Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
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FXUS62 KRAH 261958
Area Forecast Discussion
National Weather Service Raleigh NC
300 PM EST Sun Feb 26 2017
High pressure over the area will move offshore late tonight and into
the day on Monday. A warm front will approach from the south Monday
night, then move north of the region Tuesday. Warm southwesterly
flow will continue ahead of a cold front approaching from the west.
NEAR TERM /TONIGHT/...
As of 250 PM SUNDAY...
Flow aloft has become quasi-zonal in the wake of upper trough
exiting the Mid-Atlantic region. While at the surface, modified cP
parent high centered over the TN Valley this morning will shift east
over the area through the afternoon and then slip offshore late
tonight and into the day on Monday.
Batch of broken high clouds in advance of the sheared shortwave
energy ejecting eastward across the Lower MS Valley will bring an
increase in high clouds late tonight/during the predawn hours Monday
morning, which could temper low temps just a bit. Overnight lows in
.SHORT TERM /MONDAY AND MONDAY NIGHT/...
As of 250 PM SUNDAY...
Surface ridge axis will extend west into the area from the parent
high centered off the Mid-Atlantic coast. Resultant southerly-
southeasterly low-level flow will allow for moderation of cP airmass
with highs Monday a good 7 to 10 degrees warmer than Sunday. Highs
ranging from lower 60s NW to upper 60s/near 70 SE.
Strengthening southwesterly low-level WAA ahead of the sheared low-
amplitude shortwave trough moving into region will result in
increasing clouds by the late afternoon with isolated to scattered
showers possible during the evening and into the overnight hours.
The NAM is especially wet as it develops 500-750 J/Kg of elevated
instability across the area, but believe this to be a bit overdone.
Much warmer overnight lows in the upper 40s to lower/mid 50s.
.LONG TERM /Tuesday through Sunday/...
-- Changed Discussion --As of 255 PM Sunday...
Tue/Tue night: The most likely scenario is that the warm front will
be pushing NNW into the area early Tue, with the morning hours
featuring low stratus in the west, where the stable pool should
linger, and partly cloudy in the east, with isolated light precip in
the west supported by moist isentropic upglide. Models show the
entire area mixing and destabilizing during the afternoon, although
climatology would suggest that the NW CWA could stay somewhat stable
all day. Models depict marginal to moderate instability developing
mainly along and east of Highway 1 in the mid-late afternoon, with
good low level moisture streaming in, improving deep layer shear,
and decent mid level lapse rates of 7.0-7.25 C/km. Dynamics are slim
however, with a fairly flat mid level flow, and only weak upper
divergence. Will have a slight chance of showers in the afternoon in
the west, with a better chance of showers and thunderstorms in the
east, lasting into the evening before the loss of heating prompts a
trend down to small shower chances and the development of
stratocumulus overnight. These clouds should mix out with heating
Wed as we`ll be firmly in the warm sector, with a stout gusty breeze
from the SW ahead of the cold front, which should push SE through
the area Wed evening or Wed night (the GFS is several hours faster
than the ECMWF and NAM). GFS instability is a bit higher Wed
afternoon as compared to Tue, reasonable given the even greater
influx of low level moisture, and if the slight cap noted around 800
mb on GFS forecast soundings comes to fruition, in conjunction with
the GFS`s projected 7.5-8.0 C/km mid level lapse rates and
strengthening deep layer bulk shear to 55-65 kts, we could get rapid
development and intensification of convection during the afternoon,
especially over the eastern CWA, with some late-day severe storms
possible. This convection should remain supported by a pocket of
strong upper divergence as it shifts over eastern NC through late
evening, followed by frontal passage, although high clouds should
persist over the area through Wed night as the positively tilted mid
level trough will still be well off to our NW. With warm
thicknesses, expect highs of 70-77 Tue except for mid-upper 60s NW.
Morning thicknesses Wed around 1380 m, nearly 60 m above normal,
will support highs from the mid 70s NW to lower 80s SE.
Thu through Sun: A generally tranquil period with dry weather
likely. The cold front will be pushing southeast ward off the coast
Thu morning. Broad mid level troughing will persist over
central/eastern Canada and the northeast CONUS into Sat, with the
core of the polar low transferring from Hudson Bay to Newfoundland.
An initial ridge of high pressure nosing in from the WSW Thu will
bring thicknesses that are close to seasonal normals. Then, energy
that swings through the trough base across the Mid
Atlantic/Northeast on Friday will bring a reinforcing dry cold front
southeastward through the area, followed by chilly high pressure
that will build overhead by Sat morning. This should knock temps
down to below normal for Fri night/Sat, with a good chance of
orographically enhanced high clouds over the Piedmont early Sat
within fast perturbed mid level flow. Temps should moderate for Sun
as the highs drops to our SE with return flow and weak lee troughing
for the latter half of the weekend. -GIH
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.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
As of 1227 PM Sunday...
24 Hour TAF Period: There is high confidence of VFR conditions
through the 24 hour TAF period, as surface high pressure moves east
across the region, becoming positioned off the NC coast Monday
afternoon. Northwesterly winds in the 6 to 11 kt range will become
light as variable late this afternoon and evening as the high shifts
overhead, and will become south-southeasterly on Monday.
Outlook: Warm moist air advection ahead of a upper level disturbance
could produce the next chance for sub-VFR conditions Monday night
with isolated to widely scattered showers possible. Scattered
showers along with the threat of sub-VFR conditions could linger
into Tuesday and Tuesday night. A cold front passage late Wed night
or early Thu will bring a better chance for showers and possibly a
few thunderstorms. Cool high pressure building into the area
Thursday and Friday will bring a return to VFR conditions for late
in the week and into the weekend.