St. Peters, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

291
FXUS63 KLSX 090804
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
304 AM CDT Wed Apr 9 2025

.KEY MESSAGES...

- There is a low to medium (30-50%) chance for showers and weak thunderstorms
  through today for areas mainly along and north of I- 70.

- The chance (up to 70%) for showers and thunderstorms returns Thursday.
  Thunderstorms over southeastern Missouri and southwestern
  Illinois will be capable of isolated occurrences of small hail
  and gusty winds.

- A brief cool down at the end of the workweek leads to a threat
  of frost Friday night into Saturday morning.


&&

.SHORT TERM... (Through Thursday Night)
Issued at 301 AM CDT Wed Apr 9 2025

Early morning water vapor imagery shows northwesterly flow
stretching from the Northern Plains into the Ohio Valley. Within
this flow, a series of shortwaves across the Northern Plains are set
to dive southeastward into the region. The leading wave will enter
the Middle Mississippi Valley this evening, pushing a surface low
currently seen in observations over the Central Plains eastward
toward the CWA. Even at this lead time, there is still some spread
in the exact track of this low, but a majority of guidance favors it
to tracking generally along the Missouri-Iowa state line into
central Illinois. Forcing will gradually increase over the CWA
through the day as the low and its associated cold front approach.
However, the chance for scattered showers and possibly a
thunderstorm will remain low through the day and confined generally
north of I-70, as the better forcing remains northward of the CWA
and the low-levels struggle to saturate sufficiently for greater
rain chances. Despite this system and associated cloud cover, warm
air advection via southerly to southwesterly low-level flow through
the day will boost temperatures into the 60s to low 70s for most
locations. Despite northwesterly flow in the wake of the system
Wednesday night, colder air remains north of the CWA, with very
little impact to tonight`s lows and tomorrow`s temperatures
expected.

Yet another shortwave will move through the region tomorrow as a
trough begins to deepen over the eastern CONUS. Cooling temperatures
aloft will steepen lapse rates and increase instability to around
1,000 J/kg of SBCAPE per the 00z HREF mean. However, this
instability will be maximizing as deep-layer shear is decreasing,
leading to a very narrow window of strong storm potential during the
early afternoon over southeastern Missouri and southwestern
Illinois. A bulk of the instability being located in the hail growth
zone favors hail development, and inverted V soundings indicate that
downdrafts may be enhanced. However, the window over which
conditions are ideal for severe hail and damaging wind gusts is so
narrow, that confidence is very low in it materializing. Therefore,
we will forgo messaging the SPC Day 2 Marginal Risk, with our
concern currently being a low chance of small hail and gusty winds.
The chance for convection will drop notably Thursday evening as we
lose daytime heating and the forcing from the shortwave moves
southeastward.
Elmore

&&

.LONG TERM... (Friday through Tuesday)
Issued at 301 AM CDT Wed Apr 9 2025

On Friday, deep upper-level troughing will be present over the
eastern CONUS per a majority of deterministic and ensemble guidance.
The CWA will be positioned on the western periphery of this trough,
and there still may be one or two shortwaves that pass overhead in
the northwesterly flow. However, while a minority of guidance has
light QPF over and in the vicinity of the CWA Friday with these
shortwaves, an expansive area of high pressure moving into the
region and northwesterly flow will help dry the lower atmosphere out
and keep rain at bay. This northwesterly flow will usher in colder
air that will pair with cloud cover to keep temperatures about 10
degrees below normal per ensemble means, with highs topping out in
the mid to upper 50s for most locations. Friday night, a majority of
guidance has the high drifting over the Middle Mississippi that will
help calm winds and clear skies. Then, ensembles are tightly
clustered around lows in the upper to mid 30s, leading to a threat
of frost given ideal conditions for radiational cooling. Given
forecasted temperatures are borderline for frost formation, this
threat is still uncertain but bears watching given the onset of the
growing season.

Confidence is high that this cooler weather will be short lived, as
the 25th percentile of ensemble guidance for temperatures on
Saturday is several degrees warmer than the 75th percentile on
Friday. The upper-level trough will quickly phase eastward as mid to
upper-level ridging builds into the Middle Mississippi Valley,
pushing high pressure at the surface eastward and returning low-
level southerly flow to the CWA as temperatures warm aloft. This
will correlated with a 5-10 degree bump in temperatures on Saturday
compared to Friday, with temperatures warming even further on Sunday
thanks to sustained southerly to southwesterly flow.

Guidance continues to diverge on the phasing of an early week trough
and embedded shortwaves. Deterministic guidance consensus has the
leading shortwave within this trough moving through the Northern
Plains and Upper Midwest sometime Sunday into Monday. Its associated
surface low is set to follow a similar track well north of the CWA.
Despite the spread in the phasing of the trough and leading
shortwave, a majority of guidance keeps the better forcing well
north of the CWA as a front swings through sometime on Monday. This
leads to low confidence in rainfall along the front, with only about
20% of global ensemble members supporting measurable rainfall with
the FROPA.

Elmore

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1034 PM CDT Tue Apr 8 2025

Mainly dry and VFR conditions are expected through the period. The
only exception will be late tonight into Wednesday morning when
there is a low chance (20-30%) for showers between 10-15Z and
another chance (20%) of showers and possible thunderstorms as a
cold front moves through area between 21-01Z, mainly at UIN and
the St. Louis area terminals. Confidence was only high enough to
add a PROB30 at UIN between 12-15Z.  Otherwise a 40 knot low
level jet will set up over central Missouri causing LLWS at
JEF/COU. Surface winds will gradually veer from the southeast to
the west with some gusts to 20 knots at times during the day on
Wednesday.

Britt

&&

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

$$

WFO LSX