Severe Storm Outlook Narrative (AC)
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ACUS48 KWNS 230838
SPC AC 230837

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0337 AM CDT Sun Jul 23 2017

Valid 261200Z - 311200Z

Medium-range models are in general large-scale agreement at this
time, that very gradual but substantial amplification of the upper
flow field will occur over the U.S. during the medium-range period,
with eventual establishment of troughing over roughly the eastern
half of the country and ridging over the west.

The initial upper feature which looks to begin the process of
carving out eastern U.S. troughing is an upper low/trough crossing
Ontario at the start of the period.  At the surface, a low
associated with the upper system is progged to cross southern
portions of Hudson Bay/James Bay, with a trailing cold front to
cross the upper Great Lakes/upper Midwest/central Plains region of
the U.S. Day 4/Wednesday.  With stronger flow aloft to reside over
Ontario and the upper Great Lakes area, but stronger instability
likely to remain farther southwest along the front, the corridor
from roughly upper Michigan southwest to Iowa appears at this time
to be the core of greatest likelihood for isolated severe risk.

As the system shifts east toward the northeast U.S., less confidence
with respect to degree of instability which may precede the front
precludes inclusion of a Day 5 area at this time.

Meanwhile, limited/isolated severe risk may spread slowly east
across the northern Intermountain and northern Plains Days 4-6, as a
weak short-wave trough crests the developing western U.S. ridge and
eventually digs southeast into the evolving eastern U.S. trough
through the end of the period.  However, at this time the degree of
risk associated with this system does not warrant introduction of
areal highlights, mainly due to uncertainty regarding quality of the
CAPE/shear parameter space likely to precede the system.

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