SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0742 AM CDT Thu Aug 08 2024
Valid 081300Z - 091200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN PARTS OF NORTH
CAROLINA AND VIRGINIA...
...SUMMARY...
The main severe-weather threat today is for tornadoes from Tropical
Storm Debby, in parts of North Carolina and Virginia.
...Synopsis...
Mid/upper-level ridging has been suppressed considerably over the
West, as a highly amplified northern-stream pattern takes shape over
Canada and the northern tier of the CONUS. A compact but well-
defined cyclone -- evident in moisture-channel imagery over southern
MB -- will pivot southeastward then eastward over the MN/ON Boundary
Waters region, then Lake Superior and adjoining portions of
northwestern ON. As that occurs, height falls and strengthening
cyclonic flow will spread over the Great Lakes, contributing to
greater poleward advance of the deep-layer low accompanying T.S.
Debby.
At the surface, 11Z analysis shows a cold front from northwestern ON
across western Lake Superior to west-central KS and southeastern CO,
preceded by a strong outflow boundary across western MO,
northwestern/central/southwestern OK, the southern TX Panhandle, and
eastern NM just south of I-40. While supporting thunderstorm
formation today, lack of greater shear will preclude enough severe
potential near the boundary or front for an accompanying,
unconditional severe risk.
...T.S. Debby: NC/VA...
The tornado threat from T.S. Debby ramped up again overnight, with
several strong mesocyclones and a few tornadic-debris signatures
noted. Most of this activity has been occurring in response to two
internal factors that continue:
1. Stronger low-level convergence/lift over the northeast quadrant
of the system, helping to sustain both a persistent spiral band of
embedded supercells and an occasional discrete/semi-discrete
supercell outside the band;
2. Northwestward shift of favorable boundary-layer
theta-e/buoyancy, as the inland rain shield and its stabilizing
effects have shifted likewise, and as the cyclone as a whole has
made landfall and moved roughly the same way.
Though the more deeply overland path of Debby will foster gradual
weakening of the wind fields overall, per NHC wind-radii forecasts,
sufficiently strong low-level wind shear and enlarged hodographs
will remain over a favorably moist and unstable sector well
northeast of center. In that sector, both band-embedded and
discrete supercells should continue to form and move northwestward
as the system (and favorable sector) shift poleward. This will
spread the tornado threat into more of northeastern NC and into
southeastern VA with time today, perhaps reaching parts of central
VA by evening.
See SPC Tornado Watch 613 and related mesoscale discussions for
near-term coverage of the tornado threat. Refer to NHC advisories
for latest track/intensity forecasts for Debby, as well as tropical
watches/warnings.
...Southern Great Basin, southwestern Colorado Plateau...
Isolated severe gusts are possible this afternoon into early evening
from thunderstorms over portions of northern AZ and southern UT.
The northern periphery of a trough in the easterlies (equatorward of
the remaining ridge) will continue to move slowly westward over
western AZ and the lower Colorado River Valley today. This includes
a well-defined MCV -- produced by yesterday's convection over
southern AZ -- and now moving west-northwestward roughly over I-10
in western AZ. These features are expected to tighten the midlevel
flow gradient northward toward the ridge just enough to encourage
slow westward motion of thunderstorms developing this afternoon over
the Mogollon Rim, North Rim/Kaibab Plateau, and other higher-
elevation areas, where heating will preferentially remove MLCINH
soonest. Activity in the transition zone between richer moisture to
the south, and deeper mixing/dry-thunder potential to the north,
will pose a risk of isolated severe gusts from either wet or dry
downbursts, with small clusters/cold pools possibly forming also.
Peak MLCAPE in the 300-800 J/kg range should overlie a very deep
subcloud mixed layer supporting even larger vales of DCAPE, with
surface dewpoints commonly remaining in the 40s to low 50s at lower
elevations.
..Edwards/Dean.. 08/08/2024
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