Day 1 Severe Weather Outlook

Valid 141300Z – 151200Z


Widely scattered severe thunderstorms with very large hail and
damaging winds are expected across the central High Plains this
afternoon and evening. A few strong storms may be noted across the
Gulf Coast to the southern Middle Atlantic.

…Central High Plains this afternoon through tonight…
The larger-scale pattern is rather quiescent with a flat ridge over
the Rockies and moderate west-northwest flow aloft over the central
and northern High Plains. Subtle speed maxima embedded in the flow
will crest the ridge and move over the High Plains, the first and
most apparent of which will progress from central WY to the central
Plains this afternoon. In the wake of a frontal passage and
widespread convection last night across the central/southern High
Plains, surface ridging extends from western SD to the TX
Panhandle/NM border. West of the ridge, boundary-layer dewpoints in
the mid 50s have spread west to the Front Range in CO on upslope
flow, and slightly lesser moisture extends northward across eastern

Steep midlevel lapse rates above a seasonally moist boundary layer
with daytime heating will result in a corridor of MLCAPE of
2000-3000 J/kg this afternoon from northeast CO across eastern WY.
Forcing for ascent will be rather weak near the higher terrain, so
storm coverage within the corridor most favorable for supercells is
somewhat in question. Still, effective bulk shear of 40-50 kt with
relatively straight hodographs will favor supercells capable of
producing isolated very large hail. The tornado threat will likely
depend on a discrete storm interacting with the somewhat greater
low-level shear/hodograph curvature expected from western NE into
extreme northeast CO along the east edge of the greater buoyancy.
There will be some potential for storms to grow upscale into a small
MCS tonight, with the more probable corridor for upscale growth
centering near western NE and southwest SD where warm advection will
be strongest on the nose of a 25-35 kt nocturnal low-level jet, as
weak midlevel height falls occur tonight.

…Gulf Coast to the Mid-Atlantic this afternoon…
The effective surface boundary, modulated by convective outflow,
will continue to sag southward toward the Gulf Coast and
southeastward toward the southeast Atlantic and Mid-Atlantic coasts.
The richer moisture/buoyancy (PW greater than 2 inches and MLCAPE
greater than 3000 J/kg) are expected along the southern fringe of
the lingering clouds this morning, though vertical shear will be
very weak and boundary/storm mergers will largely drive isolated
downburst potential. Somewhat stronger deep-layer shear is expected
farther north toward Chesapeake Bay, which could compensate some for
the weaker buoyancy and poor midlevel lapse rates.

…Northern IN and vicinity this afternoon…
A diffuse surface front will provide a focus for thunderstorm
development this afternoon, as a midlevel trough approaches from WI.
Lingering low-level moisture and surface heating will support MLCAPE
of 1000-1500 J/kg, while deep-layer vertical shear will be at least
marginally favorable for organized/supercell storms. Very weak
low-level flow will greatly limit hodograph size/SRH magnitude, but
there will be modest vertical vorticity along the surface boundary.
A couple of storms could be strong with hail/wind near severe
criteria, and a brief/weak tornado cannot be ruled out.

..Thompson/Kerr.. 08/14/2019


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