227
FXUS63 KLSX 081725
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1125 AM CST Fri Nov 8 2024

.KEY MESSAGES...

- Patchy fog has been observed along river valleys early this
  morning and may linger until an hour or two after sunrise.

- Widespread rain is expected during the day Saturday, with a few
  rumbles of thunder possible. While amounts greater than 1 inch
  are not expected to be common, there is a roughly 30% chance of
  this occurring within the Meramec River basin. This may slow the
  recession of floodwaters slightly.

- After mild and dry conditions Sunday through Tuesday, the next
  opportunity for precipitation arrives midweek, although
  potential impacts remain uncertain.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 232 AM CST Fri Nov 8 2024

While active weather is on the way this weekend, mostly quiet
conditions prevail across the region tonight and this is largely
expected to continue through the day tomorrow as well. A few river
valley observation sites have reported fog early this morning, but
as of 1 am this has fluctuated quite a bit and fog does not appear
to be widespread. While fog has developed a little bit ahead of
schedule at these sites, particularly along the Missouri and
Mississippi River Valleys, it is not a given that it will simply
continue to increase in coverage through the early morning hours.
High clouds approaching from the west may act to impede fog
development as the night wears on, and as such, fog may be quite
variable even in areas where it has already developed. This is
somewhat uncertain, though, as the effect if high level cirrus on
fog formation is not always significant.

In any case, whatever fog remains after sunrise is likely to
diminish within an hour or two, and a steady stream of mid and high
level cloud cover will spread into the area throughout the day.
These clouds are in advance of a rather dynamic upper low that is
currently spinning across the Four Corners region, producing
impactful snow across the front range of Colorado and New Mexico,
and widespread rain and thunderstorms across the southern High &
Central Plains. Over the course of today and tomorrow, this strong
storm system will slowly begin to eject northeastward and transition
from a closed to an open low, picking up forward momentum as it
emerges into the central Plains. It`s current track forecast takes
this low across Nebraska Saturday and into Iowa by Sunday, which
leaves the lower Missouri basin largely within the warm sector until
a cold front sweeps through sometime Sunday.

Within this warm sector, a plume of rich moisture will quickly
advect northward into the region Saturday, with ensemble mean PWAT
values reaching and exceeding the 90th percentile in all major model
suites, particularly along and south of I-70 where they exceed the
99th percentile. Model guidance also suggests that a modest amount
of largely elevated CAPE will also exist in the warm sector (~100-
300 J/kg), which may help to boost rain rates within embedded
convective elements in an otherwise stratiform rain shield, and
perhaps produce a few lightning strikes. Meanwhile, this low is
expected to be progressive enough by Saturday that the duration of
this rain will be limited, likely to 12 hours or less in most areas,
and this will suppress total accumulations. Latest deterministic NBM
and WPC guidance has dipped slightly from previous forecasts, but
NBM/LREF probabilities remain very similar. Overall, the LREF has
maintained 10-20% chances of 1" of rain or more along and south
of I- 70, while the NBM is a bit more bullish with probabilities
of 30- 50%.

These probabilities wouldn`t typically suggest an impactful event,
but it deserves extra attention due to the ongoing flooding within
the Meramec River basin. While the system is expected to be
progressive and this will limit the overall accumulations, it would
be within reason for a few areas to reach near or slightly above 1"
considering the very high moisture, strong dynamics, and marginal
instability. While not likely enough to cause more flooding, it
might be enough to slow the recession of river levels. In any
case, this precipitation is expected to sweep through the area
between Saturday morning and Saturday night, and will be followed
by a modest cold front by early Sunday morning.

BRC

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 232 AM CST Fri Nov 8 2024

While the previously mentioned cold front is expected to push
through the area Sunday morning, it is not expected to usher in a
notably cold airmass, as forecast high temperatures Sunday will
likely be slightly warmer than the day before. However, this post-
frontal airmass will be much drier thanks to surface high pressure,
leading to a pleasant start to the work week with mild afternoons
and cooler morning lows. While a weak shortwave may move through
the region sometime between Monday and Tuesday, this is not
expected to produce notable weather impacts considering the dry
airmass in place.

Instead, a more significant trough remains likely to impact the
region toward the middle of the week, although there remains
considerable variability in both the timing and strength of this
system. LREF cluster analysis in this timeframe reveals very large
differences in the timing and depth of this trough, suggesting that
a wide range of potential outcomes remains. Confidence is increasing
that precipitation will occur as this system traverses the region,
or at the very least it represents the next best opportunity for it
following this weekend`s rain. Given the large ensemble spread
though, it remains a bit too early to speculate on specific amounts
or other potential impacts. This also applies to forecast
temperatures, as the variability among members has resulted in
huge forecast spreads. Generally speaking though, the slower,
deeper solutions appear to produce more precipitation, but the
effect on temperatures is less clear-cut.

BRC

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1116 AM CST Fri Nov 8 2024

VFR flight conditions are expected through much of the period
ahead of an approaching cold front. As this front approaches the
local terminals late tonight and tomorrow morning, rain chances
will increase from west to east. Rain is generally expected to be
light, but there could be scattered heavier showers that cause brief
drops in visibility, possibly producing MVFR visibilities if
direct impact occurs at terminals. There is also a low chance of
isolated thunderstorms during the day tomorrow. However, direct
impacts from thunderstorms and heavier rainfall to local terminals
remain low and have been left out of the TAFs for now. Ceilings
will also begin lowering right at the end or just beyond the
current TAF period, with gradually lowering ceilings expected just
beyond the current period.

Elmore

&&

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

$$

WFO LSX