Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KRAH 301627
Area Forecast Discussion
National Weather Service Raleigh NC
1230 PM EDT Thu Mar 30 2017
Weak high pressure will extend into the area from the north today,
then move offshore tonight. A storm system will approach from the
west tonight, cross the area Friday, then move off the coast Friday
night. High pressure will build in from the north over the weekend.
Another storm system will affect the region Monday into Tuesday.
.NEAR TERM /THROUGH TODAY/...
As of 1010 AM Thursday...
Clouds are hanging tough over much of central NC this morning, not
surprising given the deep (over 5k ft) low level moist layer noted
on the 12z GSO sounding. High-res models show this thinning as the
day progresses mainly along the edges (indirectly supported by
fairly dry low levels seen on the experimental GOES-16 low level
water vapor channel), although the moist isentropic upglide is
expected to persist through the day over the NC Foothills and W
Piedmont, so the broken to overcast skies should hold on over the NW
and far W CWA today, with partial sunshine over the far E/SE
Piedmont and Sandhills/Coastal Plain. Precip chances appear slim
until evening at the earliest, given the lack of deep moisture and
forcing for ascent until well after nightfall, so have trimmed back
pops for today, leaving just isolated in the extreme W. In areas
where the cooler/drier surface air building in from the NNE overlaps
with the thickest cloud cover, temps will be coolest this afternoon,
likely struggling to reach the 60 degree mark, while far southern
and SE sections top out near 70. -GIH
Earlier discussion from 425 AM: The southern edge of a surface high
extending down into the area from the north is bringing easterly
flow to the area but southeasterly flow along the Appalachians and
as a result a growing area of MVFR stratus is starting to build in
across the northwest Piedmont. This feature will be influential on
today`s weather when it comes to sky cover, precipitation and
ultimately temperatures. Later this morning drizzle or even light
rain will be possible across the northwest Piedmont as more moisture
flows into the area. Depending upon how thick the stratus becomes,
and model soundings show a significant cloud deck about 8 kft thick,
high temperatures under the stratus could be overdone. As a result
have slashed temperatures across the north to below guidance with
upper 50s in the Triad but a fairly tight gradient with lower 70s
across the southern counties. The triangle will be a major gamble
today with anything from lower to upper 60s possible. For now have
chosen to go on the lower end but a safe bet will be the mid 60s.-RE
.SHORT TERM /THURSDAY NIGHT AND FRIDAY/...
As of 420 AM Thursday...
Attention will turn to the southwest on Thursday night as a warm
front surges in and temperatures should actually begin to rise
overnight at most locations. Good isentropic lift in the west will
begin to encourage precipitation over the western portion of the CWA
between 0-6Z Friday. Model forecast soundings are virtually
saturated throughout the column with PW values near 1.3 inches. The
low level jet structure is fairly unorganized early Friday morning
and there are several small disturbances in the flow so expect
several smaller waves of precipitation which could be scattered in
nature at times. As the precipitation pushes eastward it could
become a bit more organized into more multicellular or broken lines
but shear will be decreasing at this time particularly in the lower
levels as winds become southwesterly and more unidirectional with
height. Instability will increase slightly with peak heating and a
dynamic push from the left exit region of a 500 mb jet could cause a
storm or two to become severe. SPC has the eastern half of the area
in a marginal risk for severe wx with damaging winds the most likely
threat. With decent moisture in this system expect rainfall totals
anywhere from a half of an inch on the low end to about an inch on
the high end.
Temperatures will be tricky on Friday given the convection but with
central NC in the warm sector for much of the day expect highs in
the low to mid 70s in most locations.
.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
As of 405 AM Thursday...
An energetic Pacific jet will send a series of significant
shortwave perturbations into the west-central U.S., where the
models indicate they will amplify toward the Four Corners/Srn
Plains region and interact with an equally energetic southern
stream. That interaction will ultimately shear the
perturbations ENEwd into the Middle Atlantic states, with
associated episodes of precipitation/convection, about every 72
hours in central NC. This a relatively persistent pattern from
recent days, which included the round of convection from this
past Mon night, and which will likely result in another one
later tonight (highlighted in the short term discussion above).
Fri night: The models are now in good agreement with the
progression of the upper low initially over the Central Plains,
to the nrn Middle Atlantic coast by late Fri night. An
accompanying surface trough axis/secondary front will cross the
ne Piedmont/nrn Coastal Plain with a lingering chance of
showers and a storm or two Fri evening. Otherwise, clearing and
cooling will occur, with lows in the lower to middle 50s.
Sat-Sun: Beneath longwave ridging bulging Nwd from the Caribbean
to central Canada, the weekend is expected to be dry, and mild,
with a reinforcing dry frontal passage (the leading edge of a
Pacific high) Sat night. Periods of stratocumulus may bubble
beneath a strong, 5 k ft subsidence inversion, but not with
enough coverage to significantly impact temperatures through the
period. Highs in the upper 60s to 70s, warmest ahead of the
reinforcing front on Sat. Lows, coolest Sun morning in the 40s,
and about a category higher in return SEly flow and increasing
deep moisture by Mon morning.
The next couple of strong shortwave perturbations will follow
on schedule and cross central NC late Mon-Mon night and again
just beyond this forecast period (Thu). The result will be
another likely probability of showers and storms late Mon-Mon
night, preceded by possible weak in-situ CAD during the day
Mon. It will otherwise be mild through the period, with
temperatures mostly in the 70s and 50s for highs and lows,
.AVIATION /18Z Thursday through Tuesday/...
As of 1230 PM Thursday...
MVFR cigs over the far W (INT/GSO) will lift to VFR during the mid
afternoon, while farther east at RDU/FAY/RWI, VFR cigs will hold for
the rest of the daylight hours. Scattered showers will move into
INT/GSO after 21z, however VFR vsbys and cigs should persist, with
only brief MVFR conditions within short-lived heavier showers
through this evening. Cigs are likely to drop back to MVFR then IFR
around midnight (04z) at INT/GSO, after 2 am (06z) at RDU/FAY, and
after 4 am (08z) at RWI, all as a storm system approaches from the W
and SW tonight. Widespread MVFR to IFR vsbys within showers and
thunderstorms are expected starting late tonight, tracking eastward
through early Fri afternoon, ahead of an eastward moving cold front.
The showers/storms and sub-VFR conditions are expected to push east
of INT/GSO after 15z Fri, and RDU around 18z. Ahead of the front,
through tonight, surface winds will be from the SSE, shifting to SW
Fri morning, then to WSW with frontal passage at western sites late
in the TAF valid period.
Looking beyond 18z Fri: The trend from MVFR to VFR will progress
eastward through RDU/RWI/FAY prior to 21z, as a cold front crosses
the area. VFR conditions are expected to dominate through Sun as
high pressure builds in from the north, although the moist low level
flow from the NE may bring a short period of MVFR cigs mainly at RWI
03z-12z Sun (Sat night). Another storm system will bring a good
chance of sub-VFR conditions and strong/shifting winds with height
starting after 06z Mon (Sun night), dominating until Tue morning,
when the system will move to our east with a trend back to VFR by
Tue afternoon. -GIH