269
FXUS63 KLSX 190838
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
338 AM CDT Sun May 19 2024

.KEY MESSAGES...

- Seasonably-warm temperatures persist through Tuesday, with some
  locations likely hitting 90 degrees both today and Tuesday.

- Mostly dry weather is forecast through early tonight, with the
  exception of a 20 percent chance of showers and thunderstorms
  from 4-7 PM in parts of northeast Missouri and west-central
  Illinois.

- An active pattern with multiple thunderstorm chances remains on
  the table for early/mid next week. The ingredients for severe
  thunderstorms are most likely to converge over our region
  Tuesday evening, but there may be another threat possible
  Wednesday afternoon depending on the speed of the cold front.

&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 324 AM CDT Sun May 19 2024

A weak cold front was located near a KMKC>>KEOK line as of 0800
UTC this morning. No precipitation has been occurring in
association with this boundary, but there has been some scattered-
broken clouds with bases around 6000 feet. The front is expected
to soon stall out, before wavering back northward as a warm front
later today. Not a whole lot has changed with respect to the forecast
specifics compared to yesterday, other than it does appear that
the front likely will be a bit further to the north (more in
northeast Missouri/west central Illinois) than it looked like
yesterday. There remains a lot of uncertainty as to if any storms
will fire along/near the boundary this afternoon due to weak
surface convergence, some capping, and building mid/upper level
heights (implying subsidence). Most CAMs do eventually develop
isolated convection, but differ on the timing/location. A minority
(hi-res NAM/HRRR included) basically do not develop any
thunderstorms at all. This is a long way to say that the threat
for thunderstorms this afternoon in parts of northeast Missouri
and west-central Illinois is pretty conditional. IF any storms do
develop and impact this region, they likely would occur from
roughly 4-7 PM. A strong to severe storm would be possible given
high instability (HREF probs >2000 J/kg are 80-90+ percent). Deep-
layer shear is quite weak though and is stronger poleward of the
retreating warm front. In our area this afternoon, model guidance
shows about 20 knots so pulse convection and/or multicellular
clusters would be the likely mode. Damaging winds in microbursts
are possible due to high DCAPE values owing to dry midlevels of
the atmosphere (entrainment) and a deep mixed layer. Cannot rule
out a marginally severe hail event or two, but this may be tougher
to come by given the expectation for non-rotating thunderstorms
and very warm temperatures helping to melt hailstones on the way
to the surface.

Speaking of temperatures, even warmer readings are expected this
afternoon due to increasing mid/upper level heights, plenty of
sunshine, and continued low-level warm air advection. Highs are
forecast to top out in the upper 80s to the low 90s across the
region. This was near the 90th percentile of available model
guidance which performed well for yesterday`s highs under fairly
similar conditions.

Strong to severe convection is forecast early tonight upstream
across the central Plains. However, as these storms head further
east, they will be heading into a more and more unfavorable
environment characterized by less instability and effective shear.
Therefore, the expectation is for fairly rapid weakening of
convection as storms head out of western Missouri toward northeast
and central parts of the state. Some showers/weak thunderstorms
probably will make it in to those locations, but coverage may only
be isolated to widely scattered in nature.

Convective trends are quite nebulous for late Monday
morning/afternoon. CAMs are all over the place, which makes some
sense given weak flow aloft and a general lack of organized
mid/upper level forcing for ascent. There is a strong signal for an
MCV to move out of Iowa into Wisconsin during the day, but this
feature should not have any impacts further south in our neck of the
woods. A couple of CAMs though have a second MCV further to the
south over central Missouri that helps initiate convection across
most of the region. There are hints of this feature also in
deterministic global guidance, though there are also timing/track
differences. Given the uncertainty of if this further south MCV will
even exist, we are not messaging any threat for strong to severe
convection at this time. Even if storms do manage to develop, the
environment likely will not be as unstable as this afternoon,
with continued marginal deep-layer shear (20-25 knots).

Monday`s highs should be about 3-4 degrees cooler for most locations
compared to this afternoon. The main reason for this is an increase
in cloud cover with at least some debris/blowoff from morning
showers/thunderstorms.

Gosselin

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 324 AM CDT Sun May 19 2024

(Monday Night - Wednesday)

The focus for showers and thunderstorms Monday night should be
mostly, if not entirely, north and west of the forecast area. This
is where the stronger mid/upper level forcing for ascent will reside
downstream of another shortwave trough. In addition, stronger low-
level moisture convergence is more toward the mid-Missouri Valley
and Upper Midwest along/north of the warm front.

Most of the daylight hours on Tuesday also are expected to be dry,
with the timing of the synoptic cold front looking about ~3 hours
slower than it did yesterday. The entire area will be within the
warm sector on Tuesday, with very warm temperatures for mid/late
May expected. Temperatures at 850 hPa approach +20C, which would
be above the 95th percentile of climatology. There probably will
be some increase in mid/high level cloudiness from west to east
late in the day, but this should be a bit too late to have any
meaningful impact on temperatures. Widespread upper 80s to low 90s
are forecast, and I couldn`t rule out some 94-95F readings
in/around metro St. Louis which would break the record for the
date (93F - 1941).

