LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 AM CDT Tue Jul 25 2023
Valid 251300Z - 261200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN PARTS OF THE
NORTHERN PLAINS/UPPER MIDWEST AND MID-ATLANTIC REGIONS...
...SUMMARY...
Scattered severe wind/hail-producing thunderstorms are possible
across parts of the Northern Plains and the Northeast this afternoon
and evening. Isolated severe gusts are also possible over portions
of the Interior West.
...Synopsis...
In mid/upper levels, a large, synoptic-scale anticyclone will remain
anchored over the southwestern CONUS, with a 598-600-decameter
central 500-mb high over northern/western NM. Ridging will extend
eastward across the southern Plains and Ozarks, and northward over
the northern High Plains. A shortwave trough -- apparent in
moisture-channel imagery over portions of MT/WY, will penetrate the
larger-scale ridge through about 18Z, then pivot southeastward,
reaching the Red River Valley of the North by 00Z, and the upper
Mississippi Valley by 12Z tomorrow. Convective enhancement is quite
probable overnight, with potential for MCS activity immediately
preceding the pre-existing vorticity lobe.
Upstream, an elongated cyclone now over coastal BC will move inland,
reaching central AB by the end of the period, with a low-amplitude/
positively tilted trough across the ID Panhandle to near AST.
Further east, a mean trough will linger over the eastern CONUS,
roughly from a persistent Hudson Bay vortex southward to the
southern Appalachians. However, this trough should deamplify
somewhat, as a strong, basal shortwave perturbation -- initially
from Lake Erie over OH, WV and northeastern TN -- ejects
northeastward to inland parts of the Mid-Atlantic States and western
New England by 00Z.
At the surface, 11Z analysis showed a warm to quasistationary front
from northwestern ND southeastward over central SD, southwestern IA,
northeastern MO, and southern IL. This boundary should move
diffusely northeastward through this evening over more of the
eastern Dakotas, southwestern MN, IA and southern/central IL. A
weak front/surface trough was drawn over central NY, east-central
PA, and central VA, with some eastward shift expected today mainly
north of VA.
...Northern Plains/Upper Midwest...
Large-scale ascent preceding the shortwave trough will be well-timed
to impinge on a favorably moist and destabilizing warm sector this
afternoon. Scattered thunderstorms are expected to develop from the
central Dakotas southward into NE near the surface fronts/trough,
with severe hail and gusts each possible. Greater coverage and
duration of convection may be with the Dakotas regime, where
low-level convergence will be strongest, and where activity may
evolve upscale and move southeastward as an MCS near the warm front.
Diurnal heating and surface dewpoints generally in the 60s, beneath
moderately steep midlevel lapse rates, will support a northward-
narrowing corridor of warm-sector MLCAPE in the 2500-3500 J/kg range
(locally/briefly higher). Low-level flow will be modest diurnally,
limiting hodograph size. However, veering with height in the warm
sector (and especially near the warm front) will boost effective-
shear magnitudes into the 30-40 kt range, suggesting a mix of
multicell and supercell modes is possible, and supporting an early
hail threat before too much upscale development occurs.
While still a conditional scenario (strongly dependent on early
storm-scale evolution and how that regulates cold-pool growth/
strength), a corridor of relatively maximized severe-wind potential
may evolve across the eastern Dakotas and portions of southern MN/
northern IA for several hours this evening, near and just to the
left of the nose of a strengthening southwesterly LLJ. If
confidence in this scenario increases based on 12Z and later
numerical guidance, and/or mesoscale trends, greater wind
probabilities may be added later today. In the meantime, the 15%/
"slight" unconditional line is extended downshear to give more room
for this potential threat to evolve and run its course tonight.
Another conditional scenario is for additional development near the
warm front over the Siouxland area and western IA this afternoon,
shifting eastward across the mid/upper Mississippi Valley ahead of
an MCV left by ongoing convection over northwestern NE and
southeastern SD. Progs are inconsistent with this in terms of
timing, coverage, and even yes/no for sustained storms. This is not
surprising, given the subtle nature of low-level forcing often found
around summertime warm fronts. Still, the marginal line has been
expanded somewhat as well to account for the possibility.
...Mid-Atlantic/New England...
Scattered to widely scattered thunderstorms are expected to develop
throughout this afternoon -- mainly over and east of the eastern rim
of the higher elevations, from the northern VA Blue Ridge/Shenandoah
regions northeastward to central ME. This will occur as low-level
lift increases due to both diurnal heating and convergence near the
surface trough, and as weak DCVA/large-scale lift spreads over
central/southern parts of the outlook area ahead of the ejecting
shortwave trough. Potential thunderstorm coverage and severe
gust/hail probabilities remain relatively maximized around the
Chesapeake Bay/Delmarva regions, where the greatest low-level
theta-e and strong inland diurnal heating will yield the largest
buoyancy available to the convective regime. Peak/preconvective
MLCAPE in the 1500-2000 J/kg range will be possible. Although
modest low/midlevel flow will limit bulk shear in roughly the lower
half of the convective profile, strong mid/upper winds (up to about
100 kt near equilibrium/anvil level) may aid in storm organization,
particularly in and near the area of greatest buoyancy.
...Northern Great Basin/Intermountain Region...
Behind the departing shortwave trough, convective coverage will be
rather uncertain near the mean ridge. However, sufficient mid/
upper-level flow will remain in place to aid in local/brief
organization of isolated to widely scattered afternoon thunderstorms
developing atop a deep, well-mixed boundary layer with very nearly
dry-adiabatic subcloud lapse rates. This will support isolated
severe-wind potential with the most vigorous cells.
..Edwards/Mosier.. 07/25/2023
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