SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0238 PM CDT Sat Sep 09 2023
Valid 092000Z - 101200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON AND EVENING ACROSS PARTS OF SOUTHWESTERN THROUGH CENTRAL
NEBRASKA...PARTS OF EASTERN WYOMING AND ADJACENT PORTIONS OF THE
GREAT PLAINS...EASTERN PENNSYLVANIA AND ADJACENT PORTIONS OF THE MID
ATLANTIC...
...SUMMARY...
Widely scattered strong to severe thunderstorm development remains
possible across parts of the northern Mid Atlantic states into
southern New England late this afternoon, and across parts of the
Great Plains to the east of the Rockies late this afternoon through
this evening.
...20Z Update...
Categorical and severe probabilistic outlook lines have been
adjusted, mostly to account for the continuing gradual progression
of synoptic and sub-synoptic scale features and their influence on
trends concerning instability.
East of the Mississippi Valley, the boundary-layer remains
seasonably moist and characterized by moderately large CAPE, along
and southeast of a weak surface frontal zone across parts of the
Allegheny Plateau into southern Maine. Southwesterly deep-layer
mean flow and shear in the vicinity of the front is rather modest
(on the order of 20-25 kt or less), but this might still be enough
to contribute to some convective organization and marginally higher
probabilities for severe wind and hail than areas elsewhere across
the east.
To the lee of the Rockies, steep lower/mid-tropospheric lapse rates
are contributing to moderately large mixed-layer CAPE, but mostly in
a rather narrow corridor east of lee surface troughing, from the
northwestern Texas Panhandle into the North Platte NE vicinity.
For more details on the severe weather potential, please refer to
the prior outlook discussion appended below, and the latest SPC
mesoscale discussions.
..Kerr.. 09/09/2023
.PREV DISCUSSION... /ISSUED 1130 AM CDT Sat Sep 09 2023/
...Nebraska/Kansas and northern High Plains...
No changes warranted, with at least widely scattered severe storms
including supercells expected across the region later today.
Deep-layer wind profiles will be conducive to discrete supercells
from late afternoon into this evening, focused along a surface front
from central Nebraska to northwest Kansas and within a post-frontal
weak low-level upslope regime to the northwest across the northern
High Plains.
A ribbon of around 1 inch PW values depicted in GOES imagery and
sampled by the 12Z LBF sounding should be maintained from western
Kansas into central Nebraska. This combined with surface
temperatures warming through the 80s along with steep mid-level
lapse rates will support a confined corridor of 1000-2000 J/kg
MLCAPE. A few to several high-based supercells should develop with
large hail as the primary hazard, along with isolated severe wind
gusts. This will initially be across central Nebraska and then
southwestward into northwest Kansas. A strengthening southerly
low-level jet during the evening may support a small cluster
evolving south over southwest Nebraska/western Kansas with a
continued wind/hail threat.
The northern High Plains portion should similarly see a few to
several supercells developing off the higher terrain in Wyoming/far
southern Montana/southwest South Dakota. While buoyancy will be
weaker here and the boundary layer not as deep, 35-45 kt effective
shear will be conducive to a few long-track cells capable of
producing sporadic hail swaths up to around golf ball size,
particularly across east-central/northeast Wyoming.
...Carolinas/Mid-Atlantic/Northeast States..
While overall regional severe coverage will be lesser after a couple
of prior active days, along with increasingly meridional and weaker
wind profiles, at least isolated damaging winds will again be
possible across much of the region. One area of focus for the
possibility of a few supercells and slow-moving multicell clustering
will be downstream of an MCV over western Pennsylvania. Here,
pockets of moderate insolation should support a plume of 1000-2000
J/kg MLCAPE centered on the Delaware Valley. As storms develop and
impinge on this buoyancy plume this afternoon, coincident with a
mesoscale belt of slightly stronger mid-level flow as sampled by
recent KCCX VWP data (30+ kt in lowest 3km AGL), the potential for
sporadic damaging winds along with isolated, marginally severe hail
should increase.
Farther south, a broad plume of weak to moderate buoyancy and
steepened low-level lapse rates will support the potential for
localized damaging wind gusts. With somewhat weaker mid-level lapse
rates and slightly less deep-layer shear relative to farther north,
potential for a more organized severe threat appears nebulous at
this time.
...South-central/southeast Texas...
Hot temperatures in excess of 100 F will again be common to the west
of overnight convection outflow (now over the western Gulf of
Mexico) and elevated thunderstorms that have redeveloped today
across east-central/southeast Texas. While some initial large
hail will be possible with storms near the coast, the primary threat
should be sporadic severe wind gusts.
...Great Basin...
Coincident with a disturbance or two embedded within low-amplitude
westerlies, and along the leading periphery of gradually increasing
PW values from the southwest, at least isolated high-based
thunderstorms are expected to develop later this afternoon. With
moderately strong mid-level winds, any of these high-based storms
will be capable of producing dry microbursts with severe-caliber
wind gusts.
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