Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 232348
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
748 PM EDT Mon Apr 23 2018

.SYNOPSIS...
An approaching storm system will bring wet weather into Tuesday,
along with windy coastal conditions, dangerous surf, and
high beach tides. Easing winds and a warming trend will follow
into mid-week in wake of this system. Another low pressure emerging
from the Gulf states, will bring rain chances Thursday into
Friday. Drying and seasonable temperatures are expected Saturday
into early next week.

&&

NEAR TERM /THROUGH TUESDAY/...
As of 300 PM Monday...A messy and complex forecast developing for
the near term and the entire area is expected to see heavy rainfall
and gusty winds this evening through early on Tuesday.

Bowling ball type upper low clearly evident across on GOES-E WV
imagery across the TN VLY is above an occluded surface low. An
occluded front extends from the low towards secondary low-
development in eastern GA, from which a triple point exists with a
warm front and cold front into the Atlantic Ocean and Gulf of
Mexico, respectively. Ahead of this warm front, moisture is pooling
and increasing across the Atlantic, and as flow becomes more
strongly onshore this evening and convergence becomes maximized, a
period of potentially very heavy rainfall is likely. Locally, the
setup looks similar to October, 2015, of course that was a 4 day
event and this will last 12 hours.

Light rain has overspread the entire CWA and will continue
periodically through the evening. This rainfall is due to a
combination of isentropic lift atop the surface high extending down
from New England, the edge of the 850mb theta-e ridge, and weak
forcing due to mid-level disturbances embedded within the flow. The
next several hours will feature periods of rainfall with QPF less
than one-quarter inch. However, this is truly the `appetizer` before
the main event which is progged to occur overnight.

As the warm front lifts northward, moisture will surge into the
Carolinas in response to a strengthening gradient driving better
moist advection onshore, and an anomalously strong LLJ of 50+ kts at
850mb surging PWATs towards 1.7 inches. These winds are forecast to
reach nearly 4 standard deviations above the mean, which when
combined with +2 sigma PWAT, and warm cloud depth of over 3km
suggest an environment which will support very heavy rainfall.
Additionally, strong diffluence both in the mid levels thanks to the
closed nature of the low to the west, and within the LFQ of an upper
jet, as well as strong FGEN within the 850mb-700mb layer along the
warm front, and low-level convergence both along the front and due
to frictional effects onshore, help drive lift and strong UVVs
despite weak to none instability. As the warm front pivots NE
overnight, all of these other ingredients will accompany this
motion, and it is expected that a band of very heavy rainfall will
lift SW to NE across the CWA overnight. While lack of instability is
a mitigating factor, the other parameters all point towards
widespread 1-3" of rainfall with locally higher amounts, especially
closer to the coast. Fortunately, we have been dry, and 12-hr FFG
guidance is 5-6", so no Flash Flood Watch is needed and flooding
should be confined to short term poor drainage type.

The main axis of rainfall will lift NE out of the area by late
morning, and the region will briefly be within the warm sector ahead
of Tuesday`s cold front which will lift NE through the aftn.
Guidance is hinting that enough forcing and residual moisture will
re-spawn some showers and maybe a tstm Tuesday aftn, but very dry
air above 700mb by that time will limit the overall coverage and
threat. By Tuesday evening the rainfall should be ending across the
CWA.

Additionally, a wind advisory is in effect for the coastal counties
as winds will steadily increase this evening and even overnight as
the LLJ impinges on the region. Attm do not expect true advisory
criteria to be met (sustained 31-39 mph or gusts 46-57 mph), but
enough mixing within the PBL should create wind gusts of 30-40 mph,
with higher gusts possible near the coast. These winds are likely to
occur during the heaviest rain as well, so for the potential for
some downed trees in saturated soils /public impact/ have extended
the wind advisory for all coastal counties through 5am. While
typically winds ease at night, progs suggest the highest wind gusts
are actually expected overnight thanks to the LLJ, and have added
gusty winds into the wx grids as well. Winds will ease slowly on
Tuesday, but will remain gusty through the aftn.

Temps have not warmed as much as expected today due to enhanced
cloud cover and onshore flow, with most places stalling in the low
to mid 60s. For this same reason, temps will be slow to fall tonight
with only a few degrees of cooling into the upper 50s/low 60s
expected. Increasing sunshine and the aftn warm sector will drive
highs on Tuesday to much warmer values more typical of late April,
into the mid 70s away from the coast.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
As of 300 PM Monday...Upper level low pressure across the southern
Appalachians Tuesday night will open up and lift northeastward
Wednesday as a pair of strong shortwaves approach from the north and
west. Surface low pressure will move northeastward and away from the
area in tandem with the weakening upper low, allowing sunshine to
return on Wednesday. Highs on Wednesday should reach the upper 70s
to near 80 degrees away from the beaches. These warm temperatures in
combination with dewpoints hanging in the 50s should develop a
substantial field of cumulus clouds during the day.

While across central and northern North Carolina colder mid-level
temperatures should allow these cumulus to grow deeper and produce
showers, a subsidence inversion between 7000-10000 feet AGL that
becomes stronger the farther south you go should prevent showers
from developing locally. I`ve got 10% PoPs across Lumberton,
Elizabethtown and Wilmington during the day Wednesday but honestly
expect most convective showers to remain farther north.

