Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 241510
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
1110 AM EDT Mon Apr 24 2017

.SYNOPSIS...
A complex and slow moving storm system will bring heavy rain
and likely flooding today through Monday night. There is a
small risk for severe thunderstorms as well. This system will
lift north northeast, away from the forecast area Tuesday with
a return to dry weather and above normal temperatures during
the mid to late week period of this week. Expanding high
pressure aloft and at the surface, will provide mainly clear
skies with warm and dry conditions thru the upcoming weekend.
High temperatures may break into the 90s away from the
immediate coast during this weekend.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 11 AM Monday...Heavy rain along with showers and a few
embedded thunderstorms continue across much of the forecast area
this morning. Low pressure aloft slowly moving southeast across
southern GA will continue pulling deep moisture into the region
today. Moisture feed extending from off the FL coast, as seen
on satellite and radar, will continue spreading over portions of
SC this morning and into early afternoon. Some of these areas
have already seen significant rainfall and have been reporting
flooding. This area of enhanced moisture should rotate north and
east as the 5h low moves into eastern GA this afternoon.
Meanwhile surface low developing north of the Bahamas, visible
as a swirl on satellite imagery, and its associated moisture
will continue moving north today. The interaction between the 5h
low over the southeast and this low/moisture will be key to the
magnitude of the afternoon/evening/overnight rain event.

The latest satellite imagery suggests at least a portion of
this moisture will be pulled in by the 5h low. The dry slot that
had been prominent on water vapor has been eroding in recent
imagery. The 12Z NAM does not depict a lot of interaction
between these features and has the bulk of the heavy rain
ending around 00Z. However, looking at the 15Z location of the
surface low north of the Bahamas in the NAM compared with
reality suggests the NAM is too quick with the movement of the
surface low. This suggests a better chance the low and its
moisture will have at least some interaction with the system
aloft. Other than tweaking the first few hours of the forecast
for latest radar/satellite trends have not made a lot of changes
to the forecast.

Previous discussion from 330 AM Monday follows:
Developing situation tonight may be more typical of mid- fall
than late- spring, but is certainly a classic setup for a heavy
rain event (think Oct 2015, Sep 2010) locally. While we are not
expecting 10-20" of rain like the 2 events mentioned, a 24-hr
period of very heavy rainfall begins now, and widespread 3-6" of
rain is being forecast, with locally heavier amounts probable.

Anomalously deep upper low currently across AL will dig SE
through this aftn before slowly lifting north to be off the SC
coast by Tuesday morning. At the surface, a weak low pressure
over SE GA will drift north across the eastern Carolinas through
today, while a secondary but very important low off FL will be
captured and absorbed into the primary low tonight.
Additionally, a weak warm front/coastal trough aligned from the
primary surface low to off the Grand Strand will waver and lift
slowly NW today.

So how does this all come together to create a flash flood threat
today? As the 500mb low pivots SE, deepens, and begins to tilt
negatively, upper diffluence increases both due to winds exiting
the upper speed max, and directional ventilation NE of the
upper feature. Beneath that, the primary surface low will aid in
creating lift, while increasing 850mb and 700mb LLJ transports
moisture from the GOM and, becoming more dominant with time, the
Atlantic Ocean. The 850mb LLJ dominates through today as strong
WAA develops on anomalously strong easterly winds, further
enhancing lift, while confluence will likely drive a pivoting
band of increasingly heavy rainfall across the region (similar
to Oct 2015). This easterly jet becomes even more important as
it pulls the surface low near FL and its associated tropical-
esque moisture (PWATs to 1.75 inches, would break the record
high for the date) into the Carolinas, and crosses orthogonally
the coastal front. A strong easterly LLJ funneling deep tropical
moisture atop a coastal front is a clear heavy rain signal
(like Sep 2010), and this will be even further aided by MLCape
of 300-500 J/kg as mid-level lapse rates steepen beneath height
falls in advance of the upper low.

