483
FXUS63 KLSX 191943
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
243 PM CDT Sun May 19 2024

.KEY MESSAGES...

- Abnormal warmth persists through Tuesday, when another run at 90
  degrees is likely (60-70% chance) for the St. Louis and
  Columbia metro areas.

- Thunderstorm chances exist through a large portion of the forecast,
  though the threat is not continuous nor uniform areawide.

- The most tangible, though conditional, threats for severe
  weather exist Tuesday (primarily in central/northeast Missouri
  and west- central Illinois) and Wednesday (along and south of
  the I-44 and I- 64 corridors in Missouri and Illinois,
  respectively).

&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 241 PM CDT Sun May 19 2024

A diffuse surface boundary is currently draped across northeast
Missouri and west-central Illinois, and its movement is largely at
the mercy of differential heating as its upper-level support is
far- removed from the region. Air on the warm/humid side of the
boundary is very unstable, with SBCAPE values exceeding 2500 J/kg
according to SPC Mesoscale Analysis. Heights aloft continue to
gradually rise with a narrow ridge axis extending from the Ark-La-
Tex region through into the Ohio River Valley. The combination of
weak surface convergence and subsidence aloft are counteracting
the positive buoyancy in place to keep convection in check. The
very strong instability may be able to overcome this subsidence,
so isolated to widely-scattered thunderstorms remain possible
through the early evening in northern Missouri and west-central
Illinois. While low, the potential for near-severe hail will exist
with any convection going into the evening.

The boundary lifts further north overnight with the low-level jet
strengthening a bit. Severe convection is poised to eject from the
central Great Plains on the nose of this LLJ and track along a
MUCAPE gradient into Iowa. As such, most of the night tonight will
be dry and warm with southerly winds gradually strengthening. At the
same time, mid-level troughing in the western CONUS becomes more
pronounced, and by Monday morning we are squarely in deep southwest
flow aloft. Temperatures once more jump well into the 80s, but may
be muted ever-so-slightly compared to today because of more abundant
cloud cover. Weak upper-level diffluence will exist as opposed to
strong subsidence, but even still the forcing mechanisms are not
very strong. There is also a question as to how much we destabilize,
with some guidance keeping the MLCAPE well-capped, and others
allowing for a more buoyant mixed layer. Without much low-level lift
to focus convection on Monday, I`m not confident we see anything
robust or strong quite yet. If convective debris lasts longer into
the morning and early afternoon, redevelopment will be inhibited. If
not, or if we have a well-defined MCV, we could see marginally-
severe thunderstorms across the forecast area. We are not currently
messaging the threat given the wide range of possibilities and low
overall threat.

Most short-range guidance relegates convection Monday night far to
our north across Iowa along the warm front/MUCAPE gradient. This
situation would leave us warm and dry once more in the open warm
sector on Monday night into Tuesday morning, though low PoPs (15-
20%) persist in northern Missouri and west-central Illinois to
account for subtle uncertainty in the exact frontal position.
Otherwise, most of the resident instability will remain untouched in
advance of the more tangible severe weather threat on Tuesday.

MRB

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 241 PM CDT Sun May 19 2024

Little has changed in the overall thinking for our severe weather
potential Tuesday, marking a return to an active weather pattern for
us. Amidst the broad upper-level trough in the western CONUS, a
shortwave will eject into the central Plains and into the
Mississippi Valley. There is still some variance in exactly where
that wave tracks, which would limit the southern extent of the
severe weather threat somewhat. That said, the mid-level jet streak
is strong (near the 90th climatological percentile) and will lend
lift and shear to the resident instability to present a severe
weather threat. It is worth noting that if we continue a northward
trend in the wave, it`s entirely possible a very small area of the
region sees severe weather. The area most likely to see severe
weather remains across the northern and western forecast area, where
the best forcing will exist during diurnal peak heating. To nod that
peak heating for a moment, Tuesday looks like another warm day with
a few spots reaching 90 degrees and widespread mid/upper 80s
otherwise.

Assuming the current presentation of the mid-level wave is
sufficient, convection still looks to initiate in a discrete mode
along a pre-frontal boundary in the open warm sector. There is a
trend to slow down the synoptic system overall compared to previous
forecasts, meaning the severe weather threat may not manifest until
close to sunset in central and northeast Missouri. By this point,
the storm mode would be in question as deep-layer shear begins to
back and become more parallel with the low-level lifting mechanism.
That said, the shear orientation is not certain and discrete
convection may persist by the time thunderstorms reach us. If the
former occurs, the highest threats would be for damaging winds and
tornadoes. If the latter occurs, all hazards will threaten the
region. Another question is how long severe convection will last and
how far south it will progress. As thunderstorms continue south and
east Tuesday evening, instability will wane and they will be further
detached from mid-level support. As such, a gradient will exist in
the threat for severe weather (illustrated well by the SPC Day 3
Convective Outlook).

The cold front attending the synoptic system will slowly march
through the region overnight Tuesday into Wednesday, but there are
no indications it will exit completely by Wednesday afternoon. As
such, a renewed threat for severe weather will exist along and ahead
of this boundary. That said, a few uncertain factors are still in
the air. The first is the effect of convection overnight Tuesday on
instability redevelopment for Wednesday. It doesn`t appear that
would be an issue, with most convection ending during the late
evening Tuesday. The second is the mid-level lift being displaced
north of the best instability and low-level forcing. While
indications are the mid-levels may exhibit some weak confluence near
the instability, there is ample upper-level jet support for deeper
convection. The area generally highlighted by the SPC Day 4
Convective Outlook is of greatest concern (essentially along and
south of the I-44/I-64 corridor). Discrete convection that does
develop will likely grow upscale quickly, posing mostly a damaging
wind and tornado threat.

The front finally passes late Wednesday into early Thursday, leading
to a brief but largely-tranquil period from Thursday into early
Friday. That said, the front doesn`t move far to our south, and its
ultimate position is unclear. Because of this uncertainty, 30-50%
PoPs linger through into the weekend. I doubt we will see a
persistent thunderstorm threat that entire time, but with fairly
large uncertainty surrounding the front position, the NBM PoPs were
used through the extended forecast. The next more appreciable rain
chance returns Friday into the weekend, at which point the CIPS/CSU
severe weather probabilities begin to inch back into the region.
Ensemble guidance hints at another amplified wave later this
weekend, but considerable timing and amplitude differences exist.

MRB

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1219 PM CDT Sun May 19 2024

VFR conditions are currently forecast through the valid TAF
period. Weak southerly winds exist at all regional terminals, with
a warm front now north of KUIN. Strong instability exists across
the region, and though the front is weak and mid-level forcing is
non-existent, we may see an errant thunderstorm near KUIN this
afternoon. Confidence in even vicinity impacts at the terminal is
low, and as such was kept out of the TAFs at this time.

Remnant thunderstorms from the central Great Plains will enter
central Missouri tonight into tomorrow morning. A few
thunderstorms may occur, but by that stage there is low confidence
in much thunder. VCSH was added, but may need to be adjusted to
include thunder if confidence increases in that. The convection
will decay further as it approaches St. Louis, and may not make it
there at all. If it does, it would be in the 17-20z timeframe.

MRB

&&

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

$$

WFO LSX