Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
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FXUS62 KRAH 242021
Area Forecast Discussion
National Weather Service Raleigh NC
420 PM EDT Mon Apr 24 2017
A strong and deep low pressure system will slowly cross the region
through Tuesday, producing wet and unsettled weather. Behind this
low, a ridge of high pressure will extend into the region for mid to
.NEAR TERM /through Tonight/...
As of 320 PM Monday...
With a few exceptions, rainfall so far has been steady and moderate,
with numerous but minor flooding confined mostly to streets and high
water in creeks. This should change over the next 9-12 hours as we
increasingly tap into a greater depth of Atlantic-source moisture
penetrating further inland, north of a surface low that will move
northward over southeastern NC tonight (although models disagree on
the exact track of this low). Low level easterly flow is expected to
strengthen this evening into tonight north of the approaching
surface low, with the strongest flow of 30-40 kts over northern
sections of central NC, mainly along and north of the Highway 64
corridor. This will continue to draw anomalously high PW values into
the area through tonight, as well as allowing the modified wedge
boundary to shift further inland. The resulting enhancement of deep
isentropic upglide will be vertically juxtaposed with focused upper
divergence to the ENE of the mid level low, fostering efficient rain
production with a warm layer nearing 3.5 km. And the additional lift
generate via elevated convective elements, most likely over our SE
and far S sections through this evening, may result in locally
heavier rain rates and higher totals. Will leave the flood watch in
place, as an additional 1.5-3.0" of rain is possible through
tonight, with higher amounts focused along bands with embedded
convection and particularly near the pivot point of any such bands
as they become more W-E oriented with time tonight as the mid level
low shifts eastward into SC. Temps should waver only a category or
so through tonight at most, with many spots seeing steady temps
through the night or a minor rise as warmer air shifts inland. -GIH
.SHORT TERM /Tuesday and Tuesday night/...
As of 340 PM Monday...
The mid level low is expected to track from NE SC across E NC Tue,
before moving NNE just off the Delmarva/NJ coast Tue night. This low
will be nearly vertically aligned with the surface low (or series of
lows rotating about one another) over E NC. The strong low level
jetting north of the surface low will shift northward into VA Tue
morning, along with the upper divergence maximum, as the mid level
dry slot punches in from the south, and this drying aloft will lead
to diminishing chances for heavy rainfall. But the surface winds
will remain from the N and be fairly light with the low to our NE,
which should help lock in the cool/damp low level stable pool a bit
longer, especially in the western Piedmont. PW values will have
dropped but remain above normal, with both low and mid level lapse
rates near or over 6 C/km, mainly over the eastern two-thirds of the
forecast area. Have retained a chance of showers and isolated
storms, higher east than west, through Tue, decreasing Tue evening
as the mid level low exits and the column stabilizes and begins to
dry further. Temps are tricky with a wide spread in model
statistical guidance, not surprising considering that any amount of
insolation will cause temps to jump up. Have held onto a scenario
that reflects a longer-lived stable pool, suggesting highs from the
mid 60s NW to the mid 70s in the far east-central CWA. Will trend
pops down and out slowly Tue night, with skies trending toward
partly cloudy everywhere except the NE, where a lingering low chance
of showers will be retained overnight. Lows 53-58. -GIH
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
As of 345 pm Monday...
The biggest story in the extended will be a transition to a
amplified western trough and eastern ridge, especially into the
Global models are in good agreement that a shortwave and associated
surface cyclone will lift out for the Central Plains and across the
Great Lakes Wednesday and Thursday, and a trailing cold front
associated will encroach upon the CWA from the west Thursday. The
removed upper forcing and lack of moisture return suggests this
waning precip with the front, which itself may never actually move
into NC as the subtropical ridge begins to expand across the
Southeast by Friday. Main impacts look to be an isolated shower or
storm in the west late Thursday or isolated storm late Thursday.
The synoptic pattern will favor near record heat by Saturday, owing
to a 590+dm H5 ridge, Bermuda highs and H7 anticyclone over SC.
Thickness are progged at 1410m Saturday, 50m above normal and
indicative of upper 80s to lower 90s. The main question is how
quickly another shortwave ejecting out of the Central/Southern
Plains, this one potentially stronger, will cause the ridge to
retreat. Heights are forecast to lower, more so across western NC,
favor low/80s west to upper 80s east Sunday and Monday, with a
chance of storms associated with the upper wave and approaching cold
front by Monday.
.AVIATION /18Z Monday through Saturday/...
As of 155 PM Monday...
High confidence in poor aviation conditions through at least 06z
tonight. Thick IFR and occasionally LIFR based clouds will persist
over central NC as a deep and strong low pressure system slowly
crosses the area through Tue, exiting to our northeast Tue night.
Periods of rain through 06z will affect all sites with occasional
MVFR to IFR vsbys, along with the mainly IFR cigs. Cigs will slowly
lift Tue morning to MVFR with decreasing vertical extent as drier
air just off the ground arrives from the S. The rain will decrease
in coverage and intensity later tonight, although scattered showers
will persist after sunrise Tue as the core of the upper low passes
overhead. Surface winds will be mainly from the NE at 7-12 kts
through tonight, occasionally gusting to 15-20 kts during the
evening and overnight hours, especially at GSO/RDU/RWI as an
easterly low level jet at 1000-2000 ft AGL extends into the area.
This low level jet may lead to a risk of low level wind shear at
INT/GSO/RDU/RWI from late evening into the overnight hours. Surface
winds will then decrease late tonight (after 09z) and become more
from the N or NW Tue.
Looking beyond 18z Tue, cigs should remain MVFR through Tue night,
with infrequent MVFR vsbys in scattered showers, although cigs may
drop back to IFR through Tue night especially at INT/GSO as a cool
stable air mass persists there. After the low shift to our NE Tue
night, high pressure ridging will take over, and we`ll see cigs rise
to MVFR then VFR through Wed morning. VFR conditions will then
dominate through Sat. -GIH
-- Changed Discussion --As of 420 PM Monday...
...Flood Watch remains in effect through 12Z Tuesday...
Much of the rainfall so far has been moderate, with most places so
far spared by the heavier rain rates. This is expected to change
through the evening hours as increasing lift and moisture lead to an
uptick in rain rates. Several creeks around central NC are
experiencing rises, with a few approaching minor flooding, and this
should persist through tonight, with retreating water levels
Tuesday. Urban and street flooding, particularly in the typical
flood-prone areas, have been common all across central NC. A few
main stem rivers are projected to approach or exceed flood stage
over the next couple of days in response to this rainfall, including
parts of the Yadkin, Haw, Little, Cape Fear, Rocky, and Tar Rivers.
Rainfall since last night has been around 1 to 3 inches, and an
additional 1-3 inches of rainfall are expected through tonight, with
locally higher totals possible. -GIH
-- End Changed Discussion --
Flood Watch through Tuesday morning for NCZ007>011-021>028-038>043-