450
FXUS63 KLSX 101145
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
545 AM CST Fri Jan 10 2025

.KEY MESSAGES...

- A winter storm will impact the region through the day, with
  widespread accumulating snowfall and travel impacts expected.

- Temperatures will remain wintry through much of the period,
  with another round of bitter cold expected early next week.


&&

.SHORT TERM... (Through Saturday Night)
Issued at 301 AM CST Fri Jan 10 2025

Water vapor imagery currently shows a deep trough crossing the
central CONUS, with embedded shortwaves located over the Southern
and Northern Plains. Even though the southern wave`s surface low
will hug the Gulf Coast as it moves eastward, moisture is still
advecting northward ahead of the northern wave, resulting in the
light to moderate snow that has been slowly spreading eastward
across the CWA overnight and early this morning.

Through the remainder of this morning, dynamic lift via the left
exit region of an upper-level jet streak and frontogenesis will
become focused across southeastern Missouri and southwestern
Illinois. Here, snow-to-liquid ratios have been increased slightly
to about 14:1-15:1 based on recent guidance trends. While some
guidance shows greater ratios, RAP soundings show the greatest lift
above the Dendritic Growth Zone, and temperatures in this portion of
the CWA will get close to freezing during the day, which will limit
snowfall potential relative to the more extreme guidance. The 00z
HREF has about 0.4" of QPF across this portion of the CWA, which
aligns well with the current forecast and yields total snow
accumulations of 4-6" with locally higher amounts. Therefore, the
current Winter Storm Warning remains on track. Further north within
the Winter Weather Advisory, forcing is overall weaker and QPF
decreases with northward extent. Still, relatively efficient
snowfall ratios (14:1) will favor widespread accumulations of 1-3".
Confidence is high in impacts to the commute this morning, with
impacts lingering into the evening commute. This is especially the
case for locations within the Winter Storm Warning, where the 00z
HREF has a 90% chance of snowfall rates reaching 0.5" an hour this
morning, leading to quickly deteriorating conditions.

While the main round of snow is expected to tapper off from west to
east through this afternoon, the chance for flurries or light snow
will continue into this evening as the axis of the upper-level
trough approaches the area. As this occurs, weak frontogenesis will
pass over the region as weak instability (roughly 20 J/kg of SBCAPE)
builds. This will lead to isolated convective pockets or streamers
(convective rolls) that will provide brief, focused bursts of light
to moderate snowfall.

In the wake of this system tonight and Saturday, subtle ridging will
quickly build in aloft, limiting cold air advection and causing low-
level winds to become southerly through Saturday morning. This
southerly low-level flow and clearing skies will help temperatures
across central and southeastern Missouri rise into the mid 30s for
highs tomorrow, with low 30s expected for most of the remainder of
the area.

Elmore

&&

.LONG TERM... (Sunday through Thursday)
Issued at 301 AM CST Fri Jan 10 2025

At the start of the long-range forecast period, guidance consensus
is that the Middle Mississippi Valley will be beneath northwesterly
flow aloft as ridging will have built across the eastern CONUS and a
trough digs to the west. At the surface, a low will be moving
somewhere through the Upper Mississippi Valley, with it`s cold front
moving toward the CWA. Southerly flow ahead of the front will
provide a brief period of warm air advection, with ensembles
clustering around mid to upper 30s for highs along and south of I-70
and low to mid 30s north of I-70. However, the cold front will make
this rise to near normal temperatures brief, with the post-frontal
airmass being Arctic in nature. Additionally with the front will be
another chance for snow. Forcing through the atmosphere over the CWA
with the FROPA is not robust and QPF is meager, but model soundings
show saturation through the Dendritic Growth Zone, favoring ice
crystal development and growth aloft. Ensemble-based probabilities
for measurable precipitation top out at 40-60% for areas north of I-
70, with less than a 20% chance for snowfall greater than 1"
assuming a 10:1 SLR. Therefore, confidence is high that for areas
that do see snow on Sunday, accumulations will be a dusting at most.

Behind the front Monday through Wednesday, varying degrees of
troughing and/or northwesterly flow will support continuous cold-
air advection, with temperatures running below normal through this
portion of the period. How much though is uncertain, as spread
within the interquartile range of ensemble guidance temperatures
ranges roughly 5-10 degrees due to differences in the handling of an
upper-level trough over the eastern CONUS. Colder solutions favor
widespread daily highs in the 20s and lows in the single digits,
while less-cold solutions have highs in the upper 20s to low 30s
with lows in the teens. Given the pattern and ensemble distribution,
the current forecast leans more toward the colder solution. This
pattern will also open the door to impacts from clippers that could
provide brief hits of snow. Right now, the best chance for such a
system is Tuesday or Wednesday per guidance consensus. Ensemble
guidance does not get over 20-30% probability for snowfall during
these two days, though this is at least in part due to timing and
spatial differences in the handling of the associated shortwaves.
Once we`re past the ongoing system, we will be able to better focus
on this part of the period and appropriately alter the forecast if
needed.

For Thursday into next weekend, uncertainty in our weather grows as
guidance becomes increasingly dispersive ahead of what looks to be a
period of significant troughing over much of the CONUS next weekend.
Ahead of this trough, guidance is roughly split between some degree
of northwesterly flow holding over the Middle Mississippi Valley and
southwesterly flow developing over the region. The former solution
favors a slower, more muted warm up, while the latter would produce
a quicker, more notable warm up. Regardless, a warming trend is
expected toward the end of the period, reflected in the current
forecast which falls in line with the median of ensemble guidance.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 525 AM CST Fri Jan 10 2025

Another winter storm is moving through the region, with impacts
expected at all local terminals through at least the first half of
the forecast period. The heaviest of the snow will occur this
morning, with snow gradually diminishing from northwest to
southeast this afternoon. While this will be the end of the more
widespread snowfall, there is a low chance for snow showers
through the evening at all local terminals. Their isolated and
brief nature makes them hard to pin down relative to the local
terminals, though if impacts are to occur, brief drops in
visibility are expected. Ceilings will bottom out this morning
before gradually improving through the remainder of the period.
How quickly remains uncertain, and some of local terminals could
see low ceilings persist or return tonight. Winds will become
northwesterly with the passage of a cold front, but remain light.

Elmore

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory until 6 PM CST this evening for Audrain
     MO-Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Jefferson MO-Lincoln MO-Marion MO-Moniteau MO-
     Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Saint
     Charles MO-Saint Louis City MO-Saint Louis MO-Shelby MO-
     Warren MO-Washington MO.

     Winter Storm Warning until 6 PM CST this evening for Iron MO-
     Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve
     MO.

IL...Winter Weather Advisory until 6 PM CST this evening 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-Saint Clair IL-Washington IL.

     Winter Storm Warning until 6 PM CST this evening for Randolph IL.

&&

$$

WFO LSX