AGNT40 KWNM 181410
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1010 AM EDT Sat Aug 18 2018

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

A front over the northern NT1 waters will gradually move south
today and Sunday, becoming anchored east-northeast off of the
Virginia and North Carolina coasts Monday into early Tuesday
before starting to move gradually back north. Surface waves will
move along the front for the early part of the week. In the near
term, outside of thunderstorms, winds should be below hazards,
and the latest observations were in concert with the gridded and
worded forecasts, on average 10 to 20 knots across the offshore
waters. For this forecast, will adjust the winds up about 5 knots
mainly over ANZ835 based on the latest observations showing
winds from 15 to 20 knots, and will adjust the waves higher by
around a foot, to around 7 feet, east of the Virginia and North
Carolina coasts in vicinity of the Gulf Stream, again, based on
the latest observations. Coolest cloud tops on satellite imagery
were located in ANZ805 and ANZ900, and lightning displays showed
a scattering of strikes from ANZ910 southwest into ANZ835.
Tonight, by around midnight, the greatest concentration of storms
should exist, according to the HRRR WRF, from ANZ910 west-
southwest to the Virginia/North Carolina border on the southern
end of a positively-tilted upper trough and in vicinity of the
surface front.

By Wednesday, as surface low pressure deepens over the lower
Great Lakes and moves northeast across far northern New England
into the Canadian Maritimes Wednesday night and Thursday, there
is some potential for gales to occur in the far eastern mid-
Atlantic offshore zones especially in and near ANZ905 and ANZ910.
The 00UTC ECMWF is faster with low pressure and just east of the
offshore zones with any concerns for gales on Thursday, while the
GFS is slower with winds around 30 knots directly in ANZ910 by
Wednesday evening. The UKMET is closer to the ECMWF currently,
including with a sharper upper trough as compared to the GFS.
925mb winds on the GFS increase to around 45 knots in ANZ905 and
ANZ910 by Wednesday evening, though the increase in winds on the
ECMWF at the same level is just east of the offshore waters for
early Thursday. Without really strong consensus, there are no
plans for changes to the current forecast for the middle of next
week for this update, but will certainly review the later model
guidance for any potential changes to that mid-week forecast.


As noted below, surge guidance would indicate levels around a
half-foot particularly off of the New Jersey and Delaware coasts
Sunday night into Monday. The ETSS is just a touch below values
of the ESTOFS, but they are very close. Keep abreast of the
latest forecasts from local National Weather Service coastal
offices into early next week.

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PREVIOUS DISCUSSION...

Over the short term, a slow-moving warm front extending from the
Gulf of Maine southeastward toward Georges Bank is still expect
to lift north of the waters today as a cold front approaches from
the northwest. The cold front will cross New England waters this
afternoon into tonight, then stall over the northern Mid-Atlantic
offshore waters Sunday, with a few weak waves of low pressure
tracking east and northeast along the stalled front. While no
hazard-level winds outside of thunderstorms can be expected into
Sunday over the waters, a shift to moderate to gusty north and
then northeast winds is still likely in wake of the front. An
ASCAT pass from last evening around 0246Z indicated southerly
winds up to 20 kt occurring off the southeast U.S. coast. The
latest GOES-IR satellite imagery and lightning density data
indicates the stronger thunderstorms currently near and east of
Long Island and near the Jersey Shore, with a few thunderstorms
also over the Gulf of Maine and well off the mid-Atlantic coast
near 36-37N and around 70W. SREF thunderstorm guidance indicates
thunderstorms increasing over many portions of the offshore
waters today into Sunday in advance of the cold front. All
mariners should use caution as the stronger thunderstorms have
the potential to produce wind gusts exceeding gale force, and
very rough seas today into Sunday. For the morning package, we
will attempt to highlight these stronger winds in the zones most
likely impacted by the stronger thunderstorms today. We will
populate winds with the 00Z Warw model guidance and cap winds at
25 kt today into Sunday over the waters. This will result in
little change from the past few OPC forecasts for the early
morning package, with confidence levels above average with very
good 00Z model agreement noted over the region through Sunday.

For Sunday night into Tuesday over the waters, the 00Z models
remain in very good agreement, with the 00Z GFS and 00Z ECMWF
remaining in excellent agreement across the region. We will
populate grids for the early morning package with a 50/50 blend
of the 00Z GFS first sigma layer winds and 00Z ECMWF winds for
Sunday into into Tuesday. This will maintain forecast continuity
over the waters, with the stalled front over the Mid-Atlantic
waters slowly weakening by Monday night and Tuesday as low
pressure which formed along the front tracks off to the east of
the offshore waters, and winds slowly diminishing across the
region. The main concern will continue to be the locally very
gusty winds and very rough seas in and near thunderstorms over
the region, with the best chance for stronger thunderstorms near
the stalled front over the Mid-Atlantic waters Sunday night into
Tuesday. For Tuesday night into Wednesday night over the region
we will trend the forecast more toward the 00Z ECMWF as it
appears be to just about in the middle of the 00Z guidance
envelope, with the 00Z GFS and 00Z Canadian models slower and the
00Z UKMET and NAVGEM models faster regarding the approach and
then passage of the next cold front over the region Wednesday
and Wednesday night. Again, no hazard-level winds appear likely
over the region for most of next week, except in and near
stronger thunderstorms. Confidence levels remain above average
over the region through Tuesday, and then become near average
thereafter as models agreement diminishes somewhat by the middle
of next week.

Seas...Once again early this morning the 00Z ECMWF WAM appears to
be verifying a little better over the region than the 00Z Wave-
watch model output, based on the latest observations and
altimeter data, with sea heights up to 6 feet noted off the
southeast U.S. coast and 4 feet over the Gulf of Maine. For the
early morning package, in order to better match the wind
forecast, we will use a 90 percent WAM, 10 percent Wavewatch
blend for today into tonight, use a 50/50 blend of these two wave
models for Sunday into Tuesday, and then something closer to the
00Z WAM guidance for Tue night into Wed night over the region.
We will adjust sea height grids slightly prior to forecast
issuance to fit initial conditions, and then in deference to
nearby coastal WFO and TAFB grids over the next few days.

Extratropical Storm Surge Guidance: No significant storm surge
events are expected through at least early next week. However,
late in the weekend and into next week, the potential exists for
a minor surge event along the southern New England and Mid
Atlantic coasts as onshore E-NE flow persists poleward of a
stalled front and developing low pressure. The 00Z ESTOFS is
still slightly higher than the 00Z ETSS, with both models
showing surge values less than 1 ft. With our forecast thinking
that the northeast winds may be slightly higher than the GFS
indicates, it would follow that the ETSS/ESTOFS are likely
slightly low with the surge. Please monitor the latest forecasts
from local National Weather Service coastal offices into early
next week.


.WARNINGS...Preliminary.

.NT1 New England Waters...
None.

.NT2 Mid-Atlantic Waters...
None.

$$

.Forecaster Figurskey/Mills. Ocean Prediction Center.