Chesterfield, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

060
FXUS63 KLSX 040436
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1036 PM CST Tue Dec 3 2024

.UPDATE...
Issued at 719 PM CST Tue Dec 3 2024

Made a few adjustments to the temperature forecast overnight
tonight which is admittedly rather complex and atypical. Currently
a thin band of 3-4000 FT clouds is progressing eastward across the
forecast area. This is the leading edge of warm/moist advection
aloft, but thus far has only amounted to these few clouds.
Outside of this band, clear skies abound. In eastern portions of
the forecast area across central Illinois, low clouds lingered
later into the day, holding temperatures down. These clouds have
departed, and winds remain very light, allowing for good cooling
conditions this evening. These areas may drop into the teens in
the coming few hours. Further west, though, low level southerly
flow remains modest, keeping the near-surface layer better mixed.
This will reduce the overall cooling potential tonight.

Meanwhile, as noted earlier, warm advection has begun aloft, and
while this warm advection is primarily being driven above the
near-surface layer, it is aiding in setting up a strong
temperature inversion, reinforcing the inversion that would
naturally set up during nocturnal cooling. Guidance suggests
temperatures will be coolest this evening before warming overnight
as warm advection continues. However, this may be a bit
overstated as guidance tends to perform poorly on nights when warm
advection helps reinforce a strong inversion. Model vertical
resolution is often not good enough to account for such strong
inversions and near-surface cooling can continue under a clear
sky while warm advection is focused aloft. However, the surface
pressure gradient is increasing overnight, which supports the idea
of a modest southerly breeze keeping the mixing depth a bit
higher, potentially high enough to tap into some of those warming
temperatures. The bottom line for most is that temperatures are
likely to be coldest this evening before they level out or rise
later in the night. Coldest temperatures will be in the east where
winds are lightest, allowing strong surface cooling. Warmer
temperatures will be in the west where the pressure gradient is
stronger and warm advection is most aggressive.

Kimble

&&

.KEY MESSAGES...

- A quick warmup to near average temperatures (mid 40s to mid 50s)
  is expected tomorrow, with gusty west winds.

- A strong cold front will bring a 30 to 40 degree drop in
  temperatures tomorrow night, with additional gusty winds.

- While temperatures Thursday will likely stay below freezing in
  most places, a steady warmup is expected Friday through the
  weekend.

- Rain is becoming increasingly likely (50 to 80% chance over 48
  hours) at some point between Sunday and Monday.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 251 PM CST Tue Dec 3 2024

Quiet conditions have settled into the area today on the heels of
yesterday`s light snow, although stubborn low level clouds persisted
longer than expected this morning. Overnight, a large and
deepening surface low will sink into the upper Great Lakes ahead
of a rather potent clipper trough. While the bulk of the
significant impacts of this system will be felt to our northeast
(at least initially), it will have an effect on local weather as
well. First, the pressure gradient will tighten significantly
overnight, leading to a strong low level jet and warm air
advection. In addition to increasing wind speeds, this is likely
to advect a bit of enhanced low level moisture into the area
overnight as well, which is likely to only amount to a round of
low to mid level cloud cover. We only bring this up because in a
few very limited deterministic forecasts from the NAM there were
hints that this could lead to a brief period of freezing drizzle
across parts of southern Illinois overnight or early tomorrow
morning. More recent runs have trended away from this, and
confidence is very low (10% or less) that this will occur, with
better chances remaining to our east.

While a pre-frontal trough will veer winds to the
west and limit warming slightly across our northern areas during the
day, there will be enough warm air advection to result in much
warmer temperatures tomorrow afternoon. The pre-frontal trough does
limit confidence in the specific values somewhat, but there is high
confidence in afternoon temperatures near average, with the
potential for readings of around 5 to 10 degrees above average
mainly across the Ozarks. This equates to highs in the mid 40s to
mid 50s, with the greatest uncertainty along the I-70 corridor.

