Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

628
FXUS63 KLSX 150852
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
352 AM CDT Thu May 15 2025

.KEY MESSAGES...

- The warmest (and most humid) day of the year this far is
  expected today with high temperatures nearing daily records.

- There is a conditional threat of isolated severe thunderstorms
  this afternoon and evening across southeastern MO and
  southwestern IL capable of all hazards.

- More widespread severe thunderstorms are likely Friday
  afternoon and early evening with all hazards, including very
  large hail and a low chance of a strong tornado. The greatest
  severe risk is along/south of I-44 in MO and I-70 in IL.


&&

.SHORT TERM...  (Through Late Friday Evening)
Issued at 351 AM CDT Thu May 15 2025

Mid-level water vapor imagery and surface observations this morning
indicate a potent upper-level trough ejecting northeastward over the
Northern Plains into Upper Midwest with a trailing "Pacific" cold
front across eastern KS. This front will continue eastward today,
reaching the CWA during the afternoon and evening. Ahead of this
front, strong low-level WAA and some clouds have kept early morning
temperatures from falling below the upper 60s to near 70 F. With
this warm start, 850-hPa temperatures nearing the maximum of
seasonal climatology, and a tendency for low-level flow to veer
(even ahead of the front) and potentially become downsloped off the
Ozark Plateau; well-above average and near daily record high
temperatures in the upper 80s to low-90s F are forecast. These
temperatures will be accompanied by breezy and humid conditions with
dewpoints flirting with 70 F ahead of the front. Even behind the
front, deeper BL mixing will offset weak CAA to maintain warm
temperatures through the daylight hours.

An EML will be present above this warm and humid airmass to yield
strong instability of 3000 to 4500 J/kg MLCAPE this afternoon but
also a strong capping inversion between 900 and 800 hPa. With 45 to
60 kt of deep-layer wind shear supportive of supercells and low-
level hodograph curvature, there is still concern for severe
thunderstorms this afternoon and evening, although whether or not
the inversion can be overcome is still uncertain. Since strongest
large-scale forcing will be passing to the north, surface-based
convective initiation would have to rely on low-level moisture
convergence along the front and heating. Around 10 to 30 percent of
HREF membership has thunderstorms developing across southeastern MO
and southwestern IL this afternoon and evening, but forecast
soundings indicate that most of these thunderstorms would be very
high based (~700 to 600 hPa/10000 to 12000 ft AGL) above the
inversion and unlikely to fully utilize the instability and
kinematics to become severe. However, one thing that will be need to
monitored is if these elevated thunderstorms can manage to cool the
capping inversion. All of these things considered, a conditional
threat of isolated severe thunderstorms is still present this
afternoon and evening across southeastern MO and southwestern IL
with all hazards possible:  very large hail, damaging winds, and
tornadoes. CAMs do show some showers and thunderstorms at times
overnight mainly along a low-level jet to the south of the CWA.

The Pacific front will lift back northward into the CWA on Friday,
mainly as a moisture discontinuity, ahead of an upper-level
shortwave trough and another associated Pacific cold front. With
relatively stronger forcing mechanisms and a weaker capping
inversion, confidence is much higher in thunderstorm initiation and
coverage on Friday. CAMs and short-term model guidance still vary on
how far north the warm front/deeper moisture reaches before the
Pacific cold front passes during the afternoon, but there is a loose
consensus on it reaching near the I-70 corridor. This feature will
be important for marking the northern extent of a potentially
volatile overlap of 2000 to 3500 J/kg MLCAPE with 50 to 70 kt of
deep-layer wind shear. The exact details of convective initiation
are not completely clear and ranges from late morning to mid-
afternoon with the highest probabilities of measurable rainfall near
and south of the I-44 (MO) and I-70 (IL) corridors. Supercells are
expected initially with a large orthogonal component of wind shear
vectors with respect to the front, but steady erosion of the capping
inversion and splits and mergers suggest a preference for upscale
growth with time. Severe thunderstorms will be capable of all
hazards, but very large hail appears to be a prominent threat with
any supercells with a large amount of CAPE in the hail growth zone
and possibly wide rotating updrafts. A strong tornado cannot be
ruled out with any uninterrupted supercells across southeastern MO
where 0-1 km SRH/low-level hodograph curvature is expected to be
greatest. Thunderstorms and the severe threat will shift to the east
of the CWA during the evening. Aside from thunderstorms, 850-hPa
temperatures will not be quite as anomalous on Friday than today but
will still support high temperatures in the 80s F.

Pfahler

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 351 AM CDT Thu May 15 2025

With upper-level quasi-zonal flow and rising heights overhead along
with Friday`s front south of the CWA, dry conditions will prevail on
Saturday. However, a return to a more active pattern is anticipated
to resume across the Mid-Mississippi River Valley on Sunday,
continuing into early next week as a series of upper-level
perturbations undercut a ridge that gives way to increasingly
southwesterly flow. As this process allows a warm front to shift
northward along with returning moisture, rounds of showers and
thunderstorms will incrementally reach further northeastward with
time. This pattern could also be favorable for mainly nocturnal MCSs
to develop on the low-level jet Sunday and Monday night, but the
highest confidence for widespread showers and thunderstorms is
currently centered on Tuesday when an upstream upper-level trough
finally ejects eastward. Probabilities of total rainfall through
this period exceeding 2" is 20 to 40 percent in the NBM, highest
across MO. The details of this pattern that would govern the threat
of heavy rainfall and additional severe thunderstorms is not yet
clear, especially considering how many rounds of convection have the
potential to modulate/influence the pattern and instability.
Although the NBM temperature distribution is clustered near or
slightly above average on Saturday, spread increases to around 10 F
into next week with the influence of the nearby wavering front and
rounds of showers and thunderstorms. This spread ranges generally
from near average to above average.

Pfahler

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1029 PM CDT Wed May 14 2025

Dry and VFR conditions are expected through the period. The only
exception is that there still is a low chance (15-20%) for
thunderstorms at STL/SUS/CPS on Thursday afternoon, mainly between
20-23Z. If thunderstorms do develop, they will likely have MVFR
or IFR visibilities and ceilings and could possibly produce hail
and wind gusts over 35 knots. Otherwise, winds will gust at times
up to 25 knots during the day on Thursday outside of
thunderstorms.

Britt

&&

.CLIMATE...
Issued at 227 PM CDT Mon May 12 2025

Record maximum temperatures and warmest minimum temperatures
for May 15.

                MAY 15

ST Louis:
MAXIMUM         94 1944
WARMEST MIN     72 2013


Columbia:
MAXIMUM         90 1944
WARMEST MIN     71 1941


Quincy:
MAXIMUM         93 1944
WARMEST MIN     69 1962


&&

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

$$

WFO LSX