079
FXUS63 KLSX 191100
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
600 AM CDT Fri Jul 19 2024

.KEY MESSAGES...

- Dry conditions with below average temperatures will continue through
  tomorrow.

- There is a chance of thunderstorms every day beginning Sunday
  and lasting through at least mid-week.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 326 AM CDT Fri Jul 19 2024

The short term can be summarized as: below average temperatures, dry
conditions, and the potential for nighttime fog. The surface high
that gave us a tranquil day yesterday will influence our sensible
weather for a couple more days before exiting the region completely
by Sunday. As such, quiet conditions will continue through Saturday,
albeit with incrementally warmer temperatures (more locations
reaching 80 degrees) and more diurnal cumulus. Low temperatures will
plummet into the upper 50s tonight amidst another stint of
radiational cooling conditions. With water temperatures well into
the 70s and 80s, steam fog is likely to occur in river valleys early
Saturday morning.

Surface high pressure will be shunted to the east as the influence
of mid-level troughing approaches the Mid-Mississippi Valley from
the northwest. Mid-level cloud cover will increase from this
direction as a result, creating a small northwest-southeast oriented
high temperature gradient on Saturday afternoon. Locations that see
more prolonged cloud cover will struggle to reach 80 degrees,
whereas locations with more sun exposure will likely see low 80s.

Jaja

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 326 AM CDT Fri Jul 19 2024

The slow moving mid-level trough or closed low won`t provide much in
terms of lift, it will bring consistent chances for showers and
thunderstorms from Sunday through at least mid-week as it meanders
over the Mid-Mississippi Valley. Its sluggish nature will allow the
system to keep slowly drawing moisture into the region each day.
With temperatures also moderating incrementally, instability will
increase. It`ll take a few days before instability reaches more
normal values for this time of year, however. Ensemble guidance
shows SBCAPE of around 100 J/kg Sunday and Monday, increasing to a
soft 1000 J/kg by Tuesday. Bulk shear values are more unimpressive.
The LREF shows values peaking at 15 kts on any given day, with the
area of the highest shear values generally decreasing as SBCAPE
increases, and/or the area of highest shear decoupled from the area
of highest SBCAPE. Soundings from the deterministic GFS show long,
skinny CAPE profiles for each day through Tuesday, suggesting slow
building updrafts. With these parameters considered, pulse
convection is the most likely mode through mid-week, and the
potential for severe weather is low at this juncture. With
increasing moisture return through mid-week, ensembles forecast PWAT
reaching the 75th percentile (using KSGF`s climatology) in our area
by Tuesday. If any thunderstorms are able to become robust, locally
heavy rainfall may be a potential hazard.

There`s uncertainty regarding when the active pattern will end.
Deterministic guidance is consistent in quickly ejecting the cutoff
low/shortwave during mid-week as it enters the mean flow. However,
another trough in the Great Lakes will follow it, and it`s influence
on our sensible weather is yet to be seen.

Jaja

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 600 AM CDT Fri Jul 19 2024

Dry and VFR conditions will prevail through the TAF period. Expect
diurnal cumulus at around 4500 to 5500 ft this afternoon. Otherwise,
winds will remain light and variable under the influence of exiting
surface high pressure.

Jaja

&&

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

$$

WFO LSX