561
FXUS63 KLSX 242337
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
637 PM CDT Mon Jun 24 2024

.KEY MESSAGES...

- The Heat Advisory remains in effect, and has been expanded into
  south-central Illinois on Tuesday.

-There remains a slight risk for strong to severe thunderstorms
 Tuesday evening and overnight, largely north of I-70. The main
 hazards will be damaging winds and large hail. This is the best
 chance for a soaking rain for the next week.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 209 PM CDT Mon Jun 24 2024

The region remains on the northeastern side of a mid-level ridge
that is centered over the southern CONUS, while in the lower levels
southwesterly flow continues warm air advection into the region.
Combined with a clear sky, temperatures are rocketing into the 90s
across the region today. Across central Missouri, where the nose
of the warm air advection is, heat index values have jumped above
100.

Overnight a shortwave will move through the quasi-zonal flow across
the Great Lakes region, producing an MCS that will march through the
Upper Mississippi Valley. How far south this convection, and any
additional isolated convection that forms along the southwest end of
the MCS, extends remains uncertain at this point. Weak forcing
across the area is not favorable for widespread thunderstorm
development. However, I cannot rule out an isolated shower or
thunderstorms moving into far northeast Missouri and west-central
Illinois tomorrow morning around sunrise. Plenty of instability will
be present (1000-3000 J/kg of MUCAPE), but shear will be weak (10-15
kts) so the stronger storms that linger across the northern CWA will
be capable of small hail.

Tuesday will be the hottest day of the week with another day of
robust warm air advection and sunshine. After warm overnight lows in
the mid to upper 70s, temperatures will warm into the mid 90s to
100, with the hottest temperatures within the St. Louis metro. The
Heat Advisory has been expanded into south central Illinois to
account for the increased temperatures and heat index values in this
area. Instability will likewise rocket tomorrow with deterministic
guidance indicating 2000-4000+ J/kg of SBCAPE will be available
tomorrow afternoon. Air mass thunderstorms will be possible across
the forecast area during the afternoon, though short-lived under
a weak flow regime with only 10-15 kts of 0-6 km shear.

The better chance for rain is overnight ahead of an
approaching cold front. An MCS is expected to develop across the
Upper Mississippi Valley and push southeastward into Missouri and
Illinois overnight, driven aloft by a shortwave embedded within the
northern periphery of the mid-level ridge. This MCS will bring much
needed rain to some parts of the mid-Mississippi Valley, while
others will remain dry. The location of the MCS and beneficial
rainfall remains uncertain with hi-res CAMs split on whether the MCS
moves through our forecast area, or moves largely to our west. Where
any outflow boundaries from tonight linger will influence where
convection forms and congeals. Instability will remain high into
the overnight hours (1500-3000 J/kg of MUCAPE), and 10-30 kts of
0-6 km shear will be able to bolster organization and maintenance
of the MCS overnight. Any thunderstorms that become strong to
severe will be capable of damaging winds and quarter sized hail.
The best location for strong to severe thunderstorms Tuesday night
will be along and north of I-70 where the best shear will be.
Uncertainty remains in how strong that shear will be, so doubt
remains that any thunderstorms will be able to strengthen to that
extent. Rainfall rates underneath thunderstorms could reach 1-2
inches over a 3 hourly period, providing a much needed soaking
rain for some.

MRM

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 209 PM CDT Mon Jun 24 2024

A cold front will follow behind the MCS, it`s attendant
surface low moving east of the Great Lakes region by Wednesday
morning. This front will slowly drag southward until another mid-
level shortwave moves through the northern stream of the westerlies
and phases with the overnight shortwave described above. This will
kick the cold front through the remainder of the forecast area
Wednesday morning. Low-level cold air advection in its wake will
bring sweet relief to the forecast area, dropping high temperatures
to near normal (mid to upper 80s) for Wednesday and Thursday. As the
low-level high shifts eastward towards the end of the work week,
southwesterly flow and associated warm air advection will
reestablish. Temperatures will warm Friday back above normal, but
this shot of hot air will be short lived. From Friday into the
weekend the region will remain on the southern edge of the
westerlies with a broad ridge stretching across the southern CONUS.
A mid-level trough will slide along the US-Canada border Friday into
Saturday, it`s surface reflection taking a similar path. An
attendant cold front is expected to move through the area during the
weekend, bringing the next best chance of showers and thunderstorms
to the area. Uncertainty remains as to the timing and strength of
this system, so who receives rain and how much remains uncertain.
Behind this system another shot of cold air advection will help cool
the region back to near normal.

MRM

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 636 PM CDT Mon Jun 24 2024

VFR conditions will persist this evening into the overnight hours
with mostly clear skies and relatively light surface flow.

There is a high degree of uncertainty heading into Tuesday.
Thunderstorms over the Upper Midwest are expected to send and
outflow boundary southward Tuesday morning, potential impacting
KUIN by mid-morning with showers and thunderstorms. The outflow
reaches metro terminals (KSTL/KSUS/KCPS) between 18z-20z before
stalling. The western reaches of the outflow flange out into a
northwest to southeast oriented boundary by the time it reaches
KCOU and KJEF in the mid-afternoon (20z or after). However, all
that being said, the weak flow environment, along with placement
and timing discrepancies in guidance give little confidence in
the spread and magnitude of thunderstorms, let alone direct
impacts. This argues to leave thunderstorms out of central MO and
metro terminals. The exception in KUIN, where thunderstorms are
slightly more certain with timing being the biggest question.

The best potential for thunderstorms likely results from new
development late Tuesday evening into Tuesday beyond the 24 hour
period. Should the complex materialize, a range of MVFR/IFR
conditions could result within the line of thunderstorms largely
due to visibility restrictions. A broader spread in stratiform
rain and thunderstorms could then play out behind the initial
line, especially from central Missouri through southwest Illinois.
Timing for this looks to be after 04z Tuesday night.

Maples

&&

.CLIMATE...
Issued at 353 AM CDT Mon Jun 24 2024

With hot temperatures forecast, 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...Heat Advisory from noon to 8 PM CDT Tuesday for Audrain MO-
     Callaway MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-
     Jefferson MO-Lincoln MO-Madison MO-Montgomery MO-Osage MO-
     Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
     City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-
     Washington MO.

     Heat Advisory until 8 PM CDT Tuesday for Boone MO-Cole MO-
     Moniteau MO.

IL...Heat Advisory from noon to 8 PM CDT Tuesday for Bond IL-Clinton
     IL-Fayette IL-Madison IL-Marion IL-Monroe IL-Randolph IL-
     Saint Clair IL-Washington IL.

&&

$$

WFO LSX