A strong, negatively tilted midlevel shortwave trough is forecast to
move through the mid-Missouri Valley into the Upper Midwest Tuesday
afternoon/early evening. While guidance agrees well with the track
of this feature, there is still some subtle timing differences which
impacts the time/location of convective initiation. Strong
forcing associated with the aforementioned shortwave trough is
expected to develop thunderstorms by early to mid afternoon near
the Missouri-Kansas border. Initial storms likely will be
discrete (supercells), with the orientation of the deep-layer
shear vector about 30 degrees off of the cold front/pre frontal
trough axis. Storms likely will then congeal into a line/QLCS
before entering parts of central and northeast Missouri early
Tuesday evening. Damaging winds and QLCS tornadoes are the main
threats. The tornado threat likely will be confined to any part of
the line that is able to surge out and orient itself more from
north/northwest to south/southeast given 0-3 km shear vectors
around 30-35 knots out of the west/southwest.

How long the line of storms/QLCS is able to stay well-organized is a
question mark. Deterministic guidance shows the strong mid/upper
level forcing rapidly moving into the Great Lakes (and further and
further removed from our area) by late Tuesday evening. In addition,
there should be a bit less deep-layer shear with east/southeast
extent and instability gradually wanes nocturnally. Therefore, the
expectation is for gradual weakening of the line/QLCS after ~0300
UTC as storms head toward east-central Missouri and south-central
Illinois. Exactly how fast/slow this weakening (and where within our
area) occurs however is far from being set in stone, but the current
SPC day 3 shows this scenario well with the enhanced risk in central
Missouri through west-central Illinois and a somewhat tight gradient
down to a marginal risk for us in parts of southeast Missouri and
southwest Illinois.

There remains more uncertainty for convective trends on Wednesday,
as the location of the front Wednesday afternoon is more variable in
guidance. Of course, the front or at least the effective front may
also be modulated by whatever convection happens late Tuesday night
into Wednesday morning. Current indications are that parts of
southeast Missouri and southwest Illinois would have the best
chances of strong to severe thunderstorms Wednesday afternoon, but
this area could shift southeast (out of the area) or get expanded to
the northwest depending on the speed of the front and how much
antecedent convection impacts instability.

Temperatures on Wednesday remain low confidence due to the
uncertainty with the speed/location of the cold front. The spread
between the 25th/75th percentile of the NBM is pretty high, on the
order of about 10 degrees across parts of the area. There should be
a pretty tight gradient from northwest to southeast. In parts of
northeast Missouri, highs may struggle to top 70 degrees. Meanwhile,
ahead of the cold front, mid to upper 80s are expected. The 75th
percentile of the NBM is around 90 degrees from St. Louis and points
south and east, so wouldn`t be shocked to see highs near 90
degrees ahead of the front if it is slower than currently forecast.


(Wednesday Night - Saturday)

Any threat for strong to severe convection in parts of southeast
Missouri and southwest Illinois should exit the area by mid-late
evening. The remainder of Wednesday night likely will be dry.
Thursday also should be a mostly dry day with a weak area of
surface high pressure settling into the mid-Mississippi Valley.

Model guidance has come into better agreement with the next midlevel
shortwave trough moving across the Mississippi Valley. This feature
should move across southern portions of the forecast area and
interact with the west-east boundary to the south of the CWA. Best
chances (30-50%) are in parts of southeast Missouri and southwest
Illinois Thursday night associated with this feature. Chances of
showers and thunderstorms linger heading into the Memorial Day
holiday weekend as additional shortwaves move through the
Plains/Upper Midwest. Forecast uncertainty however increases Friday
into Saturday as ensemble data shows more spread in the synoptic
pattern. WPC 500-hPa height clusters show mid/upper level ridging
over our area in 2 of the 4 clusters (40% of total membership from
the grand ensemble). This scenario would tend to be warmer, but
drier. Meanwhile, the other two clusters (60% of members) show zonal
or southwest flow aloft. This type of scenario likely would be more
active (better chances for showers and thunderstorms) along with
at least slightly cooler daytime high temperatures.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1020 PM CDT Sat May 18 2024

VFR conditions are expected throughout the 06Z TAF period for most
terminals, with the possible exception of fog overnight tonight
through early tomorrow morning. Patchy fog in low lying areas and
river valleys may once again impact typical fog prone terminals
like SUS and CPS with visibility reductions to MVFR. Brief periods
of lower visibility are also possible, even to LIFR, but
confidence is too low to include in the TAF at this time.

Otherwise, a limited potential for thunderstorms will exist at UIN
tomorrow afternoon and evening. A brief period of VCTS was added
to the TAF to reflect this, but confidence in direct impacts to
the terminal remain low.

BRC

&&

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

$$

WFO LSX