Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 220559
SPC AC 220558

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1258 AM CDT Tue Aug 22 2017

Valid 231200Z - 241200Z


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

Models indicate that amplified waves within the main belt of
mid-latitude westerlies (roughly centered near the Canadian/U.S.
border area) will remain at least slowly progressive through this
period.  The progressiveness of upper troughing advancing into and
across the British Columbia/Pacific Northwest coast may be a bit
greater than the large-scale troughing approaching the North
Atlantic coast, however.  While an initially deep lower/mid
tropospheric cyclone embedded within the latter feature is forecast
to weaken and accelerate across Quebec, toward Newfoundland, another
significant digging short wave impulse may reinforce the
larger-scale troughing across the Great Lakes/Ohio Valley region.

The cold front associated with the eastern troughing still appears
likely to clear much of the southern New England and northern Mid
Atlantic coast early Wednesday, while continuing to gradually
advance southward into and through the south Atlantic and Gulf Coast
states.  As it does, it may become increasingly diffuse, but the
front, and preceding convective outflow boundaries, may once again
provide a focus for scattered thunderstorm development Wednesday
through Wednesday night.  Increasingly displaced to the south of the
mid-latitude westerlies, across the Gulf states into the southern
Plains, severe weather potential appears generally limited.

...Southern Mid Atlantic Coast region...
In advance of the cold front, it does appear that a corridor of
stronger surface heating across the piedmont into coastal plain may
support moderately large CAPE on the order of 1000-2000 J/kg by late
Wednesday afternoon.  Models continue to suggest that this will
occur favorably timed with strengthening mid/upper forcing for
ascent, within increasingly cyclonic flow ahead of the approaching
upper trough axis, which should support initiation of storms.  An
upscale growing thunderstorm cluster may not be out of the question
by early Wednesday evening, perhaps enhanced by increasing vertical
shear with some strengthening of mid-level flow (to near 30 kt at
500 mb).  If this occurs, the presence of moderate CAPE and high
moisture content (with precipitable water forecast to increase to 2+
inches) may contribute to an environment conducive to an organized
potentially damaging wind threat, despite expected rather weak
ambient low-level wind fields.

..Kerr.. 08/22/2017

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