365
ACUS01 KWNS 271257
SWODY1
SPC AC 271256

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0756 AM CDT Thu Jun 27 2024

Valid 271300Z - 281200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
OF THE CENTRAL AND NORTHERN HIGH PLAINS...

...SUMMARY...
The greatest severe-thunderstorm threat is over parts of the
northern/central High Plains this afternoon and evening.  Gusts near
80 mph, damaging hail greater than 2.5 inches and a few tornadoes
are possible.

...Synopsis...
The mid/upper-level pattern will feature a progressive northern
stream, with troughs initially located over the inland Pacific
Northwest and over the Northeast.  A series of phased vorticity
maxima extend from the Northeastern trough southwestward across the
central/southern Appalachians to parts of MS/LA in recent moisture-
channel imagery.  This feature should shift eastward to the eastern
Carolinas, southern GA and coastal MS by 00Z, becoming stationary
tonight across GA and the Gulf Coast east of LA, but moving offshore
from NC and SC.

The Northwestern trough will move eastward astride the international
border, with a closed 500-mb low forming and moving over extreme
southern parts of AB/SK tonight.  By 12Z tomorrow, the trough should
extend from the southern SK low across central MT to southern ID.
Though the flow will remain nearly zonal well to its southeast over
the central High Plains, height falls are expected to extend as far
south as parts of the OK/TX Panhandles.

At the surface, the 11Z analysis showed a cold front across western
MT, central ID and northern CA, and a warm front from near HVR
across east-central MT, western/southern SD and northern NE.
Cyclogenesis will continue over MT through the day, shifting
eastward.  By 00Z, the main low should reach the MT/ND border area
roughly between GDV-DIK, with cold front southwestward across
northwestern WY, southeastern ID and central NV.  A dryline should
sharpen over the High Plains through the day, and by 00Z, extend
from near the MT/SD line across eastern CO to the western TX
Panhandle.  The warm front should extend from the low southeastward
over the western Dakotas and central parts of NE/KS, becoming
diffuse and fragmented farther southeast.  A lingering segment of a
separate front should be quasistationary and act as a surface
trough/convergence zone across southern parts of AL/GA and the
central/eastern Carolinas.

...Great Plains...
Scattered thunderstorms are expected to develop in multiple episodes
over the northern/central Plains, probably leading off with
cold-frontal initiation from late morning through afternoon across
parts of western/central MT, where large-scale lift will be
strongest with the approach of the northwestern trough aloft.  This
activity -- which may arise from the same vertical-motion field now
associated with convection in higher elevations of southwestern MT
-- should move generally eastward over the Plains, with upscale
organization possible.  Severe gusts (some exceeding 70 mph) and
sporadic hail (some exceeding 2 inches diameter) will be the main
concerns.  Subsequent development is expected closer to the surface
low, dryline and warm front over the northeastern MT/western Dakotas
area this afternoon/evening, also posing a threat for large to very
large hail and severe gusts.  Tornado potential will be relatively
maximized with any discrete or embedded supercell(s) moving into the
plume of greatest low-level moisture and backing of surface flow
near the low and warm front (east of the dryline).

Deep-layer shear should strengthen throughout the day ahead of the
cold front, across the central/northern High Plains, while heating
gradually weakens MLCINH, and regional lift increases in relation to
low-level cyclogenesis.  This will combine with a plume of favorable
low-level moisture (dewpoints generally in the 60s F by mid/late
afternoon) to support a corridor of MLCAPE in the 1000-2000 J/kg
range near and just west of the warm front.  Effective-shear
magnitudes in the 40-55-kt range (strongest on the northern areas)
will support supercell potential, with longer hodographs west of the
warm front favoring destructive-hail production.  One or more storm
clusters may evolve and grow upscale into MCS activity this evening,
further reinforcing the wind potential, with support from moisture
transport and enhanced storm-relative flow of a 40-50-kt LLJ.

Convection should be more isolated southward down the dryline and
lee trough into the western KS/OK Panhandle region, with deep shear
also weakening southward.  However, intense surface heating and
somewhat high LCL will foster boundary-layer conditions suitable for
at least isolated severe gusts and hail.  Some clustering and
mesobeta-scale cold-pool aggregation may occur to maximize
severe-wind potential for a few hours sub-regionally.

...UT and western parts of CO/WY...
Widely scattered to scattered thunderstorms are expected to form
atop a seasonally moist, well-heated and weakly capped boundary
layer this afternoon, offering locally severe gusts, isolated large
hail, and a marginal/conditional tornado threat.  This activity
should develop as height falls spread over the area related to the
Northwestern trough, along/ahead of the surface cold front.  Diurnal
heating will reduce MLCINH substantially, especially in the presence
of moisture characterized by 50s F surface dewpoints remaining in
many basins/valleys through the heating mixing cycle, while
orographic forcing also aids lift locally.  That moisture, combined
with steep low/middle-level lapse rates, will support MLCAPE of
500-1500 J/kg, atop favorable subcloud thermal profiles to support
downdraft accelerations to severe levels.  Although environmental
flow should be nearly unidirectional through most of the
troposphere, terrain-forced backing will enhanced deep shear
locally, with effective-shear magnitudes generally in the 25-40-kt
range.  Mostly multicells and a few supercells are possible.

...Southeast...
Widely scattered to scattered thunderstorms should develop this
afternoon into early evening, from the NC Coastal Plain
southwestward to southern portions of AL/GA.  Damaging to isolated
severe gusts will be the main concern.  Activity should develop most
densely near the remnant front/convergence zone as the mid/upper
trough closely approaches, as well as along sea-breeze, outflow and
differential-heating boundaries south/southeast of the front.
Despite weak midlevel lapse rates, a combination of strong diurnal
heating and rich low-level moisture (with surface dewpoints commonly
in the upper 60s to mid 70s F) should yield MLCAPE in the 1500-2000
J/kg range.  Weak low/middle-level flow will limit vertical shear
substantially, and convection should be multicellular in nature with
some clustering and local cold-pool development possible.  Though
moist-, subcloud layers nonetheless should be well-mixed, supporting
strong/locally severe downdrafts.

..Edwards/Bunting.. 06/27/2024

$$