SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0758 AM CDT Tue Aug 06 2024
Valid 061300Z - 071200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER MUCH OF
MONTANA INTO WESTERN SOUTH DAKOTA...AS WELL AS FROM PARTS OF LOWER
MICHIGAN INTO NEW JERSEY...THE COASTAL CAROLINAS...AND ACROSS PARTS
OF ARIZONA...
...SUMMARY...
Severe-thunderstorm potential exists today over much of Montana into
western South Dakota, as well as from parts of Lower Michigan into
New Jersey, the coastal Carolinas, and across parts of Arizona.
...Synopsis...
In mid/upper levels, a longstanding, large, western CONUS
anticyclone -- centered initially over southern CO -- is expected to
become more compressed on the northern and southwestern fringes,
while the center retrogrades back across the Four Corners region.
This will occur in response to shortwave developments in the
northern stream and subtropical easterlies. For the latter, a well-
defined trough is evident in moisture-channel imagery over portions
of southern NM, far west TX, and northwestern MX, with an embedded
low over west-central Chihuahua. The trough should proceed westward
through the period, reaching western Sonora and southern AZ by 12Z
tomorrow.
North of the mid/upper high, an accelerating, nearly zonal northern
stream is forecast from the interior Northwest to the northern Upper
Midwest, as heights fall south of a progressive synoptic trough over
western/central Canada. This will take an initially slow-moving
shortwave trough over coastal WA/OR and accelerate it eastward into
the northern Rockies around 00Z, then to eastern MT by the end of
the period. Another, weaker perturbation now near the CA/OR/NV
border junction should accelerate northeastward today as well,
preceding the Northwest trough in an eastward-turning path across
southern MT. This perturbation should reach the central Dakotas by
12Z. Farther downshear, a convectively enhanced shortwave trough
with embedded MCV is evident over the southern Lake Michigan region.
This feature should move eastward across Lower MI and southern ON
to the Lower Great Lakes by 00Z, then across the northern
Mid-Atlantic and most of New England by 12Z.
At the surface, 11Z analysis showed a wavy, quasistationary front
from a low near ORD east-southeastward across northern portions of
IN/OH/PA to southeastern NY and coastal southern New England. A
cold front extended from the low southwestward over northern MO,
east-central KS, to an elongated low-pressure area over the southern
KS/CO border vicinity. The low is expected to migrate eastward
along the frontal zone and weaken today, but perhaps slower than
model progs, given the presence of the MCV/shortwave trough
supporting it. By 00Z, the frontal zone should extend from near the
southern New England Coast across central/northern PA and northern
OH, while moving south as a cold front across central IN, southern
IL, the Ozarks, to central/western OK, becoming quasistationary
again to a low over southeastern CO. Surface cyclogenesis is
expected late tonight across eastern WY, effectively replacing the
prior southeastern CO low along a lee trough.
...Great Lakes to northern Mid-Atlantic...
Scattered thunderstorms -- both discrete and in clusters -- are
expected near the frontal zone today from the southern Upper Great
Lakes eastward. The initial activity is apparent over southern
Lower MI ahead of the shortwave trough. This includes an intense,
heavy-precip supercell that may maintain a tornado threat on the
north end of a favorably unstable environment in the frontal zone,
juxtaposed with strong low-level shear. Upscale growth into a
bowing clusters also may occur, following the frontal-related
instability gradient east-southeastward. See Tornado Watch 604 and
related SPC mesoscale discussions for near-term coverage.
This activity, or later/foregoing development near the front, will
encounter favorably moist and gradually more unstable/heated inflow
air today. Supercells and bowing clusters will remain a threat,
with all severe hazards possible. Surface dewpoints along and south
of the front -- as well as a sort distance to its north -- will
remain in the upper 60s to low 70s F (locally higher) over the
outlook corridor, offsetting modest mid/upper-level lapse rates to
yield MLCAPE in the 1500-2000 J/kg range. Enhancements to low-level
and deep shear are expected in the mass response immediately
preceding the shortwave trough, combining with backed surface winds
along and north of the front to contribute to 150-300 J/kg effective
SRH from southern Lower MI into northernmost OH, shifting into
western PA later this afternoon. One or two corridors of relatively
dense/greater damaging wind potential may develop within the current
15%/"slight" area, depending on timing/intensity of
convective-cluster evolution.
