Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Remove Highlighting --
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FXUS62 KRAH 211126
Area Forecast Discussion
National Weather Service Raleigh NC
726 AM EDT Fri Apr 21 2017
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.
.NEAR TERM /THROUGH TONIGHT/...
As of 350 AM Friday...
Generally Wly flow aloft will prevail through tonight, between a sub-
tropical ridge over the GOM/FL and a mid-upper low and trailing
positive tilt trough axis now centered over the upper Great Lakes and
srn Ontario. At the surface, a cold front trails from a surface low
over wrn NY this morning, SWwd to WSWwd across the lower OH
Valley/Mid-South/srn Plains; and this front will drift into the VA`s
by this evening. Meanwhile, a pre-frontal surface trough or two will
sharpen over west-central NC this afternoon, then linger through
Weak height falls aloft (10-20 M/12 hr) and low level convergence
invof the surface trough(s) will support the development of
scattered convection, within a weakly to moderately unstable and
uncapped environment over NC, owing in part to very warm
temperatures in the 80s (to near 90 degrees over the Sandhills) and
surface dewpoints in the upper 50s and lower 60s. Associated 25-30 C
surface dewpoint depressions will favor strong to marginally severe
wind gusts in mostly multi-cellular convection, as NC will remain on
the srn fringe of stronger mid level flow over the OH Valley/Middle
Atlantic states (Ie. effective shear values ranging from about 20-30
kts in central NC - strongest near the VA border). Associated
outflow will likely drift across the remainder of central NC and
provide a focus for a slight chance of showers and storms overnight,
while the synoptic frontal zone slows over srn VA. Mild, and
somewhat humid, tonight, with lows mainly 60 to 65.
.SHORT TERM /SATURDAY AND SATURDAY NIGHT/...
As of 415 AM Friday...
A strong NWly mid-upper jet from the Pac NW to the Four Corners will
help dig a trough/developing closed low into the central Plains/mid
MS Valley vicinity by early Sat. Upstream perturbations embedded
within the strong NW flow will pivot around the developing mid-upper
low in positive tilt fashion across the Mid-South/lower MS Valley
through Sat night. An associated surface cyclone will track slowly
ENEwd to Ewd across the TN valley and into the srn Appalachians by
Sun morning. The front initially over srn VA will likely sag Swd
into at least the nrn NC Piedmont Sat afternoon, with an associated
focus for diurnally-enhanced convection along and N of the front.
WSWly mid level flow will have increased into the 30-40 kt range
over central NC by Sat afternoon, downstream of the aforementioned
approaching upper low and trailing positive tilt trough. Mostly
moderate instability during the afternoon in the presence of this
enhanced mid level flow will favor semi-organized multi-cell storm
modes, and perhaps at least transient supercells in the vicinity of
the front/outflow --and enhanced low level shear-- axis.
It should again be warm south of the front Sat, with temperatures
comparable to those of Fri, while cooler 70s are expected over the
nrn Piedmont and nrn Coastal Plain.
The synoptic surface front is likely to become quasi-stationary and
bisect central NC from W to E Sat night, though outflow from earlier
convection may serve to shove an associated effective front into srn
NC by SUn morning. The focus for showers and storms, and low
overcast, will remain along and N of the synoptic/deeper front. Lows
in the low to mid 50s N to lower 60s S.
.LONG TERM /SUNDAY THROUGH FRIDAY/...
As of 335 AM Friday...
While confidence remains low concerning finer details such as the
timing and location of the frontal zone and surface low, consensus
on the track of the upper low off the South Carolina coast Monday
night does provide higher confidence in a widespread long-duration
rain event Sunday and Monday with wrap around showers lingering into
Tuesday. Storm total rainfall amounts for the entire event... with
the bulk of the rain spanning ~48 hours...should easily reach 1 to
1.25 inches, with potential for 2+ inches in the west. Duration and
intensity of the rain could cause minor flooding on creeks and
streams from Sunday onward, and if we receive much more than 2
inches, we would likely see minor flooding on the larger rivers as
early as Sunday night and into early next week.
Cool air remains dammed into the region Sunday and Sunday night,
with strong isentropic upglide...especially from the Piedmont
westward, and will increase PoPs to a minimum of 60% and downplay
the chance of thunder...which would be elevated considering the
depth of cool air and minimal instability over all but the far
southern counties. Guidance highs have been trending downward as
well, and there will likely be a strong gradient from upper 50s
north to mid or even upper 70s across the south. Mins Sunday night
will be in the 50s. The surface and upper system become better
aligned south of the area on Monday, and the ensuing deep northeast
flow, along with the rain and cloudiness, will produce highs mostly
in the mid 60s. Showers will be tapering off in the west Monday
night as the upper trof bottoms out south of Cape Lookout and begins
to lift up the coast with showers diminishing over all but the
northeast by Tuesday afternoon. Highs Tuesday will reach lower 70s
in the south, while northeast flow limits highs in the mid 60s
across the north.
Return flow overspreads the Southeast Wednesday through Friday with
dry weather and a warming trend from the mid and upper 70s
Wednesday to the low and mid 80s Friday.
.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --As of 725 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.
After that time, Sat night through Sunday night, there remains
larger than average uncertainty regarding the location of the front,
and continued focus for showers and storms along it, and showers and
low overcast to the north of it. An area of low pressure will then
develop along the front and move slowly east through, 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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