SPC Aug 29, 2017 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0746 AM CDT Tue Aug 29 2017

Valid 291300Z - 301200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
WEST-CENTRAL GULF COASTAL REGION...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK...

...SUMMARY...
A risk for a few brief tornadoes associated with Tropical Storm
Harvey will persist today into tonight from far southeast Texas
eastward, spreading toward coastal sections of Alabama.

...Synopsis...
In mid/upper levels, a seasonally high-amplitude pattern will
persist, dominated by ridging extending northward from a major
anticyclone over the Great Basin to the Arctic coastal plain of
northwestern Canada and AK.  A small cyclone over Lower MI and
northern IN, with shortwave trough extending to portions of
western/middle TN, is expected to weaken considerably today and
eject eastward across the mid/upper Ohio Valley into an area of
cyclonic flow over the Mid-Atlantic.

At the surface, the dominant feature for this forecast remains
Tropical Storm Harvey, which was centered offshore from the upper TX
coast, forecast by NHC to turn slowly northeastward toward the
southwestern LA coast through the remainder of the period.

...West-central Gulf coastal plain...
For near-term coverage of the tornado risk, refer to SPC mesoscale
discussion 1596.

Through most of the period, the tornadic supercell risk with Harvey
is expected to be temporally sporadic and spatially isolated, but
also distributed along a fairly expansive east-west extent along the
coast.  This is due to the highly asymmetric character of the
system's low-level wind distribution, and favorable boundary-layer
thermodynamic fields over the Gulf.  However, inland trajectories
persist that are characterized by travel through precip and dense
cloud cover, with weak northerly surface-wind components noted near
and even somewhat south of the I-10 corridor.  Until Harvey's center
gets very close to the coast again, this kinematic geometry and
precip character likely will continue to impede the inland progress
of a subtle but important coastal front that bounds the northern rim
of the relatively undisturbed, favorably high-theta-e
maritime/tropical air.  A narrow zone of favorable cape-shear
parameter space will lie along and south of that frontal zone,
supporting the risk for a few tornadoes in aggregate through
tonight.

...Coastal NC...
The evolution of Potential Tropical Cyclone 10 has remained slow and
poorly organized, per NHC discussions.  The time window is
shriveling rapidly for substantial tropical organization to occur
that would support:
1.  Development of more favorable, discrete/semi-discrete or banded
convective modes near shore, and
2.  Associated isallobaric forcing for boundary-layer winds that
would shift the favorable CAPE-shear parameter space across the
coastline.
While a rogue/brief tornado cannot be definitively ruled out around 
Outer Banks, the potential for a supportive convective element
moving over land has become too isolated, low and conditional to
warrant keeping a categorical risk area.

..Edwards/Smith.. 08/29/2017

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