St. Peters, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

012
FXUS63 KLSX 221120
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
520 AM CST Sun Dec 22 2024

.KEY MESSAGES...

- The upcoming week will be hallmarked by abnormal warmth,
  peaking Friday into Saturday when temperatures make a run at 60
  degrees.

- The warming temperatures accompany a more active pattern with
  multiple rain chances from Monday afternoon onward, including on
  Christmas Eve and Christmas Day.

&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 218 AM CST Sun Dec 22 2024

The upper-level ridge-trough pattern is far less amplified over
the CONUS than it was 12 hours ago. As a result, the surface
ridge axis is drifting east into the Great Lakes region, placing
the Mid-Mississippi Valley in weak southeast return flow. These
winds veer and steadily strengthen today as a northern stream
wave develops a weak surface low and cold front across the
northern Great Plains. The stronger southerly winds will begin our
warming trend this week, with the warmest temperatures coincident
with the strongest 925mb warm air advection across central and
northeast Missouri. By all accounts, moisture return into the
forecast area remains very poor today, keeping skies mostly clear.

The front gradually approaches us overnight from the northwest, but
the surface low and northern stream wave track almost due east
through the Upper Mississippi Valley and keep stronger forcing
away. Cloud cover begins to increase in the vicinity of the front
overnight, but 925mb moisture remains uniformly poor through at
least sunrise and will inhibit precipitation at first. By the late
morning, either drizzle or very light rain may blossom ahead of
the front itself generally south of I-70. With very weak forcing
aloft and meager low-level convergence, measurable rain will be a
stretch (only a 15-25% chance). There is a narrow corridor of weak
moist isentropic ascent directly ahead of the front which might
lead to drizzle instead of true rain. Regardless of the mechanism
for rain, there will be little to no impacts. Southwest winds
along front, which lags further northwest oriented along a line
generally from Jefferson City, MO to Springfield, IL, will
bolster temperatures in that area into the low 50s with others in
the upper 40s.

MRB

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 218 AM CST Sun Dec 22 2024

The pattern becomes more unsettled and active starting late Monday.
Rain chances begin to ramp up by the early hours of Christmas Eve as
the front stalls south of I-70 and a wave in the southern stream
amplifies over the southern Plains. The resultant increase in Gulf
moisture and improved upper-level ascent has a better chance (as
high as 30-50%) to force a swath of light but measurable rain in
southeast Missouri and southwest Illinois Tuesday morning. The wave
aloft amplifies to some extent, strengthening the forcing and
drawing rain chances higher still (50-70% chance) for Christmas Eve
itself. The highest chances exist along and south of I-44 in
Missouri and I-64 in Illinois, coincident with the strongest
forcing and moisture. Temperatures drop by a degree or two for
those behind the front on Tuesday, but colder air still looks
relegated north of the region and the NBM interquartile range
(IQR) remains near to slightly-above normal.

Temperatures rebound a bit for Christmas Day, though exactly how
much will depend on the northward extent of the front and overall
cloud cover. The varied scenarios surrounding the upper-level
wave`s amplitude discussed in the previous forecast still appear
in the latest ensemble guidance. Some solutions slow the system
down and extend the precipitation shield over most of the region
through late Wednesday, while other more amplified solutions eject
the wave more quickly and keep the latter half of Christmas Day
dry. Given the persistent uncertainty, little change was made to
the rain chances from the previous forecast. It`s worth noting
that, while most have at least some threat for rain from Tuesday
to Wednesday, the holiday will not be a washout. Most areas along
and north of I-44 only have upwards of a 20% chance of 0.50
inches of rain during that two-day period.

On the heels of this rain, another shortwave deepens over the
Desert Southwest late Wednesday and tracks into the Great Plains.
While the amplitude and speed of this feature vary in the cluster
analysis of available ensemble members, most solutions bring rain
back to the forecast area Thursday into early Friday. Additionally,
the NBM high temperature IQR jumps several degrees on Thursday.
The spread itself is 5-7 degrees, but even the low end of the
range is generally several degrees warmer than 24 hours before.
Yet another wave takes aim on the Mid/Lower Mississippi Valley
Friday into Saturday, which has potential to be the strongest of
the week. Not only are the trough and surface low stronger more
developed overall, there is some jet-level ascent associated with
it. Ensemble guidance varies pretty drastically by this point
though, primarily due to varying trough structure and amplitude.
That said, there are at least some chances (10-20%) of seeing at
least 1.00 inches of rain, and perhaps a few rumbles of thunder,
with this system among the global ensemble members if the more
amplified scenario comes to pass. That amplified wave would also
send temperatures into the upper 50s to perhaps 60 degrees Friday
into Saturday, though all guidance points to at least a
continuation of the abnormal warmth. CIPS Extended Analog Guidance
from Saint Louis University keeps this signal through the new
year, but early indications are that a pattern change threatens an
end to our warm, wet pattern in early January.

MRB

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 520 AM CST Sun Dec 22 2024

Dry VFR conditions are forecast through the TAF period. Winds will
gradually veer and strengthen today as a cold front approaches the
region from the northwest. Amidst this flow, low-level moisture
will gradually increase with VFR stratus pooling along and ahead
of the front tonight. Eventually, the potential for MVFR stratus
materializes at the central Missouri terminals and KUIN after this
TAF period. There is also a low threat for LLWS overnight at the
terminals, however surface winds should remain strong enough to
preclude this from materializing.

MRB

&&

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

$$

WFO LSX