As the departing surface low deepens as it moves from NJ into New
England Wednesday night, a cold front will get a renewed shove
southward and should move through the Carolinas  and off the coast
overnight. The atmosphere should remain stable for this frontal
passage with insufficient moisture for any showers.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
As of 3 PM Monday...Another area of low pressure may bring showers
to the area Thursday into Friday.  Otherwise, the long term looks
dry with temperatures near or slightly below seasonal normals.
Normals for this time of year run mid to upper 70s by day and mid
50s overnight.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
As of 00Z...Flight conditions will deteriorate with IFR likely.
Very heavy rain and possibly an isolated thunderstorm will occur
generally through 06-09Z, possibly through 12Z at KILM/KLBT.
Thunderstorms too isolated to mention in TAF. Easterly winds will be
quite strong with gusts 25 kt possible. As winds veer to the
southeast overnight, wind speeds are expected to decrease. Southeast
winds should become south as Tue morning progresses. IFR cigs should
improve to VFR 16-18Z.

Extended outlook...Tempo MVFR Thu night through Fri night. otherwise
VFR.

&&

.MARINE...
NEAR TERM /THROUGH TUESDAY/...
As of 300 PM Monday...Poor marine conditions forecast through the
period as a strong low pressure system impacts the waters. Surface
low developing across GA is draping a warm front and cold front into
the SE. This entire system will lift NE overnight into Tuesday,
while high pressure near New England slowly retreats. The gradient
between the two becomes increasingly pinched, and winds will climb
steadily this evening and overnight, becoming 20-30 kts from the
E/SE with gusts to 40 kts. A Gale Warning is in effect and will
persist into Tuesday aftn. During Tuesday conditions will slowly
improve with winds veering from SE to SW by the end of the period as
the cold front lifts across the waters. During this wind shift
speeds will gradually lower as well, becoming 15-20 kts with higher
gusts, and an SCA will likely be needed once the GLW drops. In
addition to these strong winds, a rapidly amplifying sea state is
forecast as waves build to as high as 13ft/9sec at Frying Pan Shoals
overnight. This type of swell will push coastal seas from the
current 4-8 ft, to 8-12 ft overnight, before slowly easing back to 4-
8 ft by the end of the period.

Additionally, a period of very heavy rainfall is likely across the
waters overnight. This rain will produce severely reduced visibility
to 1NM or less at times, in addition to chaotic and stronger winds.
The most likely time for these poor conditions will be overnight,
improving on Tuesday, and although lightning should not be
widespread, isolated cloud to sea lightning strikes are also
possible overnight.

SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
As of 300 PM Monday...Low pressure centered near Raleigh Tuesday
evening should drift northeastward across the Chesapeake Bay
Wednesday morning, then into New England Wednesday night. A cold
front behind the low will initially be very slow to move south, but
should get a renewed kick Wednesday night as the low deepens.
Compared to winds coming up tonight and Tuesday, winds Tuesday night
through Wednesday night should be a piece of cake: 15 knots from the
southwest Tuesday evening, then around 15 knots again with the
seabreeze on Wednesday afternoon. Wind directions should veer
northerly by late Wednesday night as the cold front moves across the
area.

Despite diminishing wind speeds, large seas will remain with us
through at least Wednesday morning, a consequence of the storminess
along the Southeast coast today through Tuesday. Nine-second
southeast swell as large as 8 feet will move through the coastal
waters Tuesday evening before diminishing to 3-6 feet Wednesday
morning. We`ll likely need to keep a Small Craft Advisory running
through Wednesday morning.

LONG TERM /THURSDAY THROUGH SATURDAY/...
As of 300 PM Monday...Expect shifting winds over the waters
Thursday and Thursday night with low pressure moving through.
NE winds Thu AM will become SE by Thu night and SW on Friday.
Speeds currently look to run about 15 KT but this could change
depending on how this next low progresses.  W to SW winds of
10 to 15 KT are expected for the remainder of the period.  Seas
of 3 to 4 FT THU will subside to 2 to 3 FT Friday and Saturday.

&&

&&

.ILM WATCHES/WARNINGS/ADVISORIES...
SC...Wind Advisory until 5 AM EDT Tuesday for SCZ054>056-058-059.
     Coastal Flood Advisory from 1 AM to 5 AM EDT Tuesday for
     SCZ054-056.
     High Rip Current Risk until 6 AM EDT Tuesday for SCZ054-056.
NC...Wind Advisory until 5 AM EDT Tuesday for NCZ105>110.
     High Rip Current Risk through Tuesday evening for NCZ106-108-
     110.
     Coastal Flood Advisory from 1 AM to 5 AM EDT Tuesday for
     NCZ106-108-110.
     Coastal Flood Advisory from 4 AM to 8 AM EDT Tuesday for
     NCZ107.
     Beach Hazards Statement until 6 AM EDT Tuesday for NCZ110.
MARINE...Gale Warning until noon EDT Tuesday for AMZ250-252-254-256.

&&

$$

NEAR TERM...JDW
SHORT TERM...TRA
LONG TERM...RAN
AVIATION...MRR


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