280
ACUS01 KWNS 251258
SWODY1
SPC AC 251257

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0757 AM CDT Sat May 25 2024

Valid 251300Z - 261200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
KANSAS...OKLAHOMA AND SOUTHWESTERN MISSOURI...

...SUMMARY...
An outbreak of severe thunderstorms is possible from late afternoon
into tonight over parts of the central and southern Plains.  A few
long-lived supercells capable of intense tornadoes may occur.  Giant
hail and destructive wind gusts are also expected.

...Synopsis...
In mid/upper levels, a synoptic-scale trough over the western CONUS
-- with several associated shortwaves and vorticity lobes -- will
shift eastward to the Great Plains States by the end of the period.
A positively tilted, basal shortwave trough -- evident in moisture-
channel imagery over parts of the lower Colorado River Valley and
southern CA -- will eject east-northeastward slightly faster than
the progress of the synoptic trough.  This shortwave trough should
reach southwestern KS, the TX Panhandle, and western/southern NM by
00Z, then extend near an MKC-CSM-HOB line by 12Z tomorrow.
Meanwhile, farther poleward, a shortwave trough now apparent over
the northwestern NV/eastern OR area should amplify and pivot across
the central Rockies, reaching eastern CO by 12Z.

A northern-stream shortwave trough now over parts of Lake Superior,
Upper MI and WI should move east-northeastward to parts of Lake
Huron and southern ON by 00Z, then break through a synoptic ridge to
its east.  By 12Z, this feature should reach parts of the St.
Lawrence Valley and northern New England.  Meanwhile, a broad fetch
of difluent mid/upper-level flow will persist over the southeastern
CONUS, with several embedded, convectively generated/augmented
vorticity lobes.

The 11Z surface analysis showed a cold front from lower MI
southwestward across southern IL and southeastern MO, becoming
quasistationary over western AR, southeastern OK, and north-central/
west-central TX.  Amid intense low-level mass response to the
approaching mid/upper trough, the boundary is forecast to move
northward across northwest TX and OK today as a warm front, becoming
more diffuse with time into evening as the airmass now to its south
approaches (and eventually moves into some of) southern KS.  A
dryline will develop and sharpen with time across the area from
west-central TX northward to western KS, as moist advection
continues to its east.  An elongated area of low pressure --
attached to a lee trough over eastern CO -- is expected to
consolidate today, with the resultant low reaching the MCK vicinity
by 00Z.  The low should move/redevelop southward into western KS
overnight then shift eastward to near SLN by 12Z.  To its southwest,
frontogenesis will occur overnight, leading to a 12Z cold-frontal
position from the low across northwestern OK and parts of the TX
Panhandle.

...Central/southern Plains...
Isolated to scattered thunderstorms should develop by midafternoon
near the low and dryline, from parts of western NE to KS and perhaps
over northwest TX and western OK.  The airmass ahead of this
activity will destabilize, moisten and get more strongly sheared
with time, supporting all severe hazards to significant levels
(tornadoes, hail, gusts).  The activity in NE/KS will occur in less
moisture than farther south, with deeper, strongly mixed subcloud
layers promoting the possibility of earlier cold-pool development
after initial evolution to supercell structures.  Relatively dense
concentrations of large to very large hail, then severe wind, are
possible -- along with potential for a band of thunderstorms to
aggregate and sweep eastward toward the lower Missouri Valley
tonight.

The low-level wind profile for much of the afternoon will support
both right- and left-moving supercells (each capable of very
large/damaging hail).  The tornado threat generally should increase
with time from midafternoon into early evening before peaking,
especially from about southern KS southward.  As the LLJ strengthens
from late afternoon into evening, greatly enlarging hodographs, and
the richest moisture spreads northward through southern/central OK,
the parameter space will become much more suitable for long-lived,
significant tornadoes.  Meanwhile the threat for large to very large
hail and severe downdrafts will continue with any relatively
discrete cells.  In parts of north TX to central OK, forecast
soundings from multiple different models reasonably superimpose
MLCAPE of 4000-5000 J/kg, upper 60s to mid 70s F dewpoints, 50-70 kt
effective-shear magnitudes, and effective SRH as high as 400-700
J/kg by early evening.  That is an uncommon (even for this time of
year) environment supporting potentially dangerous supercells, and
may spread north of I-40 as far north as southern KS.

However, some important caveats remain that preclude any greater
unconditional severe probabilities for now.  The strongest
deep-layer forcing across the central Plains will remaining
displaced north of the remnant/diffuse frontal zone, and thus north
of the richer moisture over TX and into southern OK, for much of the
day and evening.  The development of only isolated/few storms in TX
may greatly increase the threat in OK.  By contrast, dense coverage
of afternoon convection over west-central/northwest TX may affect
potential farther north -- whether directly through left-split
interactions or indirectly via outflow-related theta-e deficits
advected northward.  Also, a plume of high clouds is forecast to
develop and stream east-northeastward for hundreds of miles off the
mountains of south-central/southeastern NM as the shortwave trough
approaches, potentially limiting diurnal heating/destabilization
between about I-40 and southern KS.  North and south of that,
heating will be stronger, for longer.  Still, any supercell(s) that
can move through the high-SRH, large-buoyancy environment of north
TX to southern KS from late afternoon onward will pose a threat for
substantial tornado production.

...Lower Great Lakes/upper Ohio Valley...
Widely scattered to scattered thunderstorms are expected to develop
near the cold front, and along a nearly parallel, prefrontal
trough/convergence zone this afternoon, moving eastward to
southeastward across the outlook area and persisting into early
evening.

Aside from lift along those boundaries, activity will be supported
by diurnally weakened MLCINH, with surface dewpoints in the upper
50s to mid 60s F contributing to around 1500 J/kg MLCAPE.  Weak
low-level flow and nearly unidirectional low/middle-level wind
profiles will limit shear, except through the entire cloud layer.
Although the shortwave trough will affect very little of this area
directly, its flattening of the ridge and related enhancement to
mid/upper-level gradient winds will enable strong flow in higher
portions of the buoyant layer to aid somewhat in multicell
organization.  Damaging gusts and isolated hail will be possible
from the best-organized convection.

...Southeastern CONUS...
Multiple mesoscale boundaries -- both from ongoing convection over
the region and from later-developing outflow and differential-
heating zones -- will support isolated potential for severe gusts
and/or hail today across a broad swath of the Southeast, from the
Mid-South to GA and parts of the Carolinas.  Separately, a merger of
sea breezes over southeastern FL should prompt scattered to widely
scattered thunderstorms this afternoon.

Both areas will be characterized by rich moisture but generally weak
flow in low/middle levels, though locally backed winds on the east
and north sides of boundaries may augment vertical shear enough to
support occasional supercell structures.  MLCAPE should range from
around 3000 J/kg across the Mid-South to 1500-2000 J/kg over
outlooked parts of AL/GA/FL/Carolinas.  Damaging to isolated severe
gusts, as well as isolated large hail, will be possible, mainly from
afternoon into early evening.  Any aggregated thunderstorm clusters
in the Carolinas to AL corridor may move southeastward with
damaging-gust potential continuing into the evening.

..Edwards/Goss.. 05/25/2024

$$