Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 291732
SWODY2
SPC AC 291732

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1232 PM CDT Sat Apr 29 2017

Valid 301200Z - 011200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM MISSISSIPPI
AND ALABAMA NORTHWARD INTO THE LOWER OHIO VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
GULF COAST NORTHWARD INTO THE MIDDLE MISSISSIPPI VALLEY AND SOUTHERN
GREAT LAKES...

...SUMMARY...
Severe thunderstorms will be possible across portions of the
lower/mid Mississippi Valley eastward to Alabama Sunday into Sunday
night. Damaging winds will likely be the primary threat, but a few
tornadoes may also be possible.  A couple strong/severe storms may
also occur Sunday over parts of the Ohio Valley and southern Great
Lakes.

...Synopsis...
A large mid-level closed low will lift northeast from the
southern/central High Plains to the Midwest, as a shortwave trough
rotates around its periphery from Texas into the Ohio Valley. In
turn, expansive cyclonic flow aloft should establish over much of
the contiguous US on Sunday.

...Portions of the mid/lower Mississippi Valley and eastward...
With such an amplified system and strong meridional flow aloft,
multiple areas of convection will likely be ongoing Sunday morning.
The main influencing factor regarding timing and location of
potential severe weather will be with one or more north-south
oriented bands of storms across parts of the lower/mid Mississippi
Valleys, likely in advance of the surface cold front.  The latest
convection-allowing model guidance converges towards a scenario with
the placement of a north to south oriented squall line at 30/12z
located from western TN/eastern AR south into northern LA and
southwestern LA.  A squall line is forecast to continue eastward
across the lower Mississippi Valley into AL where a moisture-rich
airmass will support moderate buoyancy.  Despite the presence of
enlarged low-level hodographs, a linear convective mode
(nonsupercell) is forecast and will likely limit the overall tornado
risk.  Additionally, 0-3 km shear vectors with respect to projected
line segment motion will be suboptimal for a heightened mesovortex
tornado risk, although the moist low levels may support some
low-tornado threat.  Strong to severe gusts capable of wind damage
are forecast to be the primary severe hazard and could concentrate
primarily near LEWP/bowing structures in the QLCS.  This band is
forecast to gradually weaken with eastward extent as it outpaces
large-scale ascent and encounters increasing inhibition. Behind
these storms, uncertainty exists with the potential for
re-development of convection along the front (due to prior
convective overturning).

Across the mid Mississippi Valley, a highly conditional risk for
severe weather is apparent owing to destabilization concerns via
widespread convection from both clouds/early-day precip and
widespread thunderstorms located to the south during the morning.
If strong updrafts manage to develop coincident with peak heating,
storms within a portion of this region may yield a localized
wind/tornado risk before diminishing in intensity during the early
evening.

...Portions of northern OH into northwest PA...
Modest warm advection in the vicinity of lift from a west-east warm
frontal zone over the area may result in isolated storms during the
afternoon.  A couple of locally strong storms may yield the risk for
isolated marginally severe hail/damaging winds.

..Smith.. 04/29/2017

$$



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