SPC AC 261253
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0753 AM CDT Mon Jun 26 2017
Valid 261300Z - 271200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE INTERIOR NORTHWEST...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
THE CENTRAL PLAINS ACROSS MUCH OF NEW MEXICO AND THE SOUTHERN HIGH
Thunderstorms with marginally severe hail and strong wind gusts are
possible late this afternoon into early evening across parts of the
Pacific Northwest and from the southern and central Rockies eastward
into the High Plains.
In mid/upper levels, strong ridging that has dominated the western
U.S. will begin to break down and, in the case of the northern
branch of the ridge, shift eastward through the period. This will
occur as two initially separate Pacific perturbations phase into a
single synoptic-scale trough today:
1. A large, intense, northern-stream shortwave trough moving over
the southern AK Panhandle and just offshore southwestern BC,
anchored by a low/vorticity max now entering northwestern BC. The
low will proceed eastward across northern BC to northwestern AB by
12Z, with troughing southward to the ID Panhandle.
2. A less-intense but still well-defined perturbation in
moisture-channel imagery, moving northeastward toward the northern
CA and OR coasts. This trough will weaken but still link with the
northern-stream feature as it reaches western ID and northern NV
late in the period.
Meanwhile, as the downstream northern branch of the ridge moves
eastward across the northern/central Rockies, difluent northwest
flow aloft will persist over the central and southern Great Plains,
but generally not as strong as yesterday due to a lessening of the
height gradient aloft. The latter is related to the eastward shift
of troughing and related cyclonic flow across the Great lakes and
Ohio Valley. Associated cold air aloft and residual low-level
moisture, beneath strong mid/upper winds, may support small hail and
strong gusts from a few thunderstorms over parts of eastern/northern
NY and VT, but severe potential looks too low/conditional for a
categorical outlook at this time.
At the surface, the 11Z analysis showed a cold front from offshore
of the NC coast across southeastern GA, extreme southeastern LA, and
southeast TX, becoming quasistationary northwestward over
central/northwest TX, before being overtaken by an expanding
outflow/cold pol from a dissipating MCS. This front will become
quasistationary along most of its length and slowly weaken through
the period. Another frontal zone, evident from a weak low near PLN
across eastern WI, northern IA, and northern NE, is forecast to move
southward over the mid/upper Mississippi Valley region and central
Plains through the day, stalling then returning northward late
tonight over the central High Plains.
Widely scattered to scattered thunderstorms are expected to develop
this afternoon east of the Cascade crest, moving northeastward
across the outlook area into early evening, with some increase in
coverage possible due to clustering and cold-pool effects. The
potential exists for isolated severe gusts and large hail. Strong
diabatic surface heating and adequate low-level moisture will
underlie strengthening deep-layer winds and large-scale ascent aloft
in advance of the Pacific perturbation. Moisture should remain
relatively maximized in the preconvective environment from northern
ID southwestward across south-central WA and central/south-central
OR, despite effects of strong vertical mixing as afternoon heating
persists. This will support MLCAPE 500-1000 J/kg, locally higher.
To the south and north of the outlook area, despite strong surface
heating and steep low/middle-level lapse rates, moisture and
therefore CAPE will decrease.
Some of this activity also will pose a risk for dry-thunderstorm
fire ignitions, especially over eastern OR and southwestern ID;
refer to the SPC day-1 fire-weather outlook for more details.
Widely scattered thunderstorms should develop near the frontal zone
this afternoon and move southward to southeastward across this lobe
of the outlook area, offering isolated hail and gusts near severe
levels. With augmentation from evapotranspiration helping to offset
vertical mixing of the initially very shallow moist layer near
surface, dew points now mostly in the 50s F should increase somewhat
by mid afternoon, generally reaching the upper 50s to low 60s F.
Shallow but well-mixed subcloud layers should develop with diurnal
heating beneath 500-1000 J/kg MLCAPE in a generally east-west
corridor, based on modified 12Z RAOBs and forecast soundings from
Despite the eastward motion of the Great Lakes troughing, cyclonic
flow aloft with favorable deep/speed shear will remain over the
region, yielding 40-50 kt northwesterly effective-shear vectors,
aligned with a substantial component orthogonal to the frontal zone.
This will support a mix of discrete and clustered modes, and some
supercellular characteristics may be noted in the more persistent
cells. The lack of both more robust moisture/instability and
stronger low-level lift precludes a better-organized or denser
severe threat. Convection should diminish after dark while
encountering a progressively more-stable boundary layer.
...South-central High Plains, CO, NM, west TX...
A relative gap may exist between this regime and the central Plains
one described above, across parts of southwestern KS/southeastern
CO/northeastern NM/western OK Panhandle region. Intense convective
overturning and stabilization has been performed by last night's
MCS, the remnants of which now are moving south-southeastward and
weakening across a large swath of west TX, and this potential gap
area will be the slowest to recover. However, spatial precision of
such a gap, along with the tendency of the Raton Mesa and Sangre de
Cristos to initiate convection in even more marginal thermodynamic
environments than evident today, still leaves too much uncertainty
to carve those parts of the outlook area away at this time.
Farther southwest over the bulk of NM, and west TX, recycled MCS air
in the north, transitioning to a mix of MCS-processed and
unprocessed trajectories in the south (but in weaker shear/flow
aloft), will be available for afternoon development on the higher
terrain. Widely scattered to locally scattered thunderstorms are
expected to form this afternoon as diabatic heating and areas of
weak upslope flow reduce MLCINH. MLCAPE 1000-2000 J/kg should
develop atop well-mixed subcloud layers, amidst effective-shear
magnitudes ranging from around 35 kt over southeastern CO to less
than 20 kt in southern NM and far west TX. An MCV and
outflow/differential-heating boundaries related to the prior MCS
also may focus storm development/propagation over parts of southwest
and far west TX. This environment will support primarily a patchy,
slow-moving multicellular storm mode during afternoon and early
evening with isolated severe gusts and hail.
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