446
FXUS63 KLSX 110817
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
317 AM CDT Sat May 11 2024

.KEY MESSAGES...

- After a tranquil weekend, a relatively long period of showers
  and thunderstorms, widespread at times, is expected late Sunday
  night through Tuesday.

- There is another opportunity for showers and thunderstorms late
  in the week.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 315 AM CDT Sat May 11 2024

Upper-level northwesterly flow will remain in place over the
Mississippi River Valley today, with a trough traversing the Great
Lakes. A weak cold front associated with this trough is currently
passing the CWA, but will advance to the east this morning. This
front will remain dry and post-frontal low-level CAA will be weak
and brief, giving way to WAA by afternoon. The transition to WAA and
abundant insolation is expected to yield warmer, closer to average
temperatures from yesterday and in the 70s F CWA-wide. A surface SLP
ridge will arrive to the region this evening, favoring respectable
radiational cooling with light/calm winds and clear skies. As a
result, low temperatures in the upper 40s to mid-50s F are forecast.

As an upper-level closed low begins ejecting eastward from the
Intermountain West through the Central Plains on Sunday, a
transition from northwesterly to quasi-zonal flow is anticipated.
Following departure of the SLP ridge, low-level flow will become
southerly, strengthening WAA and supporting a return to above
average high temperatures in the upper 70s to low-80s F. Through
Sunday night, the strongest large-scale ascent associated with the
closed low will remain to the west of the CWA, but largely diurnally
driven isolated to scattered showers and thunderstorms are possible
(20 to 40 percent chance) in central and northeastern MO by Sunday
evening.

Pfahler

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 315 AM CDT Sat May 11 2024

The upper-level closed low is progged to continue eastward and reach
the Mid-Mississippi River Valley late Monday before finally exiting
on Tuesday. There are still slight differences in the exact timing
and evolution of the trough as well as timing and maturity of an
attendant surface cyclone tracking generally through the southern
half of the CWA.

Slow movement of the entire system will lead to a relatively long
duration of showers and thunderstorms in the CWA late Sunday
night through Tuesday, but coverage of showers and thunderstorms
should generally be greatest with a warm front lifting northward
into the CWA on Monday and behind the surface cyclone on Tuesday
within large-scale deformation. Ensemble model guidance
probabilities of total rainfall over 0.5" are higher than 60
percent across the entire CWA with a broad corridor of
probabilities of total rainfall over 1" at 40 to 60 percent now
centered on I-70, where rainfall is indicated to be most
persistent. Currently, confidence in instability and deep-layer
wind shear sufficient for severe thunderstorms is greatest in the
Lower-Mississippi River Valley on Monday, decreasing with
northward extent toward the CWA. Therefore, confidence is still
low in any strong to severe thunderstorms occurring in the CWA.
NBM temperature interquartile ranges have slowly closing with each
iteration toward cooler and near to below average temperatures on
Monday and Tuesday with widespread cloud cover and periods of
precip.

From Wednesday through the remainder of the week, global model
guidance become increasingly divergent in their depictions of the
upper-level wave pattern across much of the CONUS. That being said,
there is consensus for upper-level shortwave ridging to dominate the
Mid-Mississippi River Valley with a period of dry conditions and
moderating temperatures around Wednesday, before another upper-level
trough passes around Thursday into Friday. This trough will drive
the next opportunity for showers and thunderstorms, but there are
large differences in the structure, amplitude, and timing of the
trough. Temperatures are also expected to remain near to above
average through the end of the week.

Pfahler

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1120 PM CDT Fri May 10 2024

Dry and VFR flight conditions will persist through the TAF period. A
weak cold front passing overnight will veer winds to the northwest,
strengthening and gusting to 15 to 20 kt occasionally during the day
Saturday.

Pfahler

&&

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

$$

WFO LSX