256
FXUS63 KLSX 191905 CCA
AFDLSX

Area Forecast Discussion...CORRECTED
National Weather Service Saint Louis MO
205 PM CDT Wed Jun 19 2024

.KEY MESSAGES...

- A few showers and thunderstorms are possible this afternoon.
  While most of these storms will be weak, a few strong wind gusts
  can`t be ruled out, particularly across northeast Missouri.

- Heat will steadily build tomorrow through the weekend, with
  afternoon heat index values likely reaching between 95 and 105
  and morning temperatures in the low to mid 70s Friday and
  Saturday.

- Confidence is increasing that a round of showers and
  thunderstorms will move through the area between Saturday night
  and Sunday morning, with precipitation chances between 30 and
  70% in most areas. A few strong/severe thunderstorms will be
  possible, but this potential is less certain.

&&

.SHORT TERM... (Through Late Tonight) Issued at 159 PM CDT Wed
Jun 19 2024


Weather thus far today has been generally quiet as the mid-
Mississippi Valley region remains underneath the western
periphery of a slowly expanding upper ridge of high pressure.
However, a nearly stationary front is draped across
northwest/north central Missouri as of 1 PM, with numerous post
frontal showers and a few thunderstorms ongoing behind it. Just
ahead of this front across the warm sector, strong surface heating
within a relatively clear area has quickly eroded what little CIN
remained, and this has allowed for rapid cumulus development and
even a few weak showers. This deepening convection has developed
in two general areas as of 1 PM: across north central Missouri,
and farther southeast along a loosely defined boundary and/or
thermal ridge aligned with the I-44(MO)/I-70 (IL) corridors.

As the afternoon progresses, increasing instability (1000-1500
MLCAPE) will support slowly deepening convection, but a lack of
appreciable deep layer wind shear is likely to prevent storms from
amounting to much more than weak pulse-type convection. Meanwhile,
significant dry air aloft is likely to impede updraft growth in most
areas, and storms are likely to struggle to develop in this
environment...particularly along the southern line of convection
described above. However, areas in the vicinity of the front may
have slightly more shear to work with (near 20 kt of effective bulk
shear), and less dry air aloft, which may support a few stronger
cores capable of producing occasionally strong downburst winds.

Otherwise, temperatures have already reached the upper 80s to near
90 degrees early this afternoon, and will likely rise another few
degrees by 3 to 5 PM. While a few showers may linger in northeast
Missouri into the evening, most showers will diminish prior to
sunset. Clearing skies should allow for efficient cooling into the
upper 60s to low 70s, with a few patches of very shallow fog
possible in wind-sheltered valleys.

BRC

&&

.LONG TERM... (Thursday through Next Wednesday) Issued at 159 PM
CDT Wed Jun 19 2024

Between tomorrow and Saturday, the broad upper level ridge of high
pressure across the eastern CONUS will slowly expand westward,
gradually increasing both temperatures and subsidence aloft. This
will lead to both increasing surface temperatures and even lower
precipitation chances, although a few stray afternoon showers can`t
be completely ruled out across far northern Missouri and west-
central Illinois tomorrow afternoon.

Within this timeframe, gradually building heat is expected to be the
most noteworthy and potentially impactful weather, although current
forecast temperatures and humidity remain below levels that
typically result in significant impacts locally. Peak afternoon
temperatures remain most likely to occur on Saturday by a slim
margin, with ensemble mean 850 temperatures increasing slightly to
between 19 and 21 C, corresponding to actual surface temperatures
generally in the mid to upper 90s and heat index values near the
century mark. Temperature trends have also increased slightly on
Friday as well, and both days are now expected to see very similar
temperatures. Confidence in these temperatures is relatively high
considering the expected lack of precipitation and cloud cover that
would complicate the forecast, and narrow NBM/LREF temperature
ensemble spreads support this as well (although the NBM remains
several degrees warmer than most other available guidance).

Overnight Saturday, confidence is increasing regarding the timing of
an approaching cold front and associated round of showers and
thunderstorms. Ensemble members continue to trend toward an
overnight frontal passage, which does limit the upper end of
available instability, but in spite of this models continue to
produce just enough to support vigorous updrafts. Meanwhile, a
passing shortwave along the northern periphery of the ridge may also
provide a boost in mid-level winds, and just enough to produce
sufficient deep layer shear for updraft organization. That is the
long way of saying that a few strong or severe storms will be
possible overnight Saturday through early Sunday, with the greatest
chances across northern Missouri and west-central Illinois, although
the currently forecast parameters remain marginal.

While this cold front is expected to move into the area by Sunday,
it likely will not provide significant relief from the heat. This is
because this front is not expected to be particularly strong, and
will likely stall across the region during the day Sunday.
Meanwhile, increasing humidity may actually cause heat index values
to remain steady or even increase slightly from the previous
afternoon, unless lingering morning convection and cloud cover can
provide just enough cooling to keep us from reaching the currently
forecast values. Ensemble mean temperatures do dip a few degrees
Sunday and spreads increase slightly, so temperatures Sunday are a
bit less certain than on Saturday.

Overall though, the trend for above average temperatures will
continue into early/middle next week, as the upper ridge is expected
to retrograde into the western CONUS and amplify. Ensemble mean
temperatures in both the GEFS and NAEFS ensemble suites also
increase to between the 90th and 98th percentile by Tuesday, which
is notably higher than projections this weekend. While this may not
necessarily translate to hotter surface temperatures, the potential
exists for impactful heat early next week.

BRC

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 153 PM CDT Wed Jun 19 2024

VFR conditions will prevail at all local terminals through the
majority of the 18Z TAF cycle. A few weak showers and
thunderstorms will be possible at a few terminals this afternoon
and evening, with showers most likely to impact UIN by a slim
margin. Even here, the probability of a direct impact to the
terminal is low (around 30%), but just high enough to support a
VCTS mention in the TAF. Briefly strong wind gusts will be
possible with showers if they do manage to directly impact a
terminal.

Showers will diminish this evening, along with breezy southerly
winds. Otherwise, while some shallow patchy fog will be possible
overnight at primarily SUS, this is not expected to be widespread
and VFR conditions will persist through the remainder of the
period.

BRC

&&

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

$$

WFO LSX