855
FXUS63 KLSX 231908
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
208 PM CDT Mon Sep 23 2024

.KEY MESSAGES...

- There in a marginal risk for strong to severe thunderstorms
  this afternoon and evening mainly across southeast Missouri and
  southern Illinois. Damaging winds are the main threat.

- Conditions will clear Tuesday, leaving dry conditions for the
  mid- week period.

- Uncertainty remains in the return of rain Thursday through the
 weekend.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 207 PM CDT Mon Sep 23 2024

The region is socked in with heavy cloud cover once again today,
stunting temperatures significantly. As of 2 PM temperatures have
struggled to top 70 degrees in most areas. The mass of showers and
thunderstorms that moved into the area this morning have been
steadily moving northeast through the forecast area while another
area of showers is moving into central Missouri. A weak surface low
has developed across southwest Missouri in response to the
approaching mid-level shortwave, with its attendant stationary front
stretched across the Missouri-Arkansas border. As the mid-level
shortwave approaches additional showers and thunderstorms will spawn
across southern Missouri and Illinois this afternoon and evening.
How many develop and how strong they become depends on how much
instability is able to build, which has been limited by the
aforementioned heavy cloud cover. Current RAP analysis shows 500-
1000 J/kg of MUCAPE across the southern forecast area with higher
values across the warm sector south of the boundary. A few
thunderstorms could organize in the 30-40 kts of 0-6 km shear to
become strong to severe this afternoon. Damaging winds would be the
main concern in areas that are able to clear out and mix a little,
but an isolated case of 1" hail also cannot be ruled out.

The best environment for severe thunderstorm development will be
south of our forecast area where greater instability, as well as
enhanced lift and shear from the front will give thunderstorms an
extra boost. In this area, proximity to the front will locally
enhance the tornado potential (2%), despite ambient 10 kt 0-1 km
shear. If the front were to push northward over the next few hours,
these conditions could move into the southern CWA.

Showers and thunderstorms will linger into the overnight hours,
largely across eastern Missouri and western Illinois. With
precipitable water remaining near the climatological maximum for
this time of year (1.50-1.75") and the potential for convective
elements, heavy rain is once again a possibility with individual
thunderstorms. The 3 hourly PMM indicates up to 2 inches of
additional rainfall are possible within the areas of heaviest
precipitation. The progressive nature of the thunderstorms and 3
hour flash flood guidance still near 2-3" will keep flooding at bay,
though ponding of water in low lying areas is expected.

Showers and thunderstorms will linger into tomorrow across the area
as the mid-level trough associated with the previously mentioned
shortwave slides into the region. These are not expected to be
strong as moisture begins to erode with the approach of a surface
high into the mid-Mississippi Valley. Temperatures will begin to
rebound tomorrow from their unseasonably cool values today thanks to
increase insolation and less precipitation, highs will peak in the
low 70s.
Delia

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 207 PM CDT Mon Sep 23 2024

The mid-level trough will inch southward during the mid-week period,
with mid-level height rises and the advancing surface high keeping
conditions dry. At the same time many ensemble systems point to a
tropical system (Potential Tropical Cyclone Nine) pushing northward
into the Gulf of Mexico. By the start of the weekend the tropical
system is expected to get caught in the mid-level trough`s flow,
increasing moisture advection into the lower and mid-Mississippi
Valleys. The two systems will continue to interact while a Rex Block
sets up over the CONUS. Recent runs have shown a northward shift in
the system, the result is increasing precipitation chances for the
second half of the work week and weekend. As previously mentioned,
many of the ensemble systems depict this scenario, which is
resulting in very high PoPs (70-80%) in the extended period. These
PoPs seem too high given the uncertainty with the development,
strength, and movement of this tropical system, but confidence is
not high enough to make any significant changes.

Temperatures through the week will remain largely near normal.
Although more sunshine and less clouds and precipitation would
prompted more effective warming, particularly later in the period.

Delia

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1206 PM CDT Mon Sep 23 2024

IFR ceilings will dominate the TAF period at all terminals despite
short periods of improvement to MVFR or even VFR. IFR ceilings
will linger into tomorrow morning, lifting during the end of the
TAF period. After a brief dry period at the beginning of the
period rain and a few isolated thunderstorms will return to the
forecast area this afternoon. Another round of showers is possible
overnight. While MVFR visibility is expected with these showers,
reduced visibility is expected to linger overnight in the river
valley terminals (KJEF, KSUS, KCPS) where winds will go calm.

Winds will become northerly behind this system as a high pressure
pushes into the mid-Mississippi Valley.

Delia

&&

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

$$

WFO LSX