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AGNT40 KWNM 232023
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
423 PM EDT Mon Apr 23 2018

.FORECAST DISCUSSION...Major features/winds/seas/significant
.weather for the North Atlantic Ocean W of 50W from 30N to 50N.

The latest infrared satellite imagery showed cold cloud tops over
the southern parts of ANZ835, and although the best lightning
strikes were confined just south of that zone in AMZ111, ASCAT
data available in AWIPS showed some gale-force winds in the
southern part of ANZ835 as of 15UTC. Low pressure over the
southeast U.S. will be slow to move northeast with at least a
modest amplitude flow over the waters tonight into Tuesday as the
upper ridge axis will be overhead later tonight, with a closed
low over the Tennessee Valley. However, as the ridge axis moves
east, surface low pressure will slowly make headway northeast.
The threat of gales will spread gradually northeast with the low,
and for this forecast have changed headlines little. Will opt to
add gales into ANZ815 and ANZ825, and though it is close, will
leave out of ANZ820 for now. Coordinated with WFO Mt. Holly, and
while it may need to be added later, not confident enough in that
zone for this forecast issuance.

In later parts of the forecast, elevated instability is high
even as far north as ANZ805 and ANZ900 late Wednesday into
Wednesday night. Leaning toward the slightly slower ECMWF as
opposed to the GFS, particularly with the amplitude of the
pattern in the mid levels, have delayed the onset of gales a few
hours in ANZ905 and will leave out any mention of gales in ANZ805
and ANZ900. They may need to be added later there, too,
especially if the slightly more unstable GFS model soundings
verify, but given the very stable boundary layer of the ECMWF it
is possible any organized convection remains elevated keeping the
higher sustained winds aloft.

The latest SPC mesoanalysis shows the best downdraft CAPE just
offshore of the Georgia and South Carolina coasts, diminishing
quickly offshore. However, as low pressure slowly moves northeast
along with the surface warm front, downdraft CAPE gradually
creeps north. For the upcoming forecast, plan to mention the
potential for higher gusts in an near thunderstorms over ANZ835
tonight, and for the far southern four zones for Tuesday. The
high-resolution ECMWF does attempt to resolve some robust deep
convection there with pixels of wind around 50 knots. In a
sustained sense think gales cover things well, but mariners who
may be in the midst of storms off the Carolina coast tonight
through Tuesday should be prepared for wind gusts at or even
above 50 knots. Earlier today, in coordination with WFO
Charleston, SC, there were some convective wind gusts well above
60 mph off of the Georgia coast.

Low pressure moves northeast away from New England Thursday, and
the threat for hazards in the offshore waters diminishes Thursday
morning. From Thursday afternoon through the rest of the seven-
day period, will have no wind hazards in the forecast. The UKMET
has substantially backed off the potential for any gales
Saturday, and the ECMWF and the GFS continue to advertise none.
As surface waves move northeast, again off the southeast coast,
Saturday and Saturday night, winds should remain below gales
given the weakness of the trough. However, behind the low, in
some cold-air advection and as high pressure builds southeast
tightening the gradient, gales could develop just east and
southeast of the mid-Atlantic offshore waters late Sunday into
Monday. For now, will keep winds in the digital forecast just
below hazards especially in ANZ905, ANZ910 and ANZ920 at the end
of the period.

In summary, leaned toward the adjusted ECMWF winds tonight
through Thursday. Then followed the GFS winds adjusted for
stability from Thursday night onward. Thanks to WFOs CHS and PHI
for the collaboration today.

.SEAS...As with the winds above, leaned toward the ECMWF WAM wave
guidance for tonight through Thursday, and the Wavewatch from
Thursday night onward. Where winds are highest, especially in
vicinity of the Gulf Stream off of the mid-Atlantic and Carolina
coasts associated with deep convection, waves could reach the
lower 20s feet. Waves subside into the single digits feet
throughout the OPC offshore waters by Friday morning, returning
to around ten feet by late Sunday in the eastern mid-Atlantic
offshore zones.

.EXTRATROPICAL STORM SURGE GUIDANCE...The ETSS, and a little more
so the ESTOFS, provide for positive surge from the southern North
Carolina coast to a small part of the Georgia coast in the range
of 1.5 feet, possibly to 2 feet in local area, overnight into
Tuesday. Highest on both guidance will be overnight in areas near
the South Carolina and North Carolina border. Monitor the latest
information from coastal NWS offices in that area.

.WARNINGS...Preliminary.

.NT1 New England Waters...
.ANZ810...South of New England...
     Gale Wednesday.
     Gale Possible Wednesday night.
.ANZ815...South of Long Island...
     Gale Wednesday.

.NT2 Mid-Atlantic Waters...
.ANZ915...Hudson Canyon to the Great South Channel...
     Gale Wednesday.
     Gale Possible Wednesday night.
.ANZ920...Baltimore Canyon to the Great South Channel...
     Gale Tuesday night into Wednesday.
     Gale Possible Wednesday night.
.ANZ905...The Great South Channel to the Hague Line...
     Gale Possible Wednesday night into Thursday.
.ANZ910...East of the Great South Channel and south of 39N...
     Gale Wednesday.
     Gale Possible Wednesday night.
.ANZ825...Inner Waters from Baltimore Canyon to Cape Charles Light...
     Gale Tuesday night.
.ANZ828...Inner Waters from Cape Charles Light to Currituck Beach Light...
     Gale Tuesday into Tuesday night.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Tuesday into Wednesday.
.ANZ830...Inner Waters from Currituck Beach Light to Cape Hatteras...
     Gale Tuesday into Tuesday night.
.ANZ833...Inner Waters from Cape Hatteras to Cape Fear...
     Gale tonight into Tuesday night.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale tonight into Wednesday.
.ANZ835...Inner Waters from Cape Fear to 31N...
     Gale today into Tuesday.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale tonight into Tuesday night.

$$

.Forecaster Figurskey. Ocean Prediction Center.



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