Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 230155
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
950 PM EDT Sat Apr 22 2017

.SYNOPSIS... A cold front will exit then stall just south of central
NC overnight into Sunday. Meanwhile a slow moving low pressure system
will track along the coast of the Carolinas Sunday night through
Monday night.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 950 PM Saturday...

Colder air has begun to overspread central NC from the north.
Temperatures at mid-evening ranged from the lower 50s far north to
the mid-upper 70s far south. The cold front, which marked the
leading edge of the change in air mass, stretched west-to-east from
Albemarle to Ft Bragg to south of GSB at 01Z. This boundary should
exit our southern counties prior to midnight.

Area of showers will overspread most of central NC behind this
boundary overnight as an upper level disturbance exits the TN Valley
and crosses our region. Could see a few isolated elevated storms
though severe parameters are much weaker than compared to earlier in
the day. Still expect the bulk of the showers overnight to occur
across the Piedmont counties and the northern coastal plain where
better forcing will exists. Rainfall amounts expected tonight will
total a half inch or less, so not expect anything to set off any
flooding. The rain tonight into early Sunday will prime the soils
for the heavier, more prolonged rain expected later Sunday into
early Monday.

Main adjustment to the near term forecast was to bring the colder
air in faster than previous forecast as temperatures at mid evening
were running some 2-4 degrees colder across the north than expected.
This required a downward tweak in the minimum temperatures,
primarily across the south.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
As of 355 PM Saturday...

...Flood Watch from midnight tonight through 800 AM Tuesday...

The beginning of a long duration rain event will be underway over
the western and central Piedmont by 12z/Sunday. The mid/upper trough
is forecast to become negatively tilted during this period as it
drifts ESE from the Tennesse Valley region across the southern Mid-
Atlantic states. At the surface, a backdoor cold front will also
begin Sunday either over far southern NC or near the NC/SC border
region. This will place nearly all, if not all of central NC in the
cooler stable air. Models suggest that a surface low pressure will
be located over NW Georgia around 12z/Sunday, then move east across
eastern GA and SC Sunday PM. As the low pressure slowly strengthens
as it moves east across SC Sunday night and Monday, the low level
boundary should back NW and just inland of the coast Monday. This
still leaves central NC on the cool, wet side of the boundary with
copious amounts of very moist warm air advection from off the Gulf
Stream and western Atlantic expected to ride up and over the frontal
zone. This occurs as the mid/upper system becomes more negatively
tilted as it essentially crawls eastward, just to our south Sunday
night and Monday. The heaviest rain is expected to fall along and
north of the frontal zone through the period as the strong dynamic
forcing slowly advances east across the area. This will place the
Piedmont and Sandhills in the heaviest rain Sunday. The heaviest
rain is forecast to spread east with time, covering much of our
region Sunday night into Monday. Models are not as robust with the
elevated instability Sunday night and Monday as they are on Sunday.
However, the influx of very high precipitable waters off the Gulf
Stream and western Atlantic should will aid in heavy rainfall at
that time.

The threat of flooding will be the main hazard with this system
since our region will be in the more stable area north of the main
frontal zone. QPF storm total for the entire event is on the order
of 3-5 inches, with potentially 6 inches locally. The main time
frame for the heaviest rain should be from midnight tonight through
midnight Monday night.

Flash Flood Guidance numbers will lower with time, which are
currently running high. 6 hour FFG is on the order of 2.5 to 3
inches, and this will lower to around 1.5 inches by Sunday
afternoon. Urban areas have much lower thresholds, and they will
become an issue well before the rural locations. The soils, which
have recently been dry will become wet and saturated with the
showers/storms this evening into Sunday. This will lead to runoff
issues with the widespread heavy rain Sunday afternoon through
Monday evening. Gusty NE winds will be felt in all zones with
sustained winds of 15-25 mph, gusts to 30-35 Sunday into Monday.

Less important forecast parameters include the temperature forecast
which will be much cooler with the heavy rain and breezy NE winds.
Highs Sunday should only be in the 50s/60s NW/SE both Sunday and
Monday. Lows will be only about 5-7 degrees lower than daily maxes,
if that.

The heaviest rain should move offshore on Tuesday or Tuesday
evening, with the threat of moderate river flooding on the main stem
rivers expected into late week.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 345 PM EDT Saturday...

Much warmer air associated with a building subtropical mid level
ridge can be expected late week into the weekend when highs should
solidly get into the 80s.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
As of 810 PM Saturday...

24 Hour TAF Period: A cold front is currently settling southward
through the area this evening, with low end MVFR/IFR cigs expected
behind the front. Isolated to widely scattered showers will be
possible this evening behind the front as well at all TAF sites.
However, more widespread showers are expected to move into
northern/northwestern portions of the area overnight and affect
primary KGSO/KINT/KRDU, with on and off showers expected on Sunday.
At KRWI and KFAY, any showers will be more spotty in nature, with
the bulk of the activity (after possibly a few showers this evening)
expected Sunday afternoon. Regardless of any precip, expect low end
MVFR to IFR cigs tonight through the end of the TAF period, with a
northeasterly winds of around 9 to 14 kts, with gusts of up to 20
kts possible.

Outlook: A prolonged period of rain/showers and sub-VFR ceilings and
visibilities --in springtime cold air damming-- can be expected
through Tue, as an area of low pressure tracks east along the
aforementioned front to the south of central NC, then slowly up the
coast of the Carolinas. Conditions will be slow to improve as the
coastal low drifts only slowly away from our region, with a return
to VFR late Tue or early Wed.

&&

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

&&

$$

SYNOPSIS...MWS
NEAR TERM...WSS
SHORT TERM...Badgett
LONG TERM...Badgett
AVIATION...BSD/MWS


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