521
ACUS01 KWNS 302001
SWODY1
SPC AC 302000

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0300 PM CDT Sun Jun 30 2024

Valid 302000Z - 011200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
SOUTHERN NEW ENGLAND AND THE EASTERN MID-ATLANTIC...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN HIGH PLAINS...

...SUMMARY...
Scattered to numerous damaging wind gusts are likely across parts of
the Northeast and Mid-Atlantic States this afternoon and evening.
Large hail and severe/damaging winds should also occur from southern
Montana into western North Dakota.

...20Z Update...
The primary change with this update was to trim severe-thunderstorm
probabilities in the Northeast behind an eastward-advancing cold
front. Ahead of the front, clusters of organized storms continue
tracking eastward from the Northeast into the eastern Mid-Atlantic
states, with the primary risk being damaging winds. For additional
details, reference Mesoscale Discussions 1492/1493 and Severe
Thunderstorm Watches 489/490/491.

Within the broad Marginal Risk over the northern and central High
Plains, there may be a slightly more focused corridor of
severe-thunderstorm potential along/immediately east of the Colorado
Front Range. Around 40 kt of effective shear amid a moderately
unstable air mass will conditionally support a couple supercells
capable of large hail, before growing locally upscale and tracking
eastward. A small Slight Risk was considered, though weak
large-scale ascent and the potentially localized threat cast
uncertainty on the overall severe risk -- precluding an upgrade.

..Weinman/Jewell.. 06/30/2024

.PREV DISCUSSION... /ISSUED 1130 AM CDT Sun Jun 30 2024/

...Northeast/Mid-Atlantic into the Carolinas...
A mid-level trough over the Great Lakes and eastern Canada will
continue moving eastward through the period. An associated cold
front will likewise advance east-southeastward across New England
and much of the Mid-Atlantic through this evening. Based on area 12Z
soundings and recent surface observations, a very moist airmass is
in place ahead of the front, with surface dewpoints generally in the
low to mid 70s. Filtered daytime heating with broken cloud cover
will support weak to moderate instability through late this
afternoon, with most guidance continuing to suggest a narrow
corridor of 1000-2000 J/kg MLCAPE will be in place along/near the
I-95 corridor from southern New England to the eastern Mid-Atlantic.
Mid-level flow will increase through the day in tandem with the
upper trough, which will foster strong deep-layer shear and
organized convection.

Expectations are for thunderstorms to continue increasing in
coverage and intensity this afternoon, both along/ahead of the cold
front and a pre-frontal surface trough. Multiple rounds of intense
convection appear possible. Given a rather favorable thermodynamic
and kinematic parameter space, swaths of severe/damaging winds
generally 60-70 mph will likely occur as a mix of bowing line
segments/clusters and a few supercells sweep eastward through the
afternoon/evening. No changes have been made to the Enhanced Risk
across parts of southern New England into the Mid-Atlantic, where
the greatest concentration of damaging winds is still anticipated.
Isolated hail and perhaps a tornado may also occur with any
sustained supercell, although poor mid-level lapse rates and
modest/veered low-level flow should hinder both of these threats,
respectively. Deep-layer shear will be weaker with southward extent
across the Mid-Atlantic into the Carolinas. Convective mode should
also tend to be mainly multicellular across these regions. Even so,
steepened low-level lapse rates and ample instability should support
a threat for scattered damaging winds as thunderstorms develop and
spread eastward to the Atlantic Coast this afternoon/evening.

...Northern/Central High Plains...
A mid-level shortwave trough evident over the Northwest this morning
will progress eastward across the northern Rockies and adjacent High
Plains through tonight. A surface lee cyclone is forecast to deepen
and consolidate over northern WY/southeast MT by this evening, as
large-scale ascent preceding the upper trough overspreads the
northern High Plains. Even though low-level moisture is expected to
remain rather limited across this area, steep lapse rates and
daytime heating will contribute to at least weak instability
developing by late afternoon. Moderate to strong deep-layer shear
will support updraft organization. Cellular convection that
initially develops over the higher terrain of southwest MT and
vicinity should quickly grow upscale into a small bowing cluster as
it moves east-northeastward across central/eastern MT this evening.
Large hail may occur initially, but a transition to mainly a
severe/damaging wind threat appears likely as this mode transition
occurs. Isolated significant severe wind gusts (75-80 mph) appear
possible given the very steep/favorable low/mid-level lapse rates
expected. This wind threat may continue into parts of western ND and
vicinity tonight before convection eventually weakens.

Farther south, isolated to scattered thunderstorms should develop
along/east of the higher terrain from WY into CO. Various NAM/RAP
forecast soundings across this area show favorable shear for
organized convection, including the potential for a mix of
multicells and perhaps a couple of supercells. The southern fringe
of the stronger mid-level flow and the northwest edge of the
monsoonal moisture may overlap enough in western CO to support an
isolated threat for severe wind gusts.

...Arizona...
Low/mid-level moisture should gradually increase through the period
on the western periphery of a mid/upper-level high centered over the
southern Plains. Diurnal heating in the wake of overnight convection
should occur across the higher terrain of eastern/southeast AZ
through this afternoon, which will aid in the development of weak to
moderate instability. Expectations are for thunderstorms to
initially form over the higher terrain, and then slowly westward
late this afternoon and evening in response to around 15-20 kt of
east-southeasterly mid-level flow. A very well mixed boundary layer,
with steep lapse rates through much of the troposphere, should
support a threat for isolated strong to severe wind gusts with this
high-based convection.

$$