Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 241727
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
127 PM EDT Wed May 24 2017

.SYNOPSIS...
A warm front will lifting through North Carolina this afternoon
and into Virginia this evening. A cold front will move sweep
east across North Carolina late tonight and early Thursday
morning. A second cold front will move across the region
later Thursday. Fair and dry weather will return Thursday night
and Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1115 AM Wednesday...

...Little change in previous thinking...

...Multiple rounds of severe storms possible this afternoon
through this evening, with localized flooding possible through
tonight...

Widespread severe weather possible today and this evening, with
several factors appearing to line up ideally to support vigorous
convection, including the possibility of discrete super cells this
afternoon and a more organized line favoring bowing segments
crossing the area late afternoon into the evening. Additionally,
with saturated soil and high water in creeks, the risk of heavy
downpours producing localized flooding and high water on creeks and
rivers persists through the evening.

An upper low over the MS Valley this morning will continue to dig
south-southeastward toward the Mid-South this afternoon and evening
and then into the SE states overnight. Individual strong vort maxes
rotating about the deep cyclonic flow will impact the region through
Thursday. However, the upper wave of most immediate concern that
will impact central NC today and tonight, is currently over the
Lower MS Valley, and is forecast to lift NEWD into the Carolinas by
tonight. Deep layer lifting with the approach of the wave into the
area coupled with a strengthening deep layer wind field will support
the development of rapidly developing and robust convection.

Three noteworthy features can be seen on the latest surface
analysis: a roughly west-east wedge/CAD frontal zone across the
northern Piedmont, a warm front lifting into southeastern NC, and a
cold front to our west through the W Ohio Valley and lower Miss
Valley. The latest high-res models take the wedge front northward
gradually this morning, with expectations that the warm/maritime
tropical front will rapidly retreat and potentially merge with the
wedge front across the central/western piedmont this afternoon. And
finally, the west-to-east passage of the cold/occluded front
(featuring a triple point low tracking near or just NW of the Triad)
very late evening through the overnight hours.

Rapid destabilization as the warm sector spreads north and westward
and an improving wind field/effective bulk shear increasing to 50 to
55 kts over NC, expect development of discrete northward-moving
cells, starting in the southern CWA, by early to mid afternoon. Such
development is suggested by the NAM Nest, HRRR, and SPC`s SSEO. The
low stratus (NW CWA) and stratocu (SE CWA) will initially limit
insolation and destabilization through the morning hours, thus
limiting shower coverage early today over central NC, so will start
the day with low pops. But the wind field will improve through a
deep layer as afternoon approaches, with a 60-80 kt 500 mb jet
spreading from the FL panhandle NE through the Carolinas, and a 30-
40 kt cyclonic 850 mb SSW jet over the Southeast nosing into NC. The
GFS brings MUCAPE up to 750-1500 J/kg (NW-SE) this afternoon, likely
due in part to a batch of higher mid-level lapse rates now over the
north-central/northeast Gulf moving NE through GA/SC/NC later today,
as well as the low level heating and resident moist low levels. On
top of the increasingly favorable kinematics and thermodynamics is
the increase in mid level DPVA and upper divergence, the latter
associated with an accelerating southerly jet streak extending from
SW GA through the eastern Ohio Valley later today. All of this
supports the development and maintenance of strong to severe
convection this afternoon into this evening, with large hail
possible, along with damaging wind gusts. And, with the retreating N
Piedmont frontal zone potentially providing a rich source of low
level vorticity and enhanced 0-1km SRH of 200 to 400 ms, there will
be a chance for a tornado or two, with the greatest threat over the
N and W Piedmont.

As the cold front shifts eastward late tonight, expect a gradual
tapering down of precip chances, and a trend to mostly post-
convection stratiform rain with embedded thunder, from WSW to ENE
overnight, before ending late.

The risk of high rainfall totals possibly leading to flooding is
less certain, as the highest PW values have shifted east of the CWA,
and we`re unlikely to see the deep saturation through the column
that we saw yesterday. But given the wet ground from recent rains,
the high water levels on area creeks, and a decent chance of S-N-
oriented convective bands leading to training and streaks of high
rainfall totals, isolated to localized flooding threats will
persist. Will continue the flood watch as is, through much of
tonight, areawide. Expect additional rainfall amounts of 1-2",
highest in the NW CWA where moist upglide should be strongest.

Thicker stratus across the far N and NW CWA associated with the
wedge/CAD airmass may be hard to dislodge and thus may hold temps
down significantly for much of the day. However, across the south
and along the eastern and southern fringes of the central Piedmont,
any breaks will cause a quick rise in temps. Highs ranging from
around 70 NW to near 80 SE. Lows tonight 59-68 W-E.

&&

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
As of 115 PM Wednesday...

Another convectively active day expected on Thursday although
with a different character and a more limited coverage and
threat.

A vigorous closed mid and upper-level low and upper-level
trough axis over the eastern OH and TN valleys on Thursday
morning will become negatively tilted and cross central NC
between 18 and 00 UTC. At the surface, an initial cold front
will extend north to south across the eastern Piedmont of NC at
12 UTC and then push east reaching the coast by 18 UTC as a
second cold front reaches the Foothills of NC by 18 UTC and then
the coastal region toward 00 UTC.

