040
FXUS63 KLSX 231947
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
147 PM CST Mon Dec 23 2024

.KEY MESSAGES...

- Multiple rounds of rain are expected during the holiday week,
  with the best chances (60-100%) late Tuesday/Tuesday night
  across portions of east central and southeast Missouri as well
  as south- central Illinois.

- Above normal temperatures are forecast for the entirety of the
  holiday week, with the warmest temperatures forecast Friday
  through Sunday with highs well into the 50s each afternoon.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 137 PM CST Mon Dec 23 2024

A surface cold front is currently draped from near KMCI>>KUIN as of
1900 UTC. This front is expected to continue to sag southward
through tonight, become more ill defined, and then stall out across
southeast Missouri and southwest Illinois. Ahead of this front,
broad/weak low-level moisture convergence has helped lead to
scattered sprinkles/very light rain showers across parts of
southeast Missouri and southwest Illinois. This activity is expected
to continue sliding southeastward through the evening.

A push toward more widespread, stratiform light rain is still
expected late tonight as low-level moisture convergence strengthens
coincident with increasing mid/upper level forcing for ascent
downstream of an approaching trough. This steady light rain is
expected to reach parts of southeast Missouri just before daybreak
on Tuesday, and then spread gradually east and north through the
day. By late afternoon, the northwestern edge of the precipitation
shield is expected to be reach roughly a Jefferson City to Troy, MO
line. There remains some uncertainty as to how far northwest this
steadier light rain gets, with the 9Z RAP and 12Z HRRR about 50-75
miles further south/southeast than this forecast. However, all lower
resolution guidance as well as the remaining CAMs are quite a bit
further northwest so leaned on the consensus here. That being said,
the highest confidence in rain is in parts of southeast Missouri and
southwest Illinois where 80-100% PoPs exist late Tuesday afternoon
through much of Tuesday night.

Total rainfall amounts continue to look fairly light overall, with
the HREF LPMM only having a small area right around 0.50" in
southeast Missouri. While the duration of the rain may be fairly
long (18+ hours possibly), the rates look very light. This makes
sense given very weak forcing for ascent at mid/upper levels of the
atmosphere.

Temperatures will remain on the mild side, mainly at night given the
persistent low stratus and weak temperature advection regime
expected to be in place through the short-term forecast period. Lows
both tonight and Tuesday night are expected to be in the 30s to low
40s from north to south, or about 10 to as much as 15 degrees above
normal for late December. Highs on Tuesday will not rebound much,
likely only rising about 5-8 degrees due to the low clouds and
incoming rain from the south. Highs are forecast to be in the low to
mid 40s, just a few degrees above normal.


Gosselin

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 137 PM CST Mon Dec 23 2024

(Christmas Day - Thursday)

Steady light rain looks to end by Christmas morning as stronger low-
level moisture advection moves out of the area. In addition, the
track of the midlevel shortwave trough has trended quite a bit
further to the south, leaving the mid-Mississippi Valley more within
a shear axis than anything. This is a long way of saying that
rainfall activity on Christmas Day looks more unorganized and very
light. The best chances (30-50%) are mainly in eastern Missouri and
over into Illinois beneath this shear axis and within broad, weak
surface convergence along an inverted trough.

Christmas night into Thursday should largely be dry, though there
may be some patchy drizzle hanging around. The setup appears
favorable, with weak surface convergence, low stratus, and dry
midlevels of the atmosphere. Deterministic model guidance also has
some splotchy very light QPF - another sign pointing to the chance
of drizzle. Did not add to the forecast quite yet, but this is
something that may need to be added in future forecast updates.

Regardless of any drizzle though, the gloomy look and feel will
continue with thick, low stratus draped across the area. Similar to
earlier in the week, this will really help limit the temperature
fall Christmas night and climb each day. Lows around 40 degrees are
likely Christmas night, with highs moderating a bit by Thursday into
the low to mid 50s. Highs have been trending downward however
through midweek and there appears to be room for Thursday`s highs to
come down further as we get closer.


(Thursday Night - Next Monday)

The active pattern continues late this week heading into the weekend
as two more distinct midlevel shortwaves look to quickly pass
through the region. Each of which still has a fair amount of
uncertainty with them in terms of track/timing/uncertainty, which is
not too surprising given the very short wavelengths between each
successive wave. While the details will need to be pinned down over
the coming days, there appears to be two main windows for rain to
impact the parts if not most of the area: late Thursday night/Friday
and Saturday/Saturday night. There is more confidence in late
Thursday night/Friday, with 80-90+% of LREF members showing
measurable rainfall. Compare that to the Saturday/Saturday night
time period, where only 40-65% of members have measurable rain.

Looking at temperatures, there remains very high confidence in well-
above normal readings through this weekend and into early next week.
Highs are likely to be well into the 50s Friday through Sunday, with
some locations potentially even making a run at 60 degrees if there
is any sunshine that occurs. Lows are forecast to be mainly in the
mid to upper 40s. To put those numbers in perspective, those lows
are likely to be above normal highs for the period.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1118 AM CST Mon Dec 23 2024

A few sprinkles are possible around the St. Louis metro terminals
through this afternoon. Otherwise, dry weather is expected through
tonight. More widespread rain will move northward out of southeast
Missouri on Tuesday, but should not reach metro St. Louis until
Tuesday afternoon/evening. Further west/northwest, dry weather
should continue through the period.

Ceilings are forecast to lower tonight, with low MVFR conditions
expected late tonight spreading northward into central and east
central Missouri by Tuesday morning. IFR ceilings are possible,
but more likely south of the terminals in parts of southeast
Missouri. Some areas of fog are also expected, though there
remains a lot of uncertainty on visibilities. The best guess is
for conditions to be mainly in the 3-5SM range, so leaned toward
that. Some guidance shows IFR visibilities however, so future
forecasts may need to become more pessimistic if confidence in
those lower visibilities increases in time.

Gosselin

&&

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

$$

WFO LSX