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FXUS63 KLSX 201938
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
238 PM CDT Thu Jun 20 2024

.KEY MESSAGES...

- Steadily building heat is expected over the next several days,
  with heat likely peaking early next week. While heat may not be
  extreme on any individual day, the prolonged nature of this heat
  has the potential to amplify impacts.

- While a stray shower or weak thunderstorm will be possible
  across northeast MO and west-central IL this afternoon, better
  chances for showers and storms (30 to 70%) exist overnight
  Saturday and Sunday morning. A few strong or severe storms will
  be possible, but chances of this are low.


&&

.SHORT TERM... (Through Sunday)
Issued at 235 PM CDT Thu Jun 20 2024

Mostly benign weather conditions remain in place this afternoon as a
ridge of high pressure centered across the eastern CONUS continues
to slowly expand further into the Mississippi river valley.
Meanwhile, a stationary front remains draped near the MO/IA state
line, with widespread deepening cumulus development along and south
of the boundary across the warm sector. While warm mid-levels have
kept the environment quite stable across much of the region, some
modest MLCAPE has managed to develop across primarily northeast
Missouri and west-central Illinois as of noon, and this is also
where the deepest convective development has occurred thus far. As
the afternoon progresses, a few showers and even a weak thunderstorm
or two may emerge from this cu field, but most of this activity is
likely to remain near or just north of our northernmost counties
(Knox, Lewis, Adams, Brown). If showers do manage to form in these
areas, short bursts of heavy rain and a strike of lightning or two
will be the most likely hazards.

As we head into the weekend, the previously mentioned upper ridge
will continue to expand westward, and temperatures will slowly
increase. So far today afternoon temperatures are very similar to
yesterday, and we will likely once again reach the upper 80s to mid
90s by late afternoon, with only moderate humidity. However, this
may be the "coolest" day of the next week, and by Saturday we`ll
likely see temperatures reaching well into the mid 90s to perhaps
near the century mark. This forecast has remained rather consistent,
with ensemble mean 850 mb temperatures increasing to around 19-21 C,
along with narrow ensemble temperature spreads. The good news
leading up to Saturday is that humidity is likely to remain only
moderate, which should help to keep heat index values in check
through Saturday, and below our typical thresholds that lead to heat
impacts over short durations.

However, this may change Sunday as humidity is expected to increase
ahead of a weakening frontal boundary, with only a minimal decrease
in temperatures expected. In fact, the heat index values Sunday may
actually end up slightly higher than Saturday, even if actual
temperatures are a few degrees lower. This heat is also very likely
to persist, if not worsen, heading into early next week as well.

Before we get to that, though, the previously mentioned front is
also expected to arrive with a round of showers and thunderstorms
overnight Saturday. Confidence in the timing of this activity is
higher than average at this time range, owing to very good agreement
among ensemble members. What is less certain is just how far south
this activity will persist Sunday morning, and whether lingering
showers or precipitation will have any impact on temperatures Sunday
afternoon. At the present time, it seems unlikely that precipitation
will provide meaningful relief Sunday. Meanwhile, the
overnight/early morning timing is not climatologically favorable for
severe thunderstorms, but models continue to produce just enough
instability (500-1500 J/kg of MUCAPE) and 0-6km shear (20-25kt) to
at least consider the potential for a few strong or severe
thunderstorms in northern areas during this timeframe.

During the afternoon Sunday, this front is expected to stall
somewhere across the area and weaken. In addition to the potential
implications on heat that have already been discussed, this leaves
open the possibility of additional thunderstorms Sunday afternoon,
particularly along and south of wherever the boundary ends up.
Confidence is much lower regarding this potential and ensembles are
much less enthusiastic about precipitation during the afternoon,
particularly the NBM. However, there are a non-trivial number of
LREF members that maintain precipitation chances through Sunday
afternoon, with ample instability in place along with continued
modest deep layer shear.

BRC

&&

.LONG TERM... (Monday through Next Thursday)
Issued at 235 PM CDT Thu Jun 20 2024

On Monday, guidance consensus is that the shortwave responsible for
Sunday`s front will continue eastward and deepen as the western
upper-level ridge amplifies and tries to re-expand eastward.
However, both ensembles and deterministic guidance differ on how
quickly both of these features phase, with a slower solution keeping
northwesterly flow aloft over the Middle Mississippi Valley and
temperatures at or cooler than values expected Sunday. A quicker
solution, where the upper-level ridge builds more aggressively into
the region, favors temperatures warming above what was experienced
Sunday.

Confidence is increasing that Tuesday will likely be the hottest day
of the period. Then, guidance consensus is that 850 mb temperatures
will be around 23 degrees C, climatologically correlating with
temperatures in the upper 90s assuming deep mixing. What will give a
boost to this heat is that a majority of guidance also has
southwesterly surface flow, which will provide adiabatic warming as
air moves downslope off the Ozarks. This will support surface
temperatures around 100 degrees for most locations in the CWA,
backed by the 50th-75th percentile of global ensembles and the NBM.

As early as Tuesday night, guidance consensus is that a shortwave
will move through the Midwest, though deterministic guidance and
ensemble clusters differ on its amplitude. This impacts not only the
timing of the associated cold front, but also the potency of the
post-frontal airmass. A shallower wave will have a slower FROPA and
only take temperatures down by a few degrees, while a deeper wave
will have a quicker FROPA and greater impact on temperatures. While
how much we cool exactly is uncertain, confidence is high that we
will cool to some degree the middle of next week, as the 75th
percentile of ensemble guidance tops out in the low 90s.

Elmore

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1037 AM CDT Thu Jun 20 2024

VFR flight conditions are expected throughout the 18Z TAF cycle.
Afternoon cumulus development is expected both this afternoon and
again tomorrow afternoon, but cloud bases will remain above VFR
thresholds. A stray shower or thunderstorm can`t be completely
ruled out at Quincy late this afternoon and evening, but
probabilities are too low to include in the TAF at this time.
Showers are more likely to remain north of the terminal.
Otherwise, light south to southwest winds will persist.

BRC

&&

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

$$

WFO LSX