Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000
FXUS62 KRAH 250200
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
1000 PM EDT Mon Apr 24 2017

.SYNOPSIS...
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
late week.

&&

.NEAR TERM /Through Tonight/...
As of 1000 PM Monday...

Most of North Carolina resides in the left exit region of a jet
streak rounding the base of a compact negatively-tilted trough
across the southeastern CONUS. The center of a compact surface low
pressure can be found across far eastern South Carolina and is
moving northeastward with time.  A secondary (and stronger)
subtropical low is approaching the coast of North Carolina, and is
starting to merge with the aforementioned low pressure system to its
west.  Closer to home, the region remains in a strong CAD regime
with the wedge front delineating slightly more warm and moist air
and the CAD to its west located across the eastern CWA. As the lows
merge, continued DCVA (as well as frontogenetical forcing, and
isentropic upglide along a wedge front draped across the
southeastern CWA) will continue to enable widespread ascent. Coupled
with an anomalously moist atmosphere across the CWA that increases
toward the east (the 00z KMHX sounding reflects a PWAT of 1.83",
breaking the previous daily record by nearly nearly 0.3"),
widespread rainfall continues areawide.

Currently, the heaviest rainfall is confined to two main bands--one
across Edgecombe to Franklin counties and another across Wayne and
Johnston counties. Both bands seem to combine across Wake, Chatham
counties. While rainfall rates aren`t particularly intense within
the bands (generally a few tenths per hour approaching 0.5"/hr in
the heaviest segments), the long duration of the rainfall is leading
to 6 hourly rainfall rates of 1-2". Unsurprisingly, recent storm
reports indicate a few closed roads across the CWA with rivers
rising in the heaviest bands toward flood stage. However, some
slight drying is being noted across the far NE CWA into the NE coast
of North Carolina (likely in association with vertical mixing of a
pocket of drier air centered around 925 mb) leading to an erosion of
the northern precipitation band. As the combined low pressure system
translates northeastward across the coast of North Carolina, the
bands of heaviest precipitation will likely pivot slightly
northeastward with time. A quick peak at the latest suite of model
guidance indicates a narrow axis of MUCAPE may build into the
eastern CWA enabling slightly heavier precipitation rates as the
band moves northeastward (especially between 03-06z). Indeed, a few
flashes of lightning are occurring across the NC coast within the
aforementioned precipitation band. As the band shifts northwestward,
an additional 1-2" with isolated pockets of 3-4" seems likely across
the NE CWA (e.g. Edgecombe, Nash, Franklin counties) as well as
parts of Wake, Durham, Chatham, and Orange counties.  The latest
mesoscale discussion from the WPC lines up well with this reasoning.

&&

.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
weekend.

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 /00Z TUESDAY THROUGH SATURDAY/...
As of 820 PM Monday...

24 Hour TAF Period: Adverse aviation conditions are expected to
continue through the 24 hour TAF period (high confidence) as a deep
mid/upper level low moves across the region.

A persistent rain is expected to continue this evening across the
area into early Tuesday morning, resulting in generally IFR/low end
MVFR conditions. However, periods of LIFR cigs will be possible in
areas of light rain and/or brief lulls in the precip ahead of the
mid/upper low. Winds are expected to strengthen across northern
portions of the area this evening into early Tuesday morning, with
sustained winds in the 10 to 15 kt range and gusts of up to 20 to 25
kts possible. Will continue to keep LLWS in the TAFs at
KGSO/KINT/KRDU/RWI as easterly winds at around 2 kft are expected to
increase late this evening into early Tuesday morning and be around
50 kts. However, given this will mostly be speed shear (with
generally unidirectional flow) expect this will only create marginal
LLWS conditions (especially given some gustiness expected). Surface
winds will then decrease late tonight (after 09z) and become more
variable for Tuesday. While the main area of rain will shift
northward of the area by 12-15Z Tuesday we can still expect some
lingering showers/rain, especially moisture wrapping around the
departing low pressure system across the northeastern portions of
our area (KRWI/KRDU). While cigs should improve on Tuesday, possibly
becoming VFR temporarily, think we should generally maintain MVFR
cigs.

Outlook: Cigs should remain MVFR through Tue night, 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.

&&

.HYDROLOGY...
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

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Tuesday morning for NCZ007>011-021>028-038>043-
073>078-083>086-088-089.

&&

$$

SYNOPSIS...Hartfield
NEAR TERM...BB/MWS
SHORT TERM...Hartfield
LONG TERM...CBL/Smith
AVIATION...BSD/Hartfield
HYDROLOGY...Hartfield


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