Severe Thunderstorm Warning 160, NWS St. Louis, MO

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
600 PM CDT Sun Jun 8 2025

.KEY MESSAGES...

- Scattered thunderstorms are expected along an advancing cold
  front this afternoon and evening, and at least a few are likely
  to be strong or severe. Large hail and damaging wind gusts are
  the most likely hazards, with a low potential for a tornado or
  two.

- Dry and mild conditions are expected Monday, Tuesday, and likely
  Wednesday, but will be followed by the return of a more active
  and showery period Thursday through the end of the week.

&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 1249 PM CDT Sun Jun 8 2025

The primary concern for today is once again the potential for severe
thunderstorms this afternoon and evening, which should put a cap on
an active weekend.

Well to our north, a large closed upper low continues to slowly spin
to the east, and is currently centered near the U.S. / Canadian
border in northern Minnesota. While this is far removed from our
area, it's influence will be felt in the form of a trailing cold
front, which is oriented NE to SW and extends from northeast to
west-central Missouri. This feature is expected to slowly push
south through the afternoon and evening, and is expected to serve
as the primary initiating mechanism for scattered thunderstorms
later in the afternoon. In fact, satellite imagery already shows
developing cumulus along it, and recent CAMS are consistent in
storms developing along this boundary by early-mid afternoon 2-4
PM.

Ahead of this boundary, effective deep layer shear is expected to
steadily increase to around 40-50kt as a 500mb jet streak
approaches from the north. While low level winds once again appear
to be modest and there is little directional component to this
shear, the magnitude shear alone will be more than sufficient to
support rotating updrafts once they initiate. Model hodographs
likewise are mostly straight-line in nature, although some
increasing curvature is noted after 00Z. These profiles support
the potential for splitting supercell structures at least
initially, with perhaps a transition to linear modes supported by
shear vectors parallel to the cold front. The evolution of storm
modes will also be complicated by the surging front, which may act
to undercut developing storms and cause them to become elevated
and/or impact the merging of cold pools. Right-movers that can
stay rooted in the boundary layer, though, will stand a better
chance to remain surface based, and will pose the greatest threat
for severe hazards.

As for the specifics of those hazards, While mid-level lapse rates
degrade a bit this far northeast (6-7 C/km), this is not likely to
prevent substantial mixed-layer CAPE from developing south of the
cold front by the time storms initiate, with SPC Mesoanalysis (RAP)
indicating around 1500-2500 J/kg. This is plenty of instability to
support robust updrafts, with large hail and damaging winds all
possible (although, damaging winds will be favored as storms become
more linear). While low level shear and helicity is projected to be
modest, there will be just enough such that a tornado or two will be
possible. These storms are likely to slowly move east-southeast
through the late afternoon and evening, and gradually weaken as
instability wanes in the warm sector this evening and/or they are
undercut by the cold front. Meanwhile, scattered showers and perhaps
a few weak thunderstorms may also persist behind the front for a few
hours, although the severe potential of this activity is low.

Precipitation is expected to largely end by sunrise, which will take
us into a relatively quiet portion of the forecast period Monday and
Tuesday as dry northwest flow settles in.

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&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 1249 PM CDT Sun Jun 8 2025

As mentioned in the short term forecast discussion, Monday and
Tuesday will very likely feature dry conditions, with a modest
cooling trend expected thanks to the passing cold front and
northwest flow. Confidence is high that these two days will be dry,
and temperatures are likely dip a few degrees into the upper 70s to
low 80s.

From Wednesday through the end of the week, though, southerly flow
will gradually return to the area, bringing with it a steady influx
of rich Gulf moisture that will persist for several days. At the
same time, the upper flow pattern will feature a building longwave
ridge across the central U.S. and Mississippi Valley, with one or
more subtle subtropical lows meandering underneath it across the
central and southern Plains. These features will also gradually
drift eastward into our area, likely providing subtle forcing
mechanisms for showers and thunderstorms.

