Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 231713
SPC AC 231711

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1211 PM CDT Sun Jul 23 2017

Valid 241200Z - 251200Z



Isolated strong/severe storms may occur over portions of the
northern Plains vicinity, as well as across parts of the central
Appalachians, Mid-Atlantic, and Mid-South regions.

A broad mid/upper trough initially centered over the Great Lakes
will gradually lift northeastward into southeastern Canada while
weakening throughout the D2/Monday forecast period.  As this occurs,
another vigorous upstream disturbance centered over Alberta will
reach Manitoba overnight, while another compact and nearly
stationary mid-level low remains across northwestern California.
The Alberta disturbance will spread enhanced, cyclonically oriented
flow across areas of the northern Rockies/Plains and may provide
impetus for convective activity in the afternoon and overnight
hours.  Farther south, a broad ridge will remain nearly stationary
from the Four Corners eastward to the central/southern Plains.

At the surface, a weak low pressure area over western New York will
migrate slowly eastward throughout the day, and a trailing cold
front will migrate southward across the Ohio Valley into Kentucky
during the afternoon.  This front will become ill-defined with
westward extent across Missouri.  Farther west, a lee trough will
develop along the High Plains south of a substantial surface cyclone
over Alberta.  Late in the period, a cold front will migrate
southward across eastern Montana and eventually merge with the North
Dakota portions of the trough.

...Much of the Dakotas into Minnesota...
A complicated forecast scenario should result in at least an
isolated threat for hail and damaging wind gusts from the afternoon
onward across the Dakotas - and eventually overnight over Minnesota.
 The combination of convergence along the aforementioned lee trough
across western North Dakota/South Dakota and strong surface heating
should result in development of at least isolated high-based
convection across the region after 21Z or so.  This development
should occur to the west of deeper boundary-layer moisture and pose
a threat for severe wind gusts and hail given steep (8 deg C/km)
mid-level lapse rates and around 30-40 deg F surface dewpoint
depressions.  A separate area of development may occur just ahead of
a surface low/dryline over central South Dakota if heating/low-level
convergence can overcome weak inhibition depicted in model point
forecast soundings.  An isolated high-based supercell cannot be
ruled out in this regime given backed low-level flow ahead of the
low.  Any storms that develop in the afternoon may struggle to
persist beyond sunset, however, as inhibition increases with the
onset of nocturnal boundary layer cooling.  One exception to this
may be in northern/central North Dakota, where mid-level forcing
attendant to the Alberta trough may instigate deeper convection
persisting into the evening and overnight hours.  This scenario may
require an upgrade to 15%/Slight in later outlooks pending greater
certainty of convective coverage.

A second regime for elevated hail potential will exist farther east
into central Minnesota and vicinity.  Here, convergence on the nose
of a 50-kt 850mb jet stream centered in South Dakota, along with
steep mid-level lapse rates, will foster at least isolated storms
with hail overnight.  Again, a greater coverage of storms than
currently anticipated may necessitate an upgrade to 15%/Slight in
later outlooks.

...Portions of the Mid-South northeastward to the Appalachians and
A diurnally driven threat for isolated/sporadic damaging wind gusts
will develop in the afternoon across the region - especially where
sufficient insolation and destabilization can occur.  The area most
likely for strong destabilization will exist in portions of
Kentucky/Tennessee ahead of a southward-moving cold front, where
mid-70s F dewpoints and surface temperatures reaching into the 90s F
will foster around 3000-4000 J/kg of MUCAPE.  Deep shear is meager
in this region, however, and will likely lead to loosely organized
convective clusters in the afternoon hours.

Farther north into New York/Pennsylvania, deep shear will favor
better organization of any activity that can develop in the
afternoon, although instability should be notably weaker (around
1000-1500 J/kg MUCAPE).  Broad 5%/Marginal probabilities have been
maintained for this update to address the isolated threat.

..Cook.. 07/23/2017

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