898
ACUS01 KWNS 021253
SWODY1
SPC AC 021251

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 AM CDT Tue Jul 02 2024

Valid 021300Z - 031200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
SOUTHERN IOWA...NORTHERN MISSOURI...NORTHEASTERN KANSAS...AND
EXTREME SOUTHEASTERN NEBRASKA...

...SUMMARY...
Damaging thunderstorm winds, a few tornadoes, and large hail are
most likely this afternoon and early evening over southern Iowa,
northern Missouri, and northeastern Kansas.

...Synopsis...
Broadly cyclonic flow in mid/upper levels will persist from the
Pacific Northwest across the central/northern Rockies to the Upper
Great Lakes.  Within that, a series of shortwave troughs were
apparent, the most prominent over the CONUS being located initially
across WY and northeastern UT.  This perturbation should move
eastward to eastern SD and central NE by 00Z, cross the upper
Mississippi Valley overnight, and reach parts of WI and the Upper
Peninsula of MI by the end of the period.  As that occurs, a
persistent subtropical high over the Arklatex region will shift
eastward over the Delta region, but with attached ridging expanding
back across central and west TX to near ELP.  Another attached ridge
will shift eastward across the Ohio Valley and Lower Great Lakes.

At the surface, 11Z analysis showed a surface low near HLC, with
cold front southwestward across extreme southeastern CO and
northeastern/north-central NM.  A warm front arched from the low
across southeastern NE, then southward near the KS/MO and AR/OK
borders.  The warm front should move northeastward to the mid
Mississippi Valley today while the low moves northeastward and
weakens.  Meanwhile, by 00Z, the cold front should reach western IA,
northeastern through south-central KS, and the northern TX
Panhandle. becoming a warm front into a deepening low over
southeastern CO.  By 12Z, the cold front should reach northern IL,
central/southwestern MO, north-central OK, then into the low, which
will have sagged into the northern TX Panhandle.

...IA/MO/northeastern KS and vicinity...
A swath of decaying convection and associated cloud shield are
ongoing from Lower MI to IA and parts of southeastern NE and
northwestern MO.  This plume of clouds/precip should continue to
shift east-northeastward and erode through the day, but leave behind
an outflow/differential-heating boundary that, southwest of the
synoptic warm front, should act as the primary convective influence
ahead of the cold front.  Convergence near these boundaries, and
strong heating in the richly moist warm sector between, will support
scattered afternoon thunderstorms.  This will include the potential
for a few supercells with tornado and large-hail threats, whether
relatively discrete early in the convective cycle, or embedded in
subsequent quasi-linear evolution.  That upscale convective
organization also will maximize severe-wind potential from late
afternoon into early evening, before activity encounters a
more-stable, nocturnally cooling airmass farther east near the
warm-frontal thermal gradient.

In the preconvective warm sector, 70s F surface dewpoints should be
common, as a Gulf-origin airmass blends with areas of
evapotranspirative moisture contribution.  Though mid/upper-level
lapse rates will be modest, strong low-level destabilization will
combine with a deep troposphere to contribute to MLCAPE in the
2000-3000 J/kg range.  Effective-shear magnitudes will have a large
range -- roughly 35-55 kt based strongly on boundary effects upon
near-surface winds.  Boundary-layer shear/hodographs will be
greatest near the warm front and along any mesoscale boundaries
between the fronts, locally maximizing supercell organization in low
levels.

...Central/southern High Plains...
Widely scattered to scattered thunderstorms are forecast to develop
this afternoon over western portions of this area -- initially over
relatively high elevations of southeast WY, east-central/
southeastern CO and northeastern NM, where diurnal heating will
preferentially erode MLCINH.  This activity should move generally
eastward onto the adjoining High Plains, offering sporadic severe
gusts and hail.  The greatest concentration of convection (and
associated severe potential) still appears most probable over
southeastern CO, growing upscale as a complex and moving into parts
of southwestern KS this evening.

Vertical shear will be most favorable for supercells north of the
front, where a substantial easterly component of boundary-layer flow
will contribute to enhanced low-level shear, greater storm-relative
low-level winds, upslope lift, and moist advection (to offset
mixing-related moisture loss somewhat).  The northern lobe of the
(marginal) outlook area will be under the greatest mid/upper-level
flow and strongest deep shear, but also will have weaker low-level
moisture/theta-e and less buoyancy than farther south, with similar
mid/upper-level lapse rates under a monsoonal plume as closer to the
subtropical ridge.  In the Arkansas River Valley region, activity
moving into 50s to low 60s F surface dewpoints, with a strongly
heated boundary layer, will encounter peak diurnal MLCAPE of
500-1500 J/kg across southeastern CO, and 1000-2000 J/kg into
southwestern KS, amidst 45-55 kt effective-shear magnitudes, and
long/nearly straight low-level hodographs favoring large hail with
any discrete supercells.  However, a well-mixed subcloud layer
should support severe outflow and aggregation of outflows, leading
to upscale clustering for a few hours before activity encounters too
much CINH to remain severe over southern/central KS.  Farther south,
along and south of the front, deep-layer flow and shear will be
weaker, as will buoyancy (due to drier, deeper mixed layers), but
convection still may pose a threat for isolated severe downbursts.

..Edwards/Kerr.. 07/02/2024

$$