While true convective rainfall will likely be confined to
areas south, along, or immediately north of the coastal front,
elevated instability will allow for heavy rainfall to continue
even well north of the boundary. This will become increasingly
the case late today and tonight, and the heaviest rainfall is
likely from this aftn through the overnight hours. While a
pivoting band of heavy rainfall will likely be the primary rain
maker today, increasing coverage of showers and tstms will
develop across the area, and have carried categorical POP for
all locations. Total QPF will range from 3 to 6 inches, with
locally higher amounts probable especially just NW of the
coastal trough, or in any training convective elements. Our
soils are dry, and fuels will absorb a lot of the rain, but the
flash flood watch remains in place as extremely heavy rainfall
rates at times will produce localized flooding.

If that isn`t enough, there remains a non-zero severe threat,
especially tonight, as shear profiles suggest the chance for
storm organization. With limited instability do not anticipate
much of a wind threat, and hail chances are even smaller, but
high helicity as noted by strongly curved hodographs could
support an isolated tornado as elements cross the coastal front.
The total severe threat is low however, and flooding remains by
far the primary concern.

The upper low starts to fill and lift NE on Tuesday, causing
the surface low to weaken as it becomes a vertically stacked
system. Drier air begins to advect from the W/SW behind this,
and most of the heavy rain will be out of the area by late
morning Tuesday. With 500mb cold core of -14C to -17C moving
overhead Tuesday, diurnal showers are possible through the day,
and have carried SCHC POP for this potential. However, do not
expect these to be heavy and should be scattered at worst, and
will fade with loss of heating in the evening.

High temperatures today will be warm only south of the coastal
front, rising into the low to mid 70s along the SC coast, but
only reaching the upper 60s inland. Highs on Tuesday will be a
more uniform low 70s, although the slow erosion of cloud cover
may keep highs a bit cooler inland. Mins tonight will range from
the upper 50s well inland, to the low 60s near the coast, just
slightly above normal in what is an otherwise very abnormal
pattern.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
As of 330 AM Sunday...No POPs this period. The FA will
finally have a chance to dry out this period. The closed upper
low over northeast NC at the start of this period, will lift
north- northeast, further away from the ILM CWA, during this
period. Residual clouds early Tuesday night, will clear out by
Wednesday morning leaving only diurnal cu to affect the FA on
Wed. Ridging aloft will become the dominate upper feature
affecting the FA Wed thru Wed night and the accompanying
subsidence aloft will keep a lid on any convection that tries to
develop during Wed. Sfc ridging extending from well off the SE
U.S. coast will temporarily ridge into the area Wed and Wed
night. Will need to monitor for possible fog both Tue night and
Wed night given plenty of standing water likely due to the heavy
rains during the near term period.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 300 PM Sunday...The mid level pattern will shift
initially into broad troughing across the central U.S. to a more
amplified version of this pattern by the end of the period.
With very strong ridging expected to develop across the
southeast by the end of the period, expect dry and very warm
conditions to develop. At the surface an elongated cold front
associated with the initial broad trough makes a run to the east
but quickly loses steam and never makes it into the area and in
fact remains well to the northwest. Beyond this its all Bermuda
High pressure. Temperatures, above normal for the start of the
period quickly rise to close to ten degrees above average and
the possibility of some highs into the 90s inland.

&&

.AVIATION /14Z MONDAY THROUGH FRIDAY/...
As of 12Z...The wavering coastal front lies along the immediate
coast from just south of Bald Head Island southwest and onshore
in the vicinity of Cape Romain. Not much inland movement if at
all, is expected with this front. Overall, winds from now into
early this evening will hold from the NE to ENE north of the
front, and from the ESE to SE south of this front. The coastal
terminals may see gusts up to 20 kt depending how much into the
warm sector wavers northward.

