St. Peters, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

309
FXUS63 KLSX 211709
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1109 AM CST Thu Nov 21 2024

.KEY MESSAGES...

- Seasonably-cool weather remains in place through Friday before
  a gradual warmup this weekend.

- A cold front early next week draws cooler air back to the region
  and heralds a slightly more active weather pattern leading up to
  Thanksgiving.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 323 AM CST Thu Nov 21 2024

The expansive closed upper-level low is now centered over the
Great Lakes and a 110kt mid-level jet streak is oriented
northwest- southeast from Iowa to Tennessee. This feature is
promoting a stream of vorticity advection and deep cold air
advection into the region. A vast majority of this forcing is
relegated to northern and central Illinois, where light rain and
snow continues to fall at this hour. The strong northwest winds on
the backside of the cyclonic flow aloft is mirrored at the
surface, with gusts still peaking at 25- 35mph. These gusts will
persist, if not intensify a tad, during the day today while
stratus expands to envelop most of Illinois and eastern Missouri.
While we may experience an isolated instance of 45mph gusts in
west-central Illinois, an overwhelming majority of the day will
not see such strong winds. Mid-level forcing and a weak surface
low will track through Illinois today and threaten a 15-30% chance
for showers (and even a few snowflakes) in south-central
Illinois. Otherwise, the forecast area will stay dry and cool (low
to mid-40s). Heights begin to rise aloft this evening, which will
gradually slacken the wind gusts and halt the cold air advection.
Surface high pressure will build from the west on Friday, with
temperatures a hair warmer than today`s highs and dry conditions.

MRB

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 323 AM CST Thu Nov 21 2024

The surface ridge axis drifts to our south overnight Friday into
Saturday, which will largely calm the winds. While some remnant
cloud cover may still exist, in the places where clear skies exist
temperatures will drop efficiently. St. Louis is unlikely to see its
first sub-freezing night, but most of the region will come close if
not fall below 32 degrees. Amidst the surface return flow,
temperatures Saturday will warm back into the more normal low to mid-
50s. The warmup continues into Sunday as zonal flow aloft envelops
the central CONUS, when highs jump into the 60s. Weak diffluence
across the Rockies will support gradual cyclogenesis in the central
or southern High Plains by late Sunday evening, which will interact
with a northern stream shortwave to draw a cold front back through
the region on Monday. While exactly how this feature evolves is not
certain, nearly all available guidance suggests that sufficient
forcing, lift, and moisture will be confined to eastern Missouri and
areas east Monday into early Tuesday along the front. Temperatures
behind the front fall back into the upper 30s to upper 40s by
Tuesday, chilly values for late November but not near any record
values.

Dry weather is nearly-certain for Tuesday with high surface pressure
in place, however the potential for another mid-level wave will
arrive just in time for the busiest travel day of the year. It is
worth prefacing this aspect of the forecast with the fact that it is
FAR too early to begin speculating about timing or precipitation
type. With that said, an upper level trough will dive out of
southwest Canada on Tuesday and approach the Great Plains by
Wednesday. The wave`s amplitude and position are far from certain,
resulting in pretty varied evolution of precipitation chances for
Wednesday into Thanksgiving.  A weaker amplitude wave would not
allow for rapid cyclogenesis and most likely lead to spurious, light
precipitation. If the wave is more amplified, a more mature cyclone
would result and track through the region. This scenario is depicted
in a select few deterministic models, but is far from a certainty.
There is also the question of where the system tracks in the central
CONUS, which will hinge on where the baroclinic zone will be at that
point. Finally, on the point of precipitation types, while rain
still looks like the overwhelmingly likely hydrometeor Wednesday
into Thursday, the resident colder air aloft may allow for
snowflakes to fall across northern parts of the forecast area. While
there is still a full week for these details to change or evolve,
attention to the forecast would be prudent for those with holiday
travel interests in the central CONUS.

MRB

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1106 AM CST Thu Nov 21 2024

Stratus has become more cumuliform and should continue to lift and
scatter out this afternoon. Another round of stratus is likely
late tonight into the day on Friday coming out of northern
Illinois. Bases are expected to be between 2000 and 3000 feet AGL,
with the best chances of these MVFR ceilings at KUIN followed by
metro St. Louis sites. Central Missouri stations likely will be
too far to the west to see this area of stratus. Very gusty
northwest winds are forecast through this afternoon (gusts 30-35
knots). They will slacken by late this evening, and should cease
altogether late tonight.

Gosselin

&&

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

$$

WFO LSX