544
FXUS63 KLSX 310954
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
354 AM CST Fri Jan 31 2025

.KEY MESSAGES...

- Patchy dense fog may linger through the morning, but
  visibilities are expected to gradually improve. Some light
  showers are likely through the day.

- Dry, mild conditions are expected through the weekend, with well
  above normal temperatures expected Sunday.

- The weather turns cooler next week, though how much cooler is
  still very uncertain.

- Confidence is growing that precipitation will return near the
  middle of next week.

&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 349 AM CST Fri Jan 31 2025

In the immediate term, the primary forecast challenge continues to
be the persistence of widespread fog throughout the region, along
with another round of light to moderate rain across parts of
northeast Missouri and west-central Illinois.

At the present time, a low pressure system is in the process of
moving slowly northeast through the northern half of Missouri. In
the vicinity of the low, light winds, high humidity, and cold ground
temperatures have led to widespread fog development, with some bouts
of drizzle also ongoing. While widespread dense fog was observed
during the late evening yesterday with visibilities as low as 1/8
mile observed, there has been some modest improvement in many areas
during the very early morning hours. This improvement is most
pronounced across central/southeast Missouri so far, where a dry
slot and southwesterly surface winds are helping to erode the fog.
Visibilities are expected to gradually improve through the
morning from southwest to northeast as this dry slot expands. As
such, parts of the Dense Fog Advisory will likely be cancelled
early once it becomes clear that visibilities have improved for
good.

Meanwhile, a broad shield of mostly stratiform rain has developed
across northwest Missouri and northeast Kansas, associated with a
deformation zone along the northwest flank of the surface low. As
this band of rain slowly moves northeastward, a belt of roughly 1
to 2 inches of rain may lead to localized flooding issues across
parts of far northern Missouri, exacerbated by frozen soils in
those areas. This heavier rain is not likely to spread into our
forecast area of responsibility and local impacts are not
expected, but our northern fringes may see another round of
soaking rain through the morning hours. A Flood Watch has also
been issued just to our north (including Scotland/Clark counties
in Missouri), but we do not currently expect noteworthy local
flooding impacts farther south.

Through the day tomorrow, a cold front will push south through the
area, bringing a modest cooldown and one more round of mostly light
showers. While we don`t expect significant precipitation amounts,
this will likely keep gloomy conditions in place through the day in
most areas. There are also hints of modest instability developing in
model sounding profiles, but probabilities of more than 100 J/kg
remain very low locally (20% or less). As such, the potential for
any thunderstorm activity remains very low for tomorrow. Meanwhile,
temperatures are expected to be quite variable due to the
combination of the cold front passage and persistent cloud cover.
Areas that remain south of the cold front will likely remain within
southwest flow through the afternoon and likewise may once again
warm well into the 50s to near 60, while areas behind the front
struggle into the 40s with northwesterly surface winds.

Overnight tonight through Saturday morning, winds are expected to
weaken quickly behind the front as high pressure settles into the
area. As such, we will need to keep an eye on the potential for
another round of fog, particularly across the Ozarks there there is
a more consistent signal in high resolution model guidance. However,
tonight`s fog setup is a bit different than the one that led to this
morning`s fog, and confidence is lower overall regarding the
potential coverage and density of this second round.

BRC

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 349 AM CST Fri Jan 31 2025

By Saturday, zonal flow aloft is expected to develop across much of
the CONUS, and this is expected to persist through the middle of the
week. Meanwhile, persistent pressure falls across the central plains
will gradually re-establish southerly surface flow across the
Mississippi Valley, beginning from the southeast Saturday and
gradually veering to the southwest by Sunday. This will result in a
gradual warming trend over the weekend, particularly Sunday when
temperatures are likely to climb back into the 60s almost area-wide,
supported by narrow NBM/LREF ensemble temperature spreads.
Meanwhile, precipitation chances remain very low over the weekend,
although we may be struggle to emerge from cloud cover on Saturday.

From Monday onward, confidence in the day-to-day forecast details
begins to decrease due to a number of factors. While zonal flow
aloft is expected to persist, a passing shortwave near the US/Canada
border is expected to drive a cold front south and into our area,
where it will likely stall. Exactly where this boundary settles will
play a big role in our realized temperatures, and there remains
quite a bit of forecast variability among ensemble members regarding
exactly how far south it`ll reach. Due to there being a relatively
sharp temperature gradient along this front, forecast temperature
spreads are quite high (25th-75th percentile spreads of 10+
degrees), and this reduces confidence in our deterministic forecast
values. Still, the potential for precipitation remains low Monday
through much of the day Tuesday.

Beginning sometime late Tuesday through Wednesday, the upper flow
pattern is expected to gradually amplify as a broad trough digs into
the western CONUS, and potentially phases with another trough
digging into the Canadian Shield. This is likely to spread deep and
robust southwesterly flow aloft across much of the central CONUS and
Mississippi Valley, and transport deeper moisture into the area from
the south. In fact, all of the major ensemble suites
(NAEFS/GEFS/ECMWF) produce widespread ensemble mean PWAT values
exceeding the 90th percentile by Wednesday. Meanwhile, a cold front
remains likely to push through the area at some point mid-week,
although there remains considerable variability in timing. Between
the passing boundary and rich pre-frontal moisture in place,
confidence is increasing that we will see the return of
precipitation between late Tuesday and sometime Thursday, although
there remains very little confidence in potential timing and
amounts. Much of this uncertainty is once again wrapped up in the
timing of the cold front, and this also extends to forecast
temperatures. NBM/LREF forecast temperature spreads increase even
more during this period, with 25th-75th percentile spreads of 15-25
degrees Tuesday and Wednesday.

Beyond just the potential for precipitation, there is also a growing
signal for at least some modest instability to develop in the warm
sector, supporting at least a slight chance for thunderstorms
Wednesday through Thursday. Given the strong dynamics expected this
will need to be monitored closely, although confidence remains low
in the potential for strong/severe thunderstorms at this time.
Meanwhile, it does appear much more likely than not that the
majority of this precipitation will fall as rain, but the
considerable uncertainty in forecast temperatures does leave open a
bit of wiggle room for frozen precipitation types behind the passing
cold front.

BRC

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1056 PM CST Thu Jan 30 2025

LIFR/IFR ceilings will remain across the TAF sites through at
least midday on Friday. In the meantime, dealing with dense fog
reducing visibilities to less than half a mile. The visibilities
will gradually lift from south to north with MVFR visibilities by
09z Friday at KCOU and KJEF, by 12z Friday at St. Louis metro
area TAF sites and by 15z Friday at KUIN. Otherwise, east to
southeast winds will veer to the northwest to north during the
afternoon hours as a cold front moves through.

Byrd

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CST this morning for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln
     MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-
     Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Shelby MO-Warren MO-Washington MO.

IL...Dense Fog Advisory until 9 AM CST this morning for Adams IL-Bond
     IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-
     Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Pike IL-Randolph IL-Saint Clair IL-Washington
     IL.

&&

$$

WFO LSX