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000
AGNT40 KWNM 250200
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1000 PM EDT Tue Apr 24 2018

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

Complex low pressure over western N. Carolina and northern S.
Carolina will move NE tonight, reach the Delmarva Wed, southern
New England Wed night, and Maine on Thu. A cold front extends S
from near Cape Fear, with a stationary front extending SE from
this same area per the preliminary 00Z OPC-NCEP surface map.
Observations this evening indicate gales occurring over offshore
waters E and NE and also SE of Cape Hatteras. These gales will
likely lift NE as the low moves N and NE over the next day or
two, overspreading northern NT2 into parts of the southern NT1
or New England waters Wed into Wed night. Overall, no major
changes to the previous forecast appear needed for the evening
update and we will make only minor changes to adjust the
forecast for initial conditions and also for areas closer to the
nearby coastal WFO forecast waters and TAFB waters.

Also, for the evening update we will also attempt to highlight
the potential for stronger winds, likely exceeding gale force,
in and near thunderstorms which are currently noted by the
latest GOES-16 IR imagery and lightning density data from about
80 nm E of Cape Hatteras S and SW over the offshore waters. The
latest SREF thunderstorm guidance suggests the potential for
thunderstorms spreading further to the N and NE later tonight
through Wed, finally moving E of the area later Wed night or
early Thu, with the highest chances from Long Island southward on
Wed, moving toward the outer offshore waters Wed night.

Seas...Sea heights appear to be running very close to the
previous forecast grids with maximum seas near 21 feet off the
N. Carolina coast per the 00Z RA1 OPC sea state analysis. For
the evening update, we will not make major changes to the
previous grids and forecast. We will manually adjust grids
slightly closer to the adjacent coastal WFO waters and TAFB
waters over the next few days to create a more seamless sea
height grid/forecast.

-----------------------------------------------------------------
PREVIOUS DISCUSSION...

Impressive visible satellite images from Goes-16 this afternoon
reveal robust storm system currently centered inland over
northern SC, pulling its associated strong frontal boundaries
north across the southern OPC waters. Poleward of the warm front,
widespread E-SE onshore gales continue, confirmed by ASCAT
MetOp-A retrievals just after 15z. Strong 25-30 kt winds extend
NE from the area of gales, directly in line with the warmer and
less stable waters atop the Gulf Stream. Sporadic convection
also continues to fire over OPC waters, evidenced in a ship ob
about 75 nm SSE of Cape Lookout that reported winds to 50 kt at
18z. This was coincident with an area of deep convection at the
time. As mentioned previously, and as seen above, gusts to storm
force remain possible in or near any thunderstorms as the fronts
lift slowly N and NE overnight. Ongoing grids looked to be
representative of conditions throughout the day, and to best
incorporate some of the new guidance and timing into the
forecast, plan to blend in the new - but boosted - 12z ECMWF
wind guidance in a 1:1 ratio with ongoing grids through about
12z Thu. Realistically this will not change hazards too
drastically. There still remains a decent enough possibility that
deep convection could provide enough mixing for gales over the
stable inner waters 815/810, so will continue to include
headlines. Gale will end SW to NE throughout Wed and Wed night,
finally exiting the outer waters ANZ905 by 15z Thu.

Another series of shortwaves move across the W Atlantic Fri and
through the weekend, with subtle timing differences in passing
surface reflections and weak frontal boundaries. With the GFS
bouncing between slow and fast solutions, and the UKMET
developing a significant low pressure across the mid-Atlantic,
will opt to lean closer to the more consistent ECMWF in the
latter stages of the medium range. However to better account for
the slightly slower progression of systems in the 00z EPS and 12z
GEFS means, plan to time shift the grids about 3 hours slower.
Hazard level winds are not expected after Thu.

Seas: 12z ECMWF WAM initialized seas much closer to reality
based on ongoing buoy and ship observations. Most notably, ship
ob above reported seas to 23 feet at 18z, where the WAM had 21
feet, and the WW3 output 16 feet. Plan to use the WAM for the
duration of the forecast period ending Sun night across the W
Atlantic. Seas expected to subside to generally 8 feet or less
after 12z Fri.

Extratropical Storm Surge Guidance: 12z ESTOFS outputs higher
surge values compared to the 12z ETSS along the Eastern Seaboard
as low pressure lifts north along the coast. With persistent
strong onshore flow, would tend to favor the higher values that
approach about 1 foot in ESTOFS guidance. These may prove
slightly underdone with GFS 10m winds too low compared to ASCAT
winds.



.WARNINGS...Preliminary.

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

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

$$

.Forecaster Mills/Collins. Ocean Prediction Center.


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