During the evening and overnight tomorrow, the previously mentioned
clipper will drive another significant cold front through the area,
driving temperatures back down to well below normal values and
ushering in gusty northwesterly winds. This is expected to be a dry
frontal passage for us, with the best potential for precipitation
remaining to our northeast. However, while model QPF is very limited
(<15% chance of any accumulations per NBM ad LREF guidance), model
soundings do hint at the potential for a few sprinkles along and
behind the front, with a brief window for flurries as temperatures
drop. This is most likely across parts of Illinois, but confidence
is too low for mention in the official forecast at this time.

Meanwhile, temperatures are likely to plummet to well into the 20s
and perhaps as low as the mid teens, pending the persistence of low
level cloud cover. Ensemble mean 850 temperatures reach below the
10th percentile and near the 2nd percentile as the core of the cold
air moves through the area, but exactly how this translates to
surface temperatures will likely depend at least somewhat on cloud
cover. On the other hand, surface temperatures are being forecast in
the mid-teens in spite of persistent surface winds, so its possible
that cold air advection alone gets us close to those values
regardless of cloud cover. Meanwhile, this strong cold air
advection, persistent mixing, and steep low level lapse rates will
also help to transport gusty winds to the surface through the night,
particularly across northeast MO and south-central Illinois. Current
forecasts fall short of advisory level speeds and significant
impacts, but with 75th percentile HREF wind gust forecasts near 40
mph (both ahead of and behind the front), this isn`t completely out
of the question. Also, post-frontal winds may lead to wind chills in
the single digits to possibly near 0 degrees in the colder spots.

BRC

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 251 PM CST Tue Dec 3 2024

While wind speeds and cold air advection will gradually diminish
through the day Thursday, limited afternoon warming is expected and
highs are only likely to reach the mid 20s to low 30s. Unlike the
previous wave of cold air though, a much faster recovery is expected
over the latter half of the week as southwesterly flow quickly
resumes Friday and gradually increases through the weekend. This
should result in a rapid warming trend that brings temperatures back
to near average (mid 40s to low 50s) by Saturday, and likely above
average Sunday and possibly Monday. Confidence in forecast
temperatures does diminish Monday due to timing differences in the
approach of another cold front and the potential for precipitation,
but strong warm air advection is likely ahead of this approaching
system.

Meanwhile, the upper flow pattern will undergo a somewhat complex
evolution over the weekend and into early next week, and one that is
increasingly likely (50 to 80% over 48 hours) to result in
precipitation locally. First, a broad subtropical low will gradually
move east across the southwestern U.S. over the latter half of the
week, drawing increasing Gulf moisture into Texas. Over time, model
guidance indicates that the subtropical jet associated with this
trough will arc northward and increase in strength, and at the same
time, a significant northern stream trough will emerge from the
northern Rockies and into the Plains.

This complicated phasing of the subtropical and polar jet streams
adds some inherent uncertainty to the forecast, but on the other
hand, there is relatively good agreement among LREF clusters
regarding the timing and strength of these two troughs at this time
range. This bolsters forecast confidence in the depicted surface
response as well, which features significant warm air and moisture
advection Sunday and Monday, along with the aforementioned
increasing precipitation chances. 75th to 90th percentile NBM
dewpoint forecasts also climb well into the 50s in the warm sector
across southeast Missouri and southern Illinois during this period,
which is above the 90th percentile for this time of year and leaves
a little bit of room to consider the possibility of thunderstorms.
Given the low instability projections (90th percentile SBCAPE <200
J/kg) we will leave this potential out of the official forecast for
now, but this will need to be watched considering the potential for
very strong dynamics and anomalously rich moisture.

BRC

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1032 PM CST Tue Dec 3 2024

VFR conditions continue tonight with a steady southerly wind at
the surface. A strong westerly low level jet has developed and
will lead to low level wind shear at all sites until mixing depth
increases Wednesday morning, shifting surface winds to the west
and getting gusty. Winds shift to the northwest behind a cold
front late Wednesday afternoon at Quincy and Wednesday evening at
all other sites. Winds may be a bit stronger behind the front as
cold advection begins, with gusts over 30KT possible. It`s unclear
how much stratus will follow the front, as there is a lot of low
level cold, dry air rushing in. However, some MVFR stratus may
develop behind the front. Confidence is greatest in this at Quincy
Wednesday evening, although how extensive or long lasting it will
be is uncertain.

Kimble

&&

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

$$

WFO LSX