National Weather Service United States Department of Commerce

 


000
FXUS62 KILM 240201
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
1001 PM EDT Mon Apr 23 2018

.SYNOPSIS...
A slow moving storm system will bring wet weather into Tuesday,
along with windy conditions, dangerous surf, and coastal
flooding. Easing winds and a warming trend will follow Tuesday
and on Wednesday in the wake of this system. Low pressure
emerging from the Gulf states will bring rain chances Thursday
into Friday. Drying and seasonable temperatures are expected
Saturday and into early next week.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 900 PM Monday...A Wind Advisory along the coast remains
in effect through 5 am. Several beach/coastal hazards remain in
effect overnight and into Tue as well.

The low level jet will increase to 50+ kt down to 800 ft above
the ground overnight. The strongest winds are expected within a
few hours either side of 1 AM, but may linger across the Cape
Fear area until mid morning Tue. These strong onshore winds will
help focus moisture north of a warm front, increasing the very
moist isentropic upglide. Also, given the strength of these
winds just off the surface, we expect surface wind gusts to 35
to 40 mph, strongest along the coast with the beaches perhaps
recording a few wind gusts to 45 mph.

Bowling ball type upper low clearly evident 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
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 cloud tops continue to cool in moisture
plume just offshore. As the flow becomes strongly onshore and
convergence becomes maximized, a period of potentially very
heavy rainfall is likely as the plume becomes directed toward
the Carolinas. Locally, the setup looks similar to October,
2015, of course that was a 4 day event and this will last 12
hours.

As a 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+
kt below 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, 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 no 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.
12-hr FFG guidance is 5-6", so no Flash Flood Watch is needed
and any minor flooding that does occur will be of the low-lying
and 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 afternoon.
Guidance is hinting that enough forcing and residual moisture
will re-spawn some showers and maybe a tstm Tuesday afternoon,
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. Winds will ease slowly on Tuesday, but
will remain gusty through the aftn.

Temps will not change much through the night and lows are
expected to be in the upper 50s and lower 60s. Increasing
sunshine during Tuesday and the arrival of the aftn warm sector
will drive highs 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 300 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 /00Z TUESDAY 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 900 PM Monday...A Gale Warning is in effect. Dangerous
marine conditions overnight into Tue as strong low pressure and
its associated frontal system impacts the waters. Surface low
across GA will lift NE overnight into Tuesday, while high
pressure near New England slowly retreats. The gradient between
these two features will produce a pinched gradient, resulting in
wind speeds of 20 to 30 kt with gusts to around 40 kt. Seas will
peak in the 8 to 12 ft range and as high as 13 or 14 ft. The
wind direction overnight will be SE. Widespread showers and a
few thunderstorms into Tue morning will result in poor visibility
due to heavy rain.

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 kt with higher gusts. A
Small Craft Advisory will be required once the Gale has eased.
Seas will be subsiding during Tue, lastly across the northern
waters. Seas may not drop below 6 ft until late Tue night or on
Wed. The wave period will remain around 9 seconds.

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.

&&

.TIDES/COASTAL FLOODING...
As of 900 PM Monday...A Coastal Flood Advisory is in effect for
minor coastal flooding around the time of high tide, both at
the beaches and for portions of the lower Cape Fear River to
include downtown Wilmington. Given the duration and overnight
increase in strength of the onshore winds as well as the
increasing rainfall rates, decided to go above the available
tidal predictions. We are expecting tidal departures on the
order of 1 to 1.5 ft and as much as 2 ft. Departures of this
magnitude would bring a brief period of minor coastal flooding
to the beaches around 300 AM and downtown Wilmington around 530
AM. Significant beach erosion is possible with the overnight
high tide as well.

&&

.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...RJD/JDW
SHORT TERM...TRA
LONG TERM...RAN
AVIATION...MRR
TIDES/COASTAL FLOODING...RJD