Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 230829

Area Forecast Discussion
National Weather Service Wilmington NC
429 AM EDT Sun Apr 23 2017

A cold front will stall across the area today. A complex and
slow moving storm system will bring the potential for flooding
through Monday night. There is a small risk for severe weather
as well. This system will lift away from the area Tuesday with a
return to dry weather and above normal temperatures mid and late


As of 300 AM Sunday...Complex situation begins to unfold today
leading to what will likely be a widespread heavy rainfall event
beginning late tonight. A flash flood watch has been issued
beginning at midnight.

Cold front has sagged into the CWA and is currently nearly along the
SC/NC state line. This front will continue to slip southward into
the morning hours before stalling across our SC zones. The front
will then likely begin to lift back to the north slowly in response
to shortwave ridging ahead of a deepening mid-level closed low
dropping into GA. It is this feature that will primarily be
responsible for the deteriorating weather through Tuesday.

Initially, as the front wavers in the vicinity, it will create a
complex temperature and wind forecast locally. Locations that remain
north of the front today will be cloudy and cool, with highs only
around 70, while areas south of the boundary will warm into the low
80s with more sunshine. This tight gradient just across the CWA
leads to bust potential if the front moves differently than
currently forecast, but guidance is in pretty good agreement that
only the southern few counties will warm much today. At the same
time, the aforementioned upper low will deepen and shift SE from
western TN this morning into SE GA tonight. Ahead of this, S/SW flow
will intensify while diffluence aloft increases. Although it is late
April, this sets up an isentropic lift scenario, and rain showers
will gradually increase across the area from late morning through
early evening. While isentropic lift is usually more supportive of
stratiform rain than convective showers, enough elevated (surface)
instability will exist north (south) of the front for isolated tstms
as well. Total QPF into tonight is expected to be light however.

Things deteriorate rapidly thereafter, and the flood watch begins at
midnight. As the upper low deepens and drops further SE, LLJ at both
850mb and 700mb increase from the south causing column saturation
and PWATS rising to 1.5 inches, near the highest all-time for the
date. Additionally, increasing confluence is noted within these
layers, and moist advection will occur both from the GOM and the
western Atlantic Ocean, in an environment characterized by
increasing MLCape to 300-500 J/kg which will support very heavy
rainfall rates. High-res guidance suggests a "wall" of heavy rain
with embedded tstms will move into the western counties late,
aligned closely with the best LLJ and moist advection. QPF through
Monday morning will range from up to 1.5 inches west of i-95, to
around 0.25 inches along the Cape Fear coast, with much more
expected beyond this period.

While a lower end threat than flooding, note very high helicity
values along the wavering front this evening and tonight approaching
300 m2/s2 in the lowest 1km, especially inland. With SBCape minimal
expect a very limited tornado threat, but would not be surprised to
see some rotating updrafts in any stronger convection tonight. SPC
has added a 2% tornado prob to their MRGL risk tonight across most
of SC, noting that if buoyancy can increase above current progs, the
tornado risk may be higher, especially in the presence of the
increasing LLJ late.


As of 300 AM Sunday...A very wet period Mon and Mon night with
widespread heavy rain and scattered thunderstorms necessitates a
Flash Flood Watch for our entire CWA. We are forecasting a
storm total of 3 to 6 inches of rain with the heaviest of
rainfall ending late Mon and Mon night. Although the ground is
dry and can absorb a great deal of runoff, it is the rainfall
rates that may result in flash flooding. Urbanized areas and
areas of poor drainage will be especially prone to flash
flooding. Rivers will see dramatic rises and many will exceed
bankfull during the week.

Deep low pressure will be located across Georgia Mon morning.
Slow moving low pressure is expected to move offshore Mon eve
and then turn up the Carolina coast, following a frontal boundary
that will be stretched along the coast. The center of the low should
be in the vicinity of Cape Fear Tue morning and Cape Hatteras Tue
eve. As the low approaches, it will draw deep moisture northward
and this moisture can be seen pooling N on satellite across
portions of Cuba, Florida and the Bahamas. Precipitable water
values will reach climatological maximums and they will remain
near these levels for an extended period of time given the slow
movement of the system. Isentropic lift will be strong with deep
moisture running up and over the frontal boundary. Mixed layer
CAPE values of 500 to 800 J/kg should support convective
elements which will be capable of dumping 2 or more inches of
rain in a short amount of time. Cells will tend to train along
the front which will increase the flood threat. In addition,
cold air aloft, 500 mb temps as low as minus 19, will skirt the
area on Mon. The deep moisture should preclude any glimpses of
sunshine which would further steepen lapse rates and lead to an
increased hail and wet microburst threat. At this time, the
threat from flooding is higher than the severe weather risk.
However, the severe weather risk is non-zero and SPC has
included the area in a marginal severe weather risk on Mon. Low
pressure will be pulling away from the area on Tue and drier air
will slowly work into the region on the backside, ending the
showers. Partial clearing will likely hold off until late Tue
or Tue night as weak ridging begins to build across the
Southeast states. Of course even partial clearing will foster
low clouds and stratus to fill in overnight Tue given the
saturated soils and light winds.


