000
AGNT40 KWNM 251430
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
1030 AM EDT Sun Aug 25 2019

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

High pressure remains dominant over the New England waters this
morning while a stationary front extended NE from the southern GA
coast through the central mid Atlc offshore waters. Low pressure
was near 31N 77W with the potential to become a tropical or sub-
tropical cyclone as it moves NE over the southern outer waters
today into tonight. NHC currently has about a 60 percent chance
of developing into tropical or subtropical system within the next
48 hours. Model guidance tracks the low pressure NE over the
outer mid Atlc waters through Wed night, passing near Georges
Bank then E of the New England waters. Elsewhere low pressure was
E of the outer N mid Atlc waters along the front. The GFS is
noticeably stronger with the development of the low pressure that
moves N tonight and Mon staying E of the northern offshore
areas.

Seas...The highest seas were in the inner waters of the central
mid Atlc waters with buoy observations to 9 ft. The seas in the
NE flow are higher than guidance by about 2 to 3 ft. Buoy
observations across the southern New England offshore waters are
clearly showing showing higher values than either the WWIII WNA
and the ECMWF wam. GFE gridded values are in close agreement to
observed values and will take a closer look this afternoon on
whether or not to go any higher with seas.

For the morning update will not be making any changes for
now with prior grids looking good overall with the complicated
synoptic weather pattern affecting the offshore waters.





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PREVIOUS DISCUSSION...

A tropical or sub-tropical cyclone may develop and impact the
offshore waters off the SE and potentially Mid-Atlantic coasts
by later today or tonight, potentially into the middle of the
upcoming week. The 2 AM Tropical Weather Outlook from the
National Hurricane Center (NHC) shows that there is now a 60
percent chance of tropical or sub-tropical cyclone formation
within the next 48 hours, and a 80 percent chance through day 5,
with the system expected to impact the offshore waters beginning
later today or tonight as it tracks N and NE from central
Florida early this morning, into the southern NT2 waters by
later today or tonight. The potential then still exists for at
least some additional development and intensification through
early to middle parts of the week over especially the eastern
portion of the offshore waters. All marine interests should
continue to closely monitor the latest information from NHC, the
latest forecasts and discussions from OPC, and the coastal
National Weather Service Offices.

Over the short term, the 00Z models appear to be in rather good
agreement over the waters, and we will populate grids with a
50-50 blend of the 00Z GFS first sigma layer winds and 12Z ECMWF
winds today into Mon, and boost winds up to 10 percent or so
over the W and N walls of the gulf stream, in the NE
counterflow, just W of a stationary front that extends from the
NE coast of Georgia NE over the central NT2 waters toward 39N
65W early this morning. This will result in NE winds increasing
to 30 kt or so over portions of the central and SW NT2 waters
this afternoon and tonight. An ASCAT pass from last evening,
around 0140Z, already indicated NE winds to 25 kt or so
occurring N of the front and close to the N and W walls of the
gulf stream from 38N 68W toward the Virginia Capes and southward
off the N. Carolina coast. These winds will likely increase today
into tonight as the pressure gradient strengthens between high
pressure N of the waters and the developing system S of the
waters as well as other low pressure centers passing E and NE
along the front. The NE counterflow may relax somewhat Mon as the
high weakens, a stronger low develops and passes E of the NT1
waters, and the tropical to sub-tropical system lifts NE over
the SE part of the offshore waters. GOES-IR satellite imagery
and lightning density data from early this morning shows
scattered to numerous showers and thunderstorms persisting near
the front, and points S. Caution is advised for winds possibly
near or exceeding gale force and for very rough seas in and near
any of the stronger thunderstorms that impact mainly southern,
central and NE NT2 waters today, with additional activity likely
throughout the early and middle parts of the week. Forecast
confidence levels are now slightly above average at least in the
near term, as the 00Z models have come into slightly better
overall agreement verses 24 hours ago.

Over the medium range, Mon night through the middle parts of
the week, the forecast will continue to be highly dependent upon
the future track and intensity of the tropical or sub-tropical
system likely tracking NE over the eastern part of the NT2
waters. For the early morning package, the 00Z models have come
into somewhat better agreement Mon night into Wed over the
waters, and we will populate grids with something close to the
00Z ECMWF solution for this period, with the ECMWF in generally
good agreement with the 00Z GFS and 00Z UKMET during this time
period. Overall, the models have all trended somewhat weaker with
the possible tropical or sub-tropical system as it lifts slowly N
and NE over or just E of the NT2 offshore waters. We will,
however, continue to enhance winds up 10 percent or so closer to
the potential tropical or sub-tropical system during this time
period as well to keep winds at similar levels that were
forecast previously up to 25 to 30 kt or so, but below hazard
level at this time. For Wed night through Thu night, we will
then trend the grids more toward the 00Z GFS as the ECMWF
becomes a faster outlier in taking the potential tropical system
quicker to the NE away from the offshore waters during this time
frame, with nearly all of the remaining guidance including the
UKMET, Canadian, and NAVGEM all slower than both the fast ECMWF
and slightly slower GFS solutions. As a result, forecast
confidence quickly falls to below average confidence levels
during the medium range, and all marine interests should
continue to closely monitor the latest OPC forecast and
additional information from NHC.

.Seas...The 00Z ECMWF WAM has initialized quite well when compared
to the latest observations with seas rising up to 8-9 ft already
in the counterflow just N of the front off the Virginia Capes
and NE N. Carolina coast according to the latest observations.
Therefore, we will populate sea height grids with the 00Z WAM
for today into Wed evening across the waters and add up to 10 to
15 percent in the counterflow near the W wall of the gulf stream
into Mon night, and then further N off the Delmarva and Jersey
shore this afternoon into early Mon, where the guidance is
typically too low with this type of onshore wind. We will also
enhance seas up to 12 ft or slightly higher near the possible
tropical or sub-tropical system as it lifts NE over the eastern
portion of the NT2 waters tonight through Wed night over the
waters. We will then transition more toward the 00Z Wavewatch for
later Wed night into Thu night to better match the wind
forecast, except to continue to boost the model sea heights by a
ft or two closer to the possible tropical or sub-tropical
system. Confidence in the forecast deteriorates with time, and
all marine interests should closely monitor the latest OPC and
coastal WFO forecasts.

.EXTRATROPICAL STORM SURGE GUIDANCE...A positive surge up to 1.0
ft or slightly higher appears likely from around Long Island
southward along most of the E coast to N. Florida by later today
and persisting into Mon and Mon evening, with the 00Z ESTOFS
guidance running about 25-50 percent higher than the ETSS
guidance. At this time, we feel that the higher guidance
indicated by the ESTOFS surge model appears more reasonable
based on the higher onshore winds forecast over western sections
of the NT2 waters through Mon as described above. For the
remainder of the period the surge will become mostly dependent
on any future development and track of the tropical or sub-
tropical cyclone which could impact the waters more directly
during the next 24 hours and beyond, with this surge guidance
and possible watches or warnings likely issued by the Storm
Surge Unit from NHC, if necessary. Please see the latest
guidance and forecasts from your local National Weather Forecast
office and possibly NHC for additional information.


.WARNINGS...Preliminary.

.NT1 New England Waters...
None.

.NT2 Mid-Atlantic Waters...
None.

$$

.Forecaster Rowland/Mills. Ocean Prediction Center.