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Severe Storm Outlook Narrative (AC)
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ACUS03 KWNS 240708
SWODY3
SPC AC 240707

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0207 AM CDT Thu Aug 24 2017

Valid 261200Z - 271200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SATURDAY
AFTERNOON AND EVENING NEAR AND EAST/SOUTHEAST OF THE BLACK HILLS
INTO WEST CENTRAL NEBRASKA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
MIDDLE TEXAS COAST...

...SUMMARY...
A couple of severe storms may develop near and southeast of the
Black Hills late Saturday afternoon, before spreading into parts of
west central Nebraska Saturday evening, accompanied by a risk for
large hail and potentially damaging wind gusts.  Elsewhere, the
weakening remnants of Harvey may continue to impact parts of the
Texas coastal plain.

...Synopsis...
An increasingly prominent subtropical high center may continue to
form over the Great Basin during this period, with a building ridge
axis to the north, across the northern intermountain region and
Rockies into southern portions the western Canadian provinces.
While the stronger mid-latitude westerlies retreat further north of
the international border area, models do indicate modest
strengthening of northwesterly mid/upper flow is possible to the
east of the ridge axis, across the northern high Plains.  This is
expected to occur as troughing begins to dig within the southern
branch of splitting westerlies, across the international border area
to the west of the upper Great Lakes region.

In lower-latitudes, the circulation center of Harvey, which may
reach hurricane intensity prior to landfall, is expected to progress
inland of the lower/middle Texas coast, before stalling and
weakening.

...Parts of western/central South Dakota and Nebraska...
Low-level wind fields are expected to remain weak, but a
strengthening of flow to 30-35+ kt around 500 mb appears possible
Saturday.  This may contribute to sufficient vertical shear for
organized convective development, including an isolated supercell or
two, and perhaps the evolution of a small convective system.  Late
afternoon CAPE across the high Plains may be somewhat modest on the
order of 1000 to 1500 J/kg, but this probably will still be
sufficient to support at least some severe potential, given the
shear, and the presence of steep lapse rates.

Orographic forcing near the Black Hills may support initiation of
activity Saturday afternoon, before warm advection near a zone of
stronger differential surface heating aids development southeastward
into west central Nebraska by Saturday evening.  Stronger storms may
be accompanied by a risk for severe hail and wind, before
diminishing overnight.

...Texas coastal areas...
The extent of any tornado risk during this period remains unclear,
and may largely hinge on the lingering intensity of Harvey, and
other mesoscale developments which are still characterized by low
predictability at this extended range.  However, probabilities for
mainly isolated brief tornadoes may be highest early Saturday, when
the center of circulation may still be maintaining forward motion
inland across the lower/middle Texas coast.

..Kerr.. 08/24/2017

$$
National Weather Service Weather Forecast Office
