U.S. Severe Weather Forecast (Convective Outlook)

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, 
and vicinity... 
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. 

.Cohen.. 07/17/2017