372 
FXUS62 KILM 240816
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
416 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 330 AM Monday...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 /07Z MONDAY THROUGH FRIDAY/...
As of 06Z...Coastal front draped from near Orangeburg, SC to
off the Grand Strand coast is demarcating the IFR line tonight.
ALl terminals north of this boundary, including all the ILM 
terminals, are locked into IFR cigs, with MVFR to the south. The
wavering of this boundary will determine where IFR persists 
through the valid period, although increasing heavy rain showers
and thunderstorms will also bring IFR vsbys.

Expect all terminals to remain IFR through daybreak, with LIFR 
expected at FLO/LBT. As this front lifts slowly NW, winds at 
ILM/CRE/MYR will shift to the E/SE and may gust to 20 kts at
times, and will likely lift to MVFR except in vsby restrictions
during heavy rainfall. LBT/FLO will stay NW of the coastal 
front, and remain entrenched in IFR/LIFR through Tuesday 
morning. Have carried 2SM with +SHRA predominant for much of 
this TAF issuance at all terminals as showers and thunderstorms 
will become widespread and heavy by late morning. Any MVFR at 
the coast is expected to fall back to IFR Monday night.

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 330 AM Monday...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...JDW
SHORT TERM...DCH
LONG TERM...SHK
AVIATION...JDW