Will see convective type pcpn develop over the atlantic waters
and push northward thruout the day. Subtropical plumes of
moisture from the Atlantic may result in training of pcpn across
southeast NC and Northeast SC and across local terminals. As the
pcpn moves north of the coastal front, pcpn will transition to a
more stratiform look to it with embedded thunder. Have
basically kept IFR conditions thruout all terminals with tempo
groups to identify any thunderstorm activity and possible LIFR
conditions. Some improvement to occur during the 06Z-12Z
timeframe as the closed low moves overhead shunting the
moisture plumes east of the ILM CWA.

Extended outlook...Periods of MVFR and IFR through early Tue in
heavy rain, thunderstorms, and low stratus. IFR or lower
conditions possible in fog and stratus Tue night and early Wed.

&&

.MARINE...
NEAR TERM /THROUGH TUESDAY/...
As of 11 AM Monday...Southerly flow will continue increasing,
peaking at 20 to 25 kt this afternoon and evening as weak
surface low consolidates over southeastern SC and slowly lifts
north. Combination of enhanced wind speeds and onshore flow will
keep seas over 6 ft into tonight. Minimal changes for the
morning update.

Previous discussion from 330 AM Monday follows:
Coastal front will push inland today as SE winds increase from
10-15 kts this morning, to 15-25 kts this aftn and tonight. A
surface low pressure will drift from SE GA up to the Cape Fear
region tonight, and then weaken across the Outer Banks Tuesday.
This will cause winds to shift from SE to SW overnight into
Tuesday, and then veer further to the West late on Tuesday.
Speeds during this time may be highly variable depending on the
exact placement of the surface low, but expect a general 10-15
kts once winds turn around to the SW.

Although these winds only briefly reach SCA thresholds, the
ongoing SCA remains unchanged through 8am Tuesday. This is
due primarily for seas rising from 3-5 ft this morning, to 4-8
ft tonight, with a 7-8 sec SE wave group dominant the spectrum.
On Tuesday as the winds shift SW to W, the SE wave deamplifies
while the period lengthens, and a 6 sec SW wave develops. This
will allow wave heights to fall back to 3-5 ft and any headlines
should be allowed to expire on time Tuesday morning.

There is also likely to be widespread showers and tstms today
and tonight, with torrential rainfall creating severely
restricted visibility at times.

SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
As of 330 AM Monday...This period will be highlighted with the
closed upper low and it`s associated sfc low, lifting north-
northeast, pulling further away from the local waters. An
offshore wind will highlight Tue night into early Wed. During
Wed thru Wed night, sfc ridging from well off the Southeast U.S.
coast will extend inland just south of the local waters. This
will result in a S to SW wind regime for the end of this period.
The sfc pg remains weak or somewhat relaxed thru this period
with speeds generally 15 kt or less. Significant seas will peak
at the start of this period, followed by a slow subsiding trend.
The east-southeast ground swell at 8 to 9 second periods will
dominate the significant seas. Could see near shore locally
produced wind waves due to the sea breeze circulation during Wed
aftn thru early evening.

LONG TERM /THURSDAY THROUGH FRIDAY/...
As of 300 PM Sunday...Departing low pressure to the northeast
and troughing in the central U.S. will leave a weak pressure
gradient in place through Friday. For Thursday and Friday
expect weak south to southwest flow as the overall flow mimics
a summertime pattern. Significant seas will trend in a similar
summertime pattern direction with 1-3 feet both Thursday and
Friday.

&&

.ILM WATCHES/WARNINGS/ADVISORIES...
SC...Flash Flood Watch through Tuesday morning for SCZ017-023-024-
     032-033-039-053>056.
NC...Flash Flood Watch through Tuesday morning for NCZ087-096-099-
     105>110.
MARINE...Small Craft Advisory until midnight EDT tonight for AMZ254-256.
     Small Craft Advisory until 8 AM EDT Tuesday for AMZ250-252.

&&

$$
SYNOPSIS...DCH
NEAR TERM...III
SHORT TERM...DCH
LONG TERM...SHK
AVIATION...DCH
MARINE...



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