marine weather discussion for N Atlantic Ocean NWS ocean prediction center Washington DC 253 PM EST Tue Dec 18 2018
.Forecast discussion...Major features/winds/seas/significant .Weather for the North Atlantic ocean W of 50w from 30n to 50n.
The first major low pressure system which is producing gale force winds over most of the New England and outer portion of the northern mid-Atlantic offshore waters late today, is moving NE from the Canadian Maritimes, with the next major low pressure system, which is currently moving E into the Southern Plains states, expected to bring gale and potentially storm force winds to most of the offshore waters by Thu night into Sat as it crosses the region from the south and southwest toward the north and northeast.
Over the short term, little change appears necessary from the past few opc forecasts with very good model agreement noted in the 12z guidance. For the afternoon package for tonight, we will rely on a 90 percent previous forecast, 10 percent high resolution warw blend, with no change to the past hazards. Gale warnings will likely end from W to E over New England and the outer northern mid-Atlantic offshore waters by midnight to 0300 am EST Wed morning. An ascat overpass from around 1437z did show widespread gales occurring from northern nt2 waters N over nt1 waters, with a few wind retrievals to 45 kt noted near the north wall of the Gulf Stream. For Wed and Wed night, we will rely on the 12z warw wind guidance with no hazards likely during this time frame as high pressure moves E and se over the waters with a weak low and frontal system forming along the se U.S. Coast Wed night. Forecast confidence is above average over the short term.
Over the medium range, beginning Thu we will trend the offshore forecast more toward the 12z ECMWF winds, with the 12z ECMWF remaining quite consistent from the 00z cycle which was used for the previous few forecast packages. We still expect widespread gales to develop from S to N over the waters later Thu into Fri and Sat, as low pressure tracks N and NE from the se U.S. To the mid-Atlantic states to New England, with widespread rain and thunderstorm activity occurring over most of the region. One of the main concerns during this time frame will be the potential for storm force winds in the very strong southerly flow ahead of the low and associated cold front crossing the waters. Of note, the 12z GFS 925 mb winds increase up to 75 kt or so ahead of the cold front and low pressure system Fri and Fri night over the waters. The 12z ECMWF winds have remained generally consistent over the past few cycles, except that the placement of the storm force winds appears to be varying with each cycle, with these stronger winds potentially in and near a likely increase in thunderstorm convection near and in advance of a strong upper level shortwave, and surface cold front, crossing the waters. The 12z GFS, UKMET and Canadian guidance also show a similar trend in the sporadic placement of the stronger winds over the waters. For this reason, for the late afternoon package, we will continue to cap winds at 45 kt over the warmer SSTs near and S of the Gulf Stream, or mostly nt2 waters; and 40 kt N of the Gulf Stream, or mostly nt1 waters. This will help to maintain forecast continuity from the past few opc forecasts, with widespread gales likely late Thu into the weekend, with above average confidence levels. Storm force winds still may need to be added to the forecast at some point over the next couple of days, but for now, we are not confident enough to add them to the offshore forecast for the afternoon package due to the patchy placement of these stronger winds by most of the guidance. Please continue to closely monitor the latest opc and coastal WFO marine forecasts over the next few days as this major low pressure system has the potential for widespread impacts on navigation over and near the offshore waters later this week into the weekend. All hazards will then likely end during Sat night as the strong low passes well to the N and NE of the waters and high pressure builds E toward the region. The high will then cross nt2 waters sun into Sun night, with another cold front and weak low pressure area approaching New England waters by later Sun night or Mon.
Seas...according to the 18z ra1 opc sea state analysis sea heights range from near 21 ft over NE nt2 waters, well off the northern mid-Atlantic coast, to 3 ft just off the se U.S. Coast. To match the wind forecast as described above, over the short term, tonight into Wed night we will use a 90 percent previous sea heights grids, 10 percent 12z ECMWF wam blend, and then transition more toward the 12z wam guidance for Thu through the weekend.
.Extratropical storm surge guidance...The 12z surge guidance continues to indicate no significant positive surge amounts over the next few days, but then starts indicating an increase along the se coast later Thu into Thu evening in the strong southerly flow ahead of the next significant low pressure and frontal system expected to impact the region. A positive surge then expands N along most of the East Coast of the U.S. Thu night into Fri night, with the positive surge subsiding Sat as winds become offshore over all areas. The estofs values of up to 1 to 1.5 ft appear more reasonable at this time as the GFS and ECMWF have come into better agreement regarding the timing and strength of the low pressure system expected to impact the waters later in the week into the weekend. As the late week event is still a few days away we urge all interests to closely monitor the latest forecasts and advisories issued by local National Weather Service forecast offices for details.
.Nt1 New England waters... .anz800...Gulf of Maine... gale today into tonight. Gale possible Friday night. .Anz805...Georges Bank west of 68w... gale today into tonight. Gale possible Friday night. .Anz900...Georges Bank east of 68w... gale today into tonight. Gale possible Friday night into Saturday. .Anz810...South of New England... gale possible Friday into Friday night. .Anz815...South of Long Island... gale possible Friday into Friday night.
.Nt2 mid-Atlantic waters... .anz820...Hudson Canyon to Baltimore Canyon... gale possible Friday. .Anz915...Hudson Canyon to the great South Channel... gale today. Gale possible Friday into Friday night. .Anz920...Baltimore Canyon to the great South Channel... gale today. Gale possible Friday into Saturday. .Anz905...The great South Channel to the Hague line... gale today into tonight. Gale possible Friday night into Saturday. .Anz910...East of the great South Channel and south of 39n... gale today into tonight. Gale possible Friday into Saturday. .Anz825...Inner waters from Baltimore Canyon to Cape Charles Light... gale possible Friday. .Anz828...Inner waters from Cape Charles Light to Currituck Beach Light... gale possible Thursday night into Friday. .Anz925...Outer waters from Baltimore Canyon to Hatteras Canyon... gale possible Thursday night into Friday night. .Anz830...Inner waters from Currituck Beach Light to Cape Hatteras... gale possible Thursday night into Friday night. .Anz833...Inner waters from Cape Hatteras to Cape Fear... gale possible Thursday night into Friday night. .Anz930...Outer waters from Hatteras Canyon to Cape Fear... gale possible Thursday night into Saturday. .Anz835...Inner waters from Cape Fear to 31n... gale Thursday. Gale possible Thursday night into Friday night. .Anz935...Outer waters from Cape Fear to 31n... gale possible Thursday night into Friday night.
.Forecaster Mills. Ocean prediction center.