The end result is likely to be a return to a multi-day period of
shower and thunderstorm chances that will likely begin Thursday, and
possibly even as late Wednesday. Very little can be said about the
day to day specifics at this point, although it should be noted that
the weak upper level features do limit confidence in the potential
for sufficient wind-shear for widespread severe storms. On the other
hand, ensemble instability projections appear relatively healthy,
and there may be mesoscale features that locally enhance wind shear
at various times. Meanwhile, precipitable water values will steadily
increase, perhaps to near the 90th percentile or higher as indicated
by NAEFS/GEFS/LREF ensemble means, which at least opens the door for
rain and even locally heavy rain at times. Meanwhile, temperature
trends are less certain and will likely be interrupted by periods of
precipitation, but a return to near average values is currently most
likely.

BRC

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 555 PM CDT Sun Jun 8 2025

Primary concern remains the potential for showers and
thunderstorms during the first few hours of the TAF period. While
the initial round of thunderstorms earlier this afternoon
diminished, renewed thunderstorm activity has developed in central
and northeast Missouri at around 23 Z, and may still impact local
TAF sites. This is most likely at COU/JEF, but remains possible at
STL/SUS/CPS slightly later as well. Lightning, large hail to
quarter size, and erratic gusty winds are the primary hazards,
along with visibility reductions. The lightning threat is not
expected to last much more than 1 to 2 hours, but less impactful
showers will likely persist into the night. Precipitation should
wind down after 06Z, and VFR conditions will likely persist the
remainder of the period. Breezy west winds are expected during
the day tomorrow as well.

BRC

&&

.CLIMATE...
Issued at 227 PM CDT Fri Jun 6 2025

St Louis has still not seen 90 degrees yet in 2025. The average
first 90 degree reading is May 20. The most recent late 90-degree
start was June 10, 2021. If St Louis doesn't hit 90 by June 10,
it will be the latest first 90 degree day since at least 1995
(June 19). The latest date of first 90 degrees on record was July
4 in both 1961 and 1912.

Columbia has also not yet seen a 90 degree reading in 2025, however
the average first date is much later (May 31). The latest date of
first 90 degrees on record was July 14, 1904.

Quincy reached 90 degrees on May 15 of this year, about 3 weeks
earlier than their average first 90 degrees (June 2).

Kimble

&&

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

WFO LSX

Hazardous Weather Outlook
National Weather Service St Louis MO
1256 PM CDT Sun Jun 8 2025

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-091800-
Greene IL-Macoupin IL-Montgomery IL-Bond IL-Fayette IL-Clinton IL-
Marion IL-Washington IL-Randolph IL-Adams IL-Brown IL-Pike IL-
Calhoun IL-Jersey IL-Madison IL-Saint Clair IL-Monroe IL-Knox MO-
Lewis MO-Shelby MO-Marion MO-Monroe MO-Ralls MO-Pike MO-Boone MO-
Audrain MO-Moniteau MO-Cole MO-Osage MO-Callaway MO-Montgomery MO-
Lincoln MO-Gasconade MO-Warren MO-Saint Charles MO-Franklin MO-
Saint Louis MO-Saint Louis City MO-Jefferson MO-Crawford MO-
Washington MO-Saint Francois MO-Sainte Genevieve MO-Iron MO-
Madison MO-Reynolds MO-
1256 PM CDT Sun Jun 8 2025

This Hazardous Weather Outlook is for portions of eastern and
central Missouri as well as west central and southwest Illinois.

.DAY ONE...This afternoon and tonight.

Scattered thunderstorms are expected this afternoon and evening, and
a few of these may produce large hail, damaging wind gusts, and
possibly a tornado or two.

.DAYS TWO THROUGH SEVEN...Monday through Saturday.

Chances of thunderstorms return to the area Thursday through
Saturday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will likely be needed.

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