North Atlantic Marine Weather Discussion

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AGNT40 KWNM 180136
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
936 PM EDT Fri Aug 17 2018

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

The 00Z OPC-NCEP preliminary surface analysis shows a warm front
extending from coastal Maine southeastward to Georges Bank, with
a high pressure ridge roughly along 30N, and another high just 
SE of Nova Scotia. The latest GOES-IR satellite imagery, coastal
radar imagery, and lightning density data indicate only a few 
isolated showers and thunderstorms over the offshore waters 
early tonight, with strong thunderstorms just inland from the 
coast from northern New Jersey north and northeastward to far SW
Maine, with additional strong thunderstorms over eastern South 
Carolina and eastern Georgia. SREF thunderstorm guidance shows 
thunderstorms increasing later tonight and Sat mainly ahead of a
cold front moving E and SE over New England and the northern 
Mid-Atlantic offshore waters. Caution is advised for local wind 
gusts exceeding gale force and very rough seas in or near the 
stronger thunderstorms. For the evening update, no major changes
appear needed to the ongoing forecast. We will make a few minor 
adjustments to fit the forecast to initial conditions and over 
the far western and southern portions of the region in deference
to nearby coastal WFO and TAFB grids and forecasts.

Seas...Sea heights appear to be running up to a foot or two
higher over SW NT1 and far NW NT2 waters near Long Island and 
the Jersey Shore according to the latest observations. We will 
populate grids with the 12Z ECMWF WAM for tonight as it appears 
to have initialized these higher seas a little better. According 
to the 00Z RA1 OPC sea state analysis sea heights ranged from 4 
feet or so near and S of Long Island to 2 feet over the 
remainder of the New England and northern NT1 waters, to 3 to 4 
feet or so off the SE U.S. coast. Overall, no major changes to 
the ongoing OPC forecast appear necessary for the evening 
update. We will adjust grids slightly over the next few days in 
deference to nearby coastal WFO and TAFB grids.

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PREVIOUS DISCUSSION... 

This mornings Ascat overpasses which only covered the offshore 
waters east of about 70W, and west of 77W indicated winds 15 kt 
or less. Scattered thunderstorms, mainly multicell systems, 
continue to impact the southern NT2 zones south of about 
Hatteras Canyon, and at 18Z were generally located within 30 nm 
or so either side of a line from 31.5N 79W to 33N to 33N 74.5W 
to 35.5N 72W. Many of these systems have become outflow dominant 
and overall should diminish in coverage/intensity through the 
overnight period, which is consistent with the latest HREF and 
SREF based guidance. This lull in thunderstorm activity should 
be relatively short lived tonight, as convection should become 
more widespread early Sat into early next week as a cold front 
drops south across the New England and then stalls across the 
northern and central Mid-Atlantic waters. Based on the trends 
seen in the latest GOES-16 imagery and lightning density 
products, we will likely not be including any specific wind 
values in and near thunderstorms in the text forecasts this 
afternoon. Nevertheless, mariners should continue to exercise 
caution within and near these thunderstorms as stronger cells 
could produce locally strong to gale force winds and rough seas.

At 18Z the front across the New England waters has not yet 
started to lift north as a warm front, but should do so tonight. 
Winds could reach 20 kt in some limited areas near the front 
tonight. The 12Z models have then come into somewhat better 
agreement with the cold front expected to move south across the 
waters late Sat into early Mon and then the developing surface 
low along the stalled front across the NT2 waters Sun/Mon. 
However, there are still rather large differences with where 
exactly the front will stall and low pressure will develop, and 
also how quickly the low will move northeast across the waters.  
The 12Z ECMWF continues to be more amplified compared to the 
remaining global models. To account for some of these 
differences we plan to use a blend of the 12Z GFS and 12Z ECMWF 
for the wind grids Sun night through Mon. Then we will nudge the 
northeast winds slightly higher generally to include some winds 
to 25 kt across the New England waters Sun through Mon. Then for 
Tue through Wed, the 12Z models are fairly consistent that a 
deep upper trough will approach the east coast and support a 
cold front forecast to move offshore Wed and Wed night. The 12Z 
ECMWF, 12Z UKMET, and 12Z GFS are all in good agreement with the 
timing of the front. The GFS appears somewhat overdone with the 
winds east of the front, even showing marginal gales developing 
over the northern outer NT2 waters. We continued to favor a 
GFS/ECMWF blend for the wind grids through Wed night. We limited 
the pre- frontal winds to 25 kt Wed/Wed night.  

.Seas: The 12Z Wavewatch and 12Z ECMWF WAM are in very good 
agreement through early next week. At the end of the forecast 
period with the GFS much stronger with the southwest winds, the 
Wavewatch is about double the wave heights seen in the ECMWF 
WAM. We used the Wavewatch through Tue, but boosted guidance by 
10 to 15 percent across the northern areas where the northeast 
winds were also boosted. By Tue night, we transitioned to using 
a 75 ECMWF WAM/25 WW3 blend.   

Extratropical Storm Surge Guidance: No significant storm surge 
events are expected through early Sun. 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 12Z ESTOFS is slightly higher 
than the 12Z 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. 

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Mills/Clark. Ocean Prediction Center.