...Northern/central High Plains and adjacent foothills...
A couple rounds of strong-severe thunderstorms are possible across
the northern High Plains:
1. Convection developing by midafternoon over higher terrain of
southwestern MT, moving northeastward to eastward over
southern/central MT the remainder of the afternoon into evening.
Activity may aggregate upscale to a mainly wind-producing MCS with
sporadic severe gusts, though isolated, marginally severe hail may
occur as well. This convection should form as a zone of increasing
large-scale lift (preceding the leading/weaker shortwave trough
mentioned above) superimposes favorable diurnal heating of higher
terrain and related preferential removal of MLCINH. This convection
should move into lower elevations characterized by deep, well-
heated/mixed boundary layers, but with enough low-level moisture to
maintain and perhaps strengthen the convection as it crosses the
outlook area. Activity should encounter progressively more-stable
inflow air eastward into the western Dakotas tonight, leading to
weakening.
2. Development over the northern Rockies and adjacent foothills of
northwestern MT from late afternoon into evening, proceeding
eastward to east-southeastward across northern MT, with large hail
and locally strong-severe gusts possible. Compared to farther
south, expect somewhat more low-level moisture, in a pocket of
favorable boundary-layer theta-e undisturbed by the slightly earlier
activity to the south and southeast, as well as stronger deep shear
(45-50-kt effective-shear magnitudes) in support of supercell
potential. Peak MLCAPE of 500-1200 J/kg is expected, supported by
lower-elevation surface dewpoints commonly in the 50s F, and
residual steep low-level lapse rates from diurnal heating. Long,
relatively straight hodographs will support hail potential from any
sustained supercells -- either left- or right-moving (with splitting
storms possible). Eastward extent of the threat into tonight is
uncertain, given the potential for outflow from the higher-based,
drier-rooted southern complex to advect into the path of this
activity somewhere over central to northeastern MT.
...AZ...
Widely scattered to scattered thunderstorms are expected to form
this afternoon over the higher terrain of north-central through
southeastern AZ, roughly from east of IGM southeastward past the SAD
area, as well as on some of the smaller ranges to the south of the
Mogollon Rim. Activity should move over deserts/valleys to the west
and southwest across southern AZ, offering areas of strong-severe
gusts as outflows aggregate and additional, discretely propagating
development occurs atop the cold pools.
As the northern MX mid/upper trough shifts westward, height falls
and a somewhat tightened mid/upper gradient will shift westward over
southern AZ, leading to regional enhancement of midtropospheric
easterlies to influence convective motion and ultimate forward
propagation. Large-scale ascent/DCVA also is expected in advance of
the trough, overlying diurnally heated/deeply mixed boundary layers
to ensure steep deep-layer lapse rates. Sufficient low-level
moisture (9-13 g/kg mean mixing ratios) will remain through the
vertical-mixing process to support 1000-1500 J/kg preconvective
MLCAPE, as well as pulses of newer development when forced ascent
occurs above outflow. These processes should drive outflow, with
channels of locally severe wind, across south-central/southwestern
AZ and potentially into the lower Colorado River Valley and into
portions of southeastern CA, before activity dissipates tonight.
...Coastal Carolinas...
The threat for a few tornadoes will continue through today, and
perhaps into the evening, as T.S. Debby moves very slowly eastward
to northeastward, per NHC forecasts. With Debby potentially moving
slightly offshore, the cyclone should maintain substantial
southeasterly inflow from a large, favorably high-theta-e boundary
layer over the Gulf Stream. Within its northeastern quadrant, and
seaward of a large area of stabilizing rain a short distance inland,
the development of sporadic middle/outer-band supercells will remain
possible offshore, with tornado potential for roughly a couple
counties inland until cells encounter more prohibitively
low-buoyancy inflow air. Roughly 150-300 J/kg effective SRH and
favorably enlarged hodographs will support this threat.
See SPC Tornado Watch 603 and related mesoscale discussions for the
latest near-term guidance on tornado potential. Refer to NHC
advisories for the latest track/intensity guidance for Debby, as
well as tropical watches/warnings.
..Edwards.. 08/06/2024
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