Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KRAH 222001
Area Forecast Discussion
National Weather Service Raleigh NC
355 PM EDT Sat Apr 22 2017
.SYNOPSIS... A cold front will settle south through srn VA and
nrn NC this afternoon, then south through central NC tonight. The
front will stall over southeastern NC by Sunday, in advance of a
slow-moving area of low pressure that will track along the coast of
the Carolinas through Mon night.
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 315 PM Saturday...
Severe Thunderstorm Watch #157 in effect for the nrn Piedmont and
Coastal Plain of central NC until 9 pm EDT.
...Flood Watch from midnight tonight through 800 AM Tuesday...
A well-defined mid level vort max/low now over east-central MO will
deepen and reform sewd/across the Mid-South by 12Z Sun, as a trough
axis with embedded perturbations that trails wwd to the central
Rockies pivots and amplifies to the srn Plains and lower MS Valley.
On the se fringe of this larger-scale trough, a series of low
amplitude disturbances in WSW to SW flow aloft will track across the
srn Middle Atlantic states and Carolinas, with an associated
corridor of 20-30m/12 hr mid level height falls and periods of
enhanced forcing for ascent, through tonight. Three notable such
disturbances were located 1) near the Tri-Cities, 2) over nern GA,
and 3) over central MS - the first two of which will enhance ascent
over west and north-central NC through early this evening, and the
last later tonight.
At the surface, a synoptic cold front stretched from the mouth of
the Chesapeake Bay wwd through srn VA, to a 1008 mb Appalachian-lee
low near South Boston VA (where not coincidentally a TOR-warned
storm has recently developed), then w and swwd across central
TN/nwrn MS/n-central LA/sern TX. This front --which will become
diabatically-enhanced by convection now forming along both a
differential heating cloud band near the VA/NC state line and other
convection forced along the Blue Ridge and outflow from earlier TN
Valley convection-- will settle swd to the nrn NC Piedmont and nrn
Coastal Plain late this afternoon and early this evening (between 5
pm and 9 pm).
Strong diurnal/diabatic heating, with temperatures comparable to
those of Fri (well into the 80s, to around 90 degrees over the
Sandhills), and surface dewpoints in the middle 50s to lower 60s and
maximized along the aforementioned cold front, will yield weak, to
perhaps moderate MLCAPE immediately south of the approaching front
for the next several hours, despite only modest mid-level lapse
rates. A relative minimum in low level moisture and MLCAPE will
likely manifest over the srn Piedmont and Sandhills, as a pocket of
lower 0-1 km mean mixing ratios sampled by the 12Z FFC RAOB advects
newd, and downward. Indeed, surface dewpoints have mixed into the
middle 50s there during the past couple of hours. As such, the
greatest convective concentration and intensity will occur along and
just ahead of the approaching front, focused at least initially near
the VA/NC state line this afternoon. Elsewhere, isolated to widely
scattered activity will probably continue to percolate, but the
aforementioned mixing out of low level moisture and a lack of any
notable surface trigger, will tend limit coverage and intensity.
40-50 kts of WSWly effective bulk shear and locally backed
flow/enhanced low level shear in the vicinity of the front and
preceding the aforementioned weak lee surface wave will yield 100-
150 m2/s2 effective SRH; and the combination of both will favor
supercells amidst the aforementioned 1000-1500 J/kg MLCAPE. While
mean storm motion will be around 210-220/30kts, right-moving storm
motion vectors will be closer to 260-270 degrees and consequently be
aligned more parallel to the cold front. All severe weather hazards,
including damaging straight line winds, hail up to one and a half
inches in diameter, and perhaps a tornado or two owing to updraft
propagation contribution from both shear/dynamical influences and
surface boundary ones, which would favor storm motion and relatively
long residence times along the corridor of relatively helicity-rich
Post-frontal low overcast, showers, and gusty post-frontal NEly
winds, will develop behind the synoptic surface front as it settles
south through central NC tonight. Showers will become numerous to
widespread late tonight, particularly over the nwrn half of the
CWFA, where both influence from the aforementioned central MS
disturbance and 925-850 mb frontogenesis will become maximized.
