631
FXUS63 KLSX 211952
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
246 PM CDT Fri Jun 21 2024

.KEY MESSAGES...

- Above average temperatures will continue through at least Tuesday,
and will likely peak on either Monday or Tuesday when at least
minor heat impacts remain likely.

- Scattered showers and thunderstorms remain likely overnight
Saturday across at least northeast Missouri and west-central
Illinois, with some potential for a few damaging wind gusts and
large hail.

- Confidence is increasing that another round of showers and
thunderstorms will occur sometime between Tuesday and Wednesday,
with some modest heat relief following the front.

&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 245 PM CDT Fri Jun 21 2024

While our extended period of above average temperatures continues
this afternoon, quiet conditions are in place otherwise as high
pressure maintains its grip on the region. GOES water vapor imagery
reveals a broad upper ridge located across the southeastern and
south-central CONUS, with a very subtle impulse moving slowly across
Missouri as of 2 PM. This feature may be providing just enough of a
boost to allow isolated pulse-type showers to develop across mainly
northwest Missouri this afternoon. While a few of these cells may
briefly encroach upon central/northeast Missouri through the
afternoon, most of our forecast area will remain dry. Should any
cells develop locally there is just enough instability present (500-
1000 J/kg ML CAPE) that a stray strike of lightning or two is
possible before showers diminish this evening.

As for temperatures, so far surface observations indicate that we
are heating up at about the same rate as yesterday, and it`s likely
that we will once again reach the upper 80s to mid 90s in the next
few hours. Meanwhile, dewpoints only in the 60s will keep heat index
values from climbing much above actual temperatures, and mitigate
the potential for significant heat impacts today. This is also
likely to be the case tomorrow as well, although actual temperatures
may be a degree or two warmer. Still, this is an improvement from
previous forecasts that suggested that tomorrow would clearly be the
warmest day of the week, as it now appears that conditions will be
on par with today and yesterday.

Overnight, a weakening cold front remains likely to move through the
area, bringing with it a round of scattered showers and likely a few
thunderstorms. While the timing of the front remains very consistent
among global models/ensembles and recent CAMs, the coverage of
showers in recent CAMs has been somewhat underwhelming thus far.
While at least some precipitation chances remain likely in northeast
Missouri and west-central Illinois, we have slightly reduced
probabilities to account for this trend. As such, the current
forecast maintains sub-50% probabilities south of I-70, as
confidence low that showers will persist as they move south during
the early morning hours Sunday.

In spite of the underwhelming precipitation trends in higher-
resolution models, a few underlying ingredients for severe weather
remain in place. This includes both marginal instability (500-1500
J/kg MUCAPE) and deep layer shear (20-30kt of 0-6km shear), with
some enhance low level shear due to a relatively strong low level
jet. As such, the potential for a few strong or severe thunderstorms
remains extant, although probabilities for this will be higher to
our north. Damaging wind gusts associated with some form of linear
complex remains the most likely hazard. Considering the expected
storm mode, and also the likelihood that storms will be trending
downward as they arrive, the potential for hail and tornadoes
appears to be quite low at this time.

BRC

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 245 PM CDT Fri Jun 21 2024

By Sunday afternoon, the previously mentioned cold front will likely
settle somewhere across southern Missouri. Exactly how quickly this
front moves southwards will have a big impact on both the heat
potential and chances for redeveloping thunderstorms Sunday
afternoon. A quicker southward progression would usher in a drier
and slightly cooler airmass, providing some modest heat relief and
pushing thunderstorm chances farther south. Conversely, a slower
passage would keep a warm and humid airmass draped across the
Ozarks, and leave open the door for a few redeveloping afternoon
thunderstorms across the Ozarks. Recent model projections have
trended toward the former faster progression, though, and as such
both temperature and precipitation chances have been adjusted
slightly downward to account for this.

By Monday, an upper level ridge of high pressure will rebuild across
the western and Central Conus, while southwesterly low level flow
develops across the plains and Missouri river valley. This will
likely drive a rapid temperature rebound Monday, with ensemble mean
850 mb temperatures climbing to around 23 C (near the 99th
percentile of climatology). This is a bit sooner than in previous
model projections, as previous iterations indicated that heat would
peak on Tuesday. While this may still end up being the case, a
faster and deeper shortwave progression Tuesday makes this less
likely. In any case, these parameters support surface temperatures
climbing well into the mid to upper 90s. This is slightly lower than
the operation NBM output, which continues to forecast temperatures
reaching 100 degrees both Monday and Tuesday. The NBM has remained
the hottest of almost all available guidance in the extended period
(3+ days), and considering local climatology, it`s likely
overdoing actual temperatures a bit. In spite of this, advisory
level heat indices remain likely on Monday at least (100-107),
with some limited potential Tuesday as well. These effects may
also be amplified slightly by the fact that this will mark at
least 5 consecutive days of above average temperatures, although
the expected magnitude is not expected to produce more than minor
impacts overall.

Finally, another cold front is expected to push through the region
sometime between Tuesday and Wednesday, and confidence is increasing
that this will bring another round of showers and thunderstorms, and
likely at least some modest heat relief. While many details remain
uncertain, particularly the timing of this front, the potential for
strong/severe thunderstorms will also need to be monitored due to
the potential for strong instability and at least modest deep layer
shear (pending the strength of the passing shortwave). CSU machine-
learning guidance also notably highlights this timeframe with rather
bullish severe weather probabilities, adding at least some evidence
that this will be a day worth paying attention to in future
forecasts, at a minimum.

BRC

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1230 PM CDT Fri Jun 21 2024

VFR conditions are expected to prevail at local terminals through
the 18Z TAF cycle, with few exceptions. At COU/JEF, some weak
convection has developed in the vicinity this afternoon, but
little if any precipitation appears to have yet reached the
ground. While a shower or two can`t be ruled out, probabilities
remain low (20% or less) that either terminal will be directly
impacted, and thus showers were not initially included in the TAF.
Otherwise, southwest winds tomorrow afternoon will likely be
breezier than today, with additional afternoon VFR cumulus
expected as well.

BRC

&&

.CLIMATE...
Issued at 221 AM CDT Fri Jun 21 2024

With hot temperatures forecast next week, here are some of the
daily high temperature records:

            MON JUN 24     TUE JUN 25
ST LOUIS    102 in 1988    102 in 1954
COLUMBIA    103 in 1988    102 in 1988
QUINCY       99 in 1988    102 in 1931


&&

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

$$

WFO LSX