Day 1 Severe Weather Outlook

Valid 062000Z – 071200Z



Scattered severe thunderstorms are expected across the northern
Plains late this afternoon and evening, particularly across parts of
the central/eastern Dakotas and eastern Nebraska, where storms may
become capable of producing very large hail and damaging wind gusts.
Other storms with locally strong to severe wind gusts are expected
over a portion of the mid Mississippi Valley region.

…20Z Update…
Thunderstorms have recently developed across parts of
western/central ND along and just ahead of a surface cold front.
Current expectations are for this activity to spread southeastward
into south-central ND and north-central SD through the remainder of
the afternoon. Large hail will likely be the main threat initially
with supercells, with some potential for greater than 2 inch in
diameter hailstones with the most robust storms. Upscale growth into
a southward-moving line segment still seems probable across SD this
evening, with damaging winds perhaps becoming more likely. Based on
where storms are initiating this afternoon, there is some concern
that the corridor of greatest severe wind potential may extend
farther west into central SD where the greatest instability is
forecast to be present this evening. Have expanded the Slight Risk
for hail into more of ND to account for observational trends, and
the Enhanced Risk for severe winds has been expanded westward to
include much of central SD. See Mesoscale Discussion 1651 for more
information on the near-term severe threat across this region.

The Marginal Risk has also been extended northwestward to include
parts of northeastern MT. A storm or two may form in an area of
low-level convergence across far southwestern Saskatchewan over the
next couple of hours. Although low-level moisture remains limited
across this region (surface dewpoints generally in the mid to upper
40s), strong northwesterly flow aloft should support updraft
rotation with mainly an isolated hail risk.

Minor adjustments have been made to the Slight Risk across southern
IL and vicinity to include more of east-central/southeastern MO
based on latest radar and observational trends. The Marginal Risk
has also been trimmed across parts of the OH Valley to account for
the latest surface cold front position.

..Gleason.. 08/06/2019

.PREV DISCUSSION… /ISSUED 1126 AM CDT Tue Aug 06 2019/

…Northern and central Plains region…

A corridor of richer low-level moisture with low 60s F dewpoints
will advect northward into the northern Plains ahead of a
southeast-advancing cold front and beneath a plume of steep
mid-level lapse rates. With strong diabatic heating, these processes
will result in moderate instability from southern ND through SD into
NE with up to 2000-2500 J/kg MLCAPE expected. Most CAM solutions are
similar with evolving scenario indicating storms will initiate over
southern ND where the cold front intercepts the corridor of richer
low-level moisture and where the cap should weaken sufficiently with
deep boundary layer mixing by late this afternoon. A belt of
stronger northwesterly mid-level winds will overspread this region
contributing to 40-50 kt effective bulk shear. Supercells with the
potential for very large hail will be the initial primary threat,
though a brief tornado or two cannot be ruled out. Storms will
eventually evolve into lines/clusters as they develop south, with
primary threat transitioning to damaging wind into the evening.

…Lower Ohio Valley region…

Strong diabatic heating of a moist boundary layer with low 70s F
dewpoints will result in moderate to strong instability 2000-2500
J/kg MLCAPE in this region this afternoon. WV imagery shows a
shortwave trough embedded within a northwesterly flow regime over IA
moving southeast. At least modest forcing for ascent attending this
feature and convergence along a front and pre-frontal trough should
be sufficient to initiate storms from southern IL, southwest IN into
western KY this afternoon. Northwesterly wind profiles with 30-35 kt
above 700 mb and steepening low-level lapse rates should be
sufficient for some storms to produce locally strong to damaging
wind gusts through early or mid evening.


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