SPC AC 091253
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0753 AM CDT Thu Jul 09 2020
Valid 091300Z - 101200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF
THE CENTRAL HIGH PLAINS...SOUTHERN PLAINS TO OZARKS...AND THE UPPER
MISSISSIPPI VALLEY/UPPER MIDWEST REGION...
...SUMMARY...
Severe thunderstorms are possible today and this evening over a
portion of the central and southern Plains to Ozarks and the upper
Mississippi Valley/upper Midwest region.
...Synopsis...
In mid/upper levels, height rises are forecast across much of the
western and southern CONUS through the period. This will occur as
an anticyclone strengthens along a ridge extending from the lower
Mississippi Valley across CA and over the adjoining northeastern
Pacific. To its north, the main belt of westerlies will cover the
northern 1/3-1/4 of the CONUS, from the Northwest across the
northern Plains and Great Lakes. An associated cyclone aloft -- now
located over portions of southeastern SK and southwestern MB -- will
eject east-northeastward to the northwesternmost portions of ON by
12Z tomorrow, with a shortwave trough trailing to its south.
Meanwhile, a pronounced MCV -- now evident in satellite and
composited radar imagery over extreme western IA -- will move
eastward across IA today. This feature, and the associated 500-mb
shortwave trough, may reach the Lake Michigan vicinity by the end of
the period.
At the surface, 11Z analysis showed a diffuse cold front from
northwestern ON southwestward across northwestern MN, northeastern/
south-central SD, and the NE Panhandle, behind a series of outflow
boundaries. One such boundary was evident from northern Lower MI
across south-central Lake Michigan to central WI. A composite
outflow boundary was drawn northwestern WI across southeastern MN to
northwestern IA. Another outflow boundary, related to an MCS now
over portions of southeastern KS and adjacent states, was drawn from
southwestern IA across western MO and northeastern/north-central OK,
trailing northwestward across southwestern KS and southeastern CO.
These boundaries, rather than the cold front, should be the main
foci for severe potential today, except for the trailing potion of
the front across the central High Plains.
...Upper Mississippi Valley/Upper Midwest...
Scattered thunderstorms have developed along, and mainly behind, a
largely anafrontal composite boundary from northwestern WI across
southeastern MN and northwestern IA. With steep midlevel lapse
rates and favorable elevated moisture/buoyancy behind the boundary,
isolated large hail cannot be ruled out with this activity this
morning before a general weakening trend occurs.
Additional thunderstorms are forecast to form from midday through
this afternoon. Activity will occur in a complicated assembly of at
least partially overlapping regimes of multi-scale, deep-layer lift.
That lift mainly will be related to the MCV and associated outflow/
differential/heating boundaries, but also, the eastward-shifted
morning composite boundary and another outflow boundary across WI
from an earlier MCS. Damaging gusts and isolated large hail each
will be possible as activity increases in coverage/intensity and
shifts generally eastward over the outlook area. Rich low-level
moisture and diabatic surface heating will contribute to
preconvective/warm-sector MLCAPE commonly 1500-3000 J/kg,
potentially reaching 4000 J/kg locally.
Also, despite weak low/middle-level winds in most model-forecast
soundings, some midlevel enhancement of initially modest
environmental winds may enhance low/middle-level shear on the
mesoscale, in additional to any enrichment of low-level shear and
vorticity provided by the boundaries. As such, tornado
probabilities have been introduced, and may need further
refinement/focusing through the day as mesoscale trends warrant.
...Central High Plains...
Isolated to widely scattered thunderstorms should form this
afternoon over western parts of the outlook area, as heating of
relatively high terrain preferentially removes MLCINH. Despite
generally rising heights over the region, the environment will
include steep low/middle-level lapse rates, adequate residual
moisture for convection, and areas of relatively backed/upslope
low-level flow related to the front and MCS-outflow trajectories. A
few supercells are possible, with attendant wind/hail threat atop a
deep/well-mixed subcloud layer. Forecast soundings show modest
winds below about 600 mb, but strong veering of wind direction with
height, contributing to favorable deep shear and elongated, nearly
straight-line hodographs. As such, some storm splits may occur.
Strong upper/anvil-level flow will aid in ventilation and cloud-
layer shear as well. The moist axis expected to exist by afternoon,
from central KS northwestward across southwestern NE to near the
NE/WY line, will support peak MLCAPE ranging from 1500-2000 J/kg,
locally higher.
Upscale organization of some of this activity into a small
evening/overnight MCS is possible, and is indicated to varying
extents by both synoptic and convection-allowing guidance. This
process appears reasonable, by virtue of low-level moist advection
and lift related to the western limb of the residual outflow
boundary, as well as backed low-level winds that will enhance the
storm-relative component. Duration of such a complex is uncertain,
but the associated damaging-wind threat has been extended somewhat
southeastward to offer more room for such a process.
...Southern Plains/Ozarks...
The MCS now over portions of western MO, southeastern KS and OK
should proceed southward to southeastward through the remainder of
the morning. Although most convection currently appears sub-severe,
the strong isallobaric and theta-e perturbations evident with this
system may support at least isolated severe convection along and
just behind the gust front through the rest of the morning,
especially as it undercuts progressively warmer/higher-theta-e air
in northeastern OK. See SPC mesoscale discussion 1154 for
additional details. In addition, isolated large hail may occur from
convection rooted in the elevated, trailing low-level convergence
zone atop the cold pool -- a common feature with complexes of this
sort that still have favorable lapse rates and an inflow source of
rich moisture above the boundary layer. With time through the
afternoon, the weakening LLJ and increasingly shallow nature of the
cold-pool slab with time suggest the overall severe threat will
diminish, localized flare-ups aside.
By afternoon, the resultant outflow boundary, having stalled over OK
then retreated somewhat northward, should be a source for lift.
Intense diabatic surface heating also is expected, within and south
of its residual baroclinic zone. Severe potential is conditional
along this boundary, and its location still uncertain. Widely
scattered to scattered thunderstorms may develop in a favorably
moist air mass, with severe gusts/hail the main concerns. Although
midlevel winds will be modest, stronger upper-level flow will
enhance cloud-layer shear. Relatively maximized low-level shear
near and north of the residual boundary also may encourage supercell
characteristics, in any relatively sustained/discrete convection.
If these thunderstorms can form, some activity could become
clustered with time and may generate yet another cold pool on the
mesobeta scale, with some southeast-directed forward propagation
possible. Uncertainties exist regarding how far such a complex may
move and how severe it will become, given rising heights aloft,
boundary uncertainties, and warming midlevel temperatures.
..Edwards/Broyles.. 07/09/2020
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