774
ACUS01 KWNS 291251
SWODY1
SPC AC 291249

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0749 AM CDT Sat Jun 29 2024

Valid 291300Z - 301200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
NORTHEAST...AND OVER THE RATON MESA AND NEARBY SOUTH-CENTRAL HIGH
PLAINS...

...SUMMARY...
Damaging thunderstorm gusts and a few tornadoes are possible today
over parts of the Lower Great Lakes to upper Ohio Valley, while
severe gusts also may be focused this afternoon and evening over the
Raton Mesa and nearby south-central High Plains.

...Synopsis...
Broadly, the upper-air pattern will be characterized by a synoptic
trough moving ashore from the Pacific and over the West Coast
States, the downstream ridge moving slowly out of the northern/
central Rockies onto the adjoining High Plains, and troughing over
the Great Lakes and vicinity.  Meanwhile, strong subtropical ridging
will prevail across the Southeast, southern Plains, and NM.

A northern-stream shortwave trough -- evident in moisture-channel
imagery over northeastern MN -- will eject east-northeastward today
and weaken, while effectively replaced by an upstream perturbation
now over portions of ND/MB.  The resulting shortwave trough should
proceed eastward across the Upper Great Lakes to Lake Huron and
adjoining parts of ON by 12Z tomorrow.  To its south and southeast,
a belt of strong, cyclonically curved flow will cover the mid/upper
Ohio Valley and Northeast, with minor embedded perturbations (some
convectively generated upstream from prior/overnight activity).

At the surface, the 11Z analysis showed a low over central Lake
Superior, with cold front across central Upper MI, southern WI,
southern IA, central KS, and north-central NM.  By 00Z, the cold
front should move to northern IN, central IL, southern MO, northern
OK, the northern TX Panhandle, and northeastern/central NM.  By 12Z,
the front should extend from western parts of NY/PA across KY, the
MO Bootheel area, and northern AR, becoming quasistationary over
central/western OK, the central TX Panhandle, and east-central NM.
A prefrontal surface trough should shift eastward from the St.
Lawrence Valley/Lower Great Lakes across the northern Appalachians
and northern Mid-Atlantic States through the period.

...Northeast/Ohio Valley/Great Lakes...
Scattered thunderstorms are expected to form episodically from
midday through afternoon -- predominantly ahead of the surface cold
front.  Frontal convection over parts of Lower MI also is possible
this afternoon, with isolated severe hail/gusts.  The main severe
threat, however should be over parts of the Lower Great Lakes/upper
Ohio Valley into nearly Appalachians.

With weak capping, activity may form on a variety of foci, near the
main surface trough, outflow boundaries from morning clouds/precip,
and over areas of differential heating and orographic lift.
Activity should organize into one or more lines with embedded
supercells and bowing/LEWP segments possible.  The main threat will
be damaging to severe gusts in and near the 15%/"slight" area,
though any supercells or embedded QLCS vortices also will pose a
tornado threat.

Despite weak mid/upper-level lapse rates over most of this region,
warm-sector moisture will be characterized by mid 60s to lower 70s F
surface dewpoints.  This will combine with pockets of sustained
surface heating to yield MLCAPE in the 1000-2000 J/kg range.
Veering and strengthening of winds with height in low levels should
yield enlarged hodographs below 3 km, with effective SRH in the
150-250 J/kg range overlapping areas of favorable heating in some
parts of the upper Ohio Valley area into central PA.  Little speed
shear is apparent from the top of the boundary layer into the
mid/upper levels, however, limiting deep shear, and storm structure
may become messy rather quickly.  Overall, convection should weaken
as it approaches the Atlantic Coast this evening, and the foregoing
inflow layer becomes more stable.

...South-central Rockies/High Plains...
Isolated to scattered thunderstorms are forecast to develop this
afternoon over higher terrain from the southern Front Range area
across the Sangre de Cristos and Raton Mesa.  This will occur as
strong heating of higher terrain preferentially erodes MLCINH, in
the presence of post-frontal upslope flow in low levels, and related
moisture advection to at least partially offset moisture loss due to
mixing.  The easterly component also should aid storm-relative
boundary-layer flow and deep shear for activity propagating eastward
off the elevated terrain, with a mix of multicells and transient
supercells possible.

Despite modest low/middle-level wind speeds, forecast soundings
reasonably depict enough veering with height to yield 35-45-kt
effective-shear magnitudes, amidst 500-1500 J/kg MLCAPE and steep
boundary-layer lapse rates.  Some of the resulting convection may
coalesce into a complex of thunderstorms with a cold pool, in turn
driving eastward-directed forward propagation near the post-frontal
moist axis and across parts of southeastern CO, northeastern NM, the
adjacent Panhandles, and or southwestern KS, with a relatively
focused threat of severe gusts.

...Ozarks/south-central Plains...
Ongoing, isolated to widely scattered convection over parts of KS/MO
should weaken/dissipate through the remainder of the morning, but
may leave behind enough cloud material and (especially in MO)
outflow to augment frontal baroclinicity for afternoon convective
initiation.  Widely scattered to scattered thunderstorms should
develop on and near the front this afternoon from northern OK and
extreme southern KS to the Ozarks, in a regime of MLCINH
substantially removed by modest low-level convergence and strong
diurnal/prefrontal heating.  Deep mixing south of the front should
reduce surface dewpoints and buoyancy in the warm sector, but
adequate moisture (with 60s to low 70s F surface dewpoints) should
linger in a narrow post-frontal corridor supporting 2000-3000 J/kg
MLCAPE.

The net result should be high-based, multicell thunderstorms in
mostly weak deep shear, with pulse severe in the form of gusts and
hail.  A conditional risk exists for locally significant (65+ kt)
gusts, given the favorably deep subcloud mixed layer with large
DCAPE.  However, mesoscale foci for denser coverage of such a threat
still appear too nebulous for a 15% unconditional gust outlook
within the broader marginal area.

..Edwards/Kerr.. 06/29/2024

$$