Day 1 Severe Weather Outlook

Valid 171300Z – 181200Z


The greatest threat for severe storms appears to be over parts of
the northern and central Plains into the upper Midwest late this
afternoon into tonight.

In mid/upper levels, a highly amplified and blocky pattern inhabits
North America’s neighborhood in the Northern Hemisphere. A large,
complex cyclone will remain anchored over the north-central Pacific,
with sharp ridging to its north through east, from the Bering Sea to
waters west of both CA and Baja. Broadly cyclonic mean flow will
persist over much of Canada and the northern half of the CONUS, much
of it accompanied by seasonally cold mid/upper-level temperatures.
The most influential shortwave feature appears to be the trough now
moving eastward out of the northern Rockies region, and forecast to
amplify as it crosses MT and the Dakotas through the period. By
12Z, this perturbation should reach southeastern MB, eastern ND and
central/eastern SD. Meanwhile, convectively generated/enhanced
vorticity maxima from prior activity will aid convective potential
on the mesoscale across portions of the marginal-risk area from

At the surface, the 11Z analysis showed a surface low over extreme
northern Lower MI with cold front southwestward across northern IL,
becoming quasistationary and somewhat diffused by convective
activity over southern IA and southern NE. Baroclinicity was
stronger on an outflow arc extending from eastern MO across the
eastern KS/OK border region to west-central KS. The latter boundary
should weaken from west to east through the day, with some
northeastward retreat possible, while the frontal zone to its north
moves little. Meanwhile, a better-defined cold front was drawn from
southern MB across east-central ND and northwestern SD, to a low
over northeastern WY. This front should move eastward/southeastward
across the remainder of the northern Plains through the period. By
12Z the cold front should reach eastern MN, western/central IA,
southeastern NE, and northeastern CO.

…North-central Plains, upper Midwest…
Scattered thunderstorms should develop over western parts of the
north-central Plains this afternoon, predominantly along the cold
front and over higher terrain in and near the Black Hills and
Bighorns, offering the potential for severe hail/gusts. Supercells
are possible relatively early in the convective cycle. Then the
potential exists for some of the western SD/frontal activity to
evolve upscale toward a more wind-dominant severe risk, perhaps
aided by additional convective development to its east.

Being fairly well-removed from the ongoing extensive clouds/precip
in the eastern KS/MO region, areas of strong heating are expected in
the prefrontal warm sector. This, combined with 60s F surface dew
points and steep low/middle-level lapse rates, should render areas
of 2000-3500 J/kg late-afternoon/preconvective MLCAPE in central/
eastern SD and much of NE, decreasing westward toward the higher
terrain as moisture diminishes (but still should be sufficient to
support convection). Vertical shear should strengthen through the
afternoon and evening in the warm sector, with 40-55-kt effective-
shear magnitudes supporting supercell potential before upscale
growth occurs. The threat accordingly should transition from hail
to wind with time into the overnight period.

…IA, eastern NE, portions of KS, northern MO…
Additional scattered to numerous thunderstorms may form farther
south tonight over parts of the eastern NE/IA/northern MO area in a
zone of strong low-level warm advection and moisture transport, with
severe/significant hail potential. This regime is a good deal more
uncertain in how it may develop and evolve, including how much
influence may persist from High Plains activity persisting into the
region vs. in situ development with the warm-advection/LLJ corridor.

The extensive area of antecedent convection over KS/MO — and
related airmass effects on timing/extent of recovery farther north
this afternoon and evening — yield the bulk of uncertainty in this
forecast. The severe that occurred late over KS/MO during the past
overnight period required 12-18 hours of postconvective recovery
following extensive convection to the south. Loosely similarly, a
delayed onset of convective/severe potential into the
evening/overnight hours is probable across the outlook area again
this day-1 period. Two main differences will be:
1. Somewhat weaker but still adequate low-level moisture/theta-e,
2. Stronger but still peripheral mid/upper-level influences
southeast of the pronounced shortwave trough, in terms of
synoptic-scale lift via warm advection and weak DCVA/cooling, and
strengthening gradient winds aloft for favorable deep shear.
At this time, uncertainty remains too large to increase or areally
refine the unconditional severe probabilities.

..Edwards/Marsh.. 08/17/2019


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