Valid 081300Z – 091200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
CENTRAL HIGH PLAINS AND COLORADO FRONT RANGE FOOTHILLS CORRIDOR…
…SUMMARY…
Severe thunderstorms with strong/gusty winds and hail will be
possible across parts of the central Rockies and adjacent High
Plains this afternoon and evening.
…Synopsis…
In mid/upper levels, the main convective influence this period will
be a negatively tilted trough — now apparent in moisture-channel
imagery from near the mouth of the Columbia River across western/
southern OR, western NV, and the lower Colorado River Valley of
AZ/CA. Led by several associated small-scale vorticity lobes, this
trough should reach ID, northern UT and northern/central AZ by 00Z.
By 12Z, the trough should extend from north-central MT down the
central Rockies of WY/CO, with leading vorticity maxima ejecting
northeastward across portions of NE/Dakotas. This will occur as a
zonally elongated cyclone — now located offshore from Vancouver
Island, Canada — becomes somewhat more axisymmetric and expansive,
moving onshore of the Pacific Northwest and BC around the end of the
period.
At the surface, a cold front was analyzed from southern QC
southwestward across western Lake Erie and north-central IN,
becoming ill-defined in and overtaken by convective outflow
southwestward across northern MO and central/southern KS, where the
outflow boundaries act as the effective front. Another frontal zone
was drawn from the Hampton Roads area southwestward across eastern
NC to a weak frontal-wave low over east-central SC, then west-
northwestward over northern portions of GA/AL/MS, then north-
northwestward across the Mid-South to south-central MO, intersecting
some of the aforementioned outflow there. The combined frontal/
outflow baroclinic zone will consolidate today as a warm front
moving slowly northward across central/northern MO, northern KS,
extreme southern NE, and the northeastern CO/southwestern NE area.
Meanwhile, a currently weak lee trough will become more sharply
defined across central MT, north-central/eastern WY, and eastern CO.
…Central Rockies/High Plains…
Scattered thunderstorms will develop over the western mountains of
CO as early as midday, possibly as an extension of the same
convective/vertical-motion plume now apparent over UT. This
activity and the related/surrounding plume of large-scale ascent
should move east-northeastward to northeastward, impinging upon the
moister, diabatically destabilized Front Range/Laramie Range region
between 21-00Z. As this occurs, a mix of discrete, clustered and
quasi-linear modes is possible, with ultimate evolution of a band of
strong-severe thunderstorms across the Foothills and adjoining High
Plains from late afternoon into the evening.
Given the steep low/middle-level lapse rates anticipated, at least
marginal inflow-layer moisture content and somewhat supercell-
favorable deep shear, isolated large hail is possible, especially
from any discrete/semi-discrete cells. However, the main threat
will be severe gusts. Well-mixed subcloud/inflow layers from the
foothills across the adjoining plains will develop beneath 1000-1500
J/kg MLCAPE, leading to “inverted-v” thermodynamic profiles
supporting downdraft acceleration. This, combined with expected
coalescence of the bulk of convection into quasi-linear mode
sweeping northeastward over the outlook area, indicate wind as the
principal concern.
..Edwards/Smith.. 09/08/2019
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