SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0755 AM CDT Sun Jun 30 2024
Valid 301300Z - 011200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE I-95/NEAR-COASTAL CORRIDOR FROM THE DELAWARE VALLEY TO
SOUTHEASTERN NEW ENGLAND...INCLUDING THE NEW YORK CITY METRO...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK FROM NORTHERN NORTH CAROLINA TO NEW ENGLAND...AND ALSO
OVER PARTS OF THE NORTHERN HIGH PLAINS...
...SUMMARY...
Scattered damaging winds will be possible across parts of the
Northeast and Mid-Atlantic States this afternoon/evening. Hail and
damaging winds may also occur from eastern Montana into western
North Dakota.
...Synopsis...
In mid/upper levels, the synoptic pattern over the CONUS will remain
dominated by progressive troughs in the Northwest to northern
Rockies, and in the Great Lakes to Northeast. An anticyclone aloft,
attached to a more extensive, zonal, subtropical ridge, will remain
anchored over the southern Plains. What will become the main
shortwave perturbation associated with the Northeast trough is
apparent over portions of the Upper Great Lakes and MI, and will
amplify while moving over southern ON around 00Z. A sharp shortwave
trough should result overnight, extending from northern VT across
eastern NY to northern WV by 12Z tomorrow.
Meanwhile, a series of minor/low-amplitude shortwaves and vorticity
lobes will eject northeastward ahead of the Northwest trough, across
the central/northern Rockies. A weak, convectively induced
perturbation now near the southern AZ border should move slowly
northwestward around the subtropical ridging. The northern part of
another trough in the mid/upper easterlies is evident in moisture-
channel imagery from the MX coastline northwest of Mazatlan,
northward to at least southern Chihuahua, moving northwestward.
The 11Z surface analysis showed a low over south-central QC, with
cold front southwestward over Lake Ontario, southwestern OH,
northern/western KY, north-central OK, to east-central NM. By 00Z
the cold front should reach parts of ME, southeastern NY, northern
VA, central AL, and southern OK, becoming a quasistationary to warm
front over the TX Panhandle to northeastern NM. By the end of the
period, this boundary should move offshore north of the Outer Banks
of NC, and extend over central GA, southern AL/MS, the Red River
Valley of TX/OK, and southeastern CO. Separate cyclogenesis should
occur today over south-central/southeastern MT, with cold front
southwestward across parts of the Great Basin.
...Northern/middle Atlantic Seaboard...
Scattered thunderstorms -- mainly in lines but with some discrete
activity -- are expected to move mostly eastward across this
corridor from late morning through early evening in episodes, with
damaging winds likely, a few embedded severe gusts possible, and
isolated potential for large hail or a tornado.
Low/middle-level destabilization will occur in the preconvective/
prefrontal environment throughout the day, essentially removing
already weak MLCINH. This will occur via diurnal heating of the
surface and large-scale lift ahead of the approaching Great Lakes
trough aloft. Steepening boundary-layer lapse rates amid rich
low-level moisture, with dewpoints commonly in the upper 60s to low
70s F, will offset modest mid/upper-level lapse rates enough to
foster peak/preconvective MLCAPE in the 1500-2500 J/kg range.
Greatest buoyancy is possible over the coastal plain between the
Delmarva peninsula and eastern MA. Wind profiles will be largely
unidirectional from the southwest or west-southwest, limiting
low-level hodograph size and overall vertical shear. However,
strengthening flow aloft with northward extent will support
increasing potential for a few supercells northward from the
northern Mid-Atlantic into New England, and especially across ME. A
counterbalancing effect, however, will be diminishing buoyancy with
northward extent over New England. Farther south over much of VA
and NC, weak shear will support predominantly multicellular storm
modes, with damaging/isolated severe gusts possible in water-loaded
downdrafts.
...Northern Plains to central Rockies/High Plains...
Scattered to isolated thunderstorms should develop over the central
Rockies and higher terrain near the Four Corners and move
northeastward to eastward this afternoon -- some of it out onto the
central High Plains. Activity should produce locally strong-severe
gusts and isolated large hail from downdrafts penetrating deep,
well-mixed subcloud layers, though vertical shear this far south of
the main flow belt should remain too weak to support well-organized
severe potential.
Farther north, however, a better-organized severe threat may occur
this evening with later-developing convection in the northern
Rockies and adjacent High Plains. Increasing large-scale ascent
ahead of the Northwest trough will support this activity, along with
backed low-level flow and strengthening convergence positioned
poleward of the deepening surface low. Early-stage convection may
include a few supercells with large hail and damaging gusts, with
some convection potentially expanding upscale into one or more
clusters as it encounters greater low-level moisture content across
central/eastern MT. Large hail will remain possible -- especially
with embedded supercell(s), and the severe-gust threat should
increase with potential for mesobeta-scale cold-pool development,
especially considering the moderately high LCL. A narrow corridor
of mid 40s to mid 50s F surface dewpoints, and steep low/middle-
level lapse rates, should support MLCAPE in the 1000-1500 J/kg range
(locally/briefly higher). Veering winds with height will contribute
to effective-shear magnitudes in the 45-55-kt range in support of
storm organization.
...Central/southeastern AZ...
Cloud debris and mostly light lingering precip are evident over
southern AZ, related to the first perturbation, with warming IR
cloud tops and overall thinning of clouds noted in recent satellite
trends. This should continue and shift northwestward with the
initial perturbation through midday into early afternoon, allowing
for diurnal heating/destabilization of the boundary layer over
southeastern AZ by mid/late afternoon. This, along with favorable
low-level moisture, orographic forcing, and weak large-scale ascent
preceding the next perturbation aloft, will support widely scattered
to scattered afternoon thunderstorms, some of which may produce
severe gusts.
Valley-floor dewpoints in the 50s F largely should be maintained
through the heating/mixing cycle, beneath very steep low-level lapse
rates and a well-mixed subcloud layer. A deep southeasterly wind
profile in low and middle levels will support northwestward motion
of activity, which may aggregate their outflows to encourage both
reinforcing development and localized channels of strong-severe
wind. The somewhat delayed nature of the destabilization process,
and uncertainties about storm coverage/aggregation, preclude greater
unconditional probabilities at this time.
..Edwards/Kerr.. 06/30/2024
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