000
FXUS63 KLSX 122021
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
321 PM CDT Fri Apr 12 2024

.KEY MESSAGES...

- Elevated Fire Danger is expected again on Saturday afternoon
  into early evening across southeastern, south-central, east-
  central MO and west-central IL due to low relative humidity,
  increasing south- southwesterly winds, and dry dead vegetation.

- Rounds of showers and thunderstorms are expected next week, with the
  threat for strong to severe thunderstorms on Tuesday. However,
  details regarding the threat are still uncertain and need to be
  resolved.

- Temperatures will warm to well above average on Saturday,
  persisting through much of the first half of next week.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 317 PM CDT Fri Apr 12 2024

West-northwesterly winds with occasional gusts of 25 to 35 kt will
slacken quickly this evening upon arrival of a surface MSLP ridge
tonight. Large-scale subsidence with broad upper-level ridging will
promote clear skies tonight, but a loose MSLP gradient will preclude
pristine radiational cooling and BL decoupling with low temperatures
falling into the 40s F.

On Saturday, the MSLP ridge will shift entirely east of the CWA by
midday, allowing south-southwesterly winds to strengthen through the
day. Full insolation and the onset of appreciable low-level WAA are
anticipated to support a substantial warmup from today with high
temperatures reaching the low to mid-80s F, except across much of
Illinois where WAA onset will be later. There is some question
regarding the quality of initial moisture transport into the region,
which will be important in determining if Elevated Fire Weather
conditions are reached on Saturday as any dead vegetation continues
to dry and cure. Given that 50+ F dewpoints are currently confined
to south TX and the Gulf Coast and there could be some component of
low-level south-southwesterly flow that is downsloped off the Ozark
Mountains, latest forecast dewpoints for Saturday are closer to the
10th percentile of model guidance. As a result, Elevated Fire
Conditions are now forecast Saturday afternoon into early evening
across southeastern, south-central, east-central MO into west-
central IL.

Pfahler

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 317 PM CDT Fri Apr 12 2024

Broad upper-level ridging is still anticipated to be the dominant
feature Sunday into early Monday, but an embedded shortwave trough
is progged to traverse the Mid-Mississippi River Valley late
Saturday night into Sunday morning. A strengthening capping
inversion and lack of deeper moisture at low levels increases
confidence that this trough will not be able to yield more than
passing upper and mid-level clouds. Moisture and instability will
increase later on Sunday with a weakening cold front moving
southward into the CWA during the evening and a lift back northward
as a warm front on Monday, but resuming large-scale height
rises/subsidence and the capping inversion still present challenges
for any precip to develop. Similarly 20 percent or less of ensemble
guidance depict any measurable precip Sunday into Monday. Instead,
the main highlight Sunday and Monday will be continued high
temperatures in the 80s F to approaching 90 F, 15 to 20 F above
average and approaching daily records (see CLIMATE section), as low-
level WAA remains prevalent. Even the 10th percentile of the NBM has
widespread high temperatures in the 80s F.

Upstream of the broad upper-level ridge, an upper-level trough is
expected to eject east/northeastward from the Rocky Mountains Monday
through Tuesday, bringing a potential period of active weather
including strong to severe thunderstorms across the Central and
Southern Plains on Monday, and the Midwest and Mississippi River
Valley on Tuesday. Conceptually, the general upper-level pattern
currently depicted by deterministic and ensemble model guidance on
Tuesday; featuring a deep, negatively tilted upper-level trough
tracking from the Central Plains to the Midwest; appears favorable
for severe thunderstorms somewhere across the Mid-Mississippi River
Valley, including the CWA. On Tuesday, the CWA will be fully within
a moistening warm sector overtopped by strong (50 to 70 kt) deep-
layer wind shear and mid and low-level flow, but there are
indications that some showers and thunderstorms developing on Monday
will track into the region early Tuesday. Whether these showers and
thunderstorms continue to weaken/dissipate or restrengthen is
uncertain along with their impact on subsequent destabilization.
Ensemble model guidance probabilities of 1000 J/kg or more of SBCAPE
on Tuesday afternoon and evening are only up to 20 percent, but
these probabilities appear to be somewhat deflated by timing/spatial
differences in the unstable portion of the warm sector ahead of an
approaching dryline with many individual ensemble members containing
over 1000 J/kg of SBCAPE. The tilt and structure of the trough will
be important in determining the exact mode and evolution of
redeveloping thunderstorms. At this point, no thunderstorm hazards
can be discounted, but as the above discussion addresses, there are
still many details that are uncertain and need to be resolved that
will impact the magnitude and character of the severe thunderstorm
threat.

Global model guidance become increasingly divergent after Tuesday
with differences in how a northern stream upper-level trough
interacts with the aforementioned trough; however, around 50 percent
of ensemble model guidance include showers and thunderstorms across
the CWA around Thursday. The post-frontal airmass on Wednesday will
be mild with temperatures remaining largely above average, but NBM
interquartile temperature ranges on Thursday are as high as 25 F,
capturing uncertainty in the track and timing of a surface cyclone
associated with the trough. That being said, analog and ensemble
guidance are in reasonable agreement that a period of below average
temperatures will begin in the wake of the trough/cyclone.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1200 PM CDT Fri Apr 12 2024

Dry and VFR flight conditions will persist through the TAF period.
West-northwesterly winds with occasional gusts of 25 to 30 kt will
quickly slacken this evening and then become variable at times. On
Saturday, winds will become southerly and gradually strengthen
through midday.

Pfahler

&&

.CLIMATE...
Issued at 225 PM CDT Thu Apr 11 2024

DAILY HIGH TEMPERATURE RECORDS

          |=== 4/14 ==|=== 4/15 ==|
St. Louis | 92 (2006) | 89 (2002) |
Columbia  | 89 (2006) | 90 (1896) |
Quincy    | 86 (2006) | 88 (2002) |


&&

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

$$

WFO LSX