518
FXUS63 KLSX 050940
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
340 AM CST Wed Feb 5 2025

.KEY MESSAGES...

- Light freezing drizzle remains likely in areas north of the
  Missouri River through the day today. A light glaze may create
  hazardous roadways, particularly on bridges and overpasses,
  during both the morning and afternoon commutes.

- Elsewhere, waves of showers are expected today through tomorrow
  morning. Thunderstorm chances are low overall, but a few
  rumbles of thunder will be possible.

- Another round of showers remains likely over the weekend, which
  is likely to be primarily rain in spite of a highly uncertain
  temperature forecast.

- Another round of precipitation, including possibly frozen
  precipitation, is increasingly likely early next week.

&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 336 AM CST Wed Feb 5 2025

The primary concern in the short term continues to be the potential
for light accumulating ice throughout the day today. As expected,
temperatures have fallen to near or slightly below freezing across
mainly areas along and north of the Missouri River early this
morning behind yesterday`s passing cold front. Meanwhile, a
shortwave across the central Plains is approaching from the west,
driving a steadily increasing southwesterly low level jet that is
beginning to overrun the colder air near the surface. As this LLJ
increases, isentropic ascent will also increase, and low levels will
continue to saturate. This has been a slow process locally, but
upstream observations (radar, surface obs, and spotter reports)
indicate that light freezing drizzle has begun across western parts
of Missouri. This light drizzle will continue to spread east-
northeast through the morning, and precipitation rates are also
likely to slowly increase as well.

While drizzle will be very light at first, accumulations are likely
to begin by daylight hours (possibly sooner in a few spots), and
perhaps early enough to impact the morning commute in spots.
However, increasing precipitation rates later in the morning and
through the afternoon are likely to make this the prime time for
icing, although it will be competing with very slowly warming
surface and pavement temperatures, along with significant warming of
temperatures aloft. Between the warming temperatures and uncertainty
regarding the pavement itself, confidence remains low regarding the
ability of ice to accumulate on surfaces that are not elevated, but
it will at least be possible. Meanwhile, bridges, overpasses, and
other elevated surfaces will be most vulnerable to ice and
associated hazards. Precipitation rates are likely to diminish and
temperatures warm above freezing this evening, but the timing of
both will need to be watched closely

In areas south of the Advisory, liquid rain will be the primary
precipitation type, with a steady increase in shower activity
through the afternoon and evening. Model guidance continues to
suggest that some modest and mostly elevated instability (less than
500 J/kg MUCAPE) will also develop this afternoon and evening,
particularly as a weak surface low brings the front northward as a
warm front after sunset. While a few rumbles of thunder will be
possible, particularly across SE MO and SW IL, strong/severe storms
are not likely. Showers are likely to move east of the area
overnight, but additional showers will be possible across SE MO and
SW IL through the day Thursday along the stalled surface front.
While thunderstorms can`t be definitively ruled out, best chances
for such are likely to be to our southeast where instability is
maximized. Meanwhile, temperatures are likely to vary considerably
throughout the area Thursday, and will depend largely on how far
south the front settles by mid-day. Areas along and south of the
boundary will likely warm to well above normal values, possibly
into the 60s, while ares to the north will likely see more
seasonable values in the 40s.

BRC

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 336 AM CST Wed Feb 5 2025

By Thursday night, the cold front will finally surge back to the
south and set the stage for a cooler day Friday...likely very near
seasonal averages. Mostly dry conditions are also expected, although
there are hints that some light precipitation associated with the
next system may begin as early as Friday afternoon.

Still, the bulk of the impacts of the next system are likely to be
felt overnight Friday through Saturday, when another shortwave and
surface low is expected to move along the stalled frontal boundary.
This shortwave and associated surface low continues to appear a bit
stronger than it`s predecessor, although it is unclear whether this
will actually lead to a meaningful increase in impacts. In any
case, another round of showers is likely to develop during this
period thanks to yet another southwesterly low level jet
overrunning the surface front. Model guidance continues to suggest
that temperatures are likely to remain warm enough to support
primarily liquid rain, in spite of the very uncertain position of
the stalled front as the system arrives. NBM/LREF temperature
spreads continue to be very wide Saturday (15 to 30 degrees for
Saturday afternoon highs), and this directly caused by the
uncertainty in the position of the front among ensemble members.
Given this uncertainty, we will need to monitor the potential for
additional wintry precipitation (likely again in the form of
freezing rain/drizzle if it does occur), and mainly across
northern areas, in spite of the modest representation among
ensemble members (20% or less). Meanwhile, a continued signal for
modest, but mainly elevated instability supports at least low
chances for thunderstorms Saturday.

Following Saturday`s system, the stalled cold front is expected to
push even farther to the south, placing us more solidly on the cool
side of the boundary Sunday through early next week. In fact,
ensemble temperature spreads narrow considerably Sunday onward,
suggesting that while the timing of the front is uncertain, it is
eventually expected to clear the area to the south and remain there
for several days. Meanwhile, active zonal flow will persist aloft,
but models are beginning to hint that a broad, positively tilted
trough may finally begin to meaningfully dislodge this persistent
pattern early next week. At the same time, a growing signal for
another round of post-frontal, overrunning precipitation has
developed across the middle/lower Mississippi Valley early next
week, and considering the likelihood that colder air will be in
place, frozen precipitation is currently favored with this round.
Very little can be said about potential timing/amounts/locations
just yet, but early ensemble guidance suggests that accumulating
snow is a reasonable possibility (40% chance or greater in
NBM/LREF model output).

BRC

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1038 PM CST Tue Feb 4 2025

MVFR stratus is slowly inching northward into central Missouri,
though at this hour only KCOU has dropped to this restriction and
all others are barely VFR. The surface low has not moved much in
the last few hours, and a few waves of drizzle and light rain
across the Ozarks have struggled to reach the terminals.

A gradual degradation of conditions is still forecast from west
to east as the low-level air saturates and MVFR/fuel-alternate
conditions become more established overnight. Periods of drizzle
(and freezing drizzle at KUIN) are forecast overnight before
coming more predominantly light rain. Freezing rain is forecast to
continue at KUIN until the early evening when temperatures warm
above freezing. The other TAF sites remain forecast to be too warm
for ice to accurate.

Conditions degrade further tomorrow evening to IFR conditions
areawide and continued light rain or drizzle. Improving flight
conditions do not appear possible until at least late Thursday
morning.

MRB

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory until 6 PM CST this evening for Audrain
     MO-Boone MO-Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-
     Ralls MO-Shelby MO.

IL...Winter Weather Advisory until 6 PM CST this evening for Adams IL-
     Brown IL-Pike IL.

&&

$$

WFO LSX