SPC Apr 22, 2017 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0236 PM CDT Sat Apr 22 2017

Valid 222000Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON AND EARLY EVENING ACROSS PARTS OF SOUTHERN VIRGINIA AND
NORTHERN NORTH CAROLINA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE LOWER MISSISSIPPI INTO TENNESSEE VALLEYS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
SURROUNDING AREAS OF THE MISSISSIPPI/TENNESSEE VALLEYS INTO THE MID
ATLANTIC COAST REGION...

...SUMMARY...
A few severe thunderstorms capable of damaging wind gusts and large
hail will be possible this afternoon and evening from Louisiana to
the southeastern Virginia/North Carolina Tidewater region.  The
best-organized potential appears to be from parts of Mississippi
into western/northern Alabama and southern Tennessee, and over
southern Virginia and extreme northern North Carolina.

...20Z Outlook Update...
Mostly minor adjustments have been made to categorical and
probabilistic outlook lines to account for the latest convective
trends evident in observational data.

A gradual increase in thunderstorm coverage and intensity is ongoing
along the frontal zone near/just north of the Virginia/North
Carolina border area, and near/just ahead of the front from
central/northern Mississippi into southern middle Tennessee.  This
is occurring in the presence of modest instability characterized by
mixed layer CAPE on the order of 1000 J/kg, aided by favorable deep
layer shear beneath 30-50 kt west southwesterly 500 mb flow.  This
environment appears conducive to organized convective development
accompanied by the risk for marginally severe hail and potentially
damaging wind gusts.  And further increase and intensification of
ongoing activity seems likely while developing eastward through at
least the 00-03Z time frame.

..Kerr.. 04/22/2017

.PREV DISCUSSION... /ISSUED 1123 AM CDT Sat Apr 22 2017/

...Mississippi and Tennessee Valleys...
A well-defined vorticity maximum over Missouri is forecast by model
guidance to move eastward over the Mississippi Valley tonight as the
associated upper trough amplifies during the period.  A strong band
of mid/upper level winds is located on the southern periphery of the
system with a 60-70 kt mid-level jet streak progressing toward the
lower Mississippi Valley.

At the surface, a low near the Mississippi Delta will move slowly
eastward across northern parts of Mississippi and Alabama through
tonight, as a trailing cold front moves southeastward across the
lower Mississippi Valley. A quasistationary front extending eastward
from the low is forecast to sag slowly southward with time.

Earlier clouds over northern/central Mississippi into northern
Alabama and southern middle Tennessee have diminished substantially
permitting stronger diabatic heating to occur in advance of the
approaching cold front.  With surface temperature warming into the
70s and dew point values in the low-mid 60s, further destabilization
is likely in the pre-frontal environment with MLCAPE reaching
1000-1500 J/kg this afternoon.  CAM guidance indicates stronger
storms will develop along/ahead of the cold front during the early
afternoon with activity spreading eastward through the evening
hours.  Effective bulk shear of 35-40 kt will promote organized
storm structures including isolated supercells. Stronger cells will
be capable of producing mainly damaging wind gusts and severe hail.

...Southern Virginia/Northern North Carolina...
The environment is undergoing strong diabatic heating over North
Carolina where visible satellite imagery indicates few clouds, and
low-level warming is spreading northward into far southern Virginia
as cloud cover thins near an east-west frontal zone. Despite modest
mid-level lapse rates, surface dew points in the low-mid 60s coupled
with diurnal heating will result in further destabilization with
MLCAPE near 1500 J/kg expected this afternoon.  Storms are beginning
to develop at this time over northwest North Carolina and southwest
Virginia within a weakly-capped environment.  Activity is expected
to increase in coverage and intensity and spread/develop eastward
during the afternoon with a few severe storms possible.  See
Mesoscale Discussion 545 for more details.

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