Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 231523

Area Forecast Discussion
National Weather Service Raleigh NC
1123 AM EDT Tue May 23 2017

A series of low pressure systems will cross central NC through mid-
week, causing periods of showers and thunderstorms. Improving
weather conditions are anticipated by late in the week.


.NEAR TERM /today through tonight/...
As of 1123 AM Tuesday...

...Flood watch continues through this evening...

Mid-level perturbations, embedded within the deep cyclonic flow
around the digging upper low centered over MN/IA and associated long
wave trough encompassing much of the central/eastern CONUS, will
continue to eject northeastward, atop a quasi-stationary frontal
zone bisecting the SE states. With high PWAT of 1.5-1.8"(150 to 200
percent of normal and near daily record values) continuing to pool
along this complex frontal zone through tonight, the threat of
flooding will persist, with the next batch of moderate to heavy rain
expected to move into the area this afternoon as the next prominent
upper level disturbance currently over the Deep South/western
Florida Pan Handle. With the approach of this disturbance, models
predict a wave of low pressure will develop along the front, which
will serve to strengthen the low-level moisture convergence,
particularly along and just on the cool side of the frontal zone
where low-level f-gen will be maximized. Additional storm total
amounts of 1-2" are likely with locally higher totals probable.

Review of 24 hour rainfall totals over the past 24 hours indicate
two areas of heavy rainfall:  1)Across the Sandhills and southern
coastal plain where yesterdays convection produced an expansive area
of 1 to 2 inches with some localized higher amounts up to 3 to 3.5
inches; 2) More localized/isolated 1 to 2 inches across portions of
Forsyth and Guilford counties. In between these two areas, 24 hour
rainfall amounts have generally been around 0.50". While the two
highlighted areas above will likely be most susceptible to flooding,
heavy rainfall of 2 to 3 inches in a short period of time could
cause flooding anywhere, thus flood watch will continue for all of
central NC. The threat of the heavy rain and flooding looks to be
over aoa 06z, once the upper disturbance pushes NE of the area.

Hampered by widespread cloud cover and rain, severe threat will be
isolated and confined to the southern coastal plain(Sampson/Wayne
counties),where weak to moderate buoyancy developing east of the sfc
low combined with strengthening effective shear of 40 to 50 kts
could support an isolated tornado and straight line damaging winds.

Temps have a high error potential today, as any amount of sunshine
in the SE CWA could send readings soaring. Will go with highs from
the mid/upper 60s NW to the upper 70s/80 SE. Lows tonight 58-65.


As of 410 AM Tuesday...

Little change in forecast weather pattern and forecaster rationale
since 24 hours ago. Potential severe weather event on Wed looks very
similar to the surface evolution/pattern, storm mode, and timing
experienced a couple of weeks ago/on May 5th.

An initially (slightly) positively-tilted longwave trough from ern
Canada to the srn Plains, including an embedded mid-upper low over
the mid MS Valley/upper Midwest at 12Z Wed, will migrate ewd and
assume an increasingly negative-tilt as it pivots across the
Carolinas and middle Atlantic states by Thu and Thu night. Fast,
perturbed, and moist SSWly to Sly flow aloft will precede the

A complex pattern will result at the surface through the middle of
the week. A frontal zone will likely be draped SWwd from a series of
waves passing across and offshore the Middle Atlantic coast, marking
the srn periphery of weak ridging/CAD/low ovc extending across most
of VA/NC at the start of the day Wed, then extending NWwd as a warm
front to a low over the wrn OH Valley.

That front, edge of the CAD regime, and focus for convection, will
then likely retreat Nwd with a narrow wedge of warm sector through
srn and perhaps portions of ern NC during the day Wed, then through
the remainder of central NC early Wed night, all immediately ahead
of, and associated with, a triple point low that will develop and
migrate across the wrn Carolinas very late Wed afternoon and Wed
night. Initial CAD conditions, and subsequent convection along the
retreating warm front, will tend to slow the retreat of the unstable
warm sector and nrn bound for any severe threat through much of Wed,
such that any strong to severe potential will likely remain from
Wadesboro to Goldsboro and Rocky Mount and points SEwd. That
unstable warm sector should then surge Nwd through the remainder of
central NC early Wed night, immediately ahead of the triple point
low, yielding brief --but likely sufficient-- (weak) destabilization
to support surface-based QLCS storms capable of producing both
swaths of damaging straight line winds and isolated tornadoes along
embedded mesovortices/bows/lewps. The cold/occluded front will cause
the warm sector and associated severe threat to collapse Ewd and
through the Coastal Plain late Wed night-early Thu.

Brief drying and clearing will be probable through early midday Thu,
but clouds will redevelop with heating, with an additional band or
bands of convection --in fast,cyclonic, and CAA-bearing flow aloft--
which will spread NEwd across central NC through late afternoon, the
wrn edge of which may result in a strongly forced line of storms
along the ern periphery of focused forcing for ascent accompanying
the aforementioned upper trough/low. Cold temperatures/steep lapse
rates aloft will favor small hail in even marginally strong cores;
and a 50-70 kt SSwly speed max on ern fringe of the upper trough
will support organized storm modes, including low-topped (splitting)
supercells, given long and relatively straight forecast hodographs,
which would be capable of producing severe hail and damaging
straight line winds. Any tornado threat would be mitigated by both a
veering of the low level flow to SWly (Ie. no backing of the sfc
flow) and sfc dewpoints aob 60 degrees that would favor
evaporatively-cooled downdrafts and cool RFD`s.

Temperatures will be slow to climb into/through the 60s on Wed,
probably reaching calendar day highs at Piedmont sites between 00Z
and midnight, with afternoon highs in the 70s probable over the sern
tier or two of counties. Lows in the 60s Wed night, and highs Thu in
the upper 60s to 70s, inhibited by both variably to mostly cloudy
skies and convection during the afternoon.


As of 415 AM Tuesday...

Dry conditions will return by Thu night, as the upper trough lifts
away and weak high pressure expands across the sern states through
early Sat. A warm front will develop newd across the central
Appalachians later Sat and across the Middle Atlantic states by Sun;
with a return to warmer and more humid conditions, and a chance of
convection --some probably severe owing to 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 by the weekend-- focused along and north of the
front late Sat afternoon onward.


.AVIATION /12Z Tuesday through Saturday/...
As of 645 AM Tuesday...

Poor aviation conditions will dominate today through tonight. Areas
of showers and isolated storms will spread over central NC
throughout the day, lasting through the evening, producing mainly
MVFR to IFR conditions with low clouds and periods of moderate to
heavy rain. Details regarding the specific timing of the heavy rain
and storms is uncertain, but we do have confidence in poor aviation
conditions dominating through tonight, as low (mainly IFR) cigs are
expected to persist overnight even as the heavier rain departs from
west to east between 00z and 06z. A surface front oriented SW to NE
across central NC will hold in place today as low pressure tracks
northeastward along it, leading to erratic winds at central NC
terminals, mainly from the SE/S shifting to SW at FAY, and mainly
from the NE elsewhere through tonight.

Looking beyond 12z early Wed morning, adverse aviation conditions
are likely to hold Wed through Wed night as a deep mid level trough
and surface cold front approach from the west, bringing another
round of heavy showers and a few potentially strong thunderstorms.
Improvement to mostly VFR is expected Thu, although scattered
afternoon showers generating MVFR conditions are possible. VFR
conditions should return for Fri/Sat as a weak high pressure ridge
extends into the area. -GIH


Flood Watch through late tonight for NCZ007>011-021>028-



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