SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1258 AM CDT Mon Aug 26 2024
Valid 261200Z - 271200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR CENTRAL AND
SOUTHERN SOUTH DAKOTA...CENTRAL AND SOUTHERN MINNESOTA...AND WESTERN
WISCONSIN...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
THE CENTRAL HIGH PLAINS TOWARD THE UPPER MISSISSIPPI VALLEY...AND
OVER PORTIONS OF SOUTHERN NEW ENGLAND...
...SUMMARY...
Severe storms are expected to evolve within a zone extending from
the northern/central High Plains eastward to Wisconsin this
afternoon into tonight. Large to very large hail, a few tornadoes,
and swaths of severe, potentially damaging, gusts are expected,
especially across South Dakota into Minnesota. A few severe storms
producing strong gusts and hail are also expected across southern
New England.
...Synopsis...
A pronounced mid-level trough will eject into the Plains and
approach the Upper MS Valley while pivoting around a broader
anticyclone aloft today. Meanwhile, another mid-level trough over
New England will gradually eject into the Atlantic through the day.
Several rounds of scattered to potentially widespread thunderstorm
development are likely ahead of the upper trough over the
Plains/Upper MS Valley this afternoon and evening. Given adequate
wind shear and strong to extreme buoyancy preceding the storms,
several strong to severe thunderstorms are expected, with all severe
hazards likely, especially over SD into central and southern MN.
Strong mid-level flow and corresponding modest buoyancy pivoting
around the upper trough in the Northeast will foster some severe
potential as well.
...Northern Plains into the Upper MS Valley...
Multiple rounds of severe thunderstorms are expected across the
northern Plains into the Upper MS Valley this afternoon into
tonight. Semi-discrete supercells are likely to develop north of a
surface low centered along the SD/IA border, along a west-to-east
oriented baroclinic zone. Along this boundary, surface dewpoints in
the upper 70s F, overspread by 8-9 C/km mid-level lapse rates, will
contribute to extreme buoyancy (i.e. MLCAPE reaching at least
4000-5000 J/kg), especially over eastern SD toward the MN/IA border.
Coinciding with this extreme buoyancy are modestly curved and
elongated hodographs, with at least 35-45 kts of effective bulk
shear. As such, initial supercell structures will be capable of all
severe hazards, including hurricane-force gusts in forward-flank and
rear-flank downdraft regions, and 2+ inch diameter hail. As storm
cold pools merge, a bow echo MCS is likely across eastern SD that
will track into central/southern MN into the Upper MS Valley. This
bow echo is expected to produce a corridor of severe gusts, some of
which may exceed 75 mph. A few tornadoes also cannot be ruled out,
either with initial semi-discrete supercells, or with line-end or
leading-line mesovortices embedded in the bow echo.
Rapidly merging thunderstorm clusters across western into central SD
are poised to track near where the initial supercells/bow echo
should occur. By the time these storms reach this zone, they should
have already grown upscale into an MCS (perhaps even a bow echo)
accompanied by severe gusts (and a 75+ mph gust cannot be ruled out
with these storms as well). A Category 3/Enhanced risk has been
expanded over much of central SD to western WI to account for
multiple, organized severe wind swaths.
...Central and Northern High Plains...
Orographic lift and strong deep-layer ascent immediately preceding
the ejecting mid-level trough will support convective initiation
over the central and northern High Plains. Scattered to potentially
widespread thunderstorms will rapidly progress toward the northern
Plains (behind earlier rounds of storms), amid a dry and deep
boundary layer characterized by steep low-level lapse rates, and
overspread by 30-40 kts of effective bulk shear. Several multicells
and supercells are expected by mid to late afternoon, with severe
gusts and perhaps large hail accompanying the stronger storms.
Clusters of storms may move eastward from WY and northeastward from
eastern CO/western KS, possibly merging in the process. Should this
occur, then one or more MCSs may develop, possibly accompanied by
more focused corridors of severe-gust potential, especially over
western into central SD by evening.
...Portions of New England into the Mid Atlantic...
A 500 mb impulse/speed max will pivot around the backside of the
mid-level trough and overspread portions of the Hudson Valley today,
encouraging convective initiation in two zones. First, scattered
multicells and perhaps transient supercells should initiate over New
England and track southeast toward Cape Cod and Long Island through
the afternoon. The stronger mid-level flow will contribute to
straight/elongated hodographs and accompanying 30-40 kts of
effective bulk (speed) shear. Strong gusts and large hail will
accompany the stronger storms. The best chance for any severe
wind/hail will be over southern New England, where over 1000 J/kg
MLCAPE may overlap the stronger vertical shear to support stronger,
longer-lived updrafts. Secondly, a cluster of
multicellular/supercellular storms may develop ahead of the 500 mb
speed max over the central Appalachians and track southeast toward
the Atlantic Coast. Given modest low-level lapse rates and hodograph
elongation, strong wind gusts may accompany these storms as well.
..Squitieri/Darrow.. 08/26/2024
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