566
FXUS63 KLSX 121113
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
613 AM CDT Fri Jul 12 2024

.KEY MESSAGES...

- Isolated to scattered thunderstorms, featuring gusty winds to 40mph
  and heavy downpours, will continue to threaten portions of the
  area through early Sunday morning. Again, plenty of dry time is
  forecast in between convection.

- Abnormal heat will impact the region Sunday through Tuesday,
  with Monday being the hottest and most humid day when heat index
  values exceed 100 degrees across most of eastern Missouri and
  western Illinois.

- Multiple rain chances exist Wednesday through Friday along a
  slow-moving front, which will bring temperatures back to more
  seasonable (if not below-normal) values.

&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 321 AM CDT Fri Jul 12 2024

A west-east oriented stationary front is currently draped across the
northern fringe of the Ozark Plateau into south-central Illinois. On
the nose of a low-level jet and amidst a mid-level shortwave
impulse, strong convection blossomed late yesterday night in
southeast Kansas and southwest Missouri. As it moves east, however,
the mid-level support weakens and most convection struggled to reach
the forecast area. The stronger thunderstorms have grown upscale
into a north-south oriented line just west of Joplin, Missouri. This
feature, or at least the outflow boundary it generates, will act
with the stalled front and weakening (but still present) mid-level
forcing to promote continued isolated to scattered thunderstorms
along and south of I-70 today. With MLCAPE values still more than
sufficient for deep convection, gusty winds to 40mph and heavy
downpours are the main threat with these thunderstorms. The
continued lack of any noteworthy shear will keep nearly all
convection beneath severe strength. Otherwise, temperatures today
will remain near mid-July normals (upper 80s to low 90s).

The strong mid-level ridge centered over the western CONUS is still
forecast to expand east gradually overnight into Saturday morning,
however the returning nocturnal low-level jet will allow for
additional convection to blossom across western Missouri and drift
through the forecast area through the day on Saturday. That said,
once the LLJ weakens, height rises aloft would make it more
difficult for convection to persist into the day. That said, with
surface-based instability still in the 1500-2500J/kg range, at least
isolated thunderstorms can be expected Saturday with a weak warm
front lifting north through the region. In the wake of the warm
front, temperatures climb into the 90s areawide as we continue a
slow march to abnormal warmth by early next week.

MRB

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 321 AM CDT Fri Jul 12 2024

The weak warm front lifts north of the region by Saturday evening as
mid-level ridging expands further into the Mississippi Valley. At
the same time, a shortwave tracks along the ridge`s periphery and
sparks convection in the northern Plains into the Great Lakes
region. With the nose of the 850mb low-level jet uniformly forecast
to be closer to northeast Iowa, the bi-state area looks too far
removed from favorable forcing and instability to pose a threat for
strong to severe thunderstorms overnight Saturday into Sunday
morning. Remnant convection, possibly on the outflow of the
nocturnal MCS to our north, is possible (roughly 20% chance) Sunday
morning into the early afternoon, but it will be fighting a very
strong capping inversion and mid-level subsidence.

By this point, attention turns to the strengthening 850mb thermal
ridge and the increasing heat index values. Little has changed from
previous forecasts regarding the large-scale pattern: the eastern
periphery of the mid-level ridge expands east over us and increased
southwest flow from 850mb to the surface takes hold. Temperatures
are almost certainly (80-90% chance) to climb into the mid/upper 90s
by Sunday for all except the higher elevations of the Ozarks. While
southwest winds can act to deepen mixing and lower humidity more
than a due south wind, the recent stretch of above-normal rainfall
is keeping the topmost layer of the ground relatively moist. Amidst
these warmer temperatures, increased evapotranspiration and resident
low-level atmospheric moisture should counteract any impacts from
the Ozarks. That said, except for directly in river valleys or in
the middle of the local metropolitan areas, heat index values will
top off in the 100-105 range Sunday.

Monday looks to be the hottest day of the forecast period, with
temperatures approaching 100 degrees in the major cities and
mid/upper 90s for most of the remaining region. With similar
humidity to Sunday`s values, and 850mb temperatures/surface flow
pattern matching the patterns that historically lead to 105 degree
heat index values, the best potential for dangerous heat and
humidity exists Monday. Meanwhile, the ridge aloft begins to erode
from the north as longwave troughing from Canada and shortwaves
tracking along the cyclonic flow push a front gradually south.
Tuesday still looks abnormally warm, but there is growing potential
that either cloud cover or cooler air from the front itself would
begin to impact the region. The forecast highs (and maximum heat
index values) have cooled a few degrees compared to previous
iterations in response to this scenario becoming more prominent in
the ensemble guidance.

While some uncertainty in the longwave trough`s position and
amplitude exists, the front decidedly arrives in our north by
Wednesday. The combination of marginally-cooler air and increased
cloud cover/precipitation extent will bring temperatures back to the
mid/upper 80s Wednesday, and cooler still into the end of the work
week. There are no indications that it will rain continuously from
Wednesday onward, but exactly when and where it rains will depend on
shortwaves tracking along the stalled boundary and base of the
longwave trough (all of which is not known at this point). Each
disturbance pushes the front a bit further south, with a gradual
drying trend from Wednesday to Friday from north to south.

MRB

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 605 AM CDT Fri Jul 12 2024

Ongoing convection south of the I-70 corridor and the TAF sites is
forecast to largely remain there through the day and into the
evening, leaving the terminals dry and VFR through this forecast.
That said, there is a low (15-20%) chance that a thunderstorm
would impact the central Missouri and St. Louis metro terminals
from now through sunset. Given the low probability and current
trajectory of the convection, it was not included in the forecast.
Winds gradually turn more southerly and southwesterly with time
as a warm front lifts north tomorrow morning.

MRB

&&

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

$$

WFO LSX