SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0758 AM CDT Sat Aug 24 2019
Valid 241300Z - 251200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE CENTRAL/SOUTHERN PLAINS...
...SUMMARY...
The greatest severe-storm threat today into tonight is over parts of
the central/southern Plains, with severe wind, damaging hail and a
few tornadoes possible.
...Synopsis...
A progressive, seasonally active upper-air pattern will exist over
roughly the northern half of the CONUS through tonight. A series of
shortwaves will traverse a negatively tilted mean trough extending
from BC across the northern/central High Plains, with strongly
difluent flow across much of the central/southern Plains states.
One of those shortwaves includes an MCV -- now apparent in radar
reflectivity and satellite imagery near MCK. This feature should
move slowly northeastward across central NE to southeastern SD
through 12Z. This will occur as an upstream shortwave trough --
initially located over eastern WY and northern/central CO -- moves
southeastward across the central High Plains. The latter
perturbation should reach central KS and northwestern OK by the end
of the period, with vorticity augmentation likely by then in the
form of additional convective generation. A series of mostly
low-amplitude shortwaves will traverse the broadly cyclonic flow
field from the Pacific Northwest to the Dakotas.
At the surface, 11Z analysis showed a cold front from southern NC
and northern SC across the Tennessee Valley region to northwestern
AR, becoming quasistationary northwestward over northeastern KS,
east-central NE, central SD, and central/northwestern ND. The
Plains portion of this boundary should remain near its initial
location for most of the period, while the front decelerates and
becomes regionally quasistationary from the Ozarks eastward. A lee
trough -- analyzed from eastern MT across eastern WY and east-
central/southeastern CO, should move eastward by late afternoon to
near the western ND/SD/NE borders and eastern CO near the KS line,
while a dryline forms over CO and mixes eastward to near the trough.
...Central Plains to portions of OK...
Scattered thunderstorms are expected to develop by late afternoon
over portions of southwestern NE and northwestern KS, in a zone of
relatively sustained surface heating and enhanced low-level
convergence southwest of the MCV, and east of the dryline/surface
trough. An outflow/differential-heating corridor southwest of the
MCV appears to be the most probable focus for such development, near
a moisture/instability axis extending southeastward toward
northwestern/central KS. This activity will be in an environment
more characteristic of May or June than late August, in terms of
both synoptic support aloft, with favorable low-level thermodynamic
characteristics. The CAPE-shear parameter space will be favorable
for supercells in the first few hours of the convective cycle, with
very large/damaging hail, severe gusts and a threat for tornadoes.
Diurnal destabilization will be delayed somewhat as the
boundary-layer air mass across the region recovers from prior MCS
activity. Theta-e advection and sustained afternoon heating will
occur between the dryline/lee trough and frontal zone. Sourcing for
favorable moisture will be near an axis of 60s to low 70s surface
dew points that is in place from northern OK across central/
northwestern KS, and which should persist through the day. Steep
midlevel lapse rates will overlie this moisture plume, given
seasonally cold 500-mb temperatures in the -10 to -11 deg C range
observed upstream (northwest) of the region. This all should yield
peak preconvective MLCAPE in the 2000-3000 J/kg range, amidst
weakening MLCINH and strengthening vertical shear. Forecast
soundings suggest enough veering with height and low/midlevel flow
to support 200-300 J/kg effective SRH and 40-50-kt effective-shear
magnitudes.
With time this evening, upscale evolution of this activity into a
forward-propagational, bowing MCS is becoming more probable, with a
corridor of severe wind potentially developing. Ambient northwest
flow should strengthen in a channel located southwest of the MCV,
with the approach of the Rockies shortwave trough. This may augment
the development and maintenance of a rear-inflow jet, to support
cold-pool organization and a southeastward surge over parts of
western/central/southern KS and perhaps northern OK. Meanwhile, as
part of the low-level mass response to the trough aloft, a 40-50-kt
LLJ should develop across the Panhandles into western KS, veering
with time toward central KS. This will boost storm-relative
low-level flow, into the regime of forced ascent accompanying the
cold pool.
Additional, isolated to widely scattered convection may develop near
the dryline/lee trough this afternoon near the KS/CO line, also
moving southeastward, while offering sporadic severe hail and wind.
Longevity into the evening is more uncertain in this regime, given
somewhat stronger MLCINH and weaker large-scale support expected
with southward extent.
...ND...
Widely scattered thunderstorms should form this afternoon in a zone
of weak MLCINH, corresponding closely to a surface moist axis
analyzed now, near and west of the frontal zone, and east of the lee
trough. Additional, more-isolated convection also may form near the
trough itself. This activity will pose a threat for isolated severe
hail and gusts for a few hours into the evening before weakening.
The surface chart this morning shows a well-defined moisture plume
-- narrowed by prior MCS activity across the central High Plains --
from northern OK across central KS, then parallel to and just west
of the front into the western Dakotas. This moisture should be
maintained through the day amidst a substantial southerly low-level
flow component, combining with diurnal heating to overcome modest
midlevel lapse rates for MLCAPE around 1000 J/kg (locally near 1500
J/kg) -- atop well-mixed subcloud layers supporting maintenance of
gusts/hail to the surface. Organized multicells are likely, and
transient/marginal supercells may occur, amidst 25-35-kt effective-
shear magnitudes and areas of strong veering with height.
..Edwards/Gleason.. 08/24/2019
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