Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KRAH 211602
Area Forecast Discussion
National Weather Service Raleigh NC
1140 AM EDT Fri Apr 21 2017
-- Changed Discussion --A surface trough will strengthen over the NW Piedmont this
afternoon, and linger over the central Piedmont tonight.
Meanwhile, a cold front will approach from the northwest and move
into NC on Sat. The front will stall over a portion of
central NC Saturday into Sunday, as a slow-moving area of low
pressure tracks along the front.
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.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 1140 AM Friday...
It was sunny and warm across much of central and eastern NC late
morning. Clouds and a few showers spreading ENE across the
Mountains into the northern Piedmont of NC. This activity has been
weakening this morning, but should begin to redevelop by early
afternoon. The most likely area for convective development will be
over the NW zones (mainly the Triad region - spreading E or ENE
across the northern Piedmont late this afternoon.
The latest observational data indicated a deepening surface trough
over the Foothills of the Blue Ridge extending into the NW Piedmont.
Scattered cumulus were already developing along this trough and over
the Mountains as of late morning. The cluster of weakening showers
may get some enhancement in the next few hours as it tracks from NE
Tennessee into NW North Carolina due to the increasing instability
with heating. Additional convection is expected to develop, mainly
scattered activity along the surface trough. The latest Hi-res
models suggest the Triad east to Roxboro/Hillsborough have a good
chance of thunderstorms between 2-5 pm, possibly sinking SE into the
Triangle area late afternoon, before or as it is weakening. A few of
the storms may become strong to marginally severe with the main
threat of isolated damaging wind.
The focus then turns to the NW Piedmont between 500 PM into the
evening as additional clusters of showers/thunderstorms are expected
to develop and spread ENE out of the Mountains and Foothills (along
the previous convective outflows or surface trough). We will carry
likely POP in the aforementioned areas during the specified time.
Otherwise, only a slight to low chance of thunderstorms elsewhere as
MLCapes increase and convergence develops along another weak surface
trough expected along the I-95 corridor 3-7 PM.
Highs should top out well into the 80s, except upper 70s NW.
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 main cold front slows over NW NC and southern
VA. Lows under partly to mostly cloudy skies will be in the 60s.
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.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/...
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).