- Day Three
acus02 kwns 171701
Storm Prediction Center ac 171701
Day 2 convective outlook
National Weather Service Storm Prediction Center Norman OK
1201 PM CDT Mon Jul 17 2017
Valid 181200z - 191200z
..there is a slight risk of severe thunderstorms across the upper
Midwest/upper Mississippi Valley area...
..there is a marginal risk of severe thunderstorms across portions
of the north-central states...
Isolated to scattered severe thunderstorms will be possible on
Tuesday across the upper Midwest/upper Mississippi Valley area.
Quasi-zonal mid-level flow will remain positioned from the Pacific
northwest to the Great Lakes region, between the southern periphery
of broadly cyclonic flow over Canada and a building, west-East Ridge
from the central Great Basin to the south-central states. A surface
front associated with a speed maximum traversing the
southern/eastern rim of the broadly cyclonic flow area will track
southeastward across portions of the upper Midwest and upper
Mississippi Valley area, with this boundary trailing westward across
northern parts of the central Great Plains. An upstream,
low-amplitude impulse is expected to advance from northern California to the
northern High Plains, cresting the northern portion of a secondary
ridge building into the northern Great Plains.
..portions of the upper Midwest, upper Mississippi Valley region,
eastward-moving convection may linger near the frontal zone into the
beginning of the period, with related outflow modulating the
effective frontal position. Diurnal surface heating amid rich
low-level moisture, characterized by lower 70s surface dewpoints, to
the south of the cloud- and precipitation-reinforced boundary is
expected to support moderate to strong buoyancy by afternoon. The
boundary should be a focused zone of convective development and/or
intensification through the afternoon, with activity subsequently
spreading eastward and southeastward into the evening. The presence
of 25-40 kt of effective shear peripheral to the speed maximum will
support somewhat organized convective structures capable of
producing isolated to scattered damaging wind gusts, and perhaps
isolated marginally severe hail.
..portions of the northern and central Great Plains -- to the west
of the slight risk area...
during the afternoon, isolated to widely scattered convection may
develop along the westward-trailing segment of the front across the
vicinity of NE and SD, and within a moist, upslope-flow regime
across parts of the High Plains. This activity will spread
southeastward into the evening hours, with sufficient buoyancy
supporting isolated severe potential.
Somewhat stronger deep shear across the High Plains -- associated
with a vertically veering wind profile, and enhanced high-level flow
encouraging convective ventilation -- may support relatively more
organized convective structures with greater severe potential.
However, mid-level height rises accompanying the amplification of
the secondary ridge suggest that the severe risk may remain
For Tuesday night, modest ascent accompanying the low-amplitude
impulse may move across parts of central and eastern Mt, perhaps
supporting elevated convection. Steep mid-level lapse rates will
support sufficient elevated buoyancy for isolated severe hail with
the most intense storms.
..portions of Arizona...
A few of the most intense storms, forming in response to orographic
ascent over the higher terrain, could produce isolated strong wind
gusts during the afternoon and evening amid a deep, well-mixed
boundary layer. Present indications are that relatively modest
mid-level lapse rates and limited mid-level flow strength will limit
the severe risk.