238
FXUS63 KLSX 221913 CCA
AFDLSX

Area Forecast Discussion...CORRECTED
National Weather Service Saint Louis MO
213 PM CDT Sat Jun 22 2024

.KEY MESSAGES...

- Showers and thunderstorms are likely (60-75%) late this evening
  into the early overnight hours in parts of northeast Missouri
  and west-central Illinois along a cold front. While widespread
  severe weather is not expected, a few of the strongest storms
  may be capable of producing hail to the size of quarters and/or
  wind gusts to 60 mph.

- Above average temperatures will continue through Tuesday, and
  will likely peak on either Monday or Tuesday when heat index
  values are forecast to reach into the 100-105+F range.

- Another cold front is forecast to move through the bi-state
  region Tuesday night, bringing the best chance of more
  widespread showers and thunderstorms in several weeks. Behind
  this front, a brief (~2 days) period of cooler temperatures is
  expected but exactly how cool is uncertain.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 151 PM CDT Sat Jun 22 2024

A cold front is currently located across the mid-Missouri and Upper
Mississippi River Valleys as of 1900 UTC. Radar and GOES-16
satellite imagery shows convective initiation has begun along this
boundary. The cold front is not expected to reach into our area
until about 0300 UTC. By this time, the storms will be moving into a
less favorable environment than where they formed, both in terms of
instability as well as deep-layer shear. Probabilities at 0300 UTC
on the HREF for at least 1000 J/kg of CAPE are about 40% in parts of
northeast Missouri before gradually fading during the overnight
hours. Deep-layer shear also looks to be on the weaker side, on the
order of about 25 knots. Given this environment, would not expect
organized severe weather in our area, but a few of storms may be
capable of producing hail to the size of quarters and/or wind
gusts to 60 mph. The best window for any strong-severe storms
appears to be from about 10PM-1AM in parts of northeast Missouri
and west- central Illinois.

The coverage of storms should also gradually wane overnight tonight,
as the surface convergence weakens. The activity should become more
isolated-scattered in nature, leaving most of the area with
little/no beneficial rainfall.

Behind the front, there is not much of an air mass change. In fact,
850-hPa temperatures actually climb about 1C. However, mixing is not
nearly as deep. Instead of of mixing to 800-850 hPa like this
afternoon, the top of the mixed layer on Sunday afternoon is closer
to 875-925 hPa. Highs on Sunday for most locations should be about 3-
4F cooler than today, mainly in the upper 80s to low 90s. Dewpoints
(relative humidity) values are also expected to gradually lessen
through the afternoon thanks to persistent northwest winds around 10
mph advecting drier air into the region.

Clear and calm conditions are forecast Sunday night as a weak
surface high slides across the bi-state area. Low temperatures were
cooled down slightly from the previous forecast given the pattern
(clear sky/calm winds) into the upper 60s to near 70 degrees. Some
mid 60s are possible in the eastern Ozarks on the cooler end, while
low 70s are more likely in/around the urban heat island of
metropolitan St. Louis.


Gosselin

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 151 PM CDT Sat Jun 22 2024

(Monday - Tuesday Night)

Increasing heat and humidity continues to be the main focus for
early next week. There is higher confidence in Monday`s temperatures
due to the building mid/upper level ridge, which should keep the
area dry. While there is a subtle surface trough moving slowly
northeast across parts of the area, subsidence aloft associated with
the rising heights mentioned above and some capping should keep it
dry. High temperatures in the 90s are forecast, but dewpoints
(humidity) will be on the increase. This is particularly true
along/behind the surface trough mentioned above where dewpoints are
expected to climb into the low 70s. These dewpoints combined with
temperatures in the mid 90s are forecast to yield heat index values
of 100-105F across parts of central and southeast Missouri. A
magnitude (105+F) heat advisory is possible in these areas,
especially if temperatures and/or dewpoints are ever so slightly
higher. If the trough is a bit faster, this area of higher heat
index values could expand Monday afternoon further to the north and
east.

