Day 1 Severe Weather Outlook

Valid 291300Z – 301200Z


Severe thunderstorms appear most probable over parts of the northern
Plains this afternoon and tonight.

In mid/upper levels, the basic/large-scale pattern across the CONUS
will be simple:
1. Western troughing, anchored by a strong, slow-moving synoptic
cyclone over the interior Northwest, and
2. Ridging from a high over the western Gulf coastal plain across
the Upper Midwest, shifting eastward to the upper Great Lakes
through the period.
A broad fetch of southwesterly flow will persist between those, from
the southern Great Basin and Four Corners regions across the central
Rockies and northern Plains. The strongest associated shortwave
trough – now evident in moisture-channel imagery over southern NV
and the eastern Sierra — will eject northeastward across northern
MT overnight.

At the surface, the 11Z chart showed an elongated area of low
pressure from northeastern UT across central/eastern CO, with a warm
front eastward over northern portions of KS/MO and central IL. A
cold front was drawn from northeastern UT across southern CA. The
warm front will move/redevelop northward across the central/northern
plains through the period, while more-substantial cyclogenesis
occurs this afternoon and evening over the eastern WY/southwestern
SD region. By 00Z, the main low should be near the southwest corner
of SD, with warm front to near FSD, then southeastward across
western/southeastern IA, and cold front from eastern WY to the Four
Corners area. By 12Z, the low should reach the ABR area, with cold
front southwestward over western NE and northeastern CO, and warm
front over central MN and northern WI.

…Northern Plains…
The area with greatest confidence in storm initiation remains across
western SD, from the Black Hills northward, where low-level
convergence will be strongest, augmented by orographic assistance
from the Black Hills. Damaging gusts and large hail will be the
main concerns, though a tornado may occur. This area will reside on
the northwestern fringe of favorable boundary-layer moisture/
instability, with around 1000-2000 J/kg MLCAPE available for any
convection that develops. Where near-surface flow is most backed,
in the warm-frontal zone and east/northeast of the surface low, long
hodographs and around 50-65 kt effective-shear vectors may develop
in support of supercell potential.

A substantial fraction of the warm sector across
south-central/southeastern SD and adjoining northern/eastern NE will
have a parameter space strongly favorable for severe, including wind
and thermodynamic profiles often associated with tornadoes and very
large/damaging hail. Forecast soundings culled from multiple models
and runs consistently and reasonably depict the combination of
backed surface winds, the LLJ and strong veering with height leading
to enlarged hodographs. During the 22-01Z time span, in a zone of
2000-3000 J/kg MLCAPE supported by rich low-level moisture, they
show 0-1-km SRH reaching the 250-450 J/kg range amidst 40-50 kt
effective-shear magnitudes.

Still, regardless of how potent the parameters get, hail and
tornadoes are not happening without a storm. That is the major
concern that remains for this outlook cycle. Given the presence of
an EML for capping, and apparent lack of stronger sources for
convective-scale lift to overcome the CINH south of the warm front,
surface-based development/coverage is very questionable (but at
least remotely possible) east of the western SD/Black Hills
initiation regime discussed above. Few progs show sustained
convection in this warm-frontal/warm-sector situation, in the area
with richest low-level moisture. In many ways it represents a
“significant tornadic supercell, or nothing at all” scenario. Only
subtle profile and lift adjustments appear necessary to tip the
outcome toward one extreme or the other. As such, the southern/
eastern SD part of the outlook remains highly conditional, and
subject to considerable refinement as mesoscale diagnostic trends
and shorter-fused numerical guidance evolves between now and late

…Central/southern High Plains…
Isolated to widely scattered thunderstorms should form this
afternoon near a dryline that will extend generally southward from
the area of cyclogenesis. Isolated convection also may form over
the most strongly heated adjoining areas of the moist sector. The
most intense cells may produce severe hail and gusts before
weakening during the evening.

Well-mixed subcloud layers and a corridor of around 2000 J/kg
peak/preconvective MLCAPE should develop through the afternoon.
Veering with height in low levels and slowly strengthening
mid/upper-level winds will yield vertical shear favorable for at
least transient supercell characteristics, as well as organized
multicells. However, other than a tightening height gradient in
support of the stronger flow, mid/upper support will be weak, and
the bulk of convection should weaken through the evening.

…Southern Plains…
A plume of very rich low-level moisture will continue spreading
northward across much of central/east TX, OK, and central/eastern
KS. Within this air mass, expect a broadly confluent/convergent
flow regime, diurnal heating, and lift from horizontal rolls,
differential heating, outflow, and other localized/subtle
convergence sources. This may support isolated to scattered
warm-sector thunderstorms over parts of north TX and OK from midday
through this afternoon.

The air mass will be only weakly capped, being positioned off the
southeastern rim of the EML air in midlevels. The high theta-e and
diabatic heating in low levels will offset modest lapse rates aloft
enough to yield MLCAPE in the 2000-3000 J/kg range. Meanwhile the
southeastern fringe of mass response to the western trough will
strengthening low/middle-level winds. This should yield marginally
favorable mid/late-afternoon wind profiles, with 150-250 J/kg
effective SRH and around 35 kt effective-shear magnitudes, amidst
low LCL. Given these conditions, at least briefly supercellular
convection is possible, and low-end probabilities have been
introduced accordingly. This regime will be strongly tied to
diurnal processes and should weaken markedly after dark.

..Edwards/Broyles.. 09/29/2019


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