As of 300 PM Saturday...Fair weather is expected for the long
term period. Temperatures will be above normal.


As of 06Z...Moderate confidence forecast with IFR likely at the
start of the period, and possible again late tonight,
especially at the NC terminals.

Cold front sinking slowly southward has crossed LBT/ILM and will do
so at FLO shortly. This front may reach the Myrtles as well before
stalling, and any terminals that have FROPA overnight will also
experience a wind shift to the NE and at least TEMPO IFR
stratus. Cigs have been bouncing a bit upstream, so opted to
leave IFR at TEMPO with low-MVFR predominant, but may need AMD`s
if IFR settles in like the NAM soundings suggest. A few showers
are possible north of this front overnight as well, but most
rainfall will hold off until daybreak.

This front will waver in the vicinity during Sunday, creating a
complex ceiling forecast. Terminals that remain north of the front
may stay MVFR/IFR all aftn, while those that become south of the
boundary will likely lift to VFR. Attm expect only the Myrtles to
lift to VFR, with MVFR remaining elsewhere. Additionally, showers
and isolated tstms will become increasingly widespread, especially
Sunday night, with MVFR and possible IFR in any stronger convection.
Winds will fluctuate considerably as well, E/NE 10 kts or less north
of the front, and S/SE 10-15 kts south of the boundary. As showers
and tstms increase in coverage late in this valid period, even those
terminals that reach VFR will drop back to MVFR with cig

Extended outlook...Periods of MVFR and IFR through early
Tuesday in low stratus, showers, and thunderstorms. VFR


As of 300 AM Sunday...Cold front dropping south across the area this
morning will slow and eventually stall, creating a wind shift to the
NE with speeds increasing to 10-15 kts across only the NC waters
this morning. South of the boundary, southerly winds will remain
through the daylight hours, also around 10-15 kts. However, this
front will begin to lift back to the north tonight, moving onshore,
and turning winds around to the SE across all waters at speeds of 15-
20 kts with higher wind gusts. While current wind speeds are not
forecast to reach SCA thresholds, the combination of a 10 sec
easterly swell and an amplifying S/SE wind wave to 6ft/6sec will
drive seas up from 2-3 ft this morning to 4-6 ft late. This has
prompted an SCA to be issued, beginning at 10pm tonight and
persisting beyond the near term.

As of 300 AM Sunday...A Small Craft Advisory is in effect for
all waters through Mon and for the northern waters through Mon
night. Widespread poor visibility is expected in heavy rain and
strong to severe thunderstorms Mon and Mon night as low pressure
moves offshore of Georgia and then slowly up the Carolina coast.
The showers will end from S to N during Tue as low pressure along
the coast moves far enough to our N. Seas will be up to 4 to 7
ft Mon/Mon night. Seas will remain elevated on the backside of
the low Tue and Tue night with perhaps 5 and 6 ft seas lingering
across the northern waters. Winds Mon will be SE ahead of the
low, then the direction will become variable with low pressure
and a frontal boundary across the area. As the low pulls N of
our latitude, the wind direction will become NW late Mon night
and Tue with NW winds persisting through the end of the period.
Sustained winds will be no higher than 15 to 20 kt, but strong
to severe convection will likely produce higher winds.

As of 300 PM Saturday...Light south to southwest winds are
expected with seas 4 ft or less.


SC...Flash Flood Watch from late tonight through Tuesday morning
     for SCZ017-023-024-032-033-039-053>056.
NC...Flash Flood Watch from late tonight through Tuesday morning
     for NCZ087-096-099-105>110.
MARINE...Small Craft Advisory from 10 PM this evening to midnight EDT
     Monday night for AMZ254-256.
     Small Craft Advisory from 10 PM this evening to 8 AM EDT
     Tuesday for AMZ250-252.



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