153
FXUS63 KLSX 201859
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
159 PM CDT Sat Jul 20 2024

.KEY MESSAGES...

- Low chances of rain showers or weak thunderstorms exists
  beginning Sunday and continuing next week. Chances will tend to
  "peak" in the afternoon/early evening each day when there is a
  bit more instability.

- The activity the next several days looks spotty (much more dry
  time than not), with nothing organized/widespread expected. No
  severe weather or heavy rainfall is forecast.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 152 PM CDT Sat Jul 20 2024

The quasi-stationary area of surface high pressure is expected to
continue to weaken tonight over the Upper Midwest/western Great
Lakes. This means we will lose the predominant northeasterly winds
which were helping to maintain low dewpoints/relative humidity
values the past few days. This combined with an increase in mostly
midlevel cloud cover should yield warmer low temperatures
compared to the past few nights. Readings are expected to drop
back into the 60s, near to slightly below normal for the date. Dry
weather is also expected, though a stray shower or some sprinkles
cannot be ruled out overnight.

Isolated showers, and possibly a thunderstorm or two, are expected
late Sunday. This activity should be mainly along/west of the
Mississippi River and during the late afternoon/very early evening
hours. This is when diurnal instability reaches its peak. This
"peak" though certainly comes in a relative sense, with the HREF
only showing about 20-50% probabilities for >500 J/kg of SBCAPE
Sunday afternoon. Needless to say, those values are anomalously
low for mid/late July, and why any lightning activity should be
few and far between. This meager instability fades toward and just
after sunset Sunday, with dry weather then favored Sunday night.

Status quo is the rule for temperatures, with highs mostly in the
low 80s on Sunday and lows Sunday night in the 60s. The high
temperatures would be some 5-7 degrees below normal but lows near to
slightly below normal.


Gosselin

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 152 PM CDT Sat Jul 20 2024

There is very little change in the synoptic pattern through the
middle of the work week. The mid-Mississippi Valley is expected to
be beneath a weak mid/upper level shear axis. Isolated to scattered
showers and thunderstorms are possible each day, mainly during the
afternoon/very early evening hours when instability is highest.
Overnight periods should be dry, or at least mostly dry, due to
little/no sensible low-level jet and associated moisture
convergence. The strongest low-level jet is Monday night (~15
knots), but that is centered across the mid south.

Activity each day also should be unorganized with very little threat
for any strong to severe convection or heavy rainfall. While
instability should creep up into next week (probabilities on the
grand ensemble for >1000 J/kg above 50% beginning Wednesday
afternoon), deep-layer shear is expected to be less than 15-20
knots. In addition, precipitable water values on the NAEFS peak
out around 1.50", which would be only slightly above normal for
mid/late July. While low PoPs linger all the way through the end
of the work week, they do shift a bit more to the southeast. There
is a pretty good signal for a cold frontal passage by Thursday,
but uncertainty comes in with respect to how far south it gets
before stalling. If it stalls far enough south of the area, then
dry weather would become more likely whereas PoPs may be need to
expand if the front stalls out further north somewhere across the
CWA.

Some additional gradual warming is likely through the period, with
low temperatures warming more toward the mid 60s to near 70 degrees
(near normal) and highs in the mid to upper 80s (~2-4F below
normal). The NBM has very little spread between the 25th/75th
percentile, even at day 7 (~4F). This makes sense given the very
slowly evolving mid/upper level pattern across the CONUS and
little/no likelihood for widespread organized convection, which
could really limit daytime temperatures.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1217 PM CDT Sat Jul 20 2024

Light/variable winds with a gradual increase in mostly midlevel
cloudiness is forecast through the TAF period. An isolated shower
or sprinkle cannot be ruled out in central Missouri this
afternoon. Otherwise, dry weather is forecast through the period.
Isolated showers and possible a couple of thunderstorms are
expected to develop on Sunday afternoon, but this will be largely
just after the valid TAF package. However, the probabilities for
this activity impacting any terminal appear to be pretty low at
this early juncture.

Gosselin

&&

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

$$

WFO LSX