SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1259 AM CDT Mon Apr 28 2025
Valid 281200Z - 291200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR SOUTHERN
MINNESOTA...NORTHERN IOWA...AND WESTERN WISCONSIN...
...SUMMARY...
Multiple corridors of severe thunderstorms are expected across the
Upper Midwest later today into tonight, with a regional severe
weather outbreak possible. The most dangerous period is likely
during the late afternoon and evening when strong tornado potential
should be maximized. Scattered large to very large hail and damaging
winds are likely as well.
...Synopsis...
A positively tilted mid/upper-level shortwave trough, with several
embedded vorticity maxima, will move east-northeast through the day
across the Great Plains and Upper Midwest. The southwest portion of
the large-scale trough and an embedded midlevel cyclone will remain
in place near the Four Corners through most of the period. A strong
midlevel jet (70+ kt at 500 mb) will move from the central Plains
toward the upper Midwest/Great Lakes later today into this evening.
The primary surface cyclone will move from the eastern Dakotas
toward the upper Great Lakes, as a trailing cold front moves through
parts of the Great Plains and eventually the Upper Midwest. By late
afternoon, a dryline will extend from near the MN/IA border
southwestward into the southern High Plains.
...Upper Midwest...
A regional severe weather outbreak still appears possible across the
Upper Midwest, with a threat of strong tornadoes, very large hail,
and swaths of damaging winds. No major changes have been made to the
Moderate and Enhanced Risk areas.
Extensive elevated convection and a possible MCS will likely be
ongoing at the start of the forecast period across parts of ND/MN,
and may tend to redevelop through the day. This could slow the
northward advance of the warm front to some extent, but 60s F
dewpoints are expected reach at least central MN into northern WI by
mid/late afternoon.
South of the warm front and east of the dryline, the environment is
expected to become increasingly volatile through the afternoon, with
strong instability (MLCAPE near/above 3000 J/kg) and deep-layer
shear, and a persistently intense low-level jet that will maintain
very favorable low-level shear/SRH. This environment will be
conditionally favorable for long-track supercells with
strong/intense tornado potential.
Uncertainty remains regarding the evolution of diurnal convection
across the warm sector. Guidance generally suggests that
surface-based storms will develop near the surface low and along the
prefrontal trough/dryline across southern MN, which could evolve
quickly into a cluster or linear mode, though the environment would
still support embedded supercell potential with all severe hazards
possible. If development in this area is able to remain
semi-discrete, then a couple long-track tornadic supercells could
occur.
Farther south into IA, stronger heating/mixing is expected near the
dryline, though large-scale ascent and low-level convergence may be
somewhat weaker compared to areas farther north. While guidance
varies regarding the potential for development across IA during the
afternoon, any supercell that can initiate and be sustained could
become long-tracked within the expanding and very favorable warm
sector, posing a threat for strong to intense tornadoes and very
large hail.
As the surface low and related boundaries move eastward, organized
convection may develop in place and/or spread into parts of WI, with
a threat of all severe hazards and some strong-tornado potential.
Farther west, a couple of supercells may develop in the region
between the prefrontal trough/dryline and cold front, and pose a
threat of large hail and locally damaging gusts as they move across
northeast NE/southeast SD into parts of MN/IA.
...Parts of KS/MO into the Southern Plains...
While large-scale ascent will be comparatively weaker from KS/MO
into the southern Plains, isolated to widely scattered storm
development will be possible within a weakly capped environment.
Instability and deep-layer shear will both be moderate to strong by
late afternoon into the evening, and a few supercells may develop
near the dryline. Large to very large hail and locally damaging
winds will likely be the initial primary hazard, but increasing
low-level SRH by early evening will result in some tornado threat as
well. Additional strong to locally severe storm development will be
possible overnight near the advancing cold front, aided by a
continued nocturnal strengthening of the low-level jet.
..Dean/Weinman.. 04/28/2025
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