513
FXUS63 KLSX 042328
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
528 PM CST Wed Dec 4 2024

.KEY MESSAGES...

- A significant cold front will move through the area between
  roughly sunset and midnight, dropping temperatures by 30 to 40
  degrees overnight and perhaps producing a few brief and non-
  accumulating rain or snow showers. Gusty northwest winds are
  also expected, resulting in single digit wind chills by morning.

- Temperatures will likely remain below freezing tomorrow, but a
  quick warmup will bring us back into the 50s over the weekend.

- Shower chances are increasingly likely sometime between late
  Sunday and Monday (50 to 90% over the whole period), with best
  chances along/south of I-44.


&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 226 PM CST Wed Dec 4 2024

As expected, weather conditions today have been quite different than
in recent days thanks to a brief period of deep southwest flow ahead
of an advancing cold front. Surface temperatures are running
anywhere from 15 to 25 degrees above yesterday`s values as of 1 PM,
and these readings may have been even warmer if not for a leading
pre-frontal trough / weaker cold front that switched winds to the
west and slowed our warming. In any case, breezy conditions have
developed along with mostly clear skies as we await the passage of a
significant polar cold front.

This boundary is clearly revealed by a band of convective cloud
cover curling across southern Iowa, where it is quickly marching
southward. This front will move through the area between roughly
sunset and midnight, and will result in a 30 to as much as 40 degree
temperature drop before sunrise. Meanwhile, strong cold air
advection and downward momentum transport should bring some rather
gusty northwest winds to the surface, likely to between 35 and 40
mph and potentially higher in a few spots. While these winds will
likely add some bite to wind chills overnight, with readings
potentially reaching between 5 and -5 degrees F by early tomorrow
morning, these speeds fall just short of local wind advisory
criteria, and significant wind impacts are otherwise not
anticipated. Still, this potential will be monitored through the
night, as you don`t have to go much farther north to find wind
speeds forecast to reach these stronger speeds.

Meanwhile, while moisture content will be rather limited, the very
strong low level frontogenesis along the boundary, cold air
advection, and steep low level lapse rates may be just enough to
support some brief showers along and just behind the boundary,
followed perhaps by a few hours of sprinkles or light flurries.
Accumulations are not expected of either precipitation type.

As mentioned previously, morning temperatures will be quite cold,
with absolute values likely in the teens to low 20s, and wind chills
in the single digits...both above and below 0. Wind speeds will
weaken slowly by morning and this will continue through the day, but
not enough to allow for significant warming in spite of clearing
skies. Expect a cold day with highs only in the 20s to near
freezing for most areas.

BRC

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 226 PM CST Wed Dec 4 2024

After our quick shot of arctic air tonight and tomorrow, a synoptic
scale pattern change will bring a rapid temperature recovery to
close out the week. The large trough across the Great Lakes will
slide to the east, allowing upper heights to build across the
Mississippi Valley. Meanwhile, surface high pressure will also nudge
eastward and place us within a regime of slowly increasing
southwesterly low level flow, which will promote steady warm air
advection through the end of the week. This likely won`t be enough
to get us back to seasonal average values by Friday, but we will
likely see at least a 10 degree warmup over Thursday, and by
Saturday we will likely see temperatures in the mid 40s to low
50s...or otherwise right around average. Narrow ensemble temperature
spreads also bolster forecast confidence in this warming trend. Dry
conditions are expected on both days, although this may not last
long.

That`s because confidence continues to increase regarding the
potential for a round of showers sometime between Sunday and Monday.
The primary driver of this precipitation is likely to be a slow-
moving subtropical trough originating from the desert southwest,
which is expected to gather moisture from the Gulf of Mexico over
the latter half of the week and eventually advect it northeastward
into our area. 50th to 90th percentile NBM dewpoint forecasts
continue to range from the upper 40s to mid 50s across much of
southeast Missouri and southern Illinois during this period, which
is well above average and even exceeds the climatological 90th
percentile for this time of year. Meanwhile, LREF cluster analysis
continues to maintain relatively good agreement in the timing and
location of precipitation, although there are some relatively
minor discrepancies that are almost always present at this time
range. There also continue to be some hints of modest instability
present, although even the more bullish 90th to 95th percentile
CAPE forecasts remain below 200 J/kg. Still, it`s just enough to
add a slight chance of thunder mention across far southeastern MO
and southwest Illinois for a brief period late Sunday. As in
previous forecasts, the potential for thunderstorms appears to be
low but not completely out of the question. Probabilities for all
precipitation over the entire period range from 50 to 90%, with
chances maximized roughly along and south of the I-44 corridor.

Sometime on Monday, another cold front is likely to push through the
area, with a bit of uncertainty regarding the exact timing. This
will almost certainly lead to another cooldown, although exactly how
cold and how quickly it occurs is less certain. Ensemble mean 850 mb
anomalies are not forecast to become exceedingly cold (above the
90th percentile), which suggests that this may not be quite as
significant of a cold snap as we`ve seen in recent days. However,
forecast surface temperature spreads remain relatively high, and
confidence is low regarding day to day values. This also at least
introduces a low chance for some winter precipitation to mix in with
the rain toward the tail end of the event Monday, but as of now this
is in the significant minority of ensemble solutions and chances do
not appear high at this time.

Beyond Monday, forecast confidence decreases considerably aside from
the previously mentioned cooling trend. There are some suggestions
that we will continue with an active and unsettled flow pattern, but
there is considerable variability among ensemble members and little
can be said about day to day precipitation impacts at this time.

BRC

&&

.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 526 PM CST Wed Dec 4 2024

Confidence is high in dry and VFR flight conditions through the
forecast period at all local terminals. Winds will remain strong
through much of tomorrow as a cold front passes through the
region, but shift to out of the northwest with the passage of the
front this evening and tonight.

Elmore

&&

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

$$

WFO LSX