Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
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FXUS62 KRAH 210610
Area Forecast Discussion
National Weather Service Raleigh NC
210 AM EDT Fri Apr 21 2017
-- Changed Discussion --A pre-frontal surface trough will strengthen over west-central NC
today, then linger tonight. Meanwhile, a cold front will approach
from the northwest and move into NC on Sat. The front will settle
slowly south Sat night and Sun, in advance of a slow-moving storm
system will cross the Carolinas through Monday.
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.NEAR TERM /THROUGH TONIGHT/...
As of 10 PM Thursday...
Isolated to widely scattered showers and storms that develop along a
weak disturbance moving across the area this afternoon/evening have
generally dissipate with nocturnal stabilization. Dry conditions are
expected to generally continue with flat ridging holding over the
area, with high pressure located offshore. The next disturbance and
associated cold front will remain to the west of central NC
overnight. Thus, expect a warm night, with low temps generally in
the lower to mid 60s.
.SHORT TERM /FRIDAY THROUGH SATURDAY/...
As of 330 PM Thursday...
The aforementioned cold front is forecast to push southward into
central NC Friday night, then stall in a west to east fashion
Saturday. Models and ensembles continue to show a fair amount of
spread with the location of the front Saturday, which will determine
where the temperature/cloud gradient and the convection develops
and/or is maintained along the north of the boundary. For now, the
GFS and the EC operational solutions continue their recent trends
(with the EC indicating the front to stall closer to the NC/VA
border Saturday, and the GFS indicating the front will stall over
southern NC). The surface high to the north is weak (forecast to be
less than 1020 mb and centered well north) - suggesting the front
will not "surge" southward as if propelled by a stronger high to the
north. However, any organized pre-frontal convection may be the
driving factor of where the front ends up Saturday, which could end
up being either solution (or simply a solution in between the two),
as the ensembles suggest. This will be an important forecast as the
upstream strong system in the mid levels approaching later in the
weekend from the west will bring deep moisture/strong lift and
convergence for widespread rain and convection (see discussion
For this package, we will continue to generally broad brush this
forecast, then fine tune in later forecasts as models come into
better agreement. There will be be large precipitation and
temperature gradients with this front over the weekend - so any
adjustment by 50 miles either north or south will have huge impacts
on central NC`s sensible weather, QPF, and severe threat.
Large differences in the models start late Friday afternoon into
Friday evening, with the EC essentially dry, and the GFS very wet
and stormy associated with the initial cold front over NW NC. Most
Hi-res models suggest at least scattered thunderstorms between 400
PM and 800 PM in the Triad, and 600 PM to late evening over the
Triangle. The EC appears to be well underdone with the initial
convection QPF. We will continue to depict POP`s that show the best
chance of convection northwest and north, and no POP south late
Friday afternoon and evening. Highs Friday should reach the mid 80s,
with lower 80s NW. Forecast Mlcape suggest at least a marginal risk
of a severe storm with damaging wind the primary threat, mainly from
Raleigh west and north Friday afternoon and evening. QPF will be
around 0.50 NW, with amounts of less than 0.25 from Raleigh
southward on Friday and Friday night.
The initial convection should lay a boundary out over central NC
late Friday night and Saturday. The higher POP should be along and
just north of the exact boundary location. We will keep cooler
temps/higher POP NW-N, lower to none over the SE Coastal Plain with
temperatures well into the 80s. A marginal risk of a few severe
storms should continue, and may be enhanced later in the day with
the approaching energy from the west.
.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
As of 215 PM Thursday...
Latest model guidance showing various outcomes for this
period...leading to a low confidence forecast.
A trough extends along and just offshore the west coast, and
compromised of multiple embedded circulations/ perturbations. Some
models split this trough into two pieces by mid week sending more
troughing into the Ohio Valley. Other models hold lower heights
farther to the west. Either outcome looks reasonable but will use
continuity from previous forecast as a starting point. Using this
scenario will slow down the closed low over the Southeast early in
the week and place it further south.
An associated surface cyclone will track ENEwd to Ewd along a
frontal zone draped across the srn and central Appalachians and srn
Middle Atlantic states late Sat through Sun, then to the coast of
the Carolinas, where it will slow, by early Mon. That front, which
will have slowed over srn VA Fri night, will likely sag Swd into at
least the nrn NC Piedmont Sat, with an associated low level
convergence axis provided by both the front and the aforementioned
preceding outflow - likely near or just S of the VA state line.
Convection will become diurnally-enhanced along and N of the front,
with semi-organized storm multi-cell storm modes, and perhaps at
least transient supercells in the vicinity of the low level
convergence --and enhanced shear-- axis.
Some slight NWd retreat of the boundary will be possible Sat night-
early Sun afternoon, downstream of the aforementioned surface
cyclone approaching from the west. That would suggest the better
chances of convection would (briefly) pivot into the far NW portions
of the area through early Sun. Another round of convection will then
accompany the surface cyclone and attendant cold front as they cross
the Carolinas Sunday afternoon. As the surface low slows beneath the
aforementioned complex upper trough, deformation/stratiform rain and
low overcast are likely Sun night into Mon. Temperatures on Monday
will be in the 60s.
Drying and warming conditions will return as the low finally moves
away from the coast Tuesday, and return flow overspreads the
Southeast Tuesday through Thursday.
.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --As of 210 AM Friday...
Mainly VFR conditions are expected, at least until scattered showers
and storms develop in the vicinity of a pre-frontal surface trough
centered over the NC Piedmont this afternoon. Brief sub-VFR
conditions and gusty winds will result in/near these showers and
storms. If storms merge into clusters and form a well-developed rain-
cooled/outflow airmass in their wake, sub-VFR ceilings and
visibilities may result later tonight.
Outlook: A cold front and focus for showers and storms along and to
the north of the boundary will settle south into central NC Sat,
then southward yet to near the SC border Sat night and Sun. An area
of low pressure will develop along the front and move slowly east
across, then along the coast of, the Carolinas Sun through Mon
night. This complex system will likely result in a prolonged period
of rain and low overcast during that time (Sun-Mon night).
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