Day 1 Severe Weather Outlook

Valid 071300Z – 081200Z

…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE CENTRAL PLAINS…AND NEAR THE NORTHERN CASCADES…

…SUMMARY…
Isolated severe wind and hail are possible today from thunderstorms
over parts of the central Plains, and near the northern Cascades.

…Synopsis…
A progressive upper-air pattern will persist over the northern
1/3-1/2 of the CONUS, while continued weakening of longstanding
southwestern ridging is expected. A leading shortwave trough — now
apparent in moisture-channel imagery over portions of central MT and
western WY — will shift east-southeastward over the northern High
Plains to the Dakotas and northern NE through the period. Upstream
ridging will shift northeastward from the Pacific Coast States to
the northern/central Rockies. This will occur as a strong,
negatively tilted trough — initially located offshore from the
Pacific Coast, moves inland in the 00-06Z time frame. By the end of
the period, this trough at 500 mb should be located from western
portions of WA/OR to near LAS then down the lower Colorado River
Valley.

At the surface, a quasistationary frontal zone was analyzed from a
low over southeastern CO east-southeastward across northern parts of
OK, to portions of the Mid-South and Tennessee Valley region. This
boundary should move northward over the central Plains as a warm
front, potentially merging with another baroclinic zone evident over
southern SD.

…Central Plains across adjoining Missouri Valley…
Isolated to widely scattered convection should form late this
afternoon into this evening, within two now-conjoined corridors:
from southeastern CO across northern KS to northwestern MO, and from
western SD southeastward across central/southeastern NE. The
northern limb will correspond to a narrow, triangular, northwest/
southeast-oriented plume of favorable moisture and antecedent
destabilization, from western SD to central NE. Any sustained
thunderstorms will have the potential to become supercellular, with
large hail and strong-severe gusts possible. Any activity over CO,
KS, NE and southern SD may be surface-based, with the northern rim
of a boundary-layer moist axis impinging on relatively maximized
low-level convergence near the frontal zone, low, and an inverted
trough.

Forecast soundings suggest 1500-2000 J/kg MLCAPE by mid/late
afternoon over central NE and the eastern Sandhills, increasing to
2000-3000 J/kg toward northeastern KS (but amidst stronger capping).
This will be supported by surface dew points in the 60s F and steep
deep-layer lapse rates. Surface-based buoyancy will lessen in
magnitude and spatial width with northward extent and in over SD and
southwester extent into southeastern CO, but still may support a
severe storm or two moving southeastward to eastward obliquely
through the narrow favorable thermodynamic zone. Strong directional
shear is expected near the warm frontal zone, though weak wind
speeds will limit hodograph size in the lowest couple of km. Lack
of greater coverage of both progged convection and favorable
parameter space precludes more than a marginal unconditional severe
risk at this time. However, the potential for convection to cross
the gap between the two previous marginal risks resulted in their
union.

…Interior Northwest…
Widely scattered thunderstorms are expected to develop over and just
east of the Cascade crest this afternoon and evening, offering
isolated threats for severe hail/gusts as activity moves north-
northeastward to northeastward over the outlook area. Strong
heating of higher terrain will preferentially remove MLCINH
conterminously with strengthening large-scale ascent related to the
approaching Pacific trough. The resulting steepening of low/middle-
level lapse rates, atop a corridor of at least marginally favorable
low-level moisture remaining after diurnal mixing, will support
pockets of 500-1000 J/kg peak preconvective MLCAPE. Just as
importantly, a large-DCAPE, well-mixed boundary layer will develop
over the eastern Cascades region and part of adjoining lower
terrain, helping to maintain hail and accelerate downdrafts to the
surface. Forecast soundings suggest 25-40 kt effective-shear
vectors in support of predominantly multicellular (but potentially
isolated supercellular) storm mode.

..Edwards/Smith.. 09/07/2019

$$

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.