801
FXUS63 KLSX 131117
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
617 AM CDT Sat Jul 13 2024

.KEY MESSAGES...

- Isolated to widely-scattered thunderstorms will once again
  threaten gusty winds and locally-heavy rain in parts of the
  region today, but drier conditions take hold Sunday through
  early Tuesday.

- Heat and humidity ramp up Sunday through Tuesday, when heat
  index values will range from 100-105 degrees. Monday has the
  highest potential for widespread 105+ degree heat index values.

- A slow-moving cold front will usher in renewed rain chances and
  cooler air starting late Tuesday. While below-normal
  temperatures stick around to start the weekend, rain chances
  shift south by Friday.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 335 AM CDT Sat Jul 13 2024

On the eastern periphery of a strong upper-level ridge centered
over the Four Corners region, mid-level northwest flow and subtle
shortwave impulses continue to stream from the northern Great
Plains into the Mid-Mississippi Valley. Amidst this weak
diffluence, a broad area of surface low pressure exists in the
north-central CONUS. Isolated convection currently firing along a
weak surface boundary on the nose of a 40-50kt low-level jet will
translate north and east through the day today across the forecast
area when the boundary lifts north as a warm front. With heights
aloft increasing coincident with the upper-level ridge expanding
eastward, synoptic- scale subsidence will keep the coverage and
intensity of convection in check. That said, amidst as much as
2000J/kg+ of MUCAPE and precipitable water values over 2.00 inches
(not unusual for mid- July), gusty winds and heavy rain would
accompany any convection.

With the warm front just about clearing the forecast area to the
north by sunset, further to our north an area of severe convection
is expected to develop and grow into an MCS as it dives southeast
through the Upper Mississippi Valley. The nose of the 850mb low-
level jet is still forecast to be north and east of the region
which, combined with the strengthening mid-level ridge, bodes well
for us remaining largely-removed from impactful thunderstorms. There
are persistent signals of a composite outflow dropping south during
the early morning Sunday and reaching the northern forecast area
after dawn. While the chance for thunderstorms to fire along it on
Sunday looks low (less than 15% chance), there will more likely than
not be some convective debris lingering for part of the morning into
the early afternoon.

This debris will exist where 850mb temperatures are warmest (22-24
degrees Celsius according to most guidance), which would act to
suppress warming Sunday and keep heat index values from rising to
more dangerous values. Further south, while temperatures look less
impacted by nocturnal convection remnants, mixing may be more
efficient with a southwest wind off the Ozarks. This will mute
humidity during peak heating, throwing exact heat index values into
some degree of doubt in that part of the region. The combined heat
and humidity will send heat index values into the low 100s for most,
with perhaps the very centers of the St. Louis and Columbia
metropolitan areas touching 105 degrees. If this scenario verifies,
it falls short of our criteria for a Heat Advisory at this time. It
is worth noting that if the outflow boundary that most of this
uncertainty hinges on does not impact warming Sunday morning,
temperatures and humidity will be sufficient for more widespread
dangerous heat.

MRB

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 335 AM CDT Sat Jul 13 2024

Heat concerns remain Monday and Tuesday as the upper level ridge
continues to expand east. Current guidance shows high confidence
(90%) that 850mb temperatures will reach at least 21 degrees Celsius
with surface dew points near 70 degrees Fahrenheit. These conditions
lead to heat index values at or above 105 degrees Monday along and
north of I-70 and 100 degrees Tuesday south of I-70. However,
certainty exists on Tuesday as the timing of an approaching cold
front and cloud cover associated with convection along the front
will impact temperatures and thus heat index values. Nonetheless,
heat related illness is a concern for Monday and potentially
Tuesday if the front`s timing is slower.

The dangerous heat won`t stick around too long, as an amplifying
upper level trough aloft supporting a low pressure center near
Hudson Bay helps establish a cold front in the Upper Midwest on
Tuesday. Current guidance suggests that the cold front will enter
northern Missouri late Tuesday afternoon into the evening. With this
front, convection is expected to form and grow upscale Tuesday
afternoon through Wednesday morning along the boundary. As the front
pushes south, northerly winds and increased cloud cover will cause
temperatures to fall into the mid-80s Wednesday and Thursday.
Various rippling shortwaves within the large scale trough are
expected to help push the cold front through the CWA by Thursday
afternoon. Additional thunderstorm chances (20-30%) exist
Wednesday and Thursday with additional shortwaves and vorticity
maxima overhead. Behind the front, precipitation chances decrease
to below 10% as the upper level forcing continues east, and
temperatures are expected to remain in the mid-80s Friday and
Saturday.

Scott/MRB

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 610 AM CDT Sat Jul 13 2024

Convection is ongoing at this hour in central Missouri, within the
vicinity of KCOU and KJEF. Neither have experienced direct impacts
at the terminal, and given the cells` current trajectory, they
likely won`t this morning. With upper-level ridging slowly
building into the region from the west, convective coverage will
be less than it`s been the last few days. While isolated
thunderstorms are an outside possibility this afternoon (too low
of a chance to mention in the TAF), a majority of the expected
activity will be confined to central Missouri on the nose of a
low-level jet. When that jet weakens later this morning, largely
VFR conditions will prevail at the terminals through Sunday
morning.

MRB

&&

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

$$

WFO LSX