monday as most
models indicate a secondary trough following Sunday`s wave, but
there`s considerable variability on how strong it will be. The
operational GFS actually portrays this as the stronger of the two
troughs and is the most aggressive with continuing rain chances
into Monday. If Sunday`s trough is significantly more dominant,
then subsidence in its wake would promote a dry Memorial Day.

Kimble

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1052 PM CDT Wed May 22 2024

Showers and thunderstorms are expected to develop very early this
morning in southern Missouri, remaining generally south of the I-70
corridor as they head northeast into Illinois. Confidence in a
thunderstorm directly impacting a terminal is still too low to
warrant a prevailing or tempo group, but expect brief reduced
visibilities if this does occur. MVFR ceilings will briefly creep
into southeast Missouri late this morning, but confidence in these
impacting any site is still low given the iffy northern extent of
the lower clouds. Mentioned FEW025-027 everywhere but KCOU and KUIN
due to the possibility of brief lower ceilings. Otherwise, light and
variable winds continue tonight before picking up from the southeast
after sunrise.

Jaja

&&

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

$$

WFO LSX