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AGNT40 KWNM 240238

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1038 PM EDT Wed Aug 23 2017

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

Gale force winds are possible over NT2 waters starting Sunday
mainly due to strengthening low pressure that will develop over
the southern waters. Satellite images show fairly cloudy skies
mostly over the central waters and the lightning desnity map has
a band of dense lightning associated with these clouds. The SREF
model show up to PROB 30 for severe TSTMS threat over the waters
with highest threat inland. Most of the TSTMS are in the vicinty
of a cold front that stretches southwest from low pressure over
Canada and lies across the north and central waters. NCEP weather
map also has high pressure 1023 MB to the east near 37N46W that
has its ridge into the eastern portion of the region. Pressure
gradient is fairly relaxed across the region and observed winds
range between 10 and 20 kt with maximum at 25 kt over the far
northeast portion.

Models GFS/CMC/ECMWFHR/UKMETHR/NAM have generally initialized
well the 00Z surface observations with just minor differences
mainly on just the actual position of the cold front over the
waters. The models, though in generally good agreement in the
short term, have some differences on low pressure developing over
the southern waters. As such, will use GFS at first and then
continue with a blend of UKMET and ECMWFHR saturday through
Sunday then switch to UKMETHR through the end of forecast period.

Seas are equally small with relatively higher values over the
north waters with a peak at 9 ft. Seas range between 2 and 5 ft
elsewhere. NWWs and ECMWFWAVE fit the observed seas pattern quite
well and both models agree on keeping seas below 8 ft through
Saturday and then build seas to above 8 ft over the southern
waters in region with gale force winds. Seas will build also over
the southern portion of the noth waters in the extended period.


At 18Z the cold front extended across the Gulf of Maine and
southwest through the northern NT2 waters and inland near the
Virginia tidewater area. This mornings offshore Ascat overpasses
returned southerly winds to 20 kt east of the front. However,
there may have been slightly higher winds across the waters east
of 68W/69W where the Ascat missed. Over the next couple days the
12Z models are consistent in forecasting this cold front will
continue to slowly move southeast across the waters with at
least a pair of weak lows developing and moving northeast along
the front. The 12Z ECMWF/UKMET/GFS all indicate winds briefly
reaching 25 kt across the northern outer NT2 waters with these
features Thu into early Fri. We prefer how the ECMWF handles
these frontal waves, so will be using the 12Z ECMWF as a basis
for the wind grids through Fri, although there are only minor
differences between the ECMWF and the GFS/UKMET.

Over the last day or so, the models have generally trended
slower and weaker with the low forecast to develop off the
central/ northeast Florida coast this weekend and slowly move
northward and northeastward late this weekend through early next
week. Also, whereas earlier runs hinted that a tropical cyclone
could develop, the most recent runs unanimously indicate the low
will be baroclinic. With the strong high pressure ridge
persisting across New England, the developing low will not need
to deepen significantly to get some northeast gale force winds
across the southern and possibly central NT2 waters, especially
across the Gulf Stream, late in the weekend into next week.
Versus its previous runs, the 12Z ECMWF trended slower and does
not move the developing low into the NT2 waters until day6/Tue.
OPC preference is with the faster timing of the 12Z GFS/UKMET.
We used a blend of the 12Z UKMET and 00Z ECMWF for the wind
grids Sat through Sun night, before transitioning to the 12Z
UKMET Mon/Mon night. Then adjusted the northeast winds across
the southern NT2 waters upward to account for the unstable low
levels across the Gulf Stream as well as for the tendency for
the models to be underdone in these situations. Will also
introduce northeast gales across most of the southern/central
NT2 zones late Sun through Mon night, with slightly above
average forecast confidence.

.Seas...The 12Z Wavewatch III and 12Z ECMWF WAM are in decent
agreement through Sat, and will use a 50/50 blend for the wind
grids. Thereafter, will transition to the higher ECMWF WAM, and
will further boost its wave heights by 15 to 20 percent over the
areas with the higher northeast winds, mainly the offshore
waters south of Hatteras Canyon.

.Extratropical Storm Surge Guidance...The persistently strong
northeast winds off the southeast coast will also likely create
at least a moderate surge event, with the 12Z ESTOFS showing
surge close to 2 ft from near Cape Lookout to the northeast
Florida coast Sun through Mon night. With the ETSS and P-ETSS
limited to 96 and 102 hours, respectively, at this time only the
ESTOFS guidance captures the highest surge.


.NT1 New England Waters...

.NT2 Mid-Atlantic Waters...
.ANZ828...Inner Waters from Cape Charles Light to Currituck Beach Light...
     Gale Possible Monday night.
.ANZ925...Outer Waters from Baltimore Canyon to Hatteras Canyon...
     Gale Possible Monday night.
.ANZ830...Inner Waters from Currituck Beach Light to Cape Hatteras...
     Gale Possible Monday into Monday night.
.ANZ833...Inner Waters from Cape Hatteras to Cape Fear...
     Gale Possible Sunday into Monday night.
.ANZ930...Outer Waters from Hatteras Canyon to Cape Fear...
     Gale Possible Sunday into Monday night.
.ANZ835...Inner Waters from Cape Fear to 31N...
     Gale Possible Sunday into Monday night.
.ANZ935...Outer Waters from Cape Fear to 31N...
     Gale Possible Sunday into Monday.


.Forecaster Musonda/Clark. Ocean Prediction Center. is the U.S. government's official web portal to all federal, state and local government web resources and services.