889
FXUS63 KLSX 202045
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
345 PM CDT Fri Sep 20 2024

.KEY MESSAGES...

- An active pattern will continue through early next week. There
  is chance of showers and thunderstorms present each day through
  at least Monday.

- This period presents our best chance at beneficial rainfall in
  quite some time. Our highest chances at seeing this will be
  Saturday night into Sunday morning.


&&

.SHORT TERM... (Through Late Saturday)
Issued at 342 PM CDT Fri Sep 20 2024

A cold front is pushing through the forecast area today, though
calling it a "cold front" seems like a misnomer given the well above
average temperatures on its backside. What this front does
distinguish is the boundary between moist and less moist airmasses.
On the moist side, surface based instability is maximized with
values reaching up to 3500 J/kg. Deep layer shear of 30 - 35 kts is
also in place, presenting the possibility of strong to severe
thunderstorms later this afternoon into this evening. There are
substantial caveats to the potential for severe today. Model
soundings show a deep layer of mid-level dry air and low mid-level
lapse rates over the forecast area. Short term guidance has remained
consistent in keeping portions of southeast Missouri and southern
Illinois capped, though how far north this extends into our southern
CWA is uncertain. This is important considering the southern pushing
cold front and the fact that the only potential for thunderstorms
lies in or near this capped air. If we are not impacted by
capping, convection will be a characterized by bubbling cu
struggling against dry air entrainment, with some updrafts
prevailing over others. For this reason, isolated thunderstorm
coverage is expected. If a thunderstorm does become strong to
severe, the excess dry air aloft will make damaging winds the
primary threat. These are expected to diminish late this evening.

We`ll see a brief lull during the late night hours. During this
period, temperatures will fall into the 60s and 70s - much closer to
our average highs for mid-September. Similar dewpoint temperatures
will yield an unseasonably sticky night tonight.

On the synoptic scale, today`s southeast pushing cold front will
buckle back to the north as a warm front overnight, leaving us
within a moist warm sector. A 30 kt low-level jet is forecast to
develop over Kansas and western Missouri, sparking convection that
will move into the forecast area Saturday morning. Showers and
thunderstorms will diminish with eastward extent and with time as
they stray further from the nose of the low-level jet and as the low-
level jet diminishes, respectively. Most locations will remain dry
in the afternoon apart from some isolated to widely scattered
convection. Widespread 80s are forecast on Saturday with low 80s in
northeast and central Missouri where rain and thicker cloud cover
will be more persistent. Further east where the opposite is true,
highs will reach closer to 90 degrees.

Jaja

&&

.LONG TERM... (Saturday Night through Next Friday)
Issued at 342 PM CDT Fri Sep 20 2024

The best chance of rain and thunderstorms for our area will be
Saturday night into Sunday as a series of vorticity maxima push
through northern Missouri. The region will still be quite moist,
being in the warm sector between a retreating warm front and
advancing cold front to the northwest. At the same time, a west-
southwest oriented 40 - 45 kt low-level jet will ramp up, providing
a focus for convective initiation overnight. With PWAT values of
1.50" - 2.00", there will be plenty of moisture for the atmosphere
to work with. The same can`t be said for instability overnight, but
enough will be present for the development of some non-severe
thunderstorms. Flooding is not expected to be an issue given very
dry antecedent conditions and the progressive nature of the rain.
The only impact expected is beneficial rainfall, some of it heavy in
the luckiest spots.

The current drought monitor has our entire forecast area in at least
D0 conditions (abnormally dry), with the majority of our counties in
D1 (moderate drought). The period between today and Tuesday will be
our best chance at improving those conditions in quite some time.
Ensemble probabilities of total rainfall of 1"+ between now and
Tuesday evening are between 75 - 90% for much of the CWA. Is this
locked in? No, but it does give hope that we`ll see more substantial
rain than what we`ve gotten recently. With some locations not having
seen more than a third of an inch of rain in several weeks, any rain
is beneficial at this point.

SPC has our region in a marginal risk again for Sunday afternoon as
instability and shear increase. Currently, ensemble guidance shows
anywhere from 500 - 1500 J/kg of SBCAPE and 20 - 35 kts of bulk
shear at 1pm Sunday. What`s notable is that the instability and
shear maxima aren`t collocated well, with instability being further
south and better shear to the north, behind the passing cold front.
Throw in the possibility of lingering morning convection, and the
potential for strong to severe thunderstorms becomes even more
uncertain for Sunday afternoon.

The potential for showers and thunderstorms will continue into
Monday and Tuesday as the aforementioned cold front stalls over the
area and vorticity lobes from an approaching shortwave shoot over
it. Being generally on the backside of the front at this point,
temperatures will be much closer to normal. So although dewpoints
may still be in the 60s across much of the region, cooler
temperatures will dampen instability and thus our chances for strong
thunderstorms during this period. The front will exit the region
Tuesday as the shortwave swings through the region, ending our rain
chances through at least the end of the week.


Jaja

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1240 PM CDT Fri Sep 20 2024

The largest changes to the TAFs this issuance are the removal of
thunderstorms at the St. Louis metro terminals this afternoon and
the addition of rain at all terminals tomorrow morning. This
afternoon`s/evening`s thunderstorm potential has fallen, and any
thunderstorms that do develop are expected to remain further south
and east than previously forecast. Showers will ramp up overnight,
with the Mid-Missouri terminals having the highest likelihood at
seeing precipitation. Elsewhere confidence decreases in whether a
terminal will see rain and when exactly it will occur. Therefore,
the 4 hour VCSH window is a best estimate. Rain may start sooner
and/or end later, but rain will not be falling during the entire
duration. Otherwise, winds will be variable during most of the TAF
period as a cold front sinks south today and buckles back northward
overnight and into tomorrow.

Jaja

&&

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

$$

WFO LSX