Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
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FXUS62 KRAH 221919
Area Forecast Discussion
National Weather Service Raleigh NC
319 PM EDT Sat Apr 22 2017
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.
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.
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.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 400 AM Saturday...
This long duration rain event on tap through the short term forecast
has the potential to produce widespread 2-3 inches of rain. This
will ultimately be beneficial in alleviating the abnormally dry to
moderate drought conditions which have been gradually developing
through the late winter/spring across central NC. The heavier rain
focus shifts from the west initially Sunday to the east on Monday,
which will help mitigate the potential for widespread flooding, but
some flooding is possible on creeks and streams, and even our larger
rivers may have some minor flooding from Sunday night into early
As mentioned earlier, the initial focus for heavier rain will be in
the western Piedmont on Sunday as low level isentropic lift will be
focused on the frontal/CAD zone. Rain will ramp up over the
remainder of the forecast area later in the day and peak Sunday
night over the Piedmont as low level flow backs southeasterly as the
surface low pressure system moves slowly east across South Carolina.
The heavier rain then shifts east as the digging cutoff low phases
with the surface system Monday morning and begins to lift slowly up
the coast through Tuesday. Categorical PoPs through the period with
QPF amounts ranging from slightly >3 inches across the north to 2.5
inches in the southeast. CAD-flavored high temperatures in the Triad
will struggle into the mid and upper 50s Sunday and Monday with the
southeast reaching low 70s. Scattered non-elevated thunderstorms
will be possible around the periphery of the wedge, with lesser
potential for elevated convection heading into the deeper cool air
north and northwest.
.LONG TERM /TUESDAY THROUGH SATURDAY/...
As of 400 AM EDT Saturday...
Precip chances diminish rapidly Tuesday, with only slight chances in
the northeast by late afternoon. Shortwave ridging is expected in
the wake of the upper wave on Wednesday with dry conditions and a
warming trend through Saturday as an upper level ridge builds over
the southeast CONUS. Highs Tuesday will reach 70 to 75, with a
return to mid and upper 80s for Friday and Saturday.
.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