710
FXUS63 KLSX 311913
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
213 PM CDT Fri May 31 2024

.KEY MESSAGES...

- Rain will overspread the area tonight lasting into Saturday
  morning with potential for a little embedded thunder. Isolated
  to scattered shower and thunderstorms will then impact the area
  Saturday afternoon and early evening.

- A short period of largely dry weather will prevail from late
  Saturday evening through Sunday morning, with shower and
  thunderstorm chances return Sunday afternoon continuing through
  the middle of next week.

- After a day of cooler weather and below normal temperatures on
  Saturday, we rebound to above normal temperatures and heat more
  typical of summer starting Sunday lasting into the middle of
  next week.

&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 154 PM CDT Fri May 31 2024

A broad mid-upper trof with embedded vort maxes and currently
centered through eastern KS will move eastward into MO tonight,
with the axis moving east of the CWA by early Saturday evening.
These vort maxes along with an evolving and strengthening warm
conveyor belt featuring a veering S-SE LLJ will modulate
precipitation coverage and intensity over the next 36 hours.
Precipitation has been slow to evolve initially but as we head
into the evening the combination of the increasing LLJ, attendant
moisture transport and convergence along with larger scale
forcing should lead to widespread showers spreading and
developing northward through central and eastern MO, then shifting
east/northeast into the overnight hours. There is not much in the
way of instability forecast with the precipitation tonight, but
the appearance of negative showalter indices late evening and
overnight attendant with some mid-level cooling and better low-
level forcing would suggest a threat of some embedded elevated
thunderstorms overnight into mid-Saturday morning, especially
towards the backside of the overall precipitation shield. The
moisture transport will lead to PWs increasing to 1.5+ inches
during this time frame and given this moisture increase, slow cell
motions, and some potential for cell training we could see some
localized corridors of heavier rainfall within an SSW-NNE zone
extending from between STL-COU into west-central IL. Both the PPM
and LPPM guidance from the HREF support this idea with pockets of
1.5-3.0 inches of rainfall. Unless this falls in an hour or so or
the amounts are higher, which would probably necessitate more
convection, then the overall flood threat appears low at this
time.

The main thrust of more continuous precipitation/greater
coverage of showers should move east of the CWA by 18z or so on
Saturday as the mean mid-upper trof axis progresses east. However
diurnal destabilization in the wake of the axis passage (afternoon
MLCAPE of 1000-1500+), and weak surface convergence attendant
with the surface trof that is attendant with the upper system,
will fuel the development of additional isolated-scattered showers
and thunderstorms across eastern MO and SW IL into early evening
along/ahead of the advancing surface trof/convergence axis. The
aforementioned storms would pose a threat of heavy downpours and
lightning. All indications are any ongoing showers and storms
should dissipate rather quickly over SW IL on Saturday evening
with loss of heating/instability, and the remainder of Saturday
night should be dry and tranquil.

Glass

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 154 PM CDT Fri May 31 2024

Quasi-zonal mid-upper level flow will develop Saturday night into
early next week. This regime will initially feature lower-
amplitude disturbances through Sunday night, with more prominent
short-waves centered thru the mid-upper MS Valley traversing the
region Monday-Tuesday. These disturbances along with plentiful
instability will bring prolonged thunderstorm chances to the
region, first returning late Sunday afternoon in the western CWA.
All day rains don`t appear likely, and despite the prolonged
precipitation chances, there will be plenty of dry time. Depending
on the timing of the waves and amount of deep layer shear
present, this potential could include organized multicell strong
storms in the afternoon/evening. Also attendant with the quasi-
zonal regime will be the return of largely south-southwest low
level flow and WAA, resulting in a warmer weather and above normal
temperatures somewhat typical of June/early Summer.

Longer range guidance strongly suggests a significant pattern change
will take place starting late Tuesday and progressing through the
end of next week featuring a deepening northern stream upper trof
sliding into the eastern CONUS. This trof is eventually forecast
to be the dominant steering feature for the MS Valley eastward.
While there is generally a consensus in this change and the
development of NW flow aloft, there are substantial differences
with the ensemble systems evident in the LREF cluster analysis
regarding the details of the depth and position of the trof and
the magnitude of heights aloft. The initial short wave trof
starting the pattern change will drive a cold front through the
area, most likely in the Tuesday night-Wednesday morning time
frame. An earlier cold fropa would possibly bring a potential for
stronger storms due to greater instability. The differences in the
pattern details and heights aloft lead to lower confidence in
temperatures beyond Wednesday with the LREF IQR for highs on
Thursday on the order of 15 degrees ranging from the low-mid 70s
to upper 80s.

Glass

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1248 PM CDT Fri May 31 2024

VFR flight conditions will prevail this afternoon with gusty
southeast winds. Showers will overspread the area tonight
accompanied by gradually lowering flight conditions with all of
the terminals deteriorating to low MVFR or IFR overnight into
early Saturday morning. There could be some brief embedded thunder
accompanying the showers late this evening into Saturday but the
coverage and probability is too low to mention.

Glass

&&

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

$$

WFO LSX