Cooler, with lows in the upper 40s to around 50 degrees nw to around
60 degrees se.
-- End Changed Discussion --
.SHORT TERM /SUNDAY THROUGH TUESDAY/...
As of 355 PM Saturday...
...Flood Watch from midnight tonight through 800 AM Tuesday...
The beginning of a long duration rain event will be underway over
the western and central Piedmont by 12z/Sunday. The mid/upper trough
is forecast to become negatively tilted during this period as it
drifts ESE from the Tennesse Valley region across the southern Mid-
Atlantic states. At the surface, a backdoor cold front will also
begin Sunday either over far southern NC or near the NC/SC border
region. This will place nearly all, if not all of central NC in the
cooler stable air. Models suggest that a surface low pressure will
be located over NW Georgia around 12z/Sunday, then move east across
eastern GA and SC Sunday PM. As the low pressure slowly strengthens
as it moves east across SC Sunday night and Monday, the low level
boundary should back NW and just inland of the coast Monday. This
still leaves central NC on the cool, wet side of the boundary with
copious amounts of very moist warm air advection from off the Gulf
Stream and western Atlantic expected to ride up and over the frontal
zone. This occurs as the mid/upper system becomes more negatively
tilted as it essentially crawls eastward, just to our south Sunday
night and Monday. The heaviest rain is expected to fall along and
north of the frontal zone through the period as the strong dynamic
forcing slowly advances east across the area. This will place the
Piedmont and Sandhills in the heaviest rain Sunday. The heaviest
rain is forecast to spread east with time, covering much of our
region Sunday night into Monday. Models are not as robust with the
elevated instability Sunday night and Monday as they are on Sunday.
However, the influx of very high precipitable waters off the Gulf
Stream and western Atlantic should will aid in heavy rainfall at
The threat of flooding will be the main hazard with this system
since our region will be in the more stable area north of the main
frontal zone. QPF storm total for the entire event is on the order
of 3-5 inches, with potentially 6 inches locally. The main time
frame for the heaviest rain should be from midnight tonight through
midnight Monday night.
Flash Flood Guidance numbers will lower with time, which are
currently running high. 6 hour FFG is on the order of 2.5 to 3
inches, and this will lower to around 1.5 inches by Sunday
afternoon. Urban areas have much lower thresholds, and they will
become an issue well before the rural locations. The soils, which
have recently been dry will become wet and saturated with the
showers/storms this evening into Sunday. This will lead to runoff
issues with the widespread heavy rain Sunday afternoon through
Monday evening. Gusty NE winds will be felt in all zones with
sustained winds of 15-25 mph, gusts to 30-35 Sunday into Monday.
Less important forecast parameters include the temperature forecast
which will be much cooler with the heavy rain and breezy NE winds.
Highs Sunday should only be in the 50s/60s NW/SE both Sunday and
Monday. Lows will be only about 5-7 degrees lower than daily maxes,
The heaviest rain should move offshore on Tuesday or Tuesday
evening, with the threat of moderate river flooding on the main stem
rivers expected into late week.
.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 345 PM EDT Saturday...
Much warmer air associated with a building subtropical mid level
ridge can be expected late week into the weekend when highs should
solidly get into the 80s.
.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
As of 155 PM Saturday...
VFR conditions and a warm and breezy SW wind will prevail outside of
scattered thunderstorms that will grow in coverage and intensity, --
both greatest over the nrn Piedmont as storms over swrn VA/nwrn NC
propagate east along a cold front drifting south across srn VA--
during the next couple hours. MVFR ceilings, showers, and gusty post-
frontal NEly winds will develop behind, and with the passage of, the
front this evening through early tonight. Those conditions, which
may dip into IFR range by Sun morning at Piedmont sites, where
coverage of showers will also be greatest, will then likely continue
through the day Sun.
Outlook: A prolonged period of rain/showers and sub-VFR ceilings and
visibilities --in springtime cold air damming-- can be expected
through Tue, as an area of low pressure tracks east along the
aforementioned front, then slowly up the coast of the Carolinas.
Conditions will be slow to improve as the coastal low drifts only
slowly away from our region, with a return to VFR late Tue or
All Central NC zones from midnight tonight through 12z/Tuesday.