Day 1 Severe Weather Outlook

Valid 111200Z – 121200Z


Areas of strong to severe storms are expected over portions of the
northern through central High Plains Sunday afternoon into Sunday
night. Large hail and severe gusts are the primary threats.

…High Plains…

Early-morning water-vapor imagery depicts a well-defined mid-level
short-wave trough over the Pacific northwest, shifting east in line
with short-range model guidance. This feature is forecast to shift
into western MT/ID by 12/00z which will result in roughly 30m 12hr
500mb height falls across the northern High Plains of MT/WY. In
response to the short wave, low-level flow should remain decidedly
easterly as a surface low develops over north-central WY. With
seasonally high-PW values expected to hold across this region,
instability should be more than adequate for robust updrafts. Latest
guidance suggests strongest boundary-layer heating will be noted
across ID/southern MT/western WY and forecast soundings depict
uncapped soundings by 19-20z over the higher terrain. Latest
thinking is discrete supercells should evolve over southwestern MT
by early afternoon. This activity should then spread/develop east
toward the High Plains where low-level moisture will be more
conducive for higher buoyancy and perhaps a tornado threat.
Otherwise, damaging winds and hail are expected with storms as they
spread toward the western Dakotas.

Farther south, low-latitude disturbance is gyrating north along the
AZ/NM border around the west side of an upper ridge. This feature
should translate into south-central CO late Sunday afternoon with a
corridor of stronger mid-level flow forecast to extend downstream
into the central High Plains. Large-scale forcing ahead of this
feature should encourage convective development across the higher
terrain early in the period. It appears robust thunderstorms will
evolve along the Front Range of CO into southeast WY by 20-21z then
spread/develop east. This activity will propagate along cool side of
a pronounced surface boundary that should drape itself along the
I-70 corridor across northeast CO/KS. Forecast soundings favor
supercells, and early activity should prove discrete. However,
upward-evolving complex of storms is expected and an organized
squall line along the leading edge of an MCS should advance east as
LLJ focuses into south-central NE late. Damaging winds are possible
if this activity evolves as expected. Otherwise, hail and perhaps a
tornado may be noted with early supercells.

..Darrow/Cook.. 08/11/2019


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