The radar should be relatively quiet during the morning rush hour
although some scattered showers are possible along the initial
cold front that will be moving from the Piedmont into the
Coastal Plain. Ahead of the front, dew points will range in the
mid and even upper 60s supportive of convection. As the front
exits, dew points will fall into the mid and upper 50s.

Ahead of the approaching upper trough, large scale forcing for
ascent will increase during the day Thursday. This will be
supported by increasingly cold air aloft with 500 MB
temperatures of -18 to -20 deg C which combined with steep low
level lapse rates will lead to widespread cloudiness and the
development of scattered showers and thunderstorms very late in
the morning and into the afternoon. Cold air aloft resulting in
suppressed freezing levels and an evolving but still robust wind
field supporting shallow supercells, will support numerous
cells with small hail and a few storms producing larger hail. As
the previous shift noted, long and relatively straight forecast
hodographs would lead to some splitting supercells and the
potential for severe hail and damaging straight line winds.
Limiting factor for severe weather will be the amount of
instability while high cloud bases will reduce the tornado
threat, they will support enhanced downburst winds. The greatest
severe weather threat will be from 16 to 22Z, especially across
the northern and western Piedmont. Convection will decrease
from west to east during the late afternoon as the secondary
cold front moves through and the air mass stabilizes. High
temperatures will range in the lower 70s northwest to upper 70s
southeast.

Fair and dry conditions are expected Thursday night as heights rise
and a surface high pressure extends into the region from the south.
Low temperatures will range in the mid 50s to around 60. -Blaes

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
As of 125 PM Wednesday...

In the wake of the strong, negatively-tilted trough axis forecast to
exit the Middle Atlantic coast late Thu and Thu night, height rises
aloft/ridging and associated subsidence, will sweep to the East
Coast and yield dry and mild-warm conditions on Fri, as underlying
surface high pressure settles across the southeastern U.S.

A warm front will develop ewd across the central Appalachians and
into the srn Middle Atlantic states (VA/NC) this weekend, though
there remains uncertainty with respect to the precise location,
particularly given probable modulation by convection/outflow.
Nonetheless, there has been a consistent signal in model guidance in
recent days that would suggest a couple of episodes of convection
will likely develop along and north of the boundary Sat afternoon
through Sun, before the front likely retreats nwd in advance of a
weakening cold front forecast to settle into NC on Mon.

Some storms may be severe, given relatively strong wind fields, and
steep lapse rates related to a large plume of EML that will likely
have overspread much of the sern U.S. and srn Middle Atlantic
states, around the top of a progressive sub-tropical ridge along the
Gulf coast.

The next trough aloft will amplify and progress slowly across the
central U.S. early-mid next week, with associated surface lows
likely to lift across, and occlude over, the Great Lakes. Trailing
fronts will consequently weaken as they drift east of the
Appalachians and perhaps serve as a focus for primarily diurnal
convection. Continued warm, in the 80s.

&&

.AVIATION /17Z WEDNESDAY THROUGH SUNDAY/...
As of 630 AM Wednesday...

IFR to LIFR conditions will dominate across central NC through mid
morning, as a warm moist air mass combined with light surface winds
supports low stratus. FAY, however, will see a few periods of MVFR
to briefly VFR clouds through this morning. Cigs are expected to
rise to MVFR, with mostly VFR vsbys, at all sites by late morning
(around 15z), perhaps reaching low-end VFR by early to mid afternoon
(around 18z) as a warm front surges northward through the area with
increasing mixing. Patchy light showers are expected through early
afternoon, followed by more widespread showers/storms moving in from
the south and west during the mid to late afternoon. A period of IFR
conditions in heavy showers and storms with gusty erratic winds are
possible after 18z at INT/GSO and after 19z at RDU/FAY/RWI. Some
storms this afternoon into early evening may be strong to severe,
with large hail and intense wind gusts. Then, conditions will trend
to MVFR then VFR from west to east in the evening with decreasing
rain coverage, after 02z at INT/GSO, after 04z at RDU, and after 06z
at RWI/FAY, although patchy clouds based below 900 ft AGL may form
late tonight. Some storms this afternoon into early evening may be
strong to severe, with large hail and intense wind gusts.

Looking beyond 12z early Thu morning, cigs should rise to VFR after
13z or 14z Thu, but there will be a good chance for showers with
small graupel/hail during the day Thu as the axis of a powerful mid
level trough swings through the area. Skies will clear out late Thu
night, then VFR conditions should dominate Fri through Sun, although
there will be a chance for a few storms late Sat into Sun as a cold
front approaches. -GIH

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late tonight for NCZ007>011-021>028-
038>043-073>078-083>086-088-089.

&&

$$
SYNOPSIS...Blaes
NEAR TERM...CBL/HARTFIELD
SHORT TERM...BLAES
LONG TERM...MWS
AVIATION...HARTFIELD



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