Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Remove Highlighting --
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FXUS62 KRAH 221755
Area Forecast Discussion
National Weather Service Raleigh NC
155 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/...
As of 1145 AM Saturday...
Little change in forecast rationale and expected sensible weather
Downstream of a mid-upper low that will deepen as it tracks from
south-central MO to west-central TN/KY by 12Z Sun, a series of low
amplitude disturbances in WSW flow aloft will track ene across the
srn Middle Atlantic states and Carolinas, with an associated
corridor of steady 20-30m/12 hr mid level height falls, through
At the surface, a synoptic cold front stretched from the mouth of
the Chesapeake Bay wwd through srn VA, then swwd across central
TN/nwrn MS/nrn LA/sern TX. This front --which will become
diabatically-enhanced by convection likely to form 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 slowly swd today, to the nrn NC
Piedmont and nrn Coastal Plain late this afternoon and early this
evening (between 4 pm and 9 pm), and likely be accompanied by a weak
lee surface wave forecast to develop over swrn VA this afternoon.
Warm and breezy conditions will continue to develop through early
afternoon. Strong diurnal 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 instability immediately south of the approaching
front, by mid afternoon. 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. As such, the greatest
concentration and intensity of resultant showers and storms will
occur along the approaching front, focused initially over southside
VA, before drifting swd across the NC state line late this
afternoon. However, isolated to more widely scattered activity may
also fire along any soil/surface moisture gradients resulting from
earlier convection Fri-Fri night.
25-40 kts of WSWly mid level flow, strongest north, will favor multi-
cell storm modes, and probably a few supercells in the vicinity of
the front and preceding the aforementioned weak lee surface wave,
where locally backed low level flow and shear will become maximized
and yield perhaps 100-150 m2/s2 effective srh. 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 approaching cold front. So while strong
to marginally severe wind gusts would be favored with multi-cellular
storms, any supercells --near the VA border-- will also be capable
of producing hail, and perhaps a tornado owing to updraft
propagation contribution from both shear/dynamical influences and
surface boundary ones, which would favor storm motion roughly along
the relatively helicity-rich front.
Post-frontal low overcast, patchy light rain/showers, and 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, as a vort max now
crossing LA nears from the sw. Cooler, with lows in the upper 40s to
around 50 degrees nw to around 60 degrees se.
.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/...
-- Changed Discussion --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
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