SPC Jun 21, 2025 1730 UTC Day 2 Convective Outlook

SPC 1730Z Day 2 Outlook

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1230 PM CDT Sat Jun 21 2025

Valid 221200Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE UPPER
MISSISSIPPI VALLEY TO THE SOUTHERN HIGH PLAINS...

...SUMMARY...
Severe thunderstorms remain possible on Sunday from parts of the
central and northern Plains into the upper Midwest. Significant hail
and wind are possible across parts of eastern North Dakota and
northern Minnesota, along with some tornado potential. A few severe
storms are also possible across the Northeast.

...Synopsis...
Broad mid-level troughing will prevail across the western U.S. as an
upper ridge continues to build over the eastern CONUS tomorrow
(Sunday). A pronounced mid-level impulse/shortwave trough will eject
into the northern Plains Sunday afternoon, supporting rapid surface
cyclone development/strengthening over the central High Plains. This
surface low is poised to move northeast toward the Upper MS Valley,
encouraging the northward transport of warm, moist air to the Great
Lakes while a trailing frontal boundary slowly advances eastward
along the High Plains, down to the Rio Grande. Along the warm front
and trailing frontal boundary, scattered severe thunderstorm
development is possible. Remnant thunderstorms will also crest the
ridge and propagate into the Northeast from southeastern Ontario
Sunday morning and afternoon. These storms will traverse the eastern
periphery of moderate to strong buoyancy and modest wind shear,
which may aid in some severe potential.

...Northern Mississippi Valley...
A warm front should reach the international border by Sunday
afternoon, with 70+ F surface dewpoints beneath 8 C/km mid-level
lapse rates expected over much of the Upper MS Valley by Sunday
afternoon. This thermodynamic profile will yield widespread 3000+
J/kg MLCAPE (over 4000 J/kg in some locales), but with appreciable
MLCINH. Rapid strengthening and gradual veering of the vertical wind
profile, from the surface to about 500 mb, will support sizeable and
curved low-level hodographs (e.g. 200+ m2/s2 effective SRH), but
with modest mid to upper-level elongation. A such, if storms can
develop ahead of the frontal boundary, supercells and short bowing
segments capable of all severe hazards (including 75+ mph gusts, 2+
inch diameter hail, and a few tornadoes) are all possible. The
overall coverage of the severe risk is strongly dependent on cap
erosion and the ability for the more robust updrafts to overcome
inhibition. The best chance for this occurring is over northern MN,
closer to the warm front, where locally backed low-level flow will
also yield the best chance for tornadoes. Higher severe
probabilities may be warranted if confidence in MLCINH erosion
increases. 

...Northern High Plains...
A few thunderstorms may develop across the northern High Plains by
afternoon peak heating, behind the frontal boundary. The pronounced
mid-level shortwave trough will overspread this region, which will
experience the greatest deep-layer ascent, along with strong speed
shear (as shown via forecast soundings as elongated, straight
hodographs/50 kts of effective bulk shear). 8+ C/km low and
mid-level lapse rates will yield around 1000 J/kg MLCAPE in this
environment. A couple of multicells or transient supercells may
occur, accompanied by some risk for severe wind/hail.

...Central and southern High Plains...
Maximum daytime surface heating, and modest convergence along a
trailing surface frontal boundary, will aid in the development of at
least scattered thunderstorms amid a warm/well-mixed boundary layer.
Vertical wind shear will be weak in this region. Nonetheless, 8-9
C/km low-level lapse rates will support 1500+ J/kg SBCAPE and
associated severe gust potential with the stronger, longer-lived
multicellular complexes, especially where cold-pool mergers may
occur. A couple instances of severe hail also cannot be ruled out.

...Northeast...
A somewhat complex severe weather scenario exists for the Northeast,
from Sunday morning into the afternoon hours. Scattered strong
thunderstorms (perhaps a remnant MCS) may cross from southeastern
Ontario and progress toward the Hudson Valley region during the
morning. Should this occur, convective restrengthening is possible
along the leading edge of outflow over the Mid Atlantic, with strong
to potentially severe wind gusts likely. However, if convection
moving into the Northeast by Sunday morning is less robust,
convective re-intensification may be more likely farther north into
southern New England. A more focused severe threat would be highly
dependent on the placement and evolution of earlier convection/MCS
morphology across Ontario during the late Day 1 period, which
currently has low predictability. Despite weak vertical wind shear,
of concern is the eastward advection of a modest EML into the
Northeast, which may boost MLCAPE well over 3000 J/kg (perhaps over
4000 J/kg locally). Higher severe probabilities may be needed if
confidence in the placement and/or timing of more organized
convection increases.

..Squitieri.. 06/21/2025

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