401
FXUS63 KLSX 081737
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1137 AM CST Wed Jan 8 2025

.KEY MESSAGES...

- Arctic air will continue to dominate the region today resulting
  in well-below average temperatures and highs in the 20s.
  Temperatures will warm some on Thursday but remain below normal
  with highs in the upper 20s-upper 30s.

- Accumulating snow is likely (60-70 percent chance) across east-
  central and southeast MO and southwest IL in the Thursday
  night-Friday timeframe. There is a 40 to 60 percent chance of 2+
  inches of snowfall in southeastern MO and southwestern IL.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 322 AM CST Wed Jan 8 2025

A short-wave trof located through southern IA was producing a large
area of mid clouds across the CWA early this morning. This shortwave
will continue to dig southeastward moving into the OH/TN Valleys
later today and resulting in an overall deepening of the eastern
NOAM longwave trof. The Arctic Surface High currently centered in KS
will slide southeastward and into the region this afternoon as a
result, and mid clouds attendant with the short-wave should exit the
area by midday with lots of sun in its wake. The combination of the
encroaching surface high and snow cover will yield another cold day
with well below normal highs in the 20s.

The surface high/ridge axis will move rather quickly across the CWA
tonight resulting in light surface winds backing to southwesterly
and then southerly through the night. This is less than optimal for
good radiational cooling. Still, mostly clear skies and the aging
snow cover will make for some chilly lows in the single digits.
I`m not sure we will see any sub-zero readings, at least at
official observation sites, however it would be most possible in
sheltered areas, especially in SE MO or SRN IL where winds should
be lightest.

The deep eastern upper trof will move into the western Atlantic on
Thursday as upstream amplification ensues to our west with a
high-amplitude ridge over the eastern Pacific progressing onshore
and an elongated upper trof evolving over the western U.S. as a
result of phasing of an upper low near the Gulf of California and
short-waves digging southward on the front side of the high-
amplitude upper ridge. On Thursday this brings southwest flow
aloft back to our area and a WAA regime on the backside of the
retreating and rapidly weakening Arctic High. We should see a
decent temperature rebound on Thursday as a result. Highs will
still be below normal, but anywhere from 6-15 degrees warmer than
today with highs in the upper 20s-upper 30s.

Glass

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 322 AM CST Wed Jan 8 2025

The positively-tilted longwave trof will progress eastward into
the mid-upper Mississippi Valley Thursday Night into Friday
bringing the region the next accumulating snowfall. The primary
driver will be isentropic lift ahead of the upper trof and to a
lesser extent weak mid-level frontogenetical forcing. However the
northward extent and breadth of forcing, the greatest combined
isentropic/frontogenetic forcing, and the overall longevity are
unclear due to the structure and speed of the upper trof. There
are 2 camps of solutions in the deterministic and ensemble
prediction systems: 1) a faster shortwave which would bring a
quicker onset on Thursday evening with snow over by midday Friday,
and 2) a slower-sharper trof which would have snow starting late
Thursday evening and ending late Friday afternoon/early evening. The
EPS membership favors camp 1 and yield the lowest snowfall amounts
with most locations under 1 inch and only 30-45% probability of 2+
inches over far southern Reynolds, Iron, and Madison Counties.
Meanwhile the GEFS and CMC ensemble (CMCE) membership favor camp 2
and have higher snowfall amounts and 50-90% probability of 2+ inches
covering a much larger area from Jefferson City-Troy-Litchfield line
and south. The combined 100-member LREF still highlights SE MO and
SRN IL with a 40-60% chance of 2+ inches of snow roughly along and
south of a Viburnum to Salem line. This still looks like an event
that could cause minor impacts, especially where the higher amounts
are forecast.

In the wake of the Friday system a new broad longwave trof is
forecast to evolve over the Nation`s midsection over the weekend. A
shortwave trof moving within the longwave and through the area on
Sunday could bring light snow to northeast MO and west-central IL.
We currently don`t have any mention in the forecast but we`ll have
to watch this wave as roughly 30-50% of the LREF members have very
light measurable snow. Another big blast of cold air looks poised to
accompany this trof passage and keep below normal temperatures into
the first part of next week.

Glass

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1137 AM CST Wed Jan 8 2025

Dry and VFR flight conditions are likely at all terminals through
Thursday afternoon as a surface high pressure ridge passes the area
tonight with northwesterly winds today becoming southerly on
Thursday. Patchy river valley fog cannot be ruled out
overnight/Thursday morning, but confidence in its development is not
high enough to include at this time.

Pfahler

&&

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

$$

WFO LSX