SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0249 PM CDT Wed Aug 28 2024
Valid 282000Z - 291200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ALONG THE NORTH
DAKOTA SOUTH DAKOTA BORDER...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN PLAINS AND THE OHIO VALLEY INTO THE MID ATLANTIC...
...SUMMARY...
Severe thunderstorms are expected across parts of the Dakotas and
from the Ohio Valley to the Mid-Atlantic Coast this afternoon
through this evening.
...20z Update...
No substantial changes have been made to the prior outlook. Storm
coverage should gradually increase over parts of the OH Valley and
Mid Atlantic through this afternoon. Damaging gusts will be possible
with the stronger clusters of storms, potentially focused from
eastern OH into WV and PA.
To the west, supercell development is expected near the surface low
and along the trialing surface trough/wind shift across western
SD/ND late this afternoon. Moderate mid-level ascent overspreading a
warming and moistening air mass across the Dakotas will allow for
rapid storm development and intensification as CINH is removed.
Large hail remains the primary threat, along with damaging gusts,
given strong buoyancy and robust supercell wind profiles. However,
strong low-level turning may support a risk for a couple of
tornadoes with the more discrete supercells, especially near the
warm front. Most short-term guidance does show gradual upscale
growth this evening and into the early overnight period across the
eastern Dakotas. Should this occur, the risk for damaging winds may
increase with the greater organized/persistent clusters before they
move into more stable air across parts of western MN. See the prior
discussion for additional information.
..Lyons.. 08/28/2024
.PREV DISCUSSION... /ISSUED 1136 AM CDT Wed Aug 28 2024/
...Dakotas...
Have upgraded portions of south-central North Dakota/north-central
South Dakota to a categorical Enhanced Risk for what should be a
relatively greater likelihood and concentration of severe storms
late this afternoon through early/mid-evening.
A prominent upper-level trough centered over western Montana and
southern Alberta will continue generally eastward over the Canadian
Prairies and toward the northern High Plains, with a
trough-preceding/basal upper-level jet resulting in considerably
strengthening southwesterly winds aloft by peak heating into
tonight. This will be atop an increasingly moist boundary layer
(dewpoints increasing through the 60s F) ahead of a surface low
across South Dakota and a front extending northward into North
Dakota.
Thunderstorms should develop near the front by late afternoon/early
evening, perhaps initially across North Dakota where overall forcing
will be greater, and subsequently southward into South Dakota where
richer low-level moisture and greater buoyancy will reside. Given
steep lapse rates and the reservoir of instability, these initial
storms should be supercells capable of very large hail amidst
strengthening flow aloft. Large to very large hail and severe wind
gusts will be the most common hazards, although a tornado risk may
also exist particularly across north-central South Dakota/far
south-central North Dakota near the surface low/warm front-related
triple point where moisture/low-level SRH will be maximized. Over
time, relatively weak mid-level flow may encourage a transition to
messier, quasi-linear modes with embedded LEWP/bow formations for a
few hours. Activity should weaken overnight as it moves into
stabilizing boundary-layer air, though a 45-55-kt low-level jet may
encourage persistence of a cluster or two tonight into the eastern
Dakotas.
...Midwest/Ohio Valley to Mid-Atlantic States...
Widely scattered to scattered thunderstorms will pose a risk for
damaging winds particularly mid/late afternoon through
early/mid-evening regionally. Some hail may also occur on a more
marginal and isolated basis. This severe risk will primarily span
parts of Indiana/Ohio eastward to the coastal Mid-Atlantic/Delmarva
vicinity, although the overall extent and magnitude of today's risk
is still a bit uncertain.
The boundary layer will not be overly moist by late-summer
standards, particularly across the middle Ohio River Valley, but a
relatively richer moist axis (through the lowest 2-3 km AGL) extends
this morning from the upper Ohio Valley eastward toward the coastal
Mid-Atlantic/Northeast. The boundary layer will also not be overly
unstable later today, with severe-adequate MLCAPE generally limited
to 2000 J/kg MLCAPE.
Aside from differential heating related to last night's convection
across the Midwest, several impetuses for thunderstorm development
to the south of the primary Great Lakes front may include the
eastward transitions of MCVs across southeast Lower Michigan and
north-central Illinois, which should be influential into the Lake
Ontario/New York/northern Pennsylvania and lower/middle Ohio Valley
vicinities, respectively, later this afternoon and evening.
Effective shear will tend to be limited to 20-25 kt, but
slowly-increasing-with-height deep-layer westerlies will support
multicell-related damaging wind/some hail potential as low-level
lapse rates steepen and the boundary layer moderately destabilizes
this afternoon.
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