SPC MD 709

MD 0709 CONCERNING SEVERE POTENTIAL...WATCH UNLIKELY FOR PORTIONS OF MS AND SOUTHERN/EASTERN LA

Mesoscale Discussion 0709
NWS Storm Prediction Center Norman OK
0143 PM CDT Fri May 12 2017

Areas affected...Portions of MS and southern/eastern LA

Concerning...Severe potential...Watch unlikely 

Valid 121843Z - 122115Z

Probability of Watch Issuance...20 percent

SUMMARY...New thunderstorm development will occur through the
afternoon across parts of the lower MS Valley region, behind a
squall line crossing the north-central Gulf Coast region. Some of
this new storm development may pose an isolated severe risk as it
spreads eastward/east-southeastward into the evening hours. Present
indications are that Watch issuance will be unlikely, though
convective and environmental trends will continue to be monitored.

DISCUSSION...In the wake of a squall line crossing portions of the
north-central Gulf Coast region, air-mass recovery is underway owing
to insolation ahead of a weak cold front/trough extending from the
Ark-La-Miss region to southwest LA. Ahead of this front/trough,
relatively rich boundary-layer moisture is in place. This 
moisture -- characterized by middle 60s to lower 70s dewpoints -- is
supporting MLCAPE around 1000-1750 J/kg with little inhibition.
However, air-mass recovery across the region continues to be modest,
and low-level lapse rates have been slow to steepen thus far.
Furthermore, surface observations and VAD wind profiles suggest that
low-level convergence near and ahead of the front is weak, and the
front is rather diffuse. As such, convection should only gradually
deepen as it ingests convectively processed air, while midlevel
lapse rates and related upward convective accelerations remain
muted.

Nevertheless, a general uptick in convection will occur during the
next few hours, as initial updrafts encounter a slowly recovering
air mass featuring steepening low-level lapse rates. This activity
will be further enhanced by ascent ahead of a mobile midlevel
cyclone encouraging some destabilization, as well. Convective
intensity will likely peak in the 20Z-00Z time frame -- especially
as convection spreads across parts of south-central/southeast MS and
southeast LA. The weak magnitude of low-level ascent along the
boundary, and midlevel flow maintaining an appreciable
boundary-orthogonal component, will support discrete/semi-discrete
convective structures potentially locally evolving into small
convective clusters. The presence of 40-55 kt of effective shear may
support supercell structures capable of severe hail, especially as
midlevel temperatures cool with the progression of the cyclone.
Locally strong wind gusts may also occur, though the lack of
stronger low-level flow and more cellular convective mode will tend
to limit the severe-wind risk.

Convective and environmental trends will continue to be monitored
through the evening. However, the anticipated severe coverage is
presently expected to be sufficiently isolated/limited for Watch
issuance. Should greater destabilization yield a more substantial
severe risk, prospects for Watch issuance could increase.

..Cohen/Goss.. 05/12/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...MOB...MEG...JAN...LIX...LCH...

LAT...LON   29589143 29809180 30179179 30939169 32639095 33868988
            33788864 32618855 30948892 29739041 29589143 

Read more

from SPC Forecast Products http://ift.tt/JJKUqS