Day 1 Severe Weather Outlook

Valid 061300Z – 071200Z


Scattered severe thunderstorms are possible across the northern
Plains late this afternoon and evening, particularly across parts of
the central/eastern Dakotas and eastern Nebraska, where a few storms
may become capable of producing very large hail and strong wind

Little change is expected to the CONUS subset of the large-scale
pattern in mid/upper levels, with a western anticyclone/ridge and
eastern trough persisting. However, the gradual breakdown process
of that western ridging will begin during this period, and continue
for several more days. Minor height falls are expected over the
northern/central parts of the Rockies and Plains, as a
northern-stream cyclone digs southeastward and grows across northern
MB. A tightened height gradient is expected in northwesterly flow
from the Canadian Rockies to the mid/upper Mississippi Valley, with
several mostly low-amplitude/embedded shortwaves and vorticity
maxima. Farther east, a convectively enhanced shortwave trough is
apparent in moisture-channel imagery from southern Lake Michigan
over parts of IL and eastern MO, and is forecast to pivot eastward
across IN, southern Lower MI, and much of OH through 00Z.

At the surface, a wavy frontal zone was analyzed from the
Mid-Atlantic across the southern Appalachians, then northeastward
over the lower Great Lakes to lower MI and southern WI, through a
weak low near LSE, then arching across southeastern IA, southeastern
through north-central NE and central SD, to another low near MBG.
The western part of this boundary should shift slowly northeastward
as a warm front over eastern NE/SD today. A separate cold front —
initially analyzed from south-central SK to near the MT/AB border —
is forecast to move to northeastern/central ND by around 00Z. This
boundary will merge with or overtake a pre-existing trough/low
moving eastward from MT. By 12Z the front should extend from
northern MN to central/western SD and northern WY. Ahead of that
front, heating and strong mixing of the boundary layer should foster
the formation of a sharpening moisture gradient and effective
dryline over western parts of the Dakotas and NE Panhandle through
this afternoon.

…North-central Plains…
Thunderstorms are expected to develop mid/late this afternoon over
portions of ND and northern SD, growing in coverage and intensity
while moving southeastward. Within the first few hours of the
convective cycle, supercells will be possible, offering the threat
of large hail, sporadic severe gusts, and perhaps a tornado. Very
large-destructive hail is possible, given the favorable shear and
deep-layer thermodynamic environment. Activity will be supported by
1. Low-level convergence near the low and front, and
2. A narrow, north/south-aligned corridor of favorable boundary-
layer theta-e west and southwest of the front and east of the lee

An axis of 2000-3000 J/kg MLCAPE should develop by mid/late
afternoon from central/north-central NE across central/north-central
SD, amidst 45-50 kt effective-shear magnitudes supporting supercell
potential. Though near-surface winds will be modest, favorable
low-level shear also is evident in forecast soundings.

Upscale growth and MCS aggregation appears possible into the
overnight across eastern parts of SD and NE, with activity moving
astride the instability/moisture gradient related to the low-level
baroclinic zone. Such a complex would transition to a wind-dominant
severe threat with isolated hail, the main questions at this
juncture being transitional timing, along with most-probable path
characteristics such as length, width and specific axial track. If
these uncertainties become better-resolved via 12Z and later
guidance and mesoscale trends, additional/larger outlook
probabilities may be inserted accordingly.

…Ohio Valley to southern Great Lakes…
Widely scattered to scattered thunderstorms are expected to develop
across the outlook area this afternoon into early evening, near the
frontal zone and amidst subtle lift/destabilization provided by the
mid/upper-level shortwave perturbation. Areas of sustained heating,
away from any persistent cloud cover from this morning, should
combine with moisture evident in 60s F surface dew points to remove
MLCINH and increase buoyancy through the afternoon. Buoyancy will
vary with both those smaller-scale variations in diabatic heating
and a general northeastward decrease in mid/upper-level lapse rates
with distance from the EML. Peak preconvective MLCAPE should range
from around 2500-3000 J/kg over southeastern MO, where deep-layer
forcing will be weakest, to 1000-1500 J/kg in northeastern parts of
the outlook area. Though favorably strong anvil-level/upper-venting
flow will exist, modest low/midlevel winds will limit vertical
shear, and modes should remain predominantly multicellular. A
well-mixed subcloud layer will support the potential for
strong/isolated severe gusts region-wide.

…Northern MT…
Widely scattered thunderstorms should develop over the higher
terrain of southwestern AB, in a post-frontal/upslope regime
featuring northeasterly surface winds, moist advection and a small
area of favorable diurnal destabilization. Convection will move
southeastward toward the U.S. border and may not dissipate before
crossing it this evening. Most guidance reasonably still indicates
that any thunderstorms moving from this regime into northern MT will
be past peak intensity and coverage, encountering a boundary layer
that will be stabilizing with time and southeastward extent. As
such, most, if not all, severe potential with this activity should
remain over Canada. For now, will refrain from adding a sliver of
5%/marginal unconditional probabilities, though the situation will
continue to be monitored for evidence that some of the convection
would still offer a hail/wind risk into MT.

..Edwards/Broyles.. 08/06/2019


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