862
FXUS63 KLSX 121954
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
254 PM CDT Wed Jun 12 2024

.KEY MESSAGES...

- Temperatures are expected to run 5-15 degrees above normal for
  several days. Thursday, Sunday, and Monday continue to look
  like the warmest days with highs well into the 90s and several
  nights with lows no cooler than 70 degrees.

- Severe thunderstorms are possible Thursday evening, mainly over
  northeast Missouri and west-central Illinois. The primary
  threats will be large hail and damaging winds.

&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 249 PM CDT Wed Jun 12 2024

Surface ridging centered over the mid-Atlantic extends southwest
through Texas with light south-southwest flow over the mid-
Mississippi River Valley this afternoon. Clouds are far less
represented today than yesterday as the closest system passes over
the Upper Midwest. The southern extent of this system will send high
clouds near the Iowa/Missouri border with little impact farther
south. Local conditions are better characterized by an increase
in moisture from east to west with surface dewpoints in the
low-60s in Illinois to the mid over central Missouri. Ample
sunshine has allowed temperatures to respond in kind with
observations well into the 80s with low-90s largely confined to
urban areas.

The initial focus will be on increasing moisture and heat tonight
into Thursday. South and southwesterly flow is expected to continue
to feed moisture northward out of the western Gulf of Mexico into
the central Plains and Upper Midwest. Guidance has been consistent
with the surge of 20-25C air being transported from the central
Plains into central and northern Missouri. The warmest mid-level air
extends through central and northern Missouri, leading to a setup
with higher temperature dispersed across the northern two-thirds of
the CWA, generally along and north of I-44 in Missouri and I-64 in
Illinois. Widespread 90s are expected with the warmest reading
reaching the mid to upper 90s, especially around urban heat islands.
Heat index values approach 100-103 over central, east-central and
northeast Missouri into west-central Illinois.

Thunderstorms come into the picture late Thursday as a cold front
slowly drifts in from the north. Aside from the NAM`s occasional
north/south bounce in frontal position, guidance has been in
relatively good agreement. The front is expected to stretch east to
west through central Iowa late Thursday afternoon (20-22z). A strong
capping inversion is apparent ahead of the front, which should limit
convective initiation to the areas of best convergence along the
boundary. Guidance has been consistent in producing peak SBCAPE
indices around 4000 J/kg, mid-level lapse rate around 8C, and high
moisture content with dewpoints in the mid-70s pooling along the
front. 0-6km bulk shear has ticked up a bit in the last couple of
cycles with 40-45 knots now looking more likely. The limitations
largely stem from the northern track of the upper wave over northern
Illinois, relying on diurnal trends and surface convergence to break
through the inversion.

General agreement among the CAMs is for convective initiation to
commence north of the Iowa/Missouri border before shifting south
through the evening. The conditions suggest that once the cap
breaks, whatever is able bust through could be explosive with
initial discrete cells including large hail and damaging winds.
Mean mid-level flow runs parallel to the front suggesting discrete
development will quickly become linear and transition the primary
threat to damaging wind, some which could be strong enough to
support gusts near 70 mph. While hail could be as large as golf
ball size, this transition happens rather quickly, limiting this
threat to the far norther tier of counties in the CWA. Timing
places the front in the far northern sections of the CWA
(northeast Missouri/west- central Illinois) after 00z, well past
diurnal heating. Timing brings to question how long thunderstorms
survive given the last of upper support this far south and west of
the upper level shortwave and with the lack of a strong low level
jet. Weakening is favored as storms track south, limiting the
best potential for severe weather over northeast Missouri and
west- central Illinois with the threat all but entirely
diminishing along the I-70 corridor. The time to watch will be
00z-04z with much of the potential to low to consider much of a
threat thereafter.

Maples

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 249 PM CDT Wed Jun 12 2024

(Friday through Tuesday)

A stalling cold front is expected to extend along or just south of
the I-70 corridor early Friday morning. A remnant shower or isolated
thunderstorm from overnight convection cannot entirely be ruled out,
but most activity will have dissipated. The front drifts into
southern Missouri and Illinois by Friday afternoon with potential
for an isolated, diurnally driven shower or thunderstorm. Chances
were held below 15 percent with the lack of deep ascent.

An upper level ridge begins to translate eastward late Friday into
Saturday before an upper level shortwave ejects northeast from the
central Plains into the Upper Midwest. This eastward translation of
the upper ridge, in tandem with the upper level shortwave, will
motivate mid-level warm air advection to lift into central Missouri
Saturday morning. Meanwhile, surface high pressure over the Great
Lakes is expected to result in easterly flow at the surface with
dewpoints in the upper 50s to low-60s over the eastern half of the
CWA to near 70 from central to southeast Missouri. The surface
boundary remains stalled until later in the day, when the surface
high moves far enough east to result in return flow. Temperatures
remain warm, but relatively comfortable compared to what comes later
in the weekend. The general theme is for gradual increase in
moisture from the southwest late in the day. One question that
lingers is the chance for an isolated shower/thunderstorms Saturday
afternoon as the warm front begins to lift northeast. The core of
the upper support remains far enough north/west that chances remain
low enough to keep PoPs out of the forecast, but I would not be
surprised to see trends justify low end chances in subsequent
forecasts.

The warmest air continues to stretch from late this weekend into
early next week. It will be markedly more humid with dewpoints well
into the 60s to low-70s, pending the evolution of the mid and upper
ridge. Dp/Dt and GEFS upper level pressure contours show an ever-so-
slight southeastward shift in the ridge. While this sounds small,
shortwave energy continue to revolve around the western/northern
periphery of the upper ridge as the western side of the surface to
mid-level ridge feeds moisture northward out of the Gulf of Mexico.
If this shift continues, it may lend more questions than answers,
introducing the potential for diurnal convection late Sunday into
the early part of next week. Warm air seems to be solidly in place
on south/southwest flow with high in the 90s late this weekend into
next week. Sunday and Monday have the best promise for some mid to
upper 90s with an increasing spread heading into midweek. The caveat
will be the magnitude of warmth as it is dependent on these trends
and any shift in the ridge.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1241 PM CDT Wed Jun 12 2024

High pressure and northwesterly upper flow will result in VFR
conditions through the period. Light southerly flow will become
light and variable before returning to 5-10 knots Thursday
afternoon.

Maples

&&

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

$$

WFO LSX