964
FXUS63 KLSX 172324
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
624 PM CDT Tue Sep 17 2024

.KEY MESSAGES...

- Dry conditions are expected for most areas through the end of
  the week. Rain chances increase this weekend, but remain low
  (20-40%), with confidence low (less than 10%) in rainfall
  capable of relieving drought conditions.

- Confidence is high (90%) in above normal temperatures through
  the end of the week, with uncertainty in temperatures increasing
  this weekend and into next week.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 216 PM CDT Tue Sep 17 2024

Current water vapor imagery shows a cutoff low
centered over the Ohio Valley beneath a ridge spanning the eastern
CONUS as a trough digs equatorward over the western states. The CWA
remains situated between these two features, with high pressure at
the surface continuing to lead to calm and warm conditions.
Temperatures in most locations are currently in the mid to upper
80s, with a majority of the area expected to top out in the upper
80s over the next hour or two.

Tonight, the surface high will shift slightly eastward, returning
predominantly southerly low-level flow to the CWA. This flow is
expected to provide weak warm air advection and keep temperatures
from getting as cool as last night, with overnight lows falling to
around 60 degrees. There is a weak signal for fog across
southwestern Illinois tonight, though dew points in this area are
currently trending lower than forecasted lows, so it has been left
out of the forecast for now. Another round of valley fog can`t be
ruled out mainly in central and southeastern Missouri, but with
temperatures forecast to be slightly warmer tonight than last night,
this too has been left out of the forecast for now.

For Wednesday, the upper-level pattern will be similar to today,
though edge slightly eastward as a shortwave moves into the Northern
Plains. With southerly low-level flow, mostly clear skies, and mid-
level temperatures warming subtly, surface temperatures tomorrow
afternoon will be around to just a couple of degrees warmer than
today as highs top out around 90 degrees.

Elmore

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 216 PM CDT Tue Sep 17 2024

Starting Thursday into the weekend, guidance consensus is that an
upper-level trough will be positioned over the western CONUS as a
ridge edges into the Midwest. A shortwave within the trough will be
moving out of the Northern Plains into Canada as a portion of the
trough becomes cut off over the southwestern CONUS. As the ridge
gradually builds in, mid-level temperatures will warm, correlating
with a slight bump in high temperatures to end the week. As a
result, ensembles are clustering around 90 degrees for highs
Thursday and Friday. There is a low chance (20-40%) for rain across
northeastern Missouri and west-central Illinois Friday morning as
the low-level jet edges into the region, providing ample warm air
advection and forcing for precipitation. However, as the jet veers
into our CWA Friday morning, it will be weakening, with a downward
trend in intensity and coverage of rainfall in our CWA expected.

Temperatures and rain chances grow increasingly uncertain Saturday
and Sunday, as both deterministic and ensemble guidance vary on the
phasing of the western cutoff. The general consensus is that it will
get picked up by southwesterlies over the northern half of the
country, but how quickly this occurs and then how quickly it moves
through the Midwest varies widely among guidance. Regardless of the
exact phasing of the cutoff, moisture return over the Middle
Mississippi Valley ahead of it is expected to be meager, leading to
low rain chances (20-40%) Saturday and Sunday. Any rain that falls
is expected to be light, as ensemble medians are around 0.1-0.2" -
not nearly enough to have much effect on current drought conditions.

As for temperatures, spread among ensemble members increases notably
this weekend due to the differences in phasing of the cutoff among
guidance, with the spread between the 25th and 75th percentile
reaching 10 degrees by early next week. If temperatures remain on
the warmer end of guidance, a continued stretch of warmth
characterized by temperatures about 10 degrees above normal can be
expected. On the cooler end, temperatures would be around seasonal
normals. Confidence in either end of the spread is low, and the
going forecast represents roughly the middle ground.

Elmore

&&

.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 624 PM CDT Tue Sep 17 2024

Largely dry and VFR flight conditions are expected through the TAF
period with the exception of patchy river valley fog Wednesday
morning. Fog should be more limited in coverage than this morning
with the relatively highest chance at KSUS for a short period. Winds
will remain light as well.

Pfahler

&&

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

$$

WFO LSX