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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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.
.Nt1 New England waters... None.
.Nt2 mid-Atlantic waters... None.
.Forecaster Mills/Clark. Ocean prediction center.