Severe Storm Outlook Narrative (AC)
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000
ACUS03 KWNS 210720
SWODY3
SPC AC 210719

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0219 AM CDT Mon Aug 21 2017

Valid 231200Z - 241200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS WEDNESDAY
AFTERNOON AND EVENING ACROSS THE SOUTHERN MID ATLANTIC COAST
REGION...

...SUMMARY...
Strong thunderstorms may impact the southern Mid Atlantic Coast
region Wednesday afternoon and evening, accompanied by some risk for
severe weather.

...Synopsis...
Within the amplified main belt of mid-latitude westerlies, one
significant upper trough appears likely to remain progressive,
advancing inland across the British Columbia/Pacific Northwest coast
by late Wednesday night.  Models suggest that downstream troughing
will be reinforced across the Great Lakes/Ohio Valley region by
another significant digging shorter wavelength impulse.  However,
the embedded lower/mid tropospheric cyclone over Quebec is forecast
to weaken and accelerate toward Newfoundland.  The associated cold
front still appears likely to advance off the southern New England
and northern Mid Atlantic coast early in the day, while continuing
to gradually advance southward into/through the south Atlantic and
Gulf Coast states.  As it does, guidance suggests that it will
become increasingly diffuse.

The frontal zone, and possible preceding convective outflow
boundaries, may once again provide focus for scattered thunderstorm
activity Wednesday through Wednesday night.  However, increasingly
displaced to the south of the westerlies, severe weather potential
appears generally limited.

...Southern Mid Atlantic Coast region...
Seasonably high boundary layer moisture content, within a
pre-frontal corridor of stronger surface heating across the piedmont
into coastal plain, may contribute to moderately large CAPE on the
order of 1000-2000 J/kg by late Wednesday afternoon.  Models suggest
that this will occur favorably timed with an area of enhanced
mid/upper forcing for ascent developing across the region, within
increasingly cyclonic flow ahead of the approaching upper trough
axis.  This may provide support for the initiation of storms, and a
possible upscale growing thunderstorm cluster across eastern North
Carolina and southern Virginia by early Wednesday evening.  Coupled
with some strengthening of west/southwesterly mid-level flow
(perhaps to near 30 kt at 500 mb), it appears at least possible that
the environment could become conducive to an organized potentially
damaging wind threat.

..Kerr.. 08/21/2017

$$


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