648
FXUS63 KLSX 291119
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
619 AM CDT Sat Jun 29 2024

.KEY MESSAGES...

- Temperatures in the low to mid-90s will combine with dewpoints
  well into the 70s to result in heat index values near 105
  degrees over sections of southeast Missouri. A heat advisory
  will go into effect from 12 p.m. to 7 p.m. today.

- Isolated to widely scattered thunderstorms this afternoon into
  this evening could be strong to severe, mainly along and south
  of I-70. The primary threats are damaging winds and large hail.

- Below normal temperatures and low dewpoints Sunday and Monday
  will result in pleasant conditions. The active, warm pattern
  returns from mid to late week.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 332 AM CDT Sat Jun 29 2024

Water vapor imagery shows a broad mass of cloud cover remnant of a
decayed MCS that originated over eastern Kansas and tracked into
western Missouri overnight. The decay of this complex was a result
of rapidly decreasing MUCAPE from west to east with a notable void
in theta-e over sections of central and eastern Missouri.
Interestingly, MUCAPE increased east of the Mississippi River, where
a few showers and thunderstorms occasionally, and briefly, cropped
up on radar. Support has been rather limited, which was evident of
the short-lived updrafts earlier this morning. What remains now is
an elongated, northeast to southwest line of showers with a few
lightning strikes in the limited convective support. This activity
is expected to continue to erode with eastward travel this morning.

The main impact of this initial wave will be the cloud debris that
lingers through this morning, which may attempt to break from late
morning to early afternoon. The focus then shifts to the northwest
with the approach of a slow moving cold front that, as of 08z,
extend north to south through central Iowa into eastern Kansas. This
front will slowly sink south through the day, encountering dewpoints
well into the 70s. HREF ensembles show mean SBCAPE values recovering
back to around 2500 J/kg along and ahead of the front late this
afternoon into this evening, while some raw guidance suggests these
values could be in the 3000-3500 J/kg range. The limiting factors
may be the lack of strong, deep ascent with much of the vorticity
progs fragmented in nature and largely distributed through the mid-
levels. Additionally, 0-6km shear values are modest at 20-30 knots.
A weak surface inflection ridge along the boundary late this
afternoon into this evening, likely after 22z, setting the stage for
isolated to widely scattered showers and thunderstorms, mainly along
and south of I-70. Modeled sounding profiles show 6C to perhaps
locally 7.5C mid-level lapse rates with respectable CAPE profiles
that could utilize afternoon instability for isolated large hail
threat. The moisture rich environment, however, show little sign of
a dry layer aloft to suggest wind will be a big threat, though the
collapse of any strong updrafts could result in a localized 60 mph
gust.

This also leads to another item to address - the heat advisory over
the far southern sections of the CWA. Given the slow progress of the
front, there may be enough time for at least some sunshine. The
moisture is obviously not an issue with the uncomfortably high
dewpoints. Moisture pooling ahead of the front, combined with
temperatures in the low to mid-90s will lead to heat index value
near 105 in these areas. Therefore, the heat advisory will continue
as advertised.

The cold front will sink south late tonight into early Sunday.
Surface high pressure, which strengthens to near NAEFS
climatological maxes, will build into the region for the later half
of the weekend. This will bring refreshingly cooler temperatures and
lower dewpoints with highs that are 10-15 degrees below normal.


Maples

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 332 AM CDT Sat Jun 29 2024

Upper ridging begins to build in from the PLains as the anomalously
strong surface high centers over the Great Lakes Monday. Easterly
surface flow continues to funnel dry air into the region with HREF
ensembles showing 60-90% probabilities for dewpoints at or below 50
degrees from eastern Missouri into Illinois. The dry airmass and
highs in the upper 70s to low-80s will make a spectacular start to
the work week.

Surface flow turns out of the south from west to east late Monday
into Tuesday as the surface high moves into the New England Region.
Warm air advection will mark the end of the more pleasant conditions
with a return to the 90s Tuesday. Despite and evident and stark
increase in mid-level moisture, the rise in surface dewpoints will
be more gradual as mid to upper 50s hang on through the early half
of the day. 70 degree dewpoints edge into central and northeast
Missouri by early afternoon, while southeast Missouri and Illinois
locations remain in the low to mid-60s. Though gradually more moist,
conditions will be more bearable over eastern sections of the CWA.
The trend continues into Wednesday, when dewpoints in the 70s
encompass the entire area. Each afternoon, where air temperatures
line up in the 90s and dewpoints reach into the 70s, heat index
value could approach advisory levels, barring precipitation chances
increase heading into Wednesday.

From mid to late week, the pattern begins to look similar to that we
are experiencing late this week. The western periphery of the
surface to mid-level ridge takes on a southwest to northeast lean.
An upper ridge that extends into the Great Lake begins to flatten
with the approach of a broad upper trough. As the upper trough
closes in, a surface cold front slides underneath mid and upper
shortwaves that introduce potential for showers and thunderstorms as
soon as late Tuesday and possibly through the end of the work week.
500mb height clusters do show decent agreement in the mean pattern
through day 5 with little spread through day 7. I believe the
questions will be in the surface to mid-level synoptic layout with
regard to the timing and position of the surface front, along with
the finer details in the shortwave features. Where and when these
features line up could provide a good chance for rainfall with LREF
ensembles showing 60-70% probabilities for a return to anomalous
PWAT values at or above 2 inches.


Maples

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 618 AM CDT Sat Jun 29 2024

A cold front is beginning to drift into the region from the north
this morning with scattered showers and isolated thunderstorms
running along and well ahead of the front. Showers will be
intermittent this morning with the potential for thunder highest
south and west of the central Missouri terminals. The initial
impact will be MVFR ceilings just behind the front at KUIN around
mid-morning.

As the front shifts south this afternoon, isolated to widely
scattered thunderstorms are expected to develop, mainly around and
south of the terminals in central Missouri as well as in the metro
area. Given the lack of coverage, direct impacts are uncertain and
therefore were accounted for with a few hours of VCTS at their
respective sites. A storm or two could become strong with hail and
gusty winds with brief reductions to visibility (mainly MVFR).
However, the chances for these conditions to be realized at any
one site is low. MVFR ceilings are less likely provided timing of
the front being during and just after diurnal peak.

Once the front passes, conditions are expected to remain VFR
through the remainder of the period.

Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon today to 7 PM CDT this evening for
     Crawford MO-Iron MO-Madison MO-Reynolds MO-Saint Francois
     MO-Sainte Genevieve MO-Washington MO.

IL...None.
&&

$$

WFO LSX