Forecast uncertainty increases beginning Tuesday as the mid/upper
level ridge weakens overhead. Deterministic guidance also suggests a
midlevel shortwave moving east/southeast out of the mid-Missouri
Valley. The ECMWF is about 6 hours faster with this shortwave,
generating convection by late Tuesday morning. This in turn keeps
daytime temperatures a lot cooler than the GFS. This scenario does
not look that particularly likely however, with only about 25% of
the EPS members showing this scenario. What is more likely is that
the area will stay dry during much of the day, with the slower
shortwave helping to initiate convection along a cold front Tuesday
evening. This would mean more sunshine and another hot day with
highs in the mid to upper 90s. The low 70s dewpoints also should be
areawide, so more widespread heat index values of 100-105F are
forecast, with 105-108F readings most likely across parts of east-
central and southeast Missouri as well as southwest Illinois. A
magnitude heat advisory would be warranted if confidence in these
conditions increases over the next 24-48 hours.

While there are timing differences with the midlevel shortwave and
associated cold frontal passage, this next chance of showers and
thunderstorms does have more to work with than storms tonight. The
midlevel shortwave trough provides large-scale ascent, convergence
along the surface cold front is fairly strong throughout the night,
and low-level moisture convergence is strong (30-40 knots). In other
words, this definitely appears to be the best chance for widespread
convection (and beneficial rainfall) our area has seen in some 2-3
weeks. There also appears to be at least a slightly better chance
for strong to severe convection as well, with 50-70% probabilities
for >2000 J/kg of SBCAPE at 0300 UTC Wednesday on the GEFS. Deep-
layer shear is marginal, generally 20-25 knots, but the higher
amount of instability (and forcing for ascent throughout the column)
suggests an increased threat for severe weather. This is also
supported by the day 4 CSU machine-learning guidance.


(Wednesday - Thursday Night)

While there is high confidence that the cold front will clear the
area by late Wednesday and usher in some cooler temperatures,
exactly how cool remains a question mark. Deterministic guidance is
a bit cooler with the incoming air mass than their respective
ensemble means, which leads me to think we are headed back "closer
to normal" than a more significant change. Highs are most likely to
be in the upper 80s to low 90s Wednesday and Thursday. Humidity
levels will start off on the high end Wednesday, but dewpoints
should gradually lower through the day. By Thursday, comfortable
humidity levels are expected areawide as a surface ridge of high
pressure moves across the region. Lows Wednesday/Thursday night also
should be closer to normal, generally in the mid 60s to near 70
degrees.


(Friday - Next Saturday)

Temperatures are likely to warm back up well-above normal heading
into next weekend as mid/upper level heights rise and 850-hPa
temperatures once again approach +20C. However, confidence in
specific temperatures is low. The synoptic pattern with an elongated
west-east oriented mid/upper level ridge just to our south suggests
subtle shortwave troughs may be enough to initiate thunderstorms
during this period. If thunderstorms are widespread with convective
debris clouds, high temperatures may end up at least several degrees
cooler than forecast. The spread on the NBM 25th/75th percentiles is
quite large, generally on the order of 6-8+F which helps illustrate
this uncertainty with high temperatures during this period.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1205 PM CDT Sat Jun 22 2024

Dry, VFR conditions with gusty southwest winds are forecast this
afternoon. A cold front will approach by late evening, moving
through KUIN just after midnight and central Missouri and metro
St. Louis sites just before dawn. Scattered showers and
thunderstorms are possible along/ahead of this boundary, with best
chances in parts of northeast Missouri and west-central Illinois
including KUIN. Did not make much in the way of changes with this
TAF package, with a TEMPO at KUIN from 0400 to 0600 UTC and PROB30
groups at the metro terminals from 0700-1000 UTC. Central Missouri
sites have the least chance of any showers/storms so no mention
for those terminals. After the front moves through, look for
winds to veer to the northwest. Some guidance also hints at a
brief period (1-3 hours) of MVFR ceilings behind the front, but
the signal is too weak to add a BKN025 group anywhere. However,
did add a SCT025 mention at KUIN and the metro terminals for a few
hours behind the frontal passage.

